differential protocol analysis & api level attacks

Differential Protocol Analysis

& API Level Attacks

Mike BondComputer Security Group

Security and Protection of Information 30th Apr ‘03

Summary

• Security APIs• Hardware Security Modules• Introduction to Banking Security• Conventional Protocol Attacks• API-Level Attacks• Differential Protocol Analysis• Solution: Formal Methods & Evaluation ?• Conclusions

What is a Security API ?• A command set that uses cryptography to control

processing of and access to sensitive data, according to a certain policy

HostPC or Mainframe

Security ProcessorPCI Card or Separate Module

Security API

VDU

I/O Devs

Network

Example Security API Commands

U->C : { A }KM , { B }KM

C->U : { A+B }KM

U->C : GUESS , { ANS }KM

C->U : YES (if GUESS=ANS else NO)

U->C : { X }K1 , { K1 }KM , { K2 }KM

C->U : { X }K2

Research into API Attacks• Some work in early 90’s using prolog style search to

find attacks, but few documented atttacks• Work started in 2000 at University of Cambridge with

analysis of hardware security modules used in banks to protect PINs for ATMs

• New work found many more attacks, and produced first significant catalogue of API failures

• Scope has been broadened to include security modules used by certification authorities and also general purpose crypto libraries (eg MSCAPI, PKCS#11)

• Latest work revisiting financial APIs examining PIN generation and verification procedures…

Hardware Security Modules

• An instantiation of a security API• Often physically tamper-resistant

(epoxy potting, temperature & xray sensors)

• May have hardware crypto acceleration(not so important with speed of modern PC)

• May have special ‘trusted’ peripherals(key switches, smartcard readers, key pads)

(referred to as HSMs subsequently)

Hardware Security Modules

Who Needs Security Modules ?

• Those who need to enforce access policies to sensitive informationExamples: Granting signing permission at a Certification Authority

Enforcing split control policies on nuclear weapons & arming codes

• Those who need to protect mission critical sensitive dataExample: Protecting PIN generation keys at banks

• Those who need to protect data in hostile environmentsExamples: Protecting Token Vending Machines (Electricity, Lottery etc…)

Protecting communications keys in battlefield radios

• Those with high crypto throughput requirementsExample: SSL acceleration for webservers

Financial HSMs & API Attacks

• Attacks discovered at Cambridge– VSM Type System attack

– XOR to Null key attack

– Meet in the Middle attack

• Newly discovered attacks:– Decimalisation table attack

– PAN modification attack

• Prerequisites:– Financial security 101

– Conventional Protocol Attacks review

Why Financial Security?• Concrete and simple security policy for APIs

“Only the customer should know her PIN.”

“Keys protecting PINs may only be manipulated when authorised by two different employees.”

• API manuals are often publicly available– IBM put 4758 CCA manual on its website

– Diversity: many manufacturers have APIs performing same broad functionality – good for comparison

• ATM security was the “killer-app” that brought cryptography into the commercial mainstream – so long history of financial API development

Introduction to ATM Security

• The crucial secret is the customer PIN. The customer should be the only person that knows the value of this PIN

• PINs need to be protected from malicious insiders and outsiders

• PINs must be protected when generated, in storage, when issued to customers, when travelling via the international ATM network, and when being verified

• To this end, banks use Hardware Security Modules (HSMs) to perform cryptography and implement a policy which prevents both insiders and outsiders from gaining unauthorised access to PINS.

Security Modules in Banks

Issuing BankRegional HQ

HSM

ATM

AcquiringBank

IssuingBank

ATM Network

HSM

HSM

HSM

HSM

HSM withkeypad HSM

HSM

HSM

Start with your bank account number (PAN)

5641 8203 3428 2218

Encrypt with PIN Derivation Key(aka PMK – Pin Master Key)

22BD 4677 F1FF 34AC

Chop off the (B->1)End 2213 (D->3)

How are PINs Generated ?

decimalise

What’s a Decimalisation Table ?

• Remember encrypted result was in hexadecimal?• Encryption produces output that looks uniformly

distributed, so 0-F are all equally likely• Decimalisation Table used to map 0-F back to 0-9

digit in 0123456789ABCDEFdigit out 0123456789012345

e.g. 22BD -> 2213

• Because some numbers have several hexadecimal digits mapped to them, they are more likely to occur in issued PINs than others

0

2

4

6

8

10

12

14

16

18

20

22

24

0 1 2 3 4 5 6 7 8 9

Example Distribution : HSBC

(Sample size: 45 people)

Conventional Protocol Attacks

• The starting point for the study of API attacks• The protocol world has attacks similar to each of

those found on APIs• Each example demonstrates a different aspect of

protocol attack:– Needham Schroder Public Key

• attacker knows how to encrypt/decrypt

– TMN Protocol• attacker knows algebra of RSA

– Bleichenbacher PKCS#1 Attack• attacker knows protocol data structures & understands RSA

crypto

Protocol Attacks

ManipulationAttacks

MathematicalAttacks

Needham SchroederPublic KeyTMN Attack

Bleichenbacher Attack

Needham-Schroeder Public Key Protocol

A -> B : { NA , A}KB

B -> A : { NA , NB }KA

A -> B : { NB }KB

A -> B : { NA , A}KB

B -> D : { NA , A}KD

D -> B : { NA , ND }KA

B -> A : { NA , ND }KA

A -> B : { ND }KB

B -> D : { ND }KD

attackers abilities:Construct messages,Encrypt & decrypt

TMN Protocol

A->S: RA3 mod N

B->S: RB3 mod N

S->A: RA xor RB

C->S: RA3 * X3 mod N

D->S: RD3 mod N

S->A: RA*X xor RD

Explanation:

Cubing modulo N represents RSA encryption under public key of server S

Explanation:

Server will not accept same random number twice, so C cannot

simply replay RA3

Bleichenbacher PKCS#1 Attack

A->S: Ke mod N

S->A: {data}K

C->S: Ke * Xe mod N

S->A: {data}K*X

or

S->A: decryption error

S->A: format error

Explanation:

Alice sends key K encrypted using RSA with PKCS#1 v1.5 padding

Explanation:

Server tries to decrypt message from Charlie, and leaks information as to whether the decrypted message satisfied PKCS#1 format requirement

API Attacks & Protocol Attacks

• API Attacks already found in each category– Manipulation (VSM Type System)– Mathematical (XOR to null key)– Cryptographic (MIM attack)

(both manipulative and mathematical)

• Differences between API and Protocol Attacks:– API is a ‘dumb adversary’

– APIs are more complex – contain many protocols

– APIs (usually) have only two principals, HSM and User

XOR to Null Key Attack• Top-level crypto keys exchanged between banks in several

parts carried by separate couriers, which are recombined using the exclusive-OR function

• A single operator could feed in the same part twice, which cancels out to produce an ‘all zeroes’ test key. PINs could be extracted in the clear using this key

U->C : {KP1}KM , {KP2}KM

C->U : {KP1 xor KP2}KM

U->C : {KP1}KM , {KP1}KM

C->U : {KP1 xor KP1}KM ( = {0}KM )

(Anderson 2000)

VSM Type System Attack

• Encrypting communication keys for transfer to an ATMs used exactly the same process as calculating a customer PIN

• Customer PINs could be generated by re-labelling an account number as a communications key, and using the same encryption process

(Bond 2000)

The Visa Security Module

VSM Type Diagram

VSM Type System Attack

Type System Attack (Protocol Notation)

U->C : 5641 8203 3428 2218

C->U : {5641 8203 3428 2218}TC

U->C : {5641 8203 3428 2218}TC , { PMK }TMK

C->U : {5641 8203 3428 2218}PMK

{5641 8203 3428 2218}PMK = 22BD 4677 F1FF 34AC

So customer PIN is 22BD i.e. 2213

Car Park Analogy

• A thief walks into a car park and tries to steal a car...

• How many keys must he try?

Car Park Analogy 1900

Car Park Analogy 2000

The Meet in the Middle Attack

• Common sense statistics• Attack multiple keys in parallel • Need the same plaintext under each key• Encrypt this plaintext to get a ‘test vector’• Typical case: A 256 search for one key becomes a

240 search for 216 keys• Poor implementations of 3DES key storage allow

3DES key halves to be attacked individually

MIM Attack on DES Security Modules

40 bits16 bits

• Generate 216 keys• Encrypt test vectors

U->C : { KEY1 }KM

C->U : { 0000000000000000 }KEY1

• Do 240 searchCryptoprocessor’s Effort Search Machine’s Effort

56 bit key space

Protocol Attacks

ManipulationAttacks

DifferentialAttacks

MathematicalAttacks

Needham SchroederPublic Key

TMN Attack

Bleichenbacher Attack

Decimalisation TableAttack

PAN ModificationAttack

Decimalisation Table Attack

• Remember PINs derived from account numbers• Hexadecimal raw PIN is converted to decimal using

decimalisation table• Most APIs allow the decimalisation table to be specified

with each PIN verification command• A normal verification command eliminates one of

10,000 combinations of PIN for the attacker.• If the table is altered, whether or not the alteration

affects correct verification leaks much more information about the PIN

examples…

(Bond/Clulow 2002)

Decimalisation Table Attack (1)

PIN_Verify

Yes/No(eliminates 1 combination)

0123456789ABCDEF

0123456789012345

Trial PIN0000

PAN5641820334282218

Encrypted PMK48CCA975F4B2C8A5

1. Encrypt PANRaw PIN = 22BD2. DecimaliseNatural PIN = 22133. Verify0000 != 2213


PIN_Verify

Yes/No(eliminates all PINs containing digit 7)

0123456789ABCDEF

0000000100000000

Trial PIN0000

PAN5641820334282218


1. Encrypt PANRaw PIN = 22BD2. DecimaliseNatural PIN = 00003. Verify0000 = 0000


PIN_Verify

Yes/No(shows PIN contains digit 2)

0123456789ABCDEF

0010000000000000

Trial PIN0000

PAN5641820334282218


1. Encrypt PANRaw PIN = 22BD2. DecimaliseNatural PIN = 11003. Verify0000 != 1100


PIN_Verify

Yes/No(no information)

0123456789ABCDEF

0123456789012345

Encrypted Trial PIN{2213}KM

PAN5641820334282218




PIN_Verify

Yes/No(eliminates PINs containing digit 7)

0123456789ABCDEF

0123456089012345

Encrypted Trial PIN{2213}KM

PAN5641820334282218



PAN Modification Attack (1)

• Encrypted PINs transferred from ATM to issuing bank via ATM network using point to point encryption

• At each node PIN block must be decrypted with incoming key, and re-encrypted with outgoing key

• Common ISO standard “binds” PIN to particular customer by exclusive-ORing PAN with PIN before encryption

• Attack: specifying incorrect PAN may make deduced PIN contain hexadecimal digit ‘A’-’F’, which causes formatting error. Conditions under which formatting error arises leaks information about PIN.

(Clulow 2002)

PIN Block Formats

041234FFFFFFFFFF

xor

0000820363452239

=0412A6FC9CBADDC6

Primary Account Number (PAN)5461 8203 6345 2239

IS0-0

IS0-2

padding

PINPIN length

241234FFFFFFFFFF

Format ID


PIN_Translate

{PIN Block}AWK (or FORMAT ERROR)

Format

InfoPAN{IWK}KM

{AWK}KM {PIN Block}IWK


041234FFFFFFFFFF

xor

0000820363452239

=

0412B6FC9CBADDC6

0 1 2 3 4 5 6 7 8 9

0 0 1 2 3 4 5 6 7 8 9

1 1 0 3 2 5 4 7 6 9 8

2 2 3 0 1 6 7 4 5 A B

3 3 2 1 0 7 6 5 4 B A

4 4 5 6 7 0 1 2 3 C D

5 5 4 7 6 1 0 3 2 D C

6 6 7 4 5 2 3 0 1 E F

7 7 6 5 4 3 2 1 0 F E

8 8 9 A B C D E F 0 1

9 9 8 B A D C F E 1 0

A A B 8 9 E F C D 2 3

B B A 9 8 F E D C 3 2

C C D E F 8 9 A B 4 5

D D C F E 9 8 B A 5 4

E E F C D A B 8 9 6 7

F F E D C B A 9 8 7 6

PIN

PAN

0412B6FC9CBADDC6

xor

0000820363452239

=

041234FFFFFFFFFF

0412B6FC9CBADDC6

xor

0000720363452239

=

0412C4FFFFFFFFFF

constructionof PIN block

correct PANremoved

modified PANRemoved – PINcontains ‘C’ –error

Differential Protocol Analysis

• Phrase coined by Anderson & Bond in “Protocol Analysis, Composability and Computation” in Feb 2003

• Differential Protocol Analysis refers to attacks which use multiple runs of a protocol to gradually discover a secret key protected by the protocol

• Input differentials (pairs of inputs) are carefully chosen such that a difference in output will be observed dependent upon some secret key material

• Before this work, such attacks were rare – dumb adversaries were thought unlikely

Dectab Attack during PIN Generation

PIN_Generate

FD29DA10029726DC

0123456789ABCDEF

0123456089012345

PAN5641820334282218


1. Encrypt PANRaw PIN = 22BD2. DecimaliseNatural PIN = 22133. Store as ISO PIN Block042213FFFFFFFFFF

“DPA” - Decimalisation Table

PAN Dectab PMK Output5641820364352239

5641820364352239

0123 4567 8901 2345

1123 4567 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

5641820364352239

5641820364352239

0123 4567 8901 2345

0023 4567 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

08F8E3983E3BDF26

5641820364352239

5641820364352239

0123 4567 8901 2345

0103 4567 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

3BDF08F8E3E26983

5641820364352239

5641820364352239

0123 4567 8901 2345

0120 4567 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

726F0FD293E26F67

5641820364352239

5641820364352239

0123 4567 8901 2345

0123 0567 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

5641820364352239

5641820364352239

0123 4567 8901 2345

0123 4067 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

5641820364352239

5641820364352239

0123 4567 8901 2345

0123 4507 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

5641820364352239

5641820364352239

0123 4567 8901 2345

0123 4560 8901 2345

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

“DPA” – PAN ModificationPAN IWK AWK Output820364352239

830364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

820364352239

840364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

08F8E3983E3BDF26

820364352239

850364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

3BDF08F8E3E26983

820364352239

860364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

820364352239

870364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

820364352239

880364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

726F0FD293E26F67

820364352239

890364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

820364352239

8A0364352239

0D7604EBA10AC7F3

0D7604EBA10AC7F3

E92F67BFEADF91D9

E92F67BFEADF91D9

FD29DA10029726DC

FD29DA10029726DC

Protecting against API Attacks: Formal Methods

and Evaluation?• New protocol attacks show formal methods now cover just one of

many aspects of protocol attacks• Automated information-theoretic reasoning about protocols and

APIs could be an interesting new direction for research and evaluation tools, but is years away

• API attacks are already having an effect on the real world – especially in financial security, yet evaluation standards e.g. FIPS 140 series are not keeping up

• API attacks have defeated many different modules certified FIPS 140-1 Level 4 – why? Because the evaluation does not cover the whole system, just a few components.

• Be careful when buying entire systems to establish what has been evaluated, and what has not.

Conclusions• Protocol Analysis is far from dead• API security complements and extends the study of security

protocols• “Differential Protocol Analysis” attacks were previously

novelties, but may become important against ‘dumb adversaries’ as Security APIs become more common

• API analysis used to only be relevant to a few specialist applications such as banking security and certification authorities; TCPA and Palladium APIs when deployed will bring targets into everybody’s homes

• Formal methods tools have a long way to go to provide assurance for using systems

• Independent evaluation can protect against these attacks, but it must cover the whole system.

More Info• Academic Papers

“Decimalisation Table Attacks for PIN Cracking”Bond, Zielinski, Mar 2003

“API-Level Attacks on Embedded Systems”Bond, Anderson, Oct 2001

“The Design and Analysis of Cryptographic APIs for Security Devices”Clulow, Jan 2003

• My Webpagehttp://www.cl.cam.ac.uk/~mkb23/research.html

differential protocol analysis & api level attacks

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