implementing the mceliece

Code-basedCryptography:Implementing theMcEliece Scheme on Recon�gurableHardwareStefan Heyse

May 31, 2009

Diploma Thesis

Ruhr-University Bochum

Faculty of Electrical Engineering and Information

Technology

Chair for Embedded Security

Prof. Dr.-Ing. Christof Paar

Dr.-Ing. Tim Güneysu

Gesetzt am October 14, 2009 um 12:09 Uhr.

IIIAbstractMost advanced security systems rely on public-key schemes based either on the

factorization or discrete logarithm problem. Since both problems are known to beclosely related, a major breakthrough in cryptanalysis tackling one of those prob-

lems could render a large set of cryptosystems completely useless. The McEliecepublic-key scheme is based on the alternative security assumption that decodingunknown linear, binary codes is NP-complete. In this work, we investigate the effi-

cient implementation of the McEliece scheme on reconfigurable hardware what was– up to date – considered a challenge due to the required storage of its large keys. To

the best of our knowledge, this is the first time that the McEliece encryption schemeis implemented on a Xilinx Spartan-3 FPGA.ZusammenfassungDie meisten modernen Sicherheitssysteme basieren auf Public-Key-Verfahren dieentweder auf dem Problem der Faktorisierung oder dem diskreten Logarithmuspro-

blem beruhen. Da die beiden Probleme dafür bekannt sind, eng miteinander verbun-den zu sein, könnte ein wichtiger Durchbruch in der Kryptoanalyse, der eines dieser

Probleme löst, eine große Anzahl von Kryptosystemen völlig nutzlos machen. DasMcEliece Public-Key-System basiert auf der alternativen Annahme, dass die Deco-dierung unbekannte linearer Codes NP-vollständig ist. In dieser Arbeit untersuchen

wir die effiziente Umsetzung des McEliece Verfahren auf rekonfigurierbarer Hard-ware, was durch die notwendige Speicherung der grossen Schlüssel - bis heute - eine

Herausforderung war. Nach besten Wissen ist dies das erste Mal, dass das McElieceVerfahren auf einem Xilinx Spartan-3 FPGA implementiert wird.

V

FOR MANDY AND EMELIE

VI

Erklärung/Statement

Hiermit versichere ich, dass ich meine Diplomarbeit selbst verfaßt und keine an-

deren als die angegebenen Quellen und Hilfsmittel benutzt sowie Zitate kenntlichgemacht habe.

I hereby declare that the work presented in this thesis is my own work and thatto the best of my knowledge it is original except where indicated by references to

other authors.

Bochum, den October 14, 2009

Stefan Heyse

Contents1. Introduction 31.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2. Existing Implementations . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.4. Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52. The McEliece Crypto System 72.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2. Key Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.4. Decryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.5. Reducing Memory Requirements . . . . . . . . . . . . . . . . . . . . . . 9

2.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.6.1. Weaknesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.6.2. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.7. Side Channel Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123. Introduction to Coding Theory 153.1. Codes over Finite Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2. Goppa Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.3. Parity Check Matrix of Goppa Codes . . . . . . . . . . . . . . . . . . . 163.4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.5. Decoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.6. Solving the Key Equation . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.7. Extracting Roots of the Error Locator Polynomial . . . . . . . . . . . . 204. Recon�gurable Hardware 234.1. Introducing FPGAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.1.1. Interfacing the FPGA . . . . . . . . . . . . . . . . . . . . . . . . . 244.1.2. Buffers in Dedicated Hardware . . . . . . . . . . . . . . . . . . . 244.1.3. Secure Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.2. Field Arithmetic in Hardware . . . . . . . . . . . . . . . . . . . . . . . . 265. Designing for Area-Time-E�ciency 295.1. Extended Euclidean Algorithm . . . . . . . . . . . . . . . . . . . . . . . 29

5.1.1. Multiplication Component . . . . . . . . . . . . . . . . . . . . . 31

Contents 1

5.1.2. Component for Degree Extraction . . . . . . . . . . . . . . . . . 315.1.3. Component for Coefficient Division . . . . . . . . . . . . . . . . 32

5.1.4. Component for Polynomial Multiplication and Shifting . . . . 335.2. Computation of Polynomial Square Roots . . . . . . . . . . . . . . . . . 345.3. Computation of Polynomial Squares . . . . . . . . . . . . . . . . . . . . 356. Implementation 376.1. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.1.1. Reading the Public Key . . . . . . . . . . . . . . . . . . . . . . . 38

6.1.2. Encrypting a message . . . . . . . . . . . . . . . . . . . . . . . . 396.2. Decryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

6.2.1. Inverting the Permutation P . . . . . . . . . . . . . . . . . . . . . 426.3. Decoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6.3.1. Computing the Square Root of T(z)+z . . . . . . . . . . . . . . . 45

6.3.2. Solving the Key Equation . . . . . . . . . . . . . . . . . . . . . . 466.3.3. Computing the Error Locator Polynomial Sigma . . . . . . . . . 46

6.3.4. Searching Roots . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.4. Reverting the Substitution S . . . . . . . . . . . . . . . . . . . . . . . . . 487. Results 518. Discussion 558.1. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

8.2. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568.3. Outlook for Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . 56A. Tables 57B. Magma Functions 59C. VHDL Code Snippets 65D. Bibliography 69E. List of Figures 73F. List of Tables 75G. Listings 77H. List of Algorithms 79

1. Introduction1.1. MotivationThe advanced properties of public-key cryptosystems are required for many crypto-graphic issues, such as key establishment between parties and digital signatures. In

this context, RSA, ElGamal, and later ECC have evolved as most popular choices andbuild the foundation for virtually all practical security protocols and implementa-tions with requirements for public-key cryptography. However, these cryptosystems

rely on two primitive security assumptions, namely the factoring problem (FP) andthe discrete logarithm problem (DLP), which are also known to be closely related.

With a significant breakthrough in cryptanalysis or a major improvement of thebest known attacks on these problems (i.e., the Number Field Sieve or Index Calculus),a large number of recently employed cryptosystems may turn out to be insecure

overnight. Already the existence of a quantum computer that can provide compu-tations on a few thousand qubits would render FP and DLP-based cryptography

useless. Though quantum computers of that dimension have not been reported tobe built yet, we already want to encourage a larger diversification of cryptographicprimitives in future public-key systems. However, to be accepted as real alternatives

to conventional systems like RSA and ECC, such security primitives need to supportefficient implementations with a comparable level of security on recent computing

platforms. For example, one promising alternative are public-key schemes basedon Multivariate Quadratic (MQ) polynomials for which hardware implementationswere proposed on CHES 2008 [9]. In this work, we demonstrate the efficient im-

plementation of another public-key cryptosystem proposed by Robert J. McEliece in1978 that is based on coding theory [35]. The McEliece cryptosystem incorporates

a linear error-correcting code (namely a Goppa code) which is hidden as a gen-eral linear code. For Goppa codes, fast decoding algorithms exist when the code isknown, but decoding codewords without knowledge of the coding scheme is proven

NP-complete [3]. Contrary to DLP and FP-based systems, this makes this schemealso suitable for post-quantum era since it will remain unbroken when appropriately

chosen security parameters are used [6].

The vast majority1 of today’s computing platforms are embedded systems. Onlya few years ago, most of these devices could only provide a few hundred bytes

1Already in 2001, 98% of the microprocessors in world-wide production were assembled in embed-ded platforms.

4 Chapter 1 - Introduction

of RAM and ROM which was a tight restriction for application (and security) de-signers. Thus, the McEliece scheme was regarded impracticable on such small and

embedded systems due to the large size of the private and public keys.For examplefor a 80 bit security level, public key is 437.75 Kbyte and secret key is 377 Kbyte large.But nowadays, recent families of microcontrollers provide several hundreds of bytes

of Flash-ROM. Moreover, recent off-the-shelf hardware such as FPGAs also containdedicated memory blocks and Flash memories that support on-chip storage of up to

a few megabits of data. In particular, these memories could be used, e.g., to storethe keys of the McEliece cryptosystem.

While a microcontroller implementation already exist [16] this work present thefirst implementation of the McEliece cryptosystem on a Xilinx Spartan-3AN 1400

FPGA which is a suitable for many embedded system applications. To the bestof our knowledge, no other implementations for the McEliece scheme have been

proposed targeting embedded platforms. Fundamental operations for McEliece arebased on encoding and decoding binary linear codes in binary extension fields that,in particular, can be implemented very efficiently in dedicated hardware. Unlike FP

and DLP-based cryptosystems, operations on binary codes do not require compu-tationally expensive multi-precision integer arithmetic what is beneficial for small

computing platforms.

On quantum computers algorithms with a polynomial complexity exist whichbreak the security assumption in both cases [41]. McEliece is based on a proven

NP-complete problem and is therefore not effected by the computational power ofquantum computers. This is the primary reason why McEliece is an interesting

candidate for post quantum cryptography.

Finally, the rising market of pervasive computing, in which mobile phones, cars

and even white goods communicate with each other create a new kind of cryptog-raphy: lightweight cryptography. In the context of lightweight cryptography it is

important for crypto systems to perform fast on systems with limited memory, CPUresources and battery capacity. Therefore many symmetric and asymmetric algo-rithms like AES, DES, RSA and ECC were ported to microcontrollers and even new

algorithms like Present [10] were developed especially for embedded systems.

For these reasons it is particularly interesting how McEliece performs on embed-ded systems in comparison to commonly used public key schemes like RSA and

ECC.

1.2 Existing Implementations 51.2. Existing ImplementationsThere exist only a few McEliece software implementations [39, 40] for 32 bit archi-

tectures. Implementation [39] is written in pure i386 assembler. It encrypts with 6KBits/s and decrypts with 1.7 Kbits/s on a 16 MHz i386-CPU. A C-implementation

for 32-bit architectures exists in [40]. The code is neither formatted nor does it con-tain a single line of source code comment. Nevertheless, it was used for the opensource p2p software freenet and entropy [19, 18]. Due to the poor documentation

we are unable to give performance results for the specific code. An FPGA imple-mentation of the original McEliece crypto system appears not to exist, only in [8] a

signature scheme derived from McEliece is implemented on a FPGA.

To the best of our knowledge, this work presents the first implementation of theMcEliece public key scheme for embedded systems.1.3. GoalsGoal of this thesis is a proof-of-concept implementation of McEliece for reconfig-

urable hardware. The primary target is to solve the memory problem for the largepublic and private key data and fit all components into a low cost FPGA of the

Spartan-3 class. Simultaneously we want to achieve a high performance to allowthe McEliece scheme to become a competitor in post quantum crypto systems onembedded systems. For encryption the large public key has to be stored in a fast

accessible area and for decryption a way has to be found to reduce the private keysize, because the target FPGA has only limited memory available.1.4. OutlineFirst, the growing interest in post quantum cryptography is motivated and whyespecially embedded implementations are necessary. Next, an introduction to the

McEliece scheme is given. Subsequently the basic concepts of error correcting codes,especially Goppa codes are presented. Afterwards, an introduction to FPGAs is

given and it is shown how to perform finite field arithmetics with them. Then theencryption implementation is explained with respect to the properties of the targetplatform. Next, the implementation of decryption is described on the second target

platform. Finally, the results are presented and options for further improvement areoutlined.

2. The McEliece Crypto SystemIn this chapter an introduction to McEliece is given and the algorithms to generatethe key pairs and how to encrypt and decrypt a message are presented. After-

wards, currently known weaknesses and actual attacks on the McEliece scheme arepresented.2.1. OverviewThe McEliece crypto system consists of three algorithms which will be explained in

the following sections: A key generation algorithm which produces a public/privatekey pair, an encryption algorithm and a decryption algorithm. The public key is

a hidden generator matrix G of a binary linear code of length n and dimension k

capable of correcting up to t errors. R. McEliece suggested to use classical binarygoppa codes. The generator matrix of this code is hidden using a random k × k

binary non-singular substitution matrix S and a random n× n permutation matrixP. The matrix G = S ⋅ G ⋅ P and the error- correcting capability t forms the public

key. G itself together with matrices S and P form the secret key.

To encrypt a message, it is transformed into a codeword of the underlying code

and an error e with Hamming weight wH(e) ≤ t is added. Without knowledgeof the specific code used, errors can not be corrected and therefore the originalmessage cannot be recovered. The owner of the secret informations can reverse the

transformations of G and use the decoding algorithm of the code to correct the errorsand recover the original message.

c‘ = m ⋅ G = m ⋅ S ⋅ G ⋅ P→ c = c‘ + e = m ⋅ S ⋅ G ⋅ P + e

→ c ⋅ P−1 = m ⋅ S ⋅ G ⋅ P ⋅ P−1 + e ⋅ P−1 = m ⋅ S ⋅ G + e ⋅ P−1(2.1)

The decoding algorithm can still be used to correct the t errors and get m‘ =m ⋅ S because the equation wH(e) = wH(e ⋅ P−1) ≤ t still holds. To finally receivethe original message m a multiplication with S−1 is performed which results in

m‘ ⋅ S−1 = m ⋅ S ⋅ S−1 = m.

8 Chapter 2 - The McEliece Crypto System2.2. Key GenerationBasically the parameters of the McEliece crypto system are the parameters of theGoppa code used. After choosing the underlying Galois field GF(2m)and the errorcorrecting capability of the code t, all other parameters depend on these two values.

The original parameters suggested by McEliece in [35] are m = 10, t = 50, but incitehac, the authors noted that t = 38 is a better choice with regard to the compu-

tational complexity of the algorithm while not reducing the security level. Theseparameters nowadays only give 260 bit security [6] compared to a symmetric ciher.To achieve an appropriate level of 280-bit security at least parameters n = 211 = 2048,

t = 38 and k = n−m ⋅ t = 1751 must be chosen. Then the key generation is:

Algorithm 1 Key Generation Algorithm for McEliece Scheme

Input: Security Parameters m, t

Output: Kpub, Ksec

1: n← 2m, k← n−m ⋅ t2: C← random binary (n, k)-linear code C capable of correcting t errors

3: G← k× n generator matrix for the code C

4: S← random k× k binary non-singular matrix5: P← random n× n permutation matrix

6: G← k× n matrix S× G× P

7: return Public Key(G, t); Private Key(S, G, P).

Note that in Section 2.4 only matrices P−1 and S−1 are used. Here exist the pos-

sibility to precompute these inverse matrices and then the private (decryption) keyis redefined to (S−1, G, P−1). The permutation matrix P is very sparse. In every row

and columns exactly a single one occurs. This fact is used in the implementation tosave space when storing P.2.3. EncryptionSuppose Bob wishes to send a message m to Alice whose public key is (G, t):

McEliece encrypts a k bit message into a n bit ciphertext. With the actual parame-ters, this results in 2048 bit ciphertext for a 1751 bit message. This is an overhead of

about nk = 1.17.

2.4 Decryption 9

Algorithm 2 McEliece Message Encryption

Input: m, Kpub = (G, t)Output: Ciphertext c

1: Encode the message m as a binary string of length k

2: c‘← m ⋅ G3: Generate a random n-bit error vector z containing at most t ones

4: c = c‘ + z

5: return c2.4. DecryptionTo decrypt the ciphertext and receive the message Alice has to perform the following

steps:

Algorithm 3 McEliece Message Decryption

Input: c, Ksec = (P−1, G, S−1)Output: Plaintext m

1: c← c ⋅ P−1

2: Use a decoding algorithm for the code C to decode c to m = m ⋅ S3: m← m ⋅ S−1

4: return m

2.5. Reducing Memory RequirementsTo make McEliece-based cryptosystems more practical (i.e., reduce the key sizes),there is an ongoing research to replace the code with one that can be represented in

a more compact way. Examples for such alternative representations are quasi-cycliccodes [20], or low density parity check codes [37], but have also been broken again []

Using a naive approach, in which the code is constructed from the set of all ele-ments in Fm

2 in lexicographical order and both matrices S, P are totally random, the

public key G = S× G× P becomes a random n × k matrix. However, since P is asparse permutation matrix with only a single 1 in each row and column, it is more

efficient to store only the positions of the 1’s, resulting in an array with n ⋅m bits.

Another trick to reduce the public key size is to convert G to systematic form{Ik ∣ Q}, where Ik is the k× k identity matrix. Then, only the (k × (n− k)) matrix

Q is published [17]. But to achieve the same security level the message has to be

10 Chapter 2 - The McEliece Crypto System

converted and additional operations [17] has to be computed to avoid weaknessesdue to the know structure of Q [17].

In the last step of code decoding (Algorithm 4), the k message bits out of the n

(corrected) ciphertext bits need to be extracted. Usually, this is done by a mapping

matrix iG with G × iG = Ik. But if G is in systematic form, then this step can beomitted, since the first k bits of the corrected ciphertext corresponds to the messagebits. Unfortunately, G and G cannot both be systematic at the same time, since then

G = {Ik ∣ Q} = S × {Ik ∣ Q} × P and S would be the identity matrix which isinappropriate for use as the secret key.

For reduction of the secret key size, we chose to generate the large scrambling ma-trix S−1 on-the-fly using a cryptographic pseudo random number generator (CPRNG)and a seed. During key generation, it must be ensured that the seed does not gen-

erate a singular matrix S−1. A random binary matrix is with a probability of about33% invertible (value is empiric, not proven). Depending on the target platform and

available cryptographic accelerators, there are different options to implement such aCPRNG (e.g. AES in counter mode or a hash-based PRNG) on embedded platforms.However, to prevent the security margin to be reduced, the complexity to attack the

sequence of the CPRNG should not be significantly lower than to break the consid-ered McEliece system with a static scrambling matrix S. However, the secrecy of

S−1 is not required for hiding the secret polynomial G(z) [17]. The secret matrix Sindeed has no cryptographic function in hiding the secret goppa polynomial g(z).Today, there is no way to recover H with the knowledge of only G ⋅ P.2.6. SecurityIn the past, many researchers attempted to break the McEliece scheme [33, 32, 2] butnone of them was successful for the general case.2.6.1. WeaknessesAt Crypto’97, Berson [7] shows that McEliece has two weaknesses. It fails when

encrypting the same message twice and when encrypting a message that has aknown relation to another message. In the first case assume that c1 = m ⋅ G + e1

and c2 = m ⋅ G + e2 with e1 ∕= e2 are send which leads to c1 + c2 = e1 + e2.

Now compute two sets L0, L1 where L0 contains all positions l where c1 + c2 iszero and L1 contains the positions l where c1 + c2 is one. Due to that two errors e1, e2

are chosen independently and the transmitted messages are identical, it follows that

2.6 Security 11

for a position l ∈ L0 most probably neither c1(l) nor c2(l) is modified by an error,while for a position l ∈ L1 certainly one out of c1(l) or c2(l) is modified by an error.

For the parameters suggested by McEliece n = 1024, k = 524, t = 50 the probabilityfor a l ∈ L0 of e1(l) = e2(l) = 1 is about 0.0024 (or the other way round) for mostl ∈ L0 mainly e1(l) = e2(l) = 0. The probability pi that exactly i positions are for the

same time changed by e1 and e2 is:

pi = Pr(∣{l : e1(l) = 1} ∩ {l : e2(l) = 1}∣ = i) =( 50

1025)(974

50−i)

(102450 )

(2.2)

Therefore, the cardinality of L1 is:

E(∣L1 ∣) =50

∑i=0

(100− 2 ⋅ i) ⋅ pi ≈ 95.1 (2.3)

For example, with the cardinality ∣L1∣ = 94 it follows that ∣L0∣ = 930 and only3 entries of L0 are affected by an error. The probability to select 524 unmodified

positions from L0 is

(927525)

(930524)≈ 0.0828 (2.4)

Therefore after about 12 trials an unmodified codeword is selected which can bedecoded with the help of the public generator matrix.

In the second case where the two messages have a known linear relation the sumof the two ciphertexts becomes:

c1 + c2 = m1 ⋅ G + m2 ⋅ G + e1 + e2 (2.5)

Due to the known relation (m1 + m2) ⋅ G can be computed and subsequently c1 +c2 + (m1 +m2) ⋅ G = e1 + e2. Now, proceed like in the first case, using c1 + c2 + (m1 +m2) ⋅ G = e1 + e2 instead of c1 + c2. Remark that this attack does not reveal the secretkey.

There are several ways to make McEliece resistant against those weaknesses. Mostof them scramble or randomize the messages to destroy any relationship between

two dependent messages[17].

12 Chapter 2 - The McEliece Crypto System2.6.2. AttacksAccording to [17], there is no simple rule for choosing t with respect to n. Oneshould try to make an attack as difficult as possible using the best known attacks.

A recent paper [5] from Bernstein, Lange and Peters introduces an improved at-tack of McEliece with support of Bernsteins list decoding algorithm [4] for binary

Goppa codes. List decoding can approximately correct n −√

n ⋅ (n− 2t− 2 errorsin a length-n classical irreducible degree-t binary Goppa code, while the so far thebest known algorithm from Patterson [38] which can correct up to t errors. This

attack reduces the binary work factor to break the original McEliece scheme with a(1024, 524) Goppa code and t = 50 to 260.55 bit operations. Table 2.1 summarizes the

parameters suggested by [5] for specific security levels:

Table 2.1.: Security of McEliece Depending on Parameters

Security Level Parameters Size Kpub Size Ksec

(n, k, t), errors added in KBits (G(z), P, S) in KBits

Short-term (60 bit) (1024, 644, 38), 38 644 (0.38, 10, 405)

Mid-term (80 bit) (2048, 1751, 27), 27 3, 502 (0.30, 22, 2994)

Long-term (256 bit) (6624, 5129, 115), 117 33, 178 (1.47, 104, 25690)

For keys limited to 216, 217, 218, 219, 220 bytes, the authors propose Goppa codes

of lengths 1744, 2480, 3408, 4624, 6960 and degrees 35, 45, 67, 95, 119 respectively, with36, 46, 68, 97, 121 errors added by the sender. These codes achieve security levels84.88, 107.41, 147.94191.18, 266.94 against the attack described by the researchers.2.7. Side Channel AttacksThe susceptibility of the McEliece cryptosystem to side channel attacks has not exten-

sively been studied, yet. This is probably due to the low number of practical systemsemploying the McEliece cryptosystem. However, embedded systems can always be

subject to passive attacks such as timing analysis [30] and power/EM analysis [34].In [42], a successful timing attack on the Patterson algorithm was demonstrated. Theattack does not recover the key, but reveals the error vector z and hence allows for

efficient decryption of the message c. This implementations is not be susceptible tothis attack due to unconditional instruction execution, e.g., the implementation will

not terminate after a certain number of errors have been corrected.

2.7 Side Channel Attacks 13

For a key recovery attack on McEliece, the adversary needs to recover the secretsubstitution and permutation matrices S and P, and the Goppa code itself, either

represented by G or by the Goppa polynomial g(z) and the support L. A feasibleSPA is possible to recover c. For each bit set in c, our implementations perform oneiteration of the euclidean algorithm (EEA). Assuming the execution of EEA to be

visible in the power trace, c can easily be recovered. The attacker is able to recoverthe whole whole permutation matrix P with less than 100 chosen ciphertexts. This

powerful attack can easily be prevented by processing the bits of c in a random order.Without recovering P, power attacks on inner operations of McEliece are aggravated

due to an unknown input. Classical timing analysis, as described in [30] seemsmore realistic, as many parts of the algorithm such as EEA,Permutation and moduloreduction exhibit a data-dependent runtime. Yet, no effective timing attack has been

reported so far, and simple countermeasures are available [31].

An attacker knowing P could recover the generator polynomial G(z) and therebybreak the implementation [17]. As a countermeasure, we will randomize the execu-

tion of step 1 of Algorithm 4 due to a possible SPA recovering c, and consequentlyP−1. Differential EM/power attacks and timing attacks are impeded by the permuta-

tion and scrambling operations (P and S) obfuscating all internal states, and finally,the large key size. Yet template-like attacks [12] might be feasible if no further pro-tection is applied.

3. Introduction to Coding TheoryFor this section it is assumed that the reader is familiar with the theory of finite

fields and algebraic coding theory. A good introduction to finite fields for engineerscan be found in [36]. The following definitions are from [45] and define the crucial

building blocks for Goppa codes.3.1. Codes over Finite FieldsBecause the McEliece scheme is based on the topic of some problems in codingtheorie, a short introduction into this field based on [17] is given.

Definition 3.1.1 An (n, k)-code C over a finite Field F is a k-dimensional subvectorspace of

the vector space Fn. We call C an (n, k, d)-code if the minimum distance is d = minx,y∈Cdist(x, y),

where dist denotes a distance function, e.g. Hamming distance. The distance of x ∈ Fn to

the null-vector wt(x) := dist(0, x) is called weight of x.

Definition 3.1.2 The matrix G ∈ Fk×n is a generator matrix for the (n, k)-code C over

F, if the rows of G span C over F. The matrix H ∈ F(n−k)×n is called parity check matrix

for the code C if HT is the right kernel of C. The code generated by H is called dual code of

C and denoted by CT.

By multiplying a message with the generator matrix a codeword is formed, which

contains redundant information. This information can be used to recover the orig-inal message, even if some errors occurred, for example during transmission of thecodeword over an radio channel.3.2. Goppa CodesIntroduced by V.D. Goppa in [21] Goppa codes are a generalization of BCH- and

RS-codes. There exist good decoding algorithms e.g. [38] and [44].

Definition 3.2.1 (Goppa polynomial, Syndrome, binary Goppa Codes). Let m and t be

positive integers and let

g(z) =t

∑i=0

gizi ∈ F2m [z] (3.1)

16 Chapter 3 - Introduction to Coding Theory

be a monic polynomial of degree t called Goppa polynomial and

ℒ = {α0, ⋅ ⋅ ⋅ , αn−1} αi ∈ F2m (3.2)

a tuple of n distinct elements, called support, such that

g(αj) ∕= 0, ∀0 ≤ j ≤ n. (3.3)

For any vector c = (c0, ⋅ ⋅ ⋅ , cn−1) ∈ Fn, define the syndrome of c by

Sc(z) = −n−1

∑i=0

ci

g(αi)

g(z)− g(αi)

z− αimod g(z) (3.4)

The binary Goppa code Γ(ℒ, g(z)) over F2 is the set of all c = (c0, ⋅ ⋅ ⋅ , cn−1) ∈ Fn2

such that the indentity

Sc(z) = 0 (3.5)

holds in the polynomial ring F2m(z) or equivalent

Sc(z) =n−1

∑i=0

ci

z− αi≡ 0 mod g(z) (3.6)

If g(z) is irreducible over F2m then Γ(ℒ, g(z)) is called an irreducible binary Goppa code.3.3. Parity Check Matrix of Goppa CodesRecall equation (3.4). From there it follows that every bit of the received codewordis multiplied with

g(z)− g(αi)

g(αi) ⋅ (z− αi)(3.7)

Describing the Goppa polynomial as gs ⋅ zs + gs−1 ⋅ zs−1 ⋅ . . . ⋅+g0 one can con-struct the parity check matrix H as

H =

⎧

⎨

⎩

gs

g(α0)gs

g(α1)⋅ ⋅ ⋅ gs

g(αn−1)gs−1+gs⋅α0

g(α0)gs−1+gs⋅α0

g(α1)⋅ ⋅ ⋅ gs−1+gs⋅α0

g(αn−1)...

. . ....

g1+g2⋅α0+⋅⋅⋅+gs⋅αs−10

g(α0)g1+g2⋅α0+⋅⋅⋅+gs⋅αs−1

0g(α1)

⋅ ⋅ ⋅ g1+g2⋅α0+⋅⋅⋅+gs⋅αs−10

g(αn−1)

⎫

⎬

⎭

(3.8)

3.4 Encoding 17

This can be simplified to

H =

⎧

⎨

⎩

gs 0 ⋅ ⋅ ⋅ 0

gs−1 gs ⋅ ⋅ ⋅ 0...

. . ....

g1 g2 ⋅ ⋅ ⋅ gs

⎫

⎬

⎭

∗

⎧

⎨

⎩

1g(α0)

1g(α1)

⋅ ⋅ ⋅ 1g(αn−1)

α0g(α0)

α1g(α1)

⋅ ⋅ ⋅ αn−1

g(αn−1)...

. . ....

αs−10

g(α0)α

s−11

g(α1)⋅ ⋅ ⋅ α

s−1n−1

g(αn−1)

⎫

⎬

⎭

(3.9)

where the first part has a determinate unequal Zero and following the second partH is a equivalent parity check matrix, which has a simpler structure. By applyingthe Gaussian algorithm to the second matrix H one can bring it to systematic form

(Ik ∣ H), where Ik is the k× k identity matrix. Note that whenever a column swap isperformed, actual also a swap of the elements in the support ℒ is performed.

From the systematic parity check matrix (Ik ∣ H), now the systematic generatormatrix G can be derived as (In−k ∣ HT).3.4. EncodingTo encode a message m into a codeword c represent the message m as a binary

string of length k and multiply it with the n× k matrix G. If G is in systematic form(Ik ∣ P), where Ik is the k × k identity matrix, one only have to multiply m with

the (n− k) × k matrix P and append the result to m. This trick can not be used inthe context of McEliece without special actions (see section 2.5) because the publicgenerator matrix G is generally not in systematic form, due to multiplication with

two random matrices.3.5. DecodingHowever, decoding such a codeword r on the receiver’s side with a (possibly) ad-ditive error vector e is far more complex. For decoding, we use Patterson’s algo-rithm [38] with improvements from [43].

Since r = c+ e ≡ e mod G(z) holds, the syndrome Syn(z) of a received codeword

can be obtained from Equation (3.6) by

Syn(z) = ∑α∈GF(2m)

rα

z− α≡ ∑

α∈GF(2m)

eα

z− αmod G(z) (3.10)


To finally recover e, we need to solve the key equation σ(z) ⋅ Syn(z) ≡ ω(z)mod G(z), where σ(z) denotes a corresponding error-locator polynomial and ω(z)denotes an error-weight polynomial. Note that it can be shown that ω(z) = σ(z)′ isthe formal derivative of the error locator. By splitting σ(z) into even and odd poly-nomial parts σ(z) = a(z)2 + z ⋅ b(z)2, we finally determine the following equation

which needs to be solved to determine error positions:

Syn(z)(a(z)2 + z ⋅ b(z)2) ≡ b(z)2 mod G(z) (3.11)

To solve Equation (3.11) for a given codeword r, the following steps have to beperformed:

1. From the received codeword r compute the syndrome Syn(z) according to

Equation (3.10). This can also be done using simple table-lookups.

2. Compute an inverse polynomial T(z) with T(z) ⋅ Syn(z) ≡ 1 mod G(z) (orprovide a corresponding table). It follows that (T(z)+ z)b(z)2 ≡ a(z)2 mod G(z).

3. There is a simple case if T(z) = z ⇒ a(z) = 0 s.t. b(z)2 ≡ z ⋅ b(z)2 ⋅ Syn(z)mod G(z) ⇒ 1 ≡ z ⋅ Syn(z) mod G(z) what directly leads to σ(z) = z.Contrary, if T(z) ∕= z, compute a square root R(z) for the given polynomialR(z)2 ≡ T(z) + z mod G(z). Based on a observation by Huber [24] we can

compute the square root R(z) by:

R(z) = T0(z) + w(z) ⋅ T1(z) (3.12)

where T0(z), T1(z) are odd and even part of T(z) + z satisfying T(z) + z =T0(z)

2 + z ⋅ T1(z)2 and w(z)2 = z mod g(z) which can be precomputed for

every given code. We can then determine solutions a(z), b(z) satisfying

a(z) = b(z) ⋅ R(z) mod G(z). (3.13)

with a modified euclidean algorithm(see section 3.6). Finally, we use the identi-

fied a(z), b(z) to construct the error-locator polynomial σ(z) = a(z)2 + z ⋅ b(z)2.

4. The roots of σ(z) denote the positions of error bits(see section 3.7). If σ(αi) ≡ 0

mod G(z) with αi being the corresponding bit of a generator in GF(211), therewas an error in the position i in the received codeword that can be corrected

by bit-flipping.

This decoding process, as required in Step 2 of Algorithm 3 for message decryp-

tion, is finally summarized in Algorithm 4:

3.6 Solving the Key Equation 19

Algorithm 4 Decoding Goppa Codes

Input: Received codeword r with up to t errors, inverse generator matrix iG

Output: Recovered message m

1: Compute syndrome Syn(z) for codeword r

2: T(z) ← Syn(z)−1

3: if T(z) = z then

4: σ(z) ← z

5: else

6: R(z) ←√

T(z) + z

7: Compute a(z) and b(z) with a(z) ≡ b(z) ⋅ R(z) mod G(z)8: σ(z) ← a(z)2 + z ⋅ b(z)2

9: end if

10: Determine roots of σ(z) and correct errors in r which results in r

11: m← r ⋅ iG {Map rcor to m}

12: return m3.6. Solving the Key EquationTo solve this equation a(z) = b(z) ⋅ R(z) mod G(z) with the extended euclideanalgorithm observe the following: From σ(z) = a(z)2 + z ⋅ b(z)2 and deg(σ(z)) ≤deg(G(z)) it follows that deg(a(z)) ≤ deg(G(z))

2 and deg(b(z)) ≤ deg(G(z))−12 . During

the iterations of the extended euclidean algorithm:

ri−2(z) = 1 ⋅ R(z) + 0 ⋅ G(z)

ri−1(z) = 0 ⋅ R(z) + 1 ⋅ G(z)

...

rk = uk(z) ⋅ R(z) + vk(z) ⋅ G(z)

...

ggT(R(z), G(z)) = uL ⋅ R(z) + vL(z) ⋅ G(z)

0 = uL+1(z) ⋅ R(z) + vL+1(z) ⋅ G(z)

the following holds:

ri = ri−2− ai ⋅ ri−1 where ai =ri−2

ri−1(3.14)


and

deg(ri) < deg(ri−1) (3.15)

In addition:

deg(ui(z)) + deg(ri−1(z)) = deg(G(z)) (3.16)

It follows that deg(r(z)) is constantly decreasing after starting with deg(G(z)) anddeg(u(z)) increases after starting with zero. Using this, one can see that there is aunique point in the computation of EEA where both polynomials are just below their

bounds. The EEA can be stopped when for the first time deg(r(z)) <deg(G(z))

2 and

deg(u(z)) <deg(G(z)−1)

2 . These results are the needed polynomials a(z), b(z). Froma(z) and b(z) construct σ(z) = a(z)2 + z ⋅ b(z)2.3.7. Extracting Roots of the Error LocatorPolynomialThe roots of σ(z) indicate the error positions. If σ(αi) ≡ 0 mod G(z)1 there was an

error ei = 1 at ci in the received bit string, which can be corrected by just flippingthe bit. The roots can be found by evaluating σ(ai) for every i. A more sophisticated

method is the Chien search [13]. To get all roots αi of σ(z) the following holds for all

elements except the zero element:

σ(αi) = σs(αi)

s+σs−1(α

i)s−1

+ . . . +σ1(αi) + σ0

∼= λs,i +λs−1,i + . . . +λ1,i + λ0,i

σ(αi+1) = σs(αi+1)

s+σs−1(α

i+1)s−1

+ . . . +σ1(αi+1) + σ0

= σs(αi)

sα

s +σs−1(αi)

s−1α

s−1 + . . . +σ1(αi)α + σ0

= λs,iαs +λs−1,iα

s−1 + . . . +λ1,iα + λ0,i

∼= λs,i+1 +λs−1,i+1 + . . . +λ1,i+1 + λ0,i+1

In other words, one may define σ(αi) as the sum of a set {λj,i∣0 ≤ j ≤ s}, from

which the next set of coefficients may be derived thus:

λj,i+1 = λj,iαj (3.17)

1Note that α was a generator of GF(211)

3.7 Extracting Roots of the Error Locator Polynomial 21

Start at i = 0 with λj,0 = σj and iterate through every value of i up to s. If at anyiteration the sum:

t

∑j=0

λj,i = 0 (3.18)

then σ(αi) = 0 and αi is a root. This method is more efficient than the brute force

like method mentioned before.

If the generator matrix G is in standard form, just take the first k bits out of the

n-bit codeword c and retrieve the original message m. To get a generator matrixin this form, reorder the elements in the support of the code until G is in standardform. Note that in the computation of the syndrome and the error correction, the

i-th element in C and σ(z), respectively, corresponds with the i-th element in thesupport and not with ai. If a matrix not in standard form is used, a mapping matrix

which selects the right k bits from c, has to be found. This matrix iG must be of theform G ⋅ iG = IDk, where IDk is the k× k identity matrix. To get iG do the following:

Algorithm 5 Getting a Mapping Matrix from Codewords to Messages

Input: G

1: Select randomly k columns of G

2: Test if the resulting k× k matrix sub is invertible3: If not goto Step 1

4: Else insert the rows of sub−1 into a k × n matrix at the positions where the k

random columns come from and fill the remaining rows with zeros.5: return iG

For the Magma algorithm computing iG see Appendix B. A multiplication ofa valid codeword c with this matrix iG computes the corresponding message m

because:

c ⋅ iG = (m ⋅ G) ⋅ iG = m ⋅ IDk = m (3.19)

Now all prerequisites to start with implementing the McEliece public key schemeare given.

4. Recon�gurable HardwareThis chapter introduces FPGAs and presents the necessary additional hardware.

Furthermore, field arithmetic in hardware is introduced.4.1. Introducing FPGAsFPGA stands for Field Programmable Gate Arrays. It consists of a large amount ofLook Up Tables that can generate any logic combination with four inputs and one

output and basic storage elements based on Flip Flops.

Figure 4.1.: 4-Input LUT with FF

Two LUT’s and two FF’s are packed together into an slice and four slices into anConfigurable Logic Block. Between the CLBs is a programmable switch matrix thatcan connect the input and outputs of the CLB’s. How LUTs and CLBs are configured

is defined in a vendor specific binary file, the bitstream. Most modern FPGAs alsocontain dedicated hardware like multiplier, clock manager, and configurable block

RAM.

CLB

Blo

ck R

AM

Multip

lier

DCM

IOBs

IOBs

IOB

s

IOB

s

DCM

Blo

ck R

AM

/ M

ultip

lier

DCM

CLBs

IOBs

OBs

DCM

Figure 4.2.: Simplified Overview over an FPGA [27]

The process of building a design for a FPGA consist of several steps which are

depicted in Figure 4.3. After writing the VHDL code in an editor, it is translated

24 Chapter 4 - Reconfigurable Hardware

Figure 4.3.: VHDL Designflow[28]

to a net list. This process is called synthesis and for this implementation the toolsXST and Synplify are chosen from the hundred available. Based on the net list the

correct behavior of the design can be verified by using a simulation tool, which isin our case Modelsim. This both steps are completely hardware independent. The

next step is mapping and translating the net list into logic resources and specialresources offered by the target platform. Due to this hardware dependency, thoseand the following steps need to know the exact target hardware. The final step

place-and-route (PAR) then tries to find an optimum placement for the single logicblocks and connects them over the switching matrix. The output of PAR can now be

converted into a bitstream file and loaded into a flash memory on the FPGA board.The FPGA contains a logic block that can read this flash memory and can configurethe FPGA accordingly. On most FPGA boards this memory is located outside the

FPGA chip and can therefore be accessed by anyone. To protect the content of thebitstream, which may include intellectual property (IP) cores or, like in our case,

secret key material, the bitstream can be stored encrypted. The FPGA boot-up logic

4.1 Introducing FPGAs 25

then has to decrypt the bitstream before configuring the FPGA. Some special FPGAs,for example the Spartan3-AN series, contain large on-die flash memory, which can

only be accessed by opening the chip physically. For the decryption algorithm thebitstream file has to be protected by one of the two methods mentioned above.4.1.1. Interfacing the FPGAAside from the algorithmic part of the design, a way has to be found to get data into

the FPGA, and after computation, to read the data back. For this implementation weremain at a standard UART-interface even though interfaces with higher bandwidth

(Ethernet,USB,PCI-Express) are possible. The used UART is derived from an existingsource for the PicoBlaze published in Xilinx application note 223 [11]. A wrappercomponent was developed, that provide all necessary ports: Input en_16_x_baud

uart

C_uart

rst

rx_in

clk

tx_out

uart_tx

C_uart_tx

uart_rx

C_uart_rx

write_buffer

reset_buffer

en_16_x_baud

clk

[7:0]data_in[7:0]

serial_out

buffer_full

buffer_half_full

serial_in

read_buffer

reset_buffer

en_16_x_baud

clk

buffer_data_present

buffer_full

buffer_half_full

[7:0]data_out[7:0]

clk

reset_rx_buffer

rx_in

tx_out

Figure 4.4.: The UART component

should be pulsed HIGH for one clock cycle duration only and at a rate 16 times (orapproximately 16 times due to oversampling) faster the rate at which the serial datatransmission takes place. The receive and transmit component each contain a 16

byte buffer. Transmission is started as soon as a byte is written to the buffer. Writingto the buffer if it is full has no effect. When the receive buffer is full, all subsequent

write attempts from the host system are ignored, which will cause data loss.

26 Chapter 4 - Reconfigurable Hardware4.1.2. Bu�ers in Dedicated HardwareFor some data a structure is required, that hold some values for later processing.For example ciphertext, plaintext, bytes send and received from UART and alsoprecomputed values have to be stored inside the FPGA. Instead of building these

storage from a huge amount of registers, both target FPGA for this implementationprovide dedicated RAM. This block RAM (BRAM) is organized in blocks of 18Kbit

and can be accessed with the maximum frequency of the FPGA. Each of the BRAMsis true dual ported, allowing independent read and write access at the same time.With a Xilinx vendor tool CoreGen each block can be configured as RAM or ROM

in single or dual port configuration. Also one can select the required width anddepth of the BRAM. CoreGen then generates a VHDL instantiation template that can

be used in the VHDL code. Every type of BRAM can be initialized with predefinedcontent via the CoreGen wizard. It uses a .coe file which can contain the memorycontent in binary, decimal or hex. This is used transfer the precomputed tables and

constant polynomial w into the FPGA. Goppa polynomial G is not stored in a ROM,because we need to access it in its whole width at once.This polynomial is stored as

a large register. Because it is constant and only needed in the extended euclideanalgorithm it will be resolved to fixed connections to VCC and GND by the synthesistool. The Spartan3-200 incorporates 12 BRAMs and the Spartan3-2000 incorporates

40 BRAMs allowing up to 216 or 720 Kbits of data to be stored, respectively.4.1.3. Secure StorageAll data that is required to configure the FPGA and also constant values from VHDLcode, like ROM and RAM initialization values, has to be stored on the FPGA board.

In normal circumstances the target platform contains a flash memory chip whichholds the bitstream file. During boot-up, the FPGA reads this file and configure

himself and initialize the BRAM cells accordingly. But this flash has a standardinterface and can therefore be read by anyone. To protect the private key data, thereexist two ways. The first way, which is only possible at newer FPGAs [47], is to store

the bitstream file encrypted. The FPGA contains a hardware decryption moduleand a user defined secret key. During boot-up, the bitstream file is decrypted inside

the FPGA and then the normal configuration takes place. Our target FPGAs arenot capable of decrypting bitstream files. But the Spartan3-AN family of FPGAscontain a large on-die flash memory. Assuming that physical opening the chip is

hard, this memory can be accepted as secure on chip storage. If the designer doesnot connect the internal flash to the outside world, then the flash cannot be read

from the I/O pins. Additionally, there exist some security features which can be

4.2 Field Arithmetic in Hardware 27

Table 4.1.: Bitstream Generator Security Level Settings

Security Level Description

None Default. Unrestricted access to all configuration andReadback functions.

Level 1 Disable all Readback functions from both the SelectMAPor JTAG ports (external pins). Readback via the ICAPallowed.

Level 2 Disable all Readback operations on all ports.

Level 3 Disable all configuration and Readback functions from allconfiguration and JTAG ports. The only command (in

terms of Readback and configuration) that can be issuedand executed in Level3 is REBOOT. This erases the con-

figuration of the device. This has the same function asenabling the PROG_B pin on the device, except it is donefrom within the device.

configured during bitstream generation. Table 4.1 summarizes the different securitylevels provides by Spartan3 FPGAs [26].4.2. Field Arithmetic in HardwareAnalyzing McEliece encryption and decryption algorithms (cf. Section 3.2), the fol-

lowing arithmetic components are required supporting computations in GF(2m): amultiplier, a squaring unit, calculation of square roots, and an inverter. Furthermore,a binary matrix multiplier for encryption and a permutation element for step 2 in

Algorithm 2 are required. Many arithmetic operations in McEliece can be replacedby table lookups to significantly accelerate computations at the cost of additional

memory. Our primary goal is area and memory efficiency to fit the large keys andrequired lookup-tables into the limited on-chip memories of our embedded targetplatform.

Arithmetic operations in the underlying field GF(211) can be performed efficientlywith a combination of polynomial and exponential representation. In registers, we

store the coefficients of a value a ∈ GF(211) using a polynomial basis with naturalorder. Given an a = a10α

10 + a9α9 + a8α

8 + ⋅ ⋅ ⋅+ a0α0, the coefficient ai ∈ GF(2) is

determined by bit i of an 11 bit standard logic vector where bit 0 denotes the least sig-

28 Chapter 4 - Reconfigurable Hardware

nificant bit. In this representation, addition is fast just by performing an exclusive-oroperation on two 11 bit standard logic vectors. For more complex operations, such

as multiplication, squaring, inversion and root extraction, an exponential represen-tation is more suitable. Since every element in GF(211) can be written as a power ofsome primitive element α, all elements in the finite field can also be represented by

αi with i ∈ Z2m−1. Multiplication and squaring can then be performed by adding

the exponents of the factors over Z2m−1 such as

c = a ⋅ b = αi ⋅ αj = α

i+j ∣ a, b ∈ GF(211), 0 ≤ i, j ≤ 2m − 2. (4.1)

The inverse of a value d ∈ GF(211) in exponential representation d = αi can be

obtained from a single subtraction in the exponent d−1 = α211−1−i with a subsequent

table-lookup. Root extraction, i.e., given a value a = αi to determine r = ai/2 is

simple, when i is even and can be performed by a simple right shift on index i.

For odd values of i, perform m − 1 = 10 left shifts followed each by a reductionwith 211 − 1. To allow for efficient conversion between the two representations, we

employ two precomputed tables (so called log and antilog tables) that allow fastconversion between polynomial and exponential representation. Each table consistsof 2048 11-bit values that are stored in two of the Block Rams Cells (BRAM) in the

FPGA. For multiplication, squaring, inversion, and root extraction the operands aretransformed on-the-fly to exponential representation and reverted to the polynomial

basis after finishing the operation. To reduce routing delay every arithmetic unit canaccess their own LUT that is placed closed to the logic block.

5. Designing forArea-Time-E�ciencyMost components of an algorithm can be implemented in a way to finish as fast aspossible but then they need a lot of logic resources. On the other hand a componentcan be implemented resource efficient, but most likely it will then consume more

time to complete the task. Table 5.1 gives an estimate about how often specific partsof the McEliece algorithms will be executed on average during one encryption or

decryption with the parameters m = 11, t = 27.

From this table one can see easily that poly_EEA is the most important, time critical

and largest component in the whole design and thus is worth a few more details.5.1. Extended Euclidean AlgorithmIn algorithm 6 the extended euclidean algorithm adopted to polynomials overGF(2m)is summarized again.

Algorithm 6 Extended Euclid over GF(211) with Stop Value

Input: G(z) irreducible ∈ F211 [z], x ∈ F211 [z] with degree x < G

Output: x−1 mod G(z)1: A← G(z), B ← x(z), v ← 1, u← 0

2: while degree(A) ≥ stop do

3: q← lc(A)lc(B)

{ lc(X) is leading coefficient of X}

4: A← A− q ⋅ zk ⋅ B5: u← u + q ⋅ zk ⋅ v6: if degree(A) ≤ degree(B) then

7: A← B B← A { swap A and B}

8: u← v v← u { swap u and v}9: end if

10: end while

11: return (u(z)A0

)

We identified the following components:

30 Chapter 5 - Designing for Area-Time-Efficiency

Table 5.1.: Execution Count for Crucial Parts.

Part CountE

ncr

ypti

on

UART receive 219

UART send 256

(1751× 2048)Matrix MUL 1

(8× 8)Submatrix MUL 56064

Error Distribution 2

PRNG* 7

Dec

rypt

ion

UART receive 256

UART send 219

Permutation 1

Polynomial MUL 1

Polynomial SQRT 1

Polynomial SQR 2

EEA+ 1024 + 2

-EEA.getDegree 2 ⋅ 27 ⋅ (1024 + 2) ≈ 55, 500

-EEA.shiftPoly 2 ⋅ 27 ⋅ (1024 + 2) ≈ 55, 500

-EEA.MulCoeff 2 ⋅ 27 ⋅ 27 ⋅ (1024 + 2) ≈ 1, 500, 000

-EEA.DivCoeff 2 ⋅ 27 ⋅ (1024 + 2) ≈ 55, 500

(2048× 2048)Matrix MUL 1

(8× 8)Submatrix MUL 65, 536

PRNG- 65, 536

* One PRNG run generates 4 error positions.+ Assuming that half of the ciphertext bits are one.- One PRNG run generates one (8× 8)Submatrix.

1. get_degree: a component that determine the degree and the leading coefficient(lc) of a polynomial

2. gf_div: a component that divide two field elements

3. gf_mul: a component that multiply two field elements

5.1 Extended Euclidean Algorithm 31

4. poly_mul_shift_add: a component that computes X = X + q ⋅ zk ⋅ YWe decide to implement the extended euclidean algorithm in a full parallel way. Inother words, we are trying to compute nearly every intermediate result in one clock

cycle, which means that the design is working on a complete polynomial at once.Only in places that occur rarely during a decoding run, we compute the results in aserially, coefficient-wise way to save resources.

If all computations would be done in a serially, coefficient-wise way, this would

result in about 30 times more clock cycles. Although a serial design is smaller andshould achieve a higher clock frequency, it can not reach 30 times the frequency

of the parallel design. A Spartan3 FPGA can run at only 300 MHz and thus theadvantage of parallelism can not be outperformed when the parallel design operatesat least with 10 MHz.5.1.1. Multiplication ComponentDue to the large number of required multiplications we choose the fastest possi-ble design for this operation. Instead of performing the multiplication with the table

look-up method mentioned in Section 4.2 or implementing school-book or Karazuba-like methods, we completely unroll the multiplication to a hardwired tree of XORs

and ANDs. This tree is derived from the formal multiplication of two field elementsand includes modulo reduction mod p(α) = α

11 + α2 + 1, which is a defining poly-

nomial for GF(211). Using this a complete multiplication is finished in one clock

cycles. See Appendix C for the complete multiplication code.

After optimization by the synthesis tool the multiplier consists of an 11 bit registerfor the result and a number of XORs as shown in Table A.2 in Appendix A. Overall

one multiplier consumes 89 four-input LUT’s, but computes the product in one clockcycle.5.1.2. Component for Degree ExtractionTo allow get_degree to complete as fast as possible, first every coefficient of the input

polynomial is checked if it is Zero as shown in Listing 5.1.2.

e n t i t y gf_compare i s PORT(c o e f f _ i n : in STD_LOGIC_VECTOR(MCE_M−1 downto 0 ) ;equal : out s t d _ l o g i c

) ;end gf_compare ;


a r c h i t e c t u r e s t r u c t u r a l of gf_compare i s

begin

equal <= ’0 ’ when ( c o e f f _ i n=GF_ZERO) e lse ’ 1 ’ ;end s t r u c t u r a l ;

Listing 5.1: Architecture for gf_compare

Using gf_compare for all 27 coefficients, we construct a std_logic_vector(26 downto

0), that contains a 0 if the coefficient is zero and 1 otherwise. This construction is

shown in Listing 5.1.2.

gen_comp : for I in 0 to 26 generate

C_compare : gf_compare port map(c o e f f _ i n =>poly_in ( I ) ,equal=> i n t e r n a l _ d e g r e e ( I ) ) ;

end generate gen_comp ;

Listing 5.2: Instantiation of gf_compare

These components are unclocked and consist only of combinatorial logic. When thestart signal is driven high from this vector, degree and lc are derived in one clock

cycle.

i f i n t e r n a l _ d e g r e e ( 2 6 ) = ’1 ’ then

degree <=" 11010 " ;l c <=poly_in ( 2 6 ) ;

e l s i f i n t e r n a l _ d e g r e e (2 5 )= ’1 ’ then

degree <=" 11001 " ;l c <=poly_in ( 2 5 ) ;...

e l s i f i n t e r n a l _ d e g r e e (0 )= ’1 ’ then

degree <=" 00000 " ;l c <=poly_in ( 0 ) ;

e lse

re port " Zero polynomial de tec ted ! ! ! " s e v e r i t y e r r o r ;end i f ;

Listing 5.3: Determining the Degree of a Polynomial5.1.3. Component for Coe�cient DivisionAlso gf_div is designed for troughput. Like mentioned in section 4.2 it makes use oftwo precomputed tables in BRAM. Gf_div transforms both input coefficients into ex-

ponential representation at once with a TLU in a dual port ROM poly2exp_dualport.

5.1 Extended Euclidean Algorithm 33

The exponents are then subtracted exp_diff. This difference may become negative.To avoid an extra clock cycle for adding the modulo to bring the difference back

to the positive numbers, we extend the exp2poly table to the negative numbers asexp2polymod. It turns out that negative numbers are represented in twos complementform in hardware. So addresses 0 to 2047 contain the standard exponent-to-polynom

mapping. Addresses 2048 and 2049 contain due to twos complement form the ex-ponent −2048,−2047, which can’t occur because the maximum exponent range is

0 ⋅ ⋅ ⋅ 2046. This addresses are filled with a dummy value. The rest of the addressspace now contains the same values in the same order as at the beginning of the

table. By this, the modulo reduction is avoided resulting in about 55, 600 overallsaved clock cycles. Remark that the TLU method is chosen because these designhas finished in 3 clock cycles, whereas a dedicated arithmetic unit computing the

division need at least 11 clock cycles when assuming that one clock cycle is neededper bit operation. Figure 5.1 shows the complete division component. The look-up

tables and the substractor are highlighted in red. The surrounding signals form thecontrolling state machine.

statemachine

state[0:3]

exp_out_b_1[10:0]

valid_1_sqmuxa

un1_state[0]

e

d

e

d

e

d

e

d

un1_state_1[0]un1_state_2 un1_state_3 valid

exp_diff[11:0]

start

valid

coeff_c[10:0][10:0]

coeff_b[10:0][10:0]

coeff_a[10:0][10:0]

rst

clk =1

[10:0]

poly2exp_dualport

TLU1

exp2polymod

TLU2invert\.exp_diff_2[11:0]

+

I[1:0]

[0:3]Q[3:0]C

0R

clka

clkb

[10:0]addra[10:0]

[10:0]addrb[10:0]

[10:0]douta[10:0]

[10:0]doutb[10:0]

clka

[11:0]addra[11:0]

[10:0]douta[10:0]

[10:0]

[10:0]

[0]

[3]

0

[2]

0

[1]

1

[0]

1

Q[0][1]

D[0]

R

E

[10:0]

[11:0]

1

[11:0]Q[11:0]

[11:0]D[11:0]

R[2]

E

Figure 5.1.: The Complete Coefficient Divider

5.1.4. Component for Polynomial Multiplication and ShiftingThe poly_mul_shift_add component also is designed with priority to high perfor-mance. Therefore, the multiplication of the input polynomial with the coefficient

is completely unrolled. We use 27 multiplier in parallel of which each consists of atree of ANDs and XORs. Each of these multiplier blocks is connected to the fixed

coefficient q and one of the polynomial coefficients as shows in Figure 5.2.


Mul26 Mul22Mul23Mul24Mul25 Mul0Mul1Mul2

297

11

297

Figure 5.2.: Overview of the Polynomial Multiplier

After determining the shift value k = deg(A) − deg(B) as the difference in thedegree of both polynomials A(z), B(z) the shift step by k coefficients is accomplishedby large 297bit width 27 to 1 multiplexer to allow minimum runtime.

with k s e l e c t

poly_out <= poly_in when " 00000 " ,poly_in (2 5 downto 0)&GF_ZERO when " 00001 " ,poly_in (2 4 downto 0)&GF_ZERO&GF_ZERO when " 00010 " ,...( others=>" 00000000000 " ) when others ;

Listing 5.4: k-width Shifter

To allow maximum throughput, we decide to instantiate the poly_mul_shift_add com-ponent twice, allowing line4 and 5 of Algorithm 6 to run in parallel. For the same

reason component get_degree is instantiated twice as required in line 3 of Algo-rithm 6.

The final normalization is also handled by the poly_mul_shift_add component. Thisworks since in normal mode poly_mul_shift_add computes

u(z) = u(z) + q ⋅ zk ⋅ v(z) (5.1)

Remember that q is the result of the division of the leading coefficients of two poly-nomials. By setting u(z) = 0, q = 1

A0, k = 0 and v(z) = u(z), this equation results in

u(z) = 0 +1

A0⋅ z0 ⋅ u(z) = u(z)

A0(5.2)

5.2 Computation of Polynomial Square Roots 35

which is the required normalization. This method avoid implementation of an extranormalization component and save available slices without increasing required clock

cycles because this step can anyway be computed after the last computation whichinvolves poly_mul_shift_add.5.2. Computation of Polynomial Square RootsThe next required computation, according to Algorithm 4 line 6, is taking the squareroot of T(z) + z).Like mentioned in Section 4.2, instead of using a matrix to compute

the square root of the coefficients, we choose the method proposed by K. Huber [24].For this and a latter purpose, we need a component, which can split polynomials into

its odd and even part so that T(z) + z = T0(z)2 + z ⋅ T1(z)

2. Splitting a polynomialmeans to take the square root of all coefficients and assigning square roots from oddpositions to one result polynomials and even one to the other result polynomial.

All coefficients that origin from even positions form the even part and vise versa.Taking square roots of coefficients only occurs in this component and nowhere else

in the whole algorithm. Therefore, we choose a method that does not consume manyslices, but use the already introduced TLU method.5.3. Computation of Polynomial SquaresAnd the last required field operation is the computation of a polynomial squarein Algorithm 4 at line eight. Like the square root computation, this operation is

required only once during one decryption. Due to this low utilization we decide toimplement it in a space saving way. The square is computed coefficient by coefficient,

thus requiring only a single squaring unit. This squaring unit is simply the unrolledmultiplier from Section 5.1.1, where both input are wired together.

6. ImplementationBased on the decisions made in Chapter 4, this chapter discusses the implementationof encryption and decryption.6.1. EncryptionIn this section the implementation of the matrix multiplication and generation of the

random error plus its distribution among the ciphertext is presented. Rememberthat encryption is only c = m ⋅ G + e. But first the target platform is presented.

Because of the lower logic requirements for the encryption and since the encryp-

tion key is public and does not require confidential storage, the encryption routine isimplemented on a low cost Spartan3 FPGA, namely a XC3S200. This device is part ofa Spartan-3 development board manufactured by Digilent[14]. Aside from the FPGA

this board additional provides:

1. On-board 2Mbit Platform Flash (XCF02S)

2. 8 slide switches, 4 pushbuttons, 9 LEDs, and 4-digit seven-segment display

3. Serial port, VGA port, and PS/2 mouse/keyboard port

4. Three 40-pin expansion connectors

5. Three high-current voltage regulators (3.3V, 2.5V, and 1.2V)

6. 1Mbyte on-board 10ns SRAM (256Kb x 32)

Figure 6.2 shows an summary of the design. The single blocks will be explained

in the following.

The implementation consist of two parts which can be selected via two of thesliding switches. The first part ( selected with sw1 = 1) reads the public key from

the UART and store it in external SRAM. The second part(selected with sw0 = 1)reads the message from the UART and encrypt it with the public key.6.1.1. Reading the Public KeyThe top level component , called toplevel_encrypt controls the UART interface, reads

the external switches and writes data to the SRAM. Also this component controls

38 Chapter 6 - Implementation

(a) Board (b) Block Overview

Figure 6.1.: Spartan3-200 Development Board

PRNG

for Errors

UART

Buffer

mce_encryptI/O

SRAM

FSM

Matrix

Multiplier

Access switch

BRAM

Buffer

FS

M

8x8

Matrix

Multiplier

Access s

witch

Buffer BRAM

Row Counter

Column Counter

DE

BU

G

Mo

de

Se

lect

toplevel_encrypt

Figure 6.2.: Block Overview for Encryption

the real encryption component mce_encrypt. When a reset occurs, all buffers are

cleared, SRAM is disabled by driving SRAMce1, SRAMce2, SRAMoe high and thestate machine is set to idle state. In the moment when the UART indicates a receivedbyte, the FSM switches to read_in state, the SRAM is enabled and its address bus set

to all zero. Now, a byte is read into the lower 8 bits of an 32 bit register and theregister is shifted up by 8 bits. After reading 4 byte now, a 32 bit word is complete

which is written to SRAM. Afterwards, the address counter is incremented. This

6.1 Encryption 39

procedure is repeated until all 448, 512 bytes of G are written to SRAM. Now, theFSM returns to idle state which is indicated by driving an LED on the board. Then,

another public key could be read or, more useful, the mode can be switched toencryption by setting sw1 to 0 and sw0 to 1.

To send data from the PC to the FPGA the open source terminal program Hterm [23]

is used. Hterm can send bytes to the FPGA either from keyboard or from a file. Thestructure of the file containing the data is simply one byte in hexadecimal form (w/oleading 0x header) on a single line.

During development, debug methods where integrated, that are also in the finaldesign to allow verification of the current state and the behavior of some importantsignals. The development board contains four seven-segment displays which share

a common data bus with signals (a, b, c, d, e, f, g, DP) and are individual selectedby four separated anode control lines. The function of the signals can be seen from

Figure 6.3.

Figure 6.3.: 7 Segment Display

The common data bus is connected to a component that converts a four bit inputcharacter to the bit pattern that is necessary to display the character in hexadecimal

form. The conversion procedure is taken from the reference manual of the board and

SevenSeg

C_SevenSeg0

1dot

[3:0]char[3:0]

[0:7]data[0:7]

Figure 6.4.: Seven Segment Driver Component

is given in Appendix A.1. The decimal dot (data bus pin DB) in the display can be


misused to display data larger than 16 bit. For example, the SRAM address is 18 bitwide and therefore the two leading bits are displayed as dots in two of the segments.

Four of this components are in parallel connected to a signal called disp, which byitself can be connected to various internal signals. Which signal is displayed can beselected using the sliding switches sw2 to sw4. Table 6.1 shows the available debug

modes and how they are selected. Switch sw5 controls whether the status bit of the

Table 6.1.: Function of the Debug Switches

Signal on Seven Segment Display SW2 SW3 SW4

SRAMaddress in encrypt state 1 0 x

SRAMaddress in public key mode 0 0 x

SRAMdata low word x 0 0

SRAMdata high word x 1 0

UART or the control bits for the SRAM are displayed on the seven LEDs led0 to led6.Signal led7 is reserved and hardwired to indicate completion of transferring G to the

SRAM.6.1.2. Encrypting a messageWhen the encryption mode is entered by setting sw0 = 1, all buffers are cleared,SRAM is disabled and the state machine is set to idle state. Now every received

byte from the UART is directly feed through to the mce_encrypt component whichis depicted in Figure 6.5. This component contains the matrix multiplier and the

SRAMdata[31:0][31:0]

rst

mce_encrypt

C_mce_encrypt

clk

prng_clk

rst

start

byte_ready

[7:0]M_byte_in[0:7]

[31:0]SRAMdata[31:0]

valid

need_byte

SRAMoe

[15:0]status[15:0]

[7:0]Cipher_byte_out[0:7]

[17:0]SRAMaddress[17:0]

Figure 6.5.: Interface of the mce_encrypt Component

PRNG plus the controlling state machine.

6.1 Encryption 41

First, every byte is read into an buffer in BRAM. Because this buffer is imple-mented as BRAM of 18Kbit of which only 1752 bits are needed, we decide to addi-

tional put the cipher buffer of 2048 bit into the same BRAM block. The interface ofthis combined buffer is shown in Figure 6.6.

clk

=1

[0:7]

=0

[0:7]

YandM_buff

C_YandM_buff

clka

wea[0]

clkb

web[0]

[0:7]dina[7:0]

addra[8:0]

[0:7]dinb[7:0]

addrb[8:0]

[0:7]douta[7:0]

[0:7]doutb[7:0]

Figure 6.6.: Buffer for Plaintext and Ciphertext

After all input bytes are buffered, the FSM changes to multiplication state. Firstrow and column counter (RowCnt,ColCnt) and the SRAM address are set to zero.Note that RowCnt and ColCnt actual count blocks of (8× 8) submatrices instead

single rows and columns. Due to the (1751× 2048) dimension of the matrix thisleads to a range of 0 . . . 218 for RowCnt and 0 . . . 255 for ColCnt. The partial product

from the submatrix multiplications are summed up in a register c_work, which willcontain the resulting cipher text when the multiplication is finished. First, the lasttemporary ciphertext byte c_workis read from BRAM. In a next step, the plaintext

byte for this row is read out of BRAM and stored in a register m_work and, addition-ally, the 64 bit submatrix of G is read from SRAM into G_work prior to incrementing

the SRAM address. The partial product of this block is computed with:

gen_part_prod : for i in 0 to 7 loop

part_prod ( i )<= (M_work( 0 ) AND G_work (8∗ i +0 ) ) xor

(M_work( 1 ) AND G_work(8∗ i +1 ) ) xor






(M_work( 7 ) AND G_work(8∗ i + 7 ) ) ;end loop gen_part_prod ;

Listing 6.1: Computation of Partial Product

and the result is added to c_work. After writing c_work back to the buffer the FSM

switches to a check state. In this state ColCnt is incremented until the actual row is


finished. ColCnt also addresses the current c_work which saves an extra address reg-ister. When a new row is started the next m_work is read in. When the multiplication

is complete, the FSM changes to error distribution state.Generation and Distribution of the ErrorsTo distribute random errors among the ciphertext, the bit address of the error posi-tions is generated on-the-fly by a fast and small PRNG based on the PRESENT block

chiffre [10]. Figure 6.7 shows how Present is embedded into the PRNG. For real

prng_clk

rst

PRNG

C_PRNG

rst

clk

start

seed

valid

[63:0]data_out[63:0]

(a) Interface of the PRNG

PRNG

C_PRNG

prng_clk

rst

Present

C_present

rst

clk

start

[79:0]key[79:0]

[63:0]plain[63:0]

valid

[63:0]cipher[63:0]

rst

clk

(b) Interface of PRESENT

Figure 6.7.: Parts of the PRNG

security this should be replaced by a TRNG or at least a CSPRNG, but a hardware

design of these type of RNGs is far beyond the scope of this thesis. The PRNG isseeded by an fixed 80 bit IV. Bit 0 to 63 are used as plaintext and the whole IV isused as key. The produced ciphertext is the random output and also the plaintext

for the next iteration. The key is scheduled by rotating it one to the left and XORinga five bit counter to the lowest bits after the rotation. Figure 6.8 gives an overview

over the round function and key derivation. Due to a codeword length of 2048 bit,eleven bits are required to index each bit position. From one 64bit random wordfour errors positions can be derived by splitting the word into four 16 bit words and

using the lower eleven as index. Therefore, seven runs of PRESENT are required togenerate the 27 errors. Because the ciphertext is stored byte wise, the upper eight

bits of the error position select the byte address of c_work in the buffer and the lowerthree a bit of this byte. The bit error is induced by toggling (0 ← 1, 1 ← 0) thebit and afterwards the whole byte is written back to the buffer. After 27 errors are

distributed, mce_encrypt signals completion to the top level component by drivingsignal valid high. Now, the content of the ciphertext buffer is fed to the UART and

mce_encrypt and top_level return to idle state. Finally, the encryption is completeand either a new message can be encrypted or the FPGA can be switched over toinitialization mode and a new matrix G can be read in.

6.2 Decryption 43

PRESENT KEY

<<1

Add

Round

Counter

IVR

an

do

m

64 bit80 bit

Plain

Cipher

64 bit

80 bit

80 bit

Figure 6.8.: Construction of PRNG from PRESENT6.2. DecryptionDue to the polynomial arithmetic involved in decoding the goppa code, decryption

requires far more logic and storage than encryption. For this reason, the decryptionis implemented on a larger Spartan3 FPGA, namely an Xilinx XC3S2000. Originally,

it was planned to save the large S matrix in the internal flash of an XC3S1400AN,but it turns out that reading data from flash is possible only bitwise at 50MHzusing an SPI-Interface and therefore will become a bottleneck. Hence we developed

a method to generate S on the fly in the FPGA so that flash is only required forstoring the bitstream. The McEliece implementation (decryption configuration) for

the Spartan-3 2000 FPGA is depicted in Figure 6.9. The top_level component is nearlythe same as for encryption, except for the SRAM control and the adopted debugginginformation to the new development board. This board used for decryption is a

HW-AFX-SP3-2000 from NuHorizons [46].

As for encryption, the ciphertext is first read using the UART and fed into mce_decrypt.

Figure 6.11 shows the important components and registers of mce_decrypt. The sev-eral parts will be explained in the following sections. The mce_decrypt componentstores the ciphertext in a BRAM buffer. This buffer is shared for storing plaintext

and ciphertext. The address buses of this buffer use the same address register, ex-cept the highest bit address[8] which selects the ciphertext buffer or plaintext buffer,

respectively. See Figure 6.12 for the complete interface.


FSM

I/O

FS

M

P-1 PRNG

for S-1

(PRESENT)

Buffer

goppa_

decode

mce_decrypt

Polynomial EEA

over GF(211

)

Polynomial MUL/

SQ over GF(211

)

Access s

witch

Log Table

Anti-log Table

Syn(z) Table

Syn(z)-1

Table

Buffer BRAM

BR

AM

BR

AM

BRAM

toplevel_decryptUART

Matrix

Multiplier

Figure 6.9.: McEliece implementation on Spartan-3 2000 FPGA

Figure 6.10.: The XC3S2000 Development Board6.2.1. Inverting the Permutation PThe first step in decryption is to revert the permutation P. Like mentioned in Sec-tion 2.5, the inverse permutation matrix P−1 is not stored as matrix but as array

of 11 bit indexes into the ciphertext. P−1 is 2048 ⋅ 11 = 22.528 bit large and storedin a ROM. Because one BRAM can only hold 18 KBits, this ROM is built of two

BRAMs with a simple 11 bit address bus. The input and output buses are depictedin Figure 6.13.

From this ROM now consecutive the indexes of the permutated ciphertext areread as 11bit value perm_address. The value at every address i indicates from which

received cipher text bit j the permutated bit i is taken. Because the received cipher-text is stored byte wise, the upper 8 bits select the byte address of YandMbuff andthe lower three the corresponding bit of the byte. Listing C in the appendix should

make the process of permutation clear.

After eight iterations, one byte of the permutated ciphertext has become avail-

able,and is input to the goppa_decode component. This component can immediatelystart computing a partial syndrome, while concurrently the next permutated byte is

generated. Both components communicate with a simple hand-shake protocol with

6.2D

ecryption45

\ c ece_ _Spa ta 3\top_ e e s s 5 p 6 50 0 009

mce_decrypt

C_mce_decrypt

prng_clk

rst

clk

=0

[0:7]

=1

[0:7]

[0:6]

[1:7]

[8:2]

[0:7]

[2:0]

[7:0]

[7:0]

invP

C_invP

YandM_buff

C_YandM_buffcipher_byte[0:7]

ColCnt[7:0]

m_work[0:7]

part_prod[0:7]

perm_byte[0:7]plain_byte_out[0:7]

random[63:0]

RowCnt[7:0]

seed_prng

start_goppa

start_prng

statemachine_9

state[0:18]

t_work[0:7]

valid

PRNG

C_PRNG

goppa_decode

C_goppa_decodeY_byte_w[0:7]P_decrypt\.Y_byte_w_8[0:7]

0

1

perm_addr[10:0]

clka

[10:0]addra[10:0]

[10:0]douta[10:0]

clka

wea[0]

clkb

web[0]

[0:7]dina[7:0]

addra[8:0]

[0:7]dinb[7:0]

addrb[8:0]

[0:7]douta[7:0]

[0:7]doutb[7:0]

[0:7]Q[7:0]

[0:7]D[7:0]

R

E

[7:0]Q[7:0]

[8:1]D[7:0]

R

[0:7]Q[7:0]D[7:0]

E

[0:7]Q[7:0]

[0:7]D[7:0]

E

[0:7]Q[7:0]D[7:0]

E

[0:7]Q[7:0]

[0:7]D[7:0]

E

[63:0]Q[63:0]

[63:0]D[63:0]

E

[7:0]Q[7:0]D[7:0]

R

Q[0]D[0]

E

Q[0]D[0]

R

E

Q[0]D[0]

E

I[33:0]

[0:18]Q[18:0]C

0R

[0:7]Q[7:0]

[0:7]D[7:0]

E

Q[0]D[0]

E

rst

clk

start

seed

valid

[63:0]data_out[63:0]

clk

rst

start

byte_ready

[0:7]cipher_byte_in[0:7]

need_byte

valid

[0:7]plain_byte_out[0:7]

[0:7]Q[7:0]

[0:7]D[7:0]

E

[0:7]

[0:7][0:7]

[10:0]Q[10:0]

[10:0]D[10:0]

R

clk

prng_clk

rst

start

ext_byte_ready

[7:0] Y_byte_in[0:7]

valid

ext_need_byte

[15:0]

status[15:0]

[7:0]

plain_byte_out[0:7]

Fig

ure

6.11.:O

verv

iewo

fm

ce_decry

pt

Co

mp

on

ent


clk

=1

[0:7]

=0

[0:7]

YandM_buff

C_YandM_buff

clka

wea[0]

clkb

web[0]

[0:7]dina[7:0]

addra[8:0]

[0:7]dinb[7:0]

addrb[8:0]

[0:7]douta[7:0]

[0:7]doutb[7:0]

Figure 6.12.: Buffer for Ciphertext and Plaintext

clk

invP

C_invP

clka

[10:0]addra[10:0]

[10:0]douta[10:0]

Figure 6.13.: ROM containing the Inverse Permutation Matrix

two signals byte_ready and need_byte to indicate if mce_decrypt has a byte completed

and if goppa_decode has finished the previous byte. After the permutation is com-plete, the FSM of mce_decrypt waits until goppa_decode has finished with decoding

the permutated ciphertext. The implementation of decoding will now be explainedbased on Algorithm 4 and the decisions made in Section 5.1.6.3. DecodingThe goppa_decode component is the crucial part of the McEliece implementation in

hardware. It operates on large polynomials with up to 28 ⋅ 11 = 308 bit length.Operations on these polynomials involve assigning specific values, or shifting and

manipulating individual coefficients. This require large multiplexer, shift registers ormassive parallel mathematical operations. For details on how often which operationis performed, refer to Chapter 5.

Figure 6.14 shows the interface of this component. Like mentioned in Section 6.2.1

the ciphertext is read in byte per byte and in the same way the decoded messageis written out in a byte-wise manner. Figure 6.15 shows the different part of the

decoding component.

6.3 Decoding 47

rst

clk

goppa_decode

C_goppa_decode

clk

rst

start

byte_ready

[0:7]cipher_byte_in[0:7]

need_byte

valid

[0:7]plain_byte_out[0:7]

Figure 6.14.: Interface of goppa-decode Component

The different steps during decoding a codeword are controlled by an FSM which

enables the involved components with a start signal and awaits completion indi-cated by signal valid going high. These two signals are used in every componentexcept those who require a single clock cycle. Figure 6.16 shows the FSM for the

goppa_decode component. When goppa_decode has taken a byte it is written to a buffercipher_BRAM_buff, because after decoding the ciphertext is needed for the error cor-

rection step. Then the actual byte is scanned bit wise. If a 1 is found, the correspond-ing trailing coefficient for the polynomial that has to be inverted(see Equation (3.6))is looked up in SList-ROM. This polynomial is now fed into the poly_EEA compo-

nent (see section 5.1 for details). We recognized that there is only one time duringeach run of the EEA, when a polynomial with degree 27 occurs. This is in the first

iteration of the EEA, when polynomial A(z) is set to Goppa polynomial. To save slicesoccupied by this components, the finite state machine takes care of this special case.By resolving this case with signal first_run and manually set the degree of A(z) to 27

and lc(A(z)) = 1 without computing it, all components can be reduced in size andonly operate on polynomials of maximum degree 26. This saves 11 FFs, a complete

multiplier and one multiplexer stage in each (sub)component and additional oneclock cycle per EEA run. The resulting FSM is depicted in Figure 6.17.

In the control state called checkab state the degree and the leading coefficient ofboth polynomials A, B is computed and also the break condition deg(A) = stopdegree)is checked. For the normal EEA stopdegree is zero, but this component is also

used to solve the key equation (see Equation3.13). In this case, the EEA must be

stopped when for the first time deg(A) reaches ⌊ deg(g(z))2 ⌋ = ⌊27

2 ⌋ = 13. Assign-

ing the correct stopdegree is handled by the finite state machine of the goppa_decode

component (see Figure 6.16). In reduce1 the difference k of the degrees of A and B

and also lc(A)lc(B)

is computed. State reduce2 then waits for completion of the the two

poly_mul_shift_add components and then returns to checkab. When the break condi-tion deg(A) = stopdegree is true (depending on the actual value of stopdegree), the

state switches to normalize if stopdegree = 0 or to ready otherwise. After normaliza-

48C

hapter6

-Im

plemen

tation

goppa_decode

C_goppa_decode

mce_decrypt

C_mce_decrypt

[10:0]

[0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

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[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

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[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

=0

[2]

[2]

=0

[0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

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[10:0]

[10:0]

[10:0]

[10:0]

[10:0]

plain_byte_out[0:7]

cipher_byte[0:7]

start_split

start_mulT1w

start_EEA

start_comp_sigma

start_chien

SList

C_TLU_SLIST

InvSList

C_TLU_InvSList

cipher_BRAM_buff

C_cipher_buff

statemachine_7

state[0:18]

poly_split

C_split

poly_mulW_T1_clocked

C_mulT1W

poly_EEA

C_EEA

comp_sigma

C_comp_sig

chien_search

C_chien_search

clk

rst

start

byte_ready

[0:7]Q[7:0]

[0:7]D[7:0]

R

E

[0:7]Q[7:0]

[0:7]D[7:0]

E

Q[0][11]

D[0]

R

E

Q[0]D[0]

R

E

Q[0]D[0]

R

E

Q[0]D[0]

R

E

Q[0]D[0]

R

E

clka

[10:0]addra[10:0]

[10:0]douta[10:0]

clka

[10:0]addra[10:0]

[10:0]douta[10:0]

clka

wea[0]

[0:7]dina[7:0]

[0:7]addra[7:0]

[0:7]douta[7:0]

I[22:0]

[0:18]Q[18:0]C

0R

clk

rst

start

poly2split[296:0]

valid

[10:0]poly_odd[153:0]

poly_even[153:0]

clk

rst

start

[10:0]poly_T1[153:0]

valid

poly_WxT1[296:0]

clk

rst

start

stopdegree[4:0]

poly_x[296:0]

valid

poly_r[153:0]

poly_s[296:0]

clk

rst

start

poly_a_in[153:0]

poly_b_in[153:0]

valid

[10:0]poly_sigma[307:0]

clk

rst

start

[10:0]poly_sigma_in[307:0]

valid

ready

[10:0]root[10:0]

Fig

ure

6.15.:O

verv

iewo

fgoppa-decode

Co

mp

on

ent

6.3 Decoding 49

Figure 6.16.: FSM of the goppa_decode Component

tion the ready state is entered, in which the FSM waits until signal start drops to zero.Then, the EEA is finished and returns to idle state.

When the complete syndrome is computed, Syn(z) will be fed again into poly_EEA

to compute T(z) and z is added. In the first version of the implementation all

polynomials are kept in std_logic_vector(297 downto 0) but it turns out that synthesistool is unable to handle such large vectors. For example, in a simple XOR, both input

coefficients are placed in one corner of the FPGA and the output far away in anothercorner, leading to a large routing delay (nearly 90% of the overall delay). So thedescription of polynomials was modified to type PolyArray_t is array(26 downto 0) of

std_logic_vector(10 downto 0). Now, the synthesis tool seems to be able to identify theregular structure and place corresponding coefficents closer. This leads to a higher

achievable frequency compared to the original version.


:valid_degreea&:stop_degree!=0checkab ready

idle

reduce2reduce1

normalize :rst

:rst

:rst

:rst

:rst

:rst

:valid

_re

duce

u&

:valid

_re

duce

A&

!:rst

!:valid_reduceA&!:rst!:valid_div&!:rst

:start&!:rst

!:valid

_re

duceu&

!:rs

t

:!valid_degreea&!:rst

:degre

eA

&!s

top_

degre

e&

valid

_degre

ea

:valid_degreea&:stop_degree="0000":start&!:rst

!:valid_div&:valid_reduceu&!

Figure 6.17.: FSM of the EEA Component6.3.1. Computing the Square Root of T(z)+zThe next component handles line 6 of Algorithm 4, which is a polynomial squareroot. To compute the square root of T(z) + z, first T(z) + z is split into odd and evenpart (see Equation (3.12). In the VHDL code the odd part T0(z) is called poly_T0

and the even part T1(z) is called poly_T1. This step is also outsourced into an extracomponent called poly_split.

This component scans the input polynomial coefficient-wise and compute thesquare root of each coefficient. Depending on the origin of the coefficient (i.e odd oreven index), the square root is then appended to the odd or even result polynomial.

Note that by construction the resulting polynomials can only have up to 14 coeffi-cients, and thus only 14 ⋅ 11 = 154 bits are reserved for storing each polynomial.

After getting the odd and even part of T(z) + z its square root is computed ac-

6.3 Decoding 51

goppa_decode

C_goppa_decode

rst

clk

poly_split

C_split

clk

rst

start

[296:0]poly2split[296:0]

valid

[153:0]poly_odd[153:0]

[153:0]poly_even[153:0]

clk

rst

Figure 6.18.: Interface of the poly-split Component

cording to Equation (3.12). This step requires the only polynomial multiplication for

which a dedicated component is designed.

goppa_decode

C_goppa_decode

rst

clk


C_mulT1W

clk

rst

start

[153:0]poly_T1[153:0]

valid

[296:0]poly_WxT1[296:0]

clk

rst

Figure 6.19.: Interface of the poly-mulWT1 Component

This component is iteratively fed with the coefficients of polynomial w (recallEquation (3.12), which are read from a constant ROM in BRAM and the polynomialpoly_T1. After each iteration the intermediate result is summed up. Then poly_T1

is shifted to the left and reduced if necessary. For the reduction, the multiplier isused. If poly_T1 reaches degree 27, the goppa polynomial is multiplied with the

leading coefficient of poly_T1 and the result is added to poly_T1. Thus the leadingcoefficient of poly_T1 is set to zero, which results in the required reduction. Theoutput value is called poly_WxT1, because it is computes from w(z) ⋅ T1(z). We

don’t choose a design here, that can multiply a polynomial in one clock cycle to saveavailable slices. Since his component is used only once per decryption the slower

performance is negligible.6.3.2. Solving the Key EquationAfter computing R(z) completes, it is feed to the poly_EEA component. But now

poly_EEA runs in a special mode. Over the input value stop_EEA, one can decideif poly_EEA runs in normal mode (stop_EEA =′′ 00000′′) or in solve-key-equation-

mode (stop_EEA ∕=′′ 00000′′). In this mode, poly_EEA stops the algorithm, when the



C_mulT1W

goppa_decode

C_goppa_decode

rst

clk

poly_mul

C_poly_mul

clk

rstclk

rst

start

[10:0]coeff_mulPoly[10:0]

[307:0]poly_Mulin_extern[307:0]

valid

[307:0]poly_Mulout_extern[307:0]

clk

rst

Figure 6.20.: Interface of the poly-mul Component

degree of A(z) drops down this stop value and the normalization step is omitted. In

this step the second output polynomial of poly_EEA is required and again slices aresaved by only reserving registers for 14 coefficients because the second polynomials

can not get larger than this. See Section 3.6 for the cause of this behavior. These twopolynomials are now feed to comp_sigma component.6.3.3. Computing the Error Locator Polynomial Sigma

goppa_decode

C_goppa_decode

rst

clk

comp_sigma

C_comp_sig

clk

rst

start

[153:0]poly_a_in[153:0]

[153:0]poly_b_in[153:0]

valid

[307:0]poly_sigma[307:0]

clk

rst

Figure 6.21.: Interface of the comp-sigma Component

This component simultaneously squares the coefficients of the both input poly-

nomials by assigning both inputs of the multiplier described in Section 5.1.1 thesame value and assign the results to the correct positions of poly_sigma with is a 308

bits large std_logic_vector. Afterwards poly_sigma is fed to a component chien_search

which searches all the roots of σ(z) and indicates by signal valid and ready if one orall roots are found, respectively.6.3.4. Searching RootsFor searching roots of the error locator polynomial, an extra component was de-

signed. It reads the whole polynomial poly_sigma at once and outputs the found

6.3 Decoding 53

roots in polynomial representation of GF(2m). The port of this component is shownin Figure 6.22.

goppa_decode

C_goppa_decode

rst

clk

chien_search

C_chien_search

clk

rst

start

[307:0]poly_sigma_in[307:0]

valid

ready

[10:0]root[10:0]

clk

rst

Figure 6.22.: Interface of the chien_search Component

Remember that the Chien search initialize an array with the coefficients of poly_sigmaand updates these array according to Equation (3.17). If, at any time, the sum overthe array elements equals zero, a roots has been found. The Chien array in this com-ponent is stored in an array of std_logic_vectors to make the synthesis tool implementit as RAM.type array_t is array(27 downto 0) of std_logic_vector(10 downto 0);signal chien_array: array_t;These array is initialized with the coefficients of poly_sigma and then iteratively up-dated. After each update, the sum over all array elements is computed and if the

sum becomes zero, the actual tested field element is passed off to the goppa_decode

component. To avoid a timing based side channel attack as mentioned in [30], theiterations are not stopped when the maximum number of possible roots are found.

Instead the component continue until all field elements are checked.

Each time a new root is reported to goppa_decode, this root (in polynomial repre-sentation) is used as address for another ROM InvSList. The 11 bit value located

at this address is the ciphertext bit corresponding the the root. Similar to the per-mutation operation the upper eight bit address a byte in the ciphertext RAM ci-

pher_BRAM_buff and the lower tree bits select a bit of this byte. Due to the propertiesdescribed in Section 3.3 each root correspond with the position of an error bit. Thisbit is now corrected(by mean of toggling it) and the whole byte is written back to ci-

pher_BRAM_buff. Because checking roots by chien_search in every case requires moretime than correcting a bit error, no need for synchronizing exist and goppa_decode

can correct the error and afterwards wait for the next root.

When chien_search indicates completion, goppa_decode is also finished. Then,thetransfer of the corrected ciphertext back to mce_decrypt can be started in a byte-wise

manner.

54 Chapter 6 - Implementation6.4. Reverting the Substitution SAs mentioned in Section 2.5, we do not save the inverse scrambling matrix S−1,but generated it on-the-fly by a fast and small PRNG based on the PRESENT blockcipher [10]. Instead of storing the matrix as a whole only a small 80 bit IV is stored

in the configuration bit stream. Refer to Section 6.1.2 for details of the PRNG used.

This IV is used as seed for PRESENT. Bit 0 to 63 are used as plaintext and the

whole IV is used as key. The produced ciphertext is the random output and also

the plaintext for the next iteration. Overall ⌈17512

64 ⌉ = 47, 907 runs of PRESENT are

required the generate S−1. The output are interpreted as 8× 8 submatrices where thesingle bits are written column-wise. For example, if the random word is r0, ⋅ ⋅ ⋅ , r63

then the corresponding submatrix say, S0,0 is

S0,0 =

⎧

⎨

⎩

r0 r8 ⋅ ⋅ ⋅ r56

r1 r9 ⋅ ⋅ ⋅ r57...

. . ....

r7 r15 ⋅ ⋅ ⋅ r63

⎫

⎬

⎭

(6.1)

Actually, the complete matrix S−1 is build row wise from these submatrices. Goalof this method is to minimize memory accesses for reading and writing message

bytes of m. This way every byte of m has to be read only once and when one rowof submatrices is completed, this byte is not required anymore in the later process.

Note that the virtually generated matrix is one bit larger in each dimension thannecessary. The additional bit in each row is removed manually and the additionalcolumn is handled by padding a 0 bit to m, what does not affect the computed result.

To allow maximum throughput the PRNG has its own clock domain clk_prng. Dueto its simple and regular structure, the PRNG can be clocked with up to the double

frequency compared to the rest of the design. Nevertheless, it turned out that themultiplication procedure processes one random word in four clock cycles ( five clockcycles if a column increment occurs) and then wait about 14 clock cycles until the

PRNG has generated the next random word.

Now, the matrix multiplication reads the intermediate message byte mi,j and the

message byte mi from YandMbuff and multiply mi with Si,j. The code used here isthe same as shown in listing 6.1.2. The result is added to mi,j and afterwards mi,j iswritten back to YandMbuff and mi,j+1 is read. Figure shows a timing diagram of the

multiplication routine:

When the multiplication is finished also the decryption is completed. Now, the

content of YandMbuff is byte-by-byte feed to the UART and send to the host PC.

6.4 Reverting the Substitution S 55

waiting for PRNG MUL

Figure 6.23.: Timing Diagram of the Matrix Multiplication

After sending out the last byte the state machine switches back to idle state and is

ready for the next decryption.

7. ResultsWe now present the results the McEliece implementation providing 80 bit security

(n = 2048, k = 1751, t = 27) for an Xilinx Spartan-3 FPGA. We report performancefigures for a Xilinx Spartan-3 XC3S200-5 FPGA performing decryption and a Xilinx

Spartan-3 XC3S2000-5 FPGA performing encryption based on results obtained fromusing Xilinx ISE 10.1. The resource requirements for the FPGA implementation afterplace-and-route (PAR) are shown in Table 7.1.

Table 7.1.: Implementation results of the McEliece scheme with n = 2048, k =1751, t = 27 on a Spartan-3 FPGA after PAR

Resource With Uart and DEBUG Without Uart and DEBUG Available

En

cryp

tion

XC

3S20

0

Slices 796 (41%) 694 (36%) 1,920

LUTs 1,082 (28%) 870 (22%) 3,840

FFs 1,101 (28%) 915 (23%) 3,840

BRAMs 1 (8%) 1 (8%) 12

Flash Memory* 4,644 KBits - 16,896 Kbits

Dec

rypt

ion

XC

3S20

00

Slices 12.977 (63%) 12,443 (60%) 20,480

LUTs 17,974 (43%) 18,637 (45%) 40,960

FFs 9,985 (24%) 9,894 (23%) 40,960

BRAMs 22 (55%) 22 (55%) 40

Flash Memory 11,218 (100%) 4,644 KBits 16,896 Kbits

* For holding the bitstream file.

Table 7.2 summarizes the clock cycles needed for every part of the de- and encryp-

tion routines for the FPGA implementation.

In our FPGA design, the CPRNG to generate S−1 based on the PRESENT blockcipher turns out to be a bottleneck of our implementation since the matrix gener-ation does not meet the performance of the matrix multiplication. Since designing

CPRNGs for FPGAs is complex, this is not in the scope of this thesis. Hence, Ta-ble 7.2 gives estimates for the case that an ideal PRNG (which does not incur any

wait cycles due to throughput limitations) is available. This PRNG must be about

58 Chapter 7 - Results

Table 7.2.: Performance of McEliece implementations with n = 2048, k = 1751, t = 27on the Spartan-3 FPGA.

Aspect Spartan3-200 Spartan2-2000E

ncr

ypti

on

Maximum frequency 150 MHz 80 MHz

Load G into SRAM 180sec+ -

Encrypt c‘ = m ⋅ G 336, 606 cycles -

Inject errors 147 cycles -

Dec

rypt

ion

Undo permutation c ⋅ P−1 - combined with Syn(z)

Determine Syn(z) - 360,184 cycles

Compute T = Syn(z)−1 - 625 cycles

Compute√

T + z - 487 cycles

Solve Equation (3.13) with EEA - 312 cycles

Correct errors - 312,328 cycles

Undo scrambling m ⋅ S−1 - 1,035,684/188,306* cycles

+ Limited by UART baud rate of 19200 baud.* This figure is an estimate assuming that an ideal PRNG for generation of S−1 would be

available.

5.5 times faster than then one used by us.

The public-key cryptosystems RSA-1024 and ECC-P160 are assumed1 to roughlyachieve a similar margin of 80 bit symmetric security [15]. We finally compare our

results to published implementations of these systems that target similar platforms(i.e., Xilinx Spartan-3 FPGAs). Note that the figures for ECC are obtained from theECDSA signature scheme.

Note that all throughput figures are based on the number of plaintext bits pro-cessed by each system and do not take any message expansion in the ciphertext into

account. If our target board for encryption would be equipped with DDR2-333, wecould gain double performance. This would result in 1.07 ms/op and a throughput

of 1, 626, 517 bits/sec. Note, however, that an additional DDR2 memory controllerconsumes a significant amount of logic resources of the Spartan-3 device, but fits

into our target FPGA.

1According to [15], RSA-1248 actually corresponds to 80 bit symmetric security. However, no imple-mentation results for embedded systems are available for this key size.

59

Table 7.3.: Comparison of our McEliece designs with single-core ECC and RSA im-plementations for 80 bit security.

Method Platform Time Throughput

ms/op bits/sec

McEliece encryption Spartan-3 200-5 2.24 779, 948

McEliece decryption Spartan-3 2000-5 21.61/10.82+ 81,023/161,829+

ECC-P160 [22] Spartan-3 1000-4 5.1 31,200

RSA-1024 random [25] Spartan-3E 1500-5 51 20,275

+ These are estimates assuming that an ideal PRNG for generating S−1 would be used.

We are unable to find performance values for RSA encryption on a comparableSpartan-3 FPGA. RSA encryption can use a small exponent and should therefore be

faster than with a random exponent. Also for another important post quantum can-didate, NTRU [29], we are unable to find an encryption implementation for FPGAsof the Spartan-3 class. In citentru2, a general overview over NTRU on constrained

devices is given.

We also tried to put both encryption and decryption onto a single FPGA. Due tothe high density of the decryption routine this was only be possible on a Spartan3AN-1400. This device has a large internal flash memory and thus can store the private

key data in this protected area (see Section 4.1.3). Also the Spartan3AN family iscapable of multi-booting from different bitstream files in the Flash. Therefore both

bitstreams for encryption and decryption can be put into the internal flash and theFPGA can reconfigure itself during run-time. Because reconfiguration is slow (about50 ms)2, this is only practicable if it is not executed too often. The encryption func-

tion achieves the same performance level as the Spartan3-200 FPGA, while decryp-tion is about 5MHz slower. The reason for this behavior is the lower amount (11, 264

instead 20, 480)of slices in the Spartan3AN-1400, which leads to longer critical pathsduring PAR and therefore to a lower maximum frequency.

To compare how the performance increases when using high performance FPGAsthe design was implemented on Virtex4 and Virtex5 FPGAs. Table 7.4 summarizes

the resource requirements including UART and the same debug components as forthe Spartan3 FPGAs.

This would result in a throughput of 1.3 MBits/sec for encryption on a Virtex4 and

2TBITLOAD=(Bitstream Length in bits)/((Clock Frequency in Hz)*(Configuration Port Width inbits)) [1]

60 Chapter 7 - Results

Table 7.4.: Implementation results of the McEliece scheme with n = 2048, k =1751, t = 27 Virtex FPGAs after PAR.

Resource Encryption Decryption Available


Vir

tex4

-lx4

0

Slices 1,056 ( 5%) 13,162 (72%) 18,432

LUTs 1,204 ( 3%) 19,343 (52%) 36,864

FFs 1,158 ( 3%) 10,861 (29%) 36,864

BRAMs 1 ( 1%) 22 (22%) 96

Flash Memory* 1,497 KBits 1,497 KBits –


Vir

tex5

-lx5

0

Slices 369 ( 5%) 4,896 (68%) 7,200

LUTs 944 ( 3%) 13,227 (46%) 28,800

FFs 973 ( 3%) 10,051 (34%) 28,800

BRAMs 1 ( 2%) 12 (25%) 48

Flash Memory* 1,533 KBits 1,533 KBits –

* For holding the bitstream file.

284 KBit/sec for decryption. The Virtex5 can achieve 1.82 MBits/sec for encryptionand 304 KBit/sec for decryption.

8. DiscussionFinally, we like to summarize our findings of this thesis. Furthermore, open issues

and unconsidered aspects will be stated in Section 8.3 to point out the scope forfuture research.

8.1. ConclusionsWe have implemented the McEliece crypto system with special attention to memorylimitations found in all modern FPGAs. This implementation is 2,5 times faster in

encryption than ECC and 23 times faster than RSA. The decryption is about twotimes slower than ECC, but 5 times faster than RSA. When taking the output rateinto account this implementation encrypts about 25 times faster than ECC160 with

respect to the plaintext size and decrypts at a five times higher rate. In comparisonto RSA1024 this implementation achieves 38 time the output rate in encryption.

Decryption is eight times faster then RSA1024. Additionally we developed a methodto significantly reduce the secret key size, what is very important since secret keymaterial has to be stored in a protected area.

Although the underlying algorithms consist matrix multiplication, polynomialfield arithmetic and the extended Euclidean algorithm, due to the large polynomials

and huge data to be stored and processed, we were unable to reach the performancewe expected. For encryption wider RAMs are required to read in as much data aspossible at once. Decryption would benefit from faster PRNGs and shorter polyno-

mials. When the size of the underlying field is increased, less errors are requiredfor the same level of security. The polynomials can then have a lower degree and

thus an shorter total bit length. This should result in a more effective routing, whichmakes about 80% of the delay in our implementation.

Nevertheless, we believe with growing memories in embedded systems, ongo-

ing research and optimizations, McEliece can evolve to a suitable and quantumcomputer- resistant alternative to RSA and ECC that have been extensively stud-

ied for years.

62 Chapter 8 - Discussion8.2. ProblemsDepending on the chosen parameters for PAR the maximum achievable frequency

is in the range of 70 to 80 MHz on the XC3S2000 FPGA. The critical path is alwaysin the EEA between the get_degree and poly_mul_shift_add component. Inserting a

pipeline stage flipflop into one of these components would double the number ofclock cycles required and do not lead to a double frequency. About 80% of the signaldelay are due to routing. We found out that synthesis always had problems placing

logical connected parts also close together on the chip. For example, the log/antilogBRAM was placed in one edge of the chip, while the logic working with the output

was placed in the opponent edge. However, this issue could not be corrected bymanual placement due to other constraints, i.e. often signal paths would becomelonger, resulting in even worser critical paths.8.3. Outlook for Further WorkAs mentioned in Section 2.5 some research was done in reducing the public key size.

These approaches reduce the public key size at the cost of additional computationsduring de- and encryption. But for our purposes, reducing the secret key size is

more important, which should be further investigated.

Also the impact of different field sizes for the same security level is still a fieldfor further research. Especially for implementations that do the field arithmetic with

table lookups, it can be an advantage, if the size of the tables fit exactly into thememory and has a word size that is a multiple of the register size of the target.

Furthermore the ongoing research to replace Goppa codes with other, more com-

pactly representable codes, should be intensified. Note that all proposed replace-ments for the Goppa Codes were broken.

In a future implementation, other ways to compute the parity check matrix should

be tested. And finally we plan to implement the Niederreiter scheme, which isclosely related to McEliece to compare the efficiency of both systems at the same

level of security.

A. TablesTable A.1.: Display Characters and Resulting Control Values

Character Control Value

(abcdefg)

0 (0000001)

1 (1001111)

2 (0010010)

3 (0000110)

4 (1001100)

5 (0100100)

6 (0100000)

7 (0001111)

8 (0000000)

9 (0000100)

A (0001000)

B (1100000)

C (0110001)

D (1000010)

E (0110000)

F (0111000)

64 Chapter A - Tables

Table A.2.: Number of XORs for Multiplication.

Type of XOR Count

XOR2 2

XOR3 2

XOR4 1

XOR5 2

XOR6 2

XOR7 2

XOR8 2

XOR9 2

XOR10 1

XOR11 2

XOR12 2

B. Magma FunctionsListing B.1: Construction of the Mapping Matrix

function image ( Ghat )n := Ncols (G) ;k :=Nrows(G) ;count : = 1 ;repea t

sub := ColumnSubmatrix (G, count , k ) ;count := count +1;

u n t i l ( I sU ni t ( sub ) or ( count eq ( n−k ) ) ) ;count := count−1;sub := sub^−1;iG := Zero ( KMatrixSpace (GF( 2 ) ,n , k ) ) ;I n s e r t B l o c k (~ iG , sub , count , 1 ) ;return iG ;

end function ;

Listing B.2: Present Block Cipher in Magma

F := F i n i t e F i e l d ( 2 ) ;plainV := VectorSpace ( F , 6 4 ) ;keyV := VectorSpace ( F , 8 0 ) ;

//REMARK! ! !//magma i n d i c a t e s from (1 to n ) while C i n d i c a t e s from ( n−1 downto 0 )function sbox ( nibb le )

out := case <nibble |[ 0 , 0 , 0 , 0 ] : [ 0 , 0 , 1 , 1 ] ,

[ 1 , 0 , 0 , 0 ] : [ 1 , 0 , 1 , 0 ] ,[ 0 , 1 , 0 , 0 ] : [ 0 , 1 , 1 , 0 ] ,[ 1 , 1 , 0 , 0 ] : [ 1 , 1 , 0 , 1 ] ,[ 0 , 0 , 1 , 0 ] : [ 1 , 0 , 0 , 1 ] ,[ 1 , 0 , 1 , 0 ] : [ 0 , 0 , 0 , 0 ] ,[ 0 , 1 , 1 , 0 ] : [ 0 , 1 , 0 , 1 ] ,[ 1 , 1 , 1 , 0 ] : [ 1 , 0 , 1 , 1 ] ,[ 0 , 0 , 0 , 1 ] : [ 1 , 1 , 0 , 0 ] ,[ 1 , 0 , 0 , 1 ] : [ 0 , 1 , 1 , 1 ] ,[ 0 , 1 , 0 , 1 ] : [ 1 , 1 , 1 , 1 ] ,[ 1 , 1 , 0 , 1 ] : [ 0 , 0 , 0 , 1 ] ,[ 0 , 0 , 1 , 1 ] : [ 0 , 0 , 1 , 0 ] ,[ 1 , 0 , 1 , 1 ] : [ 1 , 1 , 1 , 0 ] ,

66 Chapter B - Magma Functions

[ 0 , 1 , 1 , 1 ] : [ 1 , 0 , 0 , 0 ] ,[ 1 , 1 , 1 , 1 ] : [ 0 , 1 , 0 , 0 ] ,d e f a u l t : [ 0 , 0 , 0 , 0 ] > ;

return out ;end function ;

function perm( data )out := Zero ( plainV ) ;for i in [0 . . 6 3 ] do

out [ ( ( i mod 4 ) ∗16 + Floor ( i /4) ) +1 ] := data [ i + 1 ] ;end for ;return out ;

end function ;

function present ( pla in , key )// p r i n t " p la in : " , Reverse ( E l t s e q ( p la in ) ) ;data := pla in ;for count in [1 . . 3 1 ] do

// p r i n t "\nround : " , count ;keyseq := E l t s e q ( key ) ;//key := keyV ! keyseq ;roundkey := plainV ! keyseq [ 1 7 . . 8 0 ] ;p r i n t " Roundkey : " , Reverse ( E l t s e q ( roundkey ) ) ;// p r i n t " Data a f t e r . . . " ;//add roundkeydata := data + roundkey ;// p r i n t " key xor : " , Reverse ( E l t s e q ( data ) ) ;dataseq := E l t s e q ( data ) ;//sboxfor i in [ 0 . . 1 5 ] do

subst_data := sbox ( dataseq [4∗ i +1 . . 4∗ i +4 ] ) ;for j in [ 1 . . 4 ] do

data [4∗ i + j ] : = subst_data [ j ] ;end for ;

end for ;// p r i n t " Sbox : " , Reverse ( E l t s e q ( data ) ) ;//permdata :=perm ( data ) ;// p r i n t " pbox : " , Reverse ( E l t s e q ( data ) ) ;//compute roundkey// p r i n t " Next key : " ;key := Rotate ( key , 6 1 ) ;// p r i n t " a f t e r r o t : " , Reverse ( E l t s e q ( key ) ) ;keyseq := E l t s e q ( key ) ;key_nibble := keyseq [7 7 . . 8 0 ] ;subst_key := sbox ( key_nibble ) ;for j in [ 1 . . 4 ] do

key [76+ j ] : = subst_key [ j ] ;

67

end for ;// p r i n t " a f t e r sbox : " , Reverse ( E l t s e q ( key ) ) ;tmp:= count ;i f ( tmp ge 1 6 ) then

tmp:=tmp−16;key [ 2 0 ] : = key [ 2 0 ] + 1 ;

end i f ;i f ( tmp ge 8 ) then

tmp:=tmp−8;key [ 1 9 ] : = key [ 1 9 ] + 1 ;


tmp:=tmp−4;key [ 1 8 ] : = key [ 1 8 ] + 1 ;


tmp:=tmp−2;key [ 1 7 ] : = key [ 1 7 ] + 1 ;


tmp:=tmp−1;key [ 1 6 ] : = key [ 1 6 ] + 1 ;

end i f ;// p r i n t " a f t e r s a l t : " , Reverse ( E l t s e q ( key ) ) ;

end for ;keyseq := E l t s e q ( key ) ;roundkey := plainV ! keyseq [ 1 7 . . 8 0 ] ;p r i n t " f i n a l roundkey : " , Reverse ( E l t s e q ( roundkey ) ) ;// p r i n t " Data a f t e r . . . " ;//add roundkeydata := data + roundkey ;// p r i n t " f i n a l key xor : " , Reverse ( E l t s e q ( data ) ) ;return data ;

end function ;

Listing B.3: PRNG based on Present in Magma

load " l o c a l /present . mag" ;

function i n i t _ p r n g ( IV )key := IV ;IVseq := E l t s e q ( IV ) ;p la in := plainV ! IVseq [ 1 . . 6 4 ] ;count : = 0 ;return <plain , key , count > ;

end function ;

function prng ( s t a t e )


pla in := s t a t e [ 1 ] ;key := s t a t e [ 2 ] ;count := s t a t e [ 3 ] ;p la in := present ( pla in , key ) ;key := Rotate ( key , 1 ) ;tmp:= count ;

i f ( tmp ge 1 6 ) then

tmp:=tmp−16;key [ 5 ] : = key [ 5 ] + 1 ;


tmp:=tmp−8;key [ 4 ] : = key [ 4 ] + 1 ;


tmp:=tmp−4;key [ 3 ] : = key [ 3 ] + 1 ;


tmp:=tmp−2;key [ 2 ] : = key [ 2 ] + 1 ;


tmp:=tmp−1;key [ 1 ] : = key [ 1 ] + 1 ;

end i f ;count : = ( count +1) mod 2^5;return <plain , key , count > ;

end function ;

Listing B.4: Generation and Testing for the Substitution Matrix

function gen_S ( dim )versuch : = 1 ;count : = 0 ;BlockBound:= Cei l ing ( dim/8) ;BitBound :=8∗BlockBound ;repea t

RowCnt : = 0 ;ColCnt : = 0 ;BlockRowCnt : = 0 ;BlockColCnt : = 0 ;p r i n t f " Versuch %o\n " , versuch ;IV :=Random( keyV ) ;s t a t e := i n i t _ p r n g ( IV ) ;s t a t e := prng ( s t a t e ) ;versuch := versuch +1;S := Zero ( KMatrixSpace ( F , BitBound , BitBound ) ) ;

69

// f i l l S with 8x8 blocks , each block colums per columsfor i in [1 . . BitBound ^2] do

count := count +1;S [8∗BlockRowCnt+RowCnt +1 ] [8∗BlockColCnt+ColCnt +1 ] :=

s t a t e [ 1 ] [ count ] ;//reload prngi f ( count eq 6 4 ) then

s t a t e := prng ( s t a t e ) ;count : = 0 ;

end i f ;i f (RowCnt l t 7 ) then

RowCnt :=RowCnt +1;e lse

RowCnt : = 0 ;i f ( ColCnt l t 7 ) then

ColCnt := ColCnt +1;e lse

ColCnt : = 0 ;i f ( BlockColCnt l t BlockBound−1)

then

BlockColCnt := BlockColCnt +1;e lse

BlockColCnt : = 0 ;BlockRowCnt := BlockRowCnt +1;p r i n t "Now working in

BlockRow " , BlockRowCnt ;end i f ;

end i f ;end i f ;

end for ;u n t i l I sU ni t ( Submatrix ( S , 1 , 1 , dim , dim ) ) ;return Submatrix ( S , 1 , 1 , dim , dim ) , S , IV ;

end function ;

function build_S ( IV , dim )BlockBound := Cei l ing ( dim/8) ;BitBound :=8∗BlockBound ;count : = 0 ;RowCnt : = 0 ;ColCnt : = 0 ;BlockRowCnt : = 0 ;BlockColCnt : = 0 ;s t a t e := i n i t _ p r n g ( IV ) ;s t a t e := prng ( s t a t e ) ;S := Zero ( KMatrixSpace ( F , BitBound , BitBound ) ) ;// f i l l S with 8x8 blocks , each block colums per columsfor i in [1 . . BitBound ^2] do

count := count +1;


// p r i n t f" S[%o][%o]\n" ,8∗BlockRowCnt+RowCnt , 8∗ BlockColCnt+ColCnt ;

S [8∗BlockRowCnt+RowCnt +1 ] [8∗BlockColCnt+ColCnt +1 ] :=s t a t e [ 1 ] [ count ] ;

//reload prngi f ( count eq 6 4 ) then

s t a t e := prng ( s t a t e ) ;count : = 0 ;

end i f ;i f (RowCnt l t 7 ) then

RowCnt :=RowCnt +1;e lse

RowCnt : = 0 ;i f ( ColCnt l t 7 ) then

ColCnt := ColCnt +1;e lse

ColCnt : = 0 ;i f ( BlockColCnt l t BlockBound−1) then

BlockColCnt := BlockColCnt +1;e lse

BlockColCnt : = 0 ;BlockRowCnt := BlockRowCnt +1;p r i n t "Now working in BlockRow

" , BlockRowCnt ;end i f ;

end i f ;end i f ;

end for ;p r i n t I sU ni t ( Submatrix ( S , 1 , 1 , dim , dim ) ) ;return Submatrix ( S , 1 , 1 , dim , dim ) ;

end function ;

C. VHDL Code SnippetsListing C.1: Complete Unrolled Field Multiplication

c ( 1 0 ) <=(a ( 0 ) AND b ( 1 0 ) ) XOR ( a ( 1 ) AND b ( 9 ) ) XOR ( a ( 2 ) AND b ( 8 ) ) XOR ( a ( 3 )AND b ( 7 ) ) XOR ( a ( 4 ) AND b ( 6 ) ) XOR ( a ( 5 ) AND b ( 5 ) ) XOR ( a ( 6 ) AND b ( 4 ) )XOR ( a ( 7 ) AND b ( 3 ) ) XOR ( a ( 8 ) AND b ( 2 ) ) XOR ( a ( 9 ) AND b ( 1 ) ) XOR ( a ( 9 )AND b ( 1 0 ) ) XOR ( a ( 1 0 ) AND b ( 0 ) ) XOR ( a ( 1 0 ) AND b ( 9 ) ) ;

c ( 9 ) <=(a ( 0 ) AND b ( 9 ) ) XOR ( a ( 1 ) AND b ( 8 ) ) XOR ( a ( 2 ) AND b ( 7 ) ) XOR ( a ( 3 )AND b ( 6 ) ) XOR ( a ( 4 ) AND b ( 5 ) ) XOR ( a ( 5 ) AND b ( 4 ) ) XOR ( a ( 6 ) AND b ( 3 ) )XOR ( a ( 7 ) AND b ( 2 ) ) XOR ( a ( 8 ) AND b ( 1 ) ) XOR ( a ( 8 ) AND b ( 1 0 ) ) XOR ( a ( 9 )AND b ( 0 ) ) XOR ( a ( 9 ) AND b ( 9 ) ) XOR ( a ( 1 0 ) AND b ( 8 ) ) XOR ( a ( 1 0 ) AND

b ( 1 0 ) ) ;

c ( 8 ) <=(a ( 0 ) AND b ( 8 ) ) XOR ( a ( 1 ) AND b ( 7 ) ) XOR ( a ( 2 ) AND b ( 6 ) ) XOR ( a ( 3 )AND b ( 5 ) ) XOR ( a ( 4 ) AND b ( 4 ) ) XOR ( a ( 5 ) AND b ( 3 ) ) XOR ( a ( 6 ) AND b ( 2 ) )XOR ( a ( 7 ) AND b ( 1 ) ) XOR ( a ( 7 ) AND b ( 1 0 ) ) XOR ( a ( 8 ) AND b ( 0 ) ) XOR ( a ( 8 )AND b ( 9 ) ) XOR ( a ( 9 ) AND b ( 8 ) ) XOR ( a ( 9 ) AND b ( 1 0 ) ) XOR ( a ( 1 0 ) AND

b ( 7 ) ) XOR ( a ( 1 0 ) AND b ( 9 ) ) ;

c ( 7 ) <=(a ( 0 ) AND b ( 7 ) ) XOR ( a ( 1 ) AND b ( 6 ) ) XOR ( a ( 2 ) AND b ( 5 ) ) XOR ( a ( 3 )AND b ( 4 ) ) XOR ( a ( 4 ) AND b ( 3 ) ) XOR ( a ( 5 ) AND b ( 2 ) ) XOR ( a ( 6 ) AND b ( 1 ) )XOR ( a ( 6 ) AND b ( 1 0 ) ) XOR ( a ( 7 ) AND b ( 0 ) ) XOR ( a ( 7 ) AND b ( 9 ) ) XOR ( a ( 8 )AND b ( 8 ) ) XOR ( a ( 8 ) AND b ( 1 0 ) ) XOR ( a ( 9 ) AND b ( 7 ) ) XOR ( a ( 9 ) AND b ( 9 ) )XOR ( a ( 1 0 ) AND b ( 6 ) ) XOR ( a ( 1 0 ) AND b ( 8 ) ) ;

c ( 6 ) <=(a ( 0 ) AND b ( 6 ) ) XOR ( a ( 1 ) AND b ( 5 ) ) XOR ( a ( 2 ) AND b ( 4 ) ) XOR ( a ( 3 )AND b ( 3 ) ) XOR ( a ( 4 ) AND b ( 2 ) ) XOR ( a ( 5 ) AND b ( 1 ) ) XOR ( a ( 5 ) AND b ( 1 0 ) )XOR ( a ( 6 ) AND b ( 0 ) ) XOR ( a ( 6 ) AND b ( 9 ) ) XOR ( a ( 7 ) AND b ( 8 ) ) XOR ( a ( 7 )AND b ( 1 0 ) ) XOR ( a ( 8 ) AND b ( 7 ) ) XOR ( a ( 8 ) AND b ( 9 ) ) XOR ( a ( 9 ) AND b ( 6 ) )XOR ( a ( 9 ) AND b ( 8 ) ) XOR ( a ( 1 0 ) AND b ( 5 ) ) XOR ( a ( 1 0 ) AND b ( 7 ) ) ;

c ( 5 ) <=(a ( 0 ) AND b ( 5 ) ) XOR ( a ( 1 ) AND b ( 4 ) ) XOR ( a ( 2 ) AND b ( 3 ) ) XOR ( a ( 3 )AND b ( 2 ) ) XOR ( a ( 4 ) AND b ( 1 ) ) XOR ( a ( 4 ) AND b ( 1 0 ) ) XOR ( a ( 5 ) AND b ( 0 ) )XOR ( a ( 5 ) AND b ( 9 ) ) XOR ( a ( 6 ) AND b ( 8 ) ) XOR ( a ( 6 ) AND b ( 1 0 ) ) XOR ( a ( 7 )AND b ( 7 ) ) XOR ( a ( 7 ) AND b ( 9 ) ) XOR ( a ( 8 ) AND b ( 6 ) ) XOR ( a ( 8 ) AND b ( 8 ) )XOR ( a ( 9 ) AND b ( 5 ) ) XOR ( a ( 9 ) AND b ( 7 ) ) XOR ( a ( 1 0 ) AND b ( 4 ) ) XOR

( a ( 1 0 ) AND b ( 6 ) ) ;

c ( 4 ) <=(a ( 0 ) AND b ( 4 ) ) XOR ( a ( 1 ) AND b ( 3 ) ) XOR ( a ( 2 ) AND b ( 2 ) ) XOR ( a ( 3 )AND b ( 1 ) ) XOR ( a ( 3 ) AND b ( 1 0 ) ) XOR ( a ( 4 ) AND b ( 0 ) ) XOR ( a ( 4 ) AND b ( 9 ) )XOR ( a ( 5 ) AND b ( 8 ) ) XOR ( a ( 5 ) AND b ( 1 0 ) ) XOR ( a ( 6 ) AND b ( 7 ) ) XOR ( a ( 6 )AND b ( 9 ) ) XOR ( a ( 7 ) AND b ( 6 ) ) XOR ( a ( 7 ) AND b ( 8 ) ) XOR ( a ( 8 ) AND b ( 5 ) )XOR ( a ( 8 ) AND b ( 7 ) ) XOR ( a ( 9 ) AND b ( 4 ) ) XOR ( a ( 9 ) AND b ( 6 ) ) XOR ( a ( 1 0 )

72 Chapter C - VHDL Code Snippets

AND b ( 3 ) ) XOR ( a ( 1 0 ) AND b ( 5 ) ) ;

c ( 3 ) <=(a ( 0 ) AND b ( 3 ) ) XOR ( a ( 1 ) AND b ( 2 ) ) XOR ( a ( 2 ) AND b ( 1 ) ) XOR ( a ( 2 )AND b ( 1 0 ) ) XOR ( a ( 3 ) AND b ( 0 ) ) XOR ( a ( 3 ) AND b ( 9 ) ) XOR ( a ( 4 ) AND b ( 8 ) )XOR ( a ( 4 ) AND b ( 1 0 ) ) XOR ( a ( 5 ) AND b ( 7 ) ) XOR ( a ( 5 ) AND b ( 9 ) ) XOR ( a ( 6 )AND b ( 6 ) ) XOR ( a ( 6 ) AND b ( 8 ) ) XOR ( a ( 7 ) AND b ( 5 ) ) XOR ( a ( 7 ) AND b ( 7 ) )XOR ( a ( 8 ) AND b ( 4 ) ) XOR ( a ( 8 ) AND b ( 6 ) ) XOR ( a ( 9 ) AND b ( 3 ) ) XOR ( a ( 9 )AND b ( 5 ) ) XOR ( a ( 1 0 ) AND b ( 2 ) ) XOR ( a ( 1 0 ) AND b ( 4 ) ) ;

c ( 2 ) <=(a ( 0 ) AND b ( 2 ) ) XOR ( a ( 1 ) AND b ( 1 ) ) XOR ( a ( 1 ) AND b ( 1 0 ) ) XOR ( a ( 2 )AND b ( 0 ) ) XOR ( a ( 2 ) AND b ( 9 ) ) XOR ( a ( 3 ) AND b ( 8 ) ) XOR ( a ( 3 ) AND b ( 1 0 ) )XOR ( a ( 4 ) AND b ( 7 ) ) XOR ( a ( 4 ) AND b ( 9 ) ) XOR ( a ( 5 ) AND b ( 6 ) ) XOR ( a ( 5 )AND b ( 8 ) ) XOR ( a ( 6 ) AND b ( 5 ) ) XOR ( a ( 6 ) AND b ( 7 ) ) XOR ( a ( 7 ) AND b ( 4 ) )XOR ( a ( 7 ) AND b ( 6 ) ) XOR ( a ( 8 ) AND b ( 3 ) ) XOR ( a ( 8 ) AND b ( 5 ) ) XOR ( a ( 9 )AND b ( 2 ) ) XOR ( a ( 9 ) AND b ( 4 ) ) XOR ( a ( 1 0 ) AND b ( 1 ) ) XOR ( a ( 1 0 ) AND

b ( 3 ) ) XOR ( a ( 1 0 ) AND b ( 1 0 ) ) ;

c ( 1 ) <=(a ( 0 ) AND b ( 1 ) ) XOR ( a ( 1 ) AND b ( 0 ) ) XOR ( a ( 2 ) AND b ( 1 0 ) ) XOR ( a ( 3 )AND b ( 9 ) ) XOR ( a ( 4 ) AND b ( 8 ) ) XOR ( a ( 5 ) AND b ( 7 ) ) XOR ( a ( 6 ) AND b ( 6 ) )XOR ( a ( 7 ) AND b ( 5 ) ) XOR ( a ( 8 ) AND b ( 4 ) ) XOR ( a ( 9 ) AND b ( 3 ) ) XOR ( a ( 1 0 )AND b ( 2 ) ) ;

c ( 0 ) <=(a ( 0 ) AND b ( 0 ) ) XOR ( a ( 1 ) AND b ( 1 0 ) ) XOR ( a ( 2 ) AND b ( 9 ) ) XOR ( a ( 3 )AND b ( 8 ) ) XOR ( a ( 4 ) AND b ( 7 ) ) XOR ( a ( 5 ) AND b ( 6 ) ) XOR ( a ( 6 ) AND b ( 5 ) )XOR ( a ( 7 ) AND b ( 4 ) ) XOR ( a ( 8 ) AND b ( 3 ) ) XOR ( a ( 9 ) AND b ( 2 ) ) XOR ( a ( 1 0 )AND b ( 1 ) ) XOR ( a ( 1 0 ) AND b ( 1 0 ) ) ;

Listing C.2: Performing the Permutation

WHEN perm2=>−−put t h e nex t b i t i n t o p erm _ by t e

case perm_number(2 downto 0 ) i s

WHEN " 000 "=>perm_byte<=perm_byte (1 to 7 ) & Y_byte_r ( 0 ) ;...

WHEN " 111 "=>perm_byte<=perm_byte (1 to 7 ) & Y_byte_r ( 7 ) ;end case ;perm_addr<=perm_addr+1;s t a t e <=perm3 ;

WHEN perm3=>i f ( perm_addr(2 downto 0 )=" 000 " ) then

−− f e e d t h e nex t b y t e t o t h e goppa d e c o d e rstart_goppa <= ’1 ’ ;c ipher_byte <=perm_byte ;byte_ready <= ’1 ’ ;

73

i f ( need_byte = ’1 ’ ) then−−goppa r e a d y t o t a k ewait4take <= ’1 ’ ;

e lse−−dont need or t a k e n ?

i f ( wait4 take = ’1 ’ ) then−−t ak en , c o n t i n u eb u i l d i n g t h e nex t b y t e

wait4take <= ’0 ’ ;byte_ready <= ’0 ’ ;s t a t e <=perm1 ;−− e l s e dont need , w a i t i n g

end i f ;end i f ;

e lse−−c o n t i n u e b u i l d i n g t h e c u r r e n t b y t ebyte_ready <= ’0 ’ ;s t a t e <=perm1 ;

end i f ;

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http://entropy.stop1984.com

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www.der-hammer.info/terminal/

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http://www.xilinx.com/support/documentation/white_papers/wp266.pdf

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http://toolbox.xilinx.com/docsan/xilinx10/books/manuals.pdf

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[29] J. H. S. Jeffrey Hoffstein, Jill Pipher. NTRU: A Ring-Based Public KeyCryptosystem. Algorithmic Number Theory (ANTS III), 1423:267–288, 1998.http://www.ntru.com/cryptolab/pdf/ANTS97.pdf.

[30] P. C. Kocher. Timing Attacks on Implementations of Diffie-Hellman, RSA, DSS,

and Other Systems. In Advances in Cryptology — CRYPTO ’96, volume 1109 ofLNCS, pages 104–113. Springer Verlag, 1996.

[31] B. Köpf and M. Dürmuth. A Provably Secure And Efficient CountermeasureAgainst Timing Attacks. Cryptology ePrint Archive, Report 2009/089, 2009.http://eprint.iacr.org/2009/089.pdf.

[32] V. Korzhik and A. Turkin. Cryptanalysis of McElieces Public-Key Cryptosystem,pages 68–70. Springer Verlag Berlin / Heidelberg, 1991.

[33] P. Lee and E. Brickell. An Observation on the Security of McElieces Public-Key

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[34] S. Mangard, E. Oswald, and T. Popp. Power Analysis Attacks: Revealing the Secrets

of Smartcards. Springer-Verlag, 2007.

[35] R. J. McEliece. A public-key cryptosystem based on algebraic coding theory.Deep Space Network Progress Report, 44:114–116, Jan. 1978.

[36] R. J. McEliece. Finite Fields for Computer Scientists and Engineers. Kluwer Aca-

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[37] C. Monico, J. Rosenthal, and A. Shokrollahi. Using low density parity check

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[38] N. Patterson. The algebraic decoding of Goppa codes. Information Theory, IEEE

Transactions on, 21:203–207, 1975.

[39] B. Preneel, A. Bosselaers, R. Govaerts, and J. Vandewalle. A Software Imple-mentation of the McEliece Public-Key Cryptosystem. In Proceedings of the 13th

Symposium on Information Theory in the Benelux, Werkgemeenschap voor Informatie

en Communicatietheorie, pages 119–126. Springer-Verlag, 1992.

[40] Prometheus. Implementation of McEliece Cryptosystem for 32-bit microproces-sors (c-source). http://www.eccpage.com/goppacode.c, 2009.

[41] P. W. Shor. Polynomial-time algorithms for prime factorization and discrete

logarithms on a quantum computer. quant-ph/9508027, Aug. 1995. SIAM J. Sci.Statist. Comput. 26 (1997) 1484.

[42] F. Strenzke, E. Tews, H. Molter, R. Overbeck, and A. Shoufan. Side Channelsin the McEliece PKC. In 2nd Workshop on Post-Quantum Cryptography, pages

216–229. Springer, 2008.

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[43] Y. Sugiyama, M. Kasahara, S. Hirasawa, and T. Namekawa. A Method forSolving Key Equation for Decoding Goppa Codes. Information and Control, IEEE


[44] Y. Sugiyama, M. Kasahara, S. Hirasawa, and T. Namekawa. An erasures-and-errors decoding algorithm for goppa codes (corresp.). Information Theory, IEEE


[45] H. C. van Tilborg. Fundamentals of Cryptology. Kluwer Academic Publishers,2000.

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E. List of Figures4.1. 4-Input LUT with FF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2. Simplified Overview over an FPGA [27] . . . . . . . . . . . . . . . . . . 234.3. VHDL Designflow[28] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.4. The UART component . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.1. The Complete Coefficient Divider . . . . . . . . . . . . . . . . . . . . . 335.2. Overview of the Polynomial Multiplier . . . . . . . . . . . . . . . . . . 33

6.1. Spartan3-200 Development Board . . . . . . . . . . . . . . . . . . . . . . 376.2. Block Overview for Encryption . . . . . . . . . . . . . . . . . . . . . . . 376.3. 7 Segment Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

6.4. Seven Segment Driver Component . . . . . . . . . . . . . . . . . . . . . 396.5. Interface of the mce_encrypt Component . . . . . . . . . . . . . . . . . . 39

6.6. Buffer for Plaintext and Ciphertext . . . . . . . . . . . . . . . . . . . . . 406.7. Parts of the PRNG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.8. Construction of PRNG from PRESENT . . . . . . . . . . . . . . . . . . 41

6.9. McEliece implementation on Spartan-3 2000 FPGA . . . . . . . . . . . 426.10. The XC3S2000 Development Board . . . . . . . . . . . . . . . . . . . . . 42

6.11. Overview of mce_decrypt Component . . . . . . . . . . . . . . . . . . . 426.12. Buffer for Ciphertext and Plaintext . . . . . . . . . . . . . . . . . . . . . 436.13. ROM containing the Inverse Permutation Matrix . . . . . . . . . . . . . 43

6.14. Interface of goppa-decode Component . . . . . . . . . . . . . . . . . . . . 436.15. Overview of goppa-decode Component . . . . . . . . . . . . . . . . . . . 44

6.16. FSM of the goppa_decode Component . . . . . . . . . . . . . . . . . . . 446.17. FSM of the EEA Component . . . . . . . . . . . . . . . . . . . . . . . . 446.18. Interface of the poly-split Component . . . . . . . . . . . . . . . . . . . 45

6.19. Interface of the poly-mulWT1 Component . . . . . . . . . . . . . . . . 456.20. Interface of the poly-mul Component . . . . . . . . . . . . . . . . . . . 46

6.21. Interface of the comp-sigma Component . . . . . . . . . . . . . . . . . 466.22. Interface of the chien_search Component . . . . . . . . . . . . . . . . . . 476.23. Timing Diagram of the Matrix Multiplication . . . . . . . . . . . . . . . 49

F. List of Tables2.1. Security of McEliece Depending on Parameters . . . . . . . . . . . . . 12

4.1. Bitstream Generator Security Level Settings . . . . . . . . . . . . . . . . 26

5.1. Execution Count for Crucial Parts. . . . . . . . . . . . . . . . . . . . . . 30

6.1. Function of the Debug Switches . . . . . . . . . . . . . . . . . . . . . . 39

7.1. Implementation results of the McEliece scheme with n = 2048, k =1751, t = 27 on a Spartan-3 FPGA after PAR . . . . . . . . . . . . . . . 51

7.2. Performance of McEliece implementations with n = 2048, k = 1751, t =27 on the Spartan-3 FPGA. . . . . . . . . . . . . . . . . . . . . . . . . . . 52

7.3. Comparison of our McEliece designs with single-core ECC and RSA

implementations for 80 bit security. . . . . . . . . . . . . . . . . . . . . . 537.4. Implementation results of the McEliece scheme with n = 2048, k =

1751, t = 27 Virtex FPGAs after PAR. . . . . . . . . . . . . . . . . . . . . 54

A.1. Display Characters and Resulting Control Values . . . . . . . . . . . . 57A.2. Number of XORs for Multiplication. . . . . . . . . . . . . . . . . . . . . 58

G. Listings5.1. Architecture for gf_compare . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2. Instantiation of gf_compare . . . . . . . . . . . . . . . . . . . . . . . . . 325.3. Determining the Degree of a Polynomial . . . . . . . . . . . . . . . . . 325.4. k-width Shifter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.1. Computation of Partial Product . . . . . . . . . . . . . . . . . . . . . . . 40

B.1. Construction of the Mapping Matrix . . . . . . . . . . . . . . . . . . . . 59

B.2. Present Block Cipher in Magma . . . . . . . . . . . . . . . . . . . . . . 59B.3. PRNG based on Present in Magma . . . . . . . . . . . . . . . . . . . . . 61B.4. Generation and Testing for the Substitution Matrix . . . . . . . . . . . 62

C.1. Complete Unrolled Field Multiplication . . . . . . . . . . . . . . . . . . 65C.2. Performing the Permutation . . . . . . . . . . . . . . . . . . . . . . . . . 66

H. List of Algorithms1. Key Generation Algorithm for McEliece Scheme . . . . . . . . . . . . . 8

2. McEliece Message Encryption . . . . . . . . . . . . . . . . . . . . . . . 93. McEliece Message Decryption . . . . . . . . . . . . . . . . . . . . . . . . 9

4. Decoding Goppa Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5. Getting a Mapping Matrix from Codewords to Messages . . . . . . . . 21

6. Extended Euclid over GF(211) with Stop Value . . . . . . . . . . . . . . 29

implementing the mceliece

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