crypt 21 cipher
TRANSCRIPT
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Sidra Malik
MS(Computer and Communication Security)
A Novel Key-based Transposition
Scheme for Text Encryption
NUST School of Electrical Engineering and Computer
Science
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Outline
y Basic Idea
y Characteristics of Proposed Idea
y Description of a Single Round Cipher
y Encryption Exampley Decryption Example
y Making Iterative
y Strengths and Weaknesses
y Conclusion and Future Directions
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Basic Idea
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y What is a Transposition Cipher?
y a method of encryption by which the positions held by units of
plaintext (the text needed to be hidden) are shifted, so that the
ciphertext constitutes a permutation of the plaintext.
y In other words,y Simply the order of text is changed.
y Contribution
y A new transposition/ordering scheme is proposed which is based on
graphical pattern of key letters
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General Working of a Cipher
y Plaintext of size n is fed into the encryption
scheme
y Which is a process which consists of
substitution and transposition mainly.
These methods can be applied in any orderfollowed.
y one or more rounds of encryption (if the
block cipher is iterated) to attain the
desired working of your cipher
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Characteristics of Crypt21y substitution concept borrowed from
classical ciphers
y transposition based on key matrix and
merges it with the working phenomenonof iterated block ciphers
y The Cipher consists of mainly three
steps: KeyExtension, Substitution andTransposition.
Find permutations of
key and generate a new
key
C=f(Plaintext, key)
Transformation using
key pattern
Select key
New key
ciphertext1
Ciphertext
plaintext
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Description of a Single Roundy Key Extension
y Select a key, and make permutations of this key by ROT function.Rotate previous shifted again, and store all the combinations of key.Calculate new extended key, Nk wherey N
k= Sum(Key) mod Keylength
y Out of the previously stored combinations of key, select Nk
keys.Append them to make one long extended key.
y Substitution
y In this step, ciphertext is computed by using new key and plaintext byformula given belowy C1 = P + K mod 26
y where C1 is the obtained ciphertext, P is the plaintext as input and K is theextended key derived in the previous step
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Description of a Single Round
Transposition Based on Key
y create l(key length) times a 3*3 matrix and fill each with a pattern of
1s and 0s depicting each key letter.
y next step is to fill in these matrices where there is 1 with letters of
intermediate ciphertext
y The last step is to read the same 1 positioned values row wise to
generate final ciphertext.
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Description of a Single Round
Example (Encryption)y Plaintext:Mary had a little key it is all she could export
y Key: excess
1) Step 1,Key Extension:
E(4) + X(23) + C(2) + E(4) + S(18) + S(18) = 69
69 mod 6 (keylength) = 3
y Use the above answer to rotate row wise with a shift of 1 towards left to getpermutations of key:
XCESSE
CESSEX
ESSEXCSSEXCE
SEXCES
y Now consider rst three as 69 mod 6 equals 3 and concatenate them.
y NEWKEY: XCESSECESSEXESSEXC
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Description of a Single Round
Example (Encryption)
Step 2, Substitution:
Substitution of plaintext using extended key is done
as follows
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Description of a Single Round
Example (Encryption)Step 3, Transposition based on key:
y the ciphertext received in Step 2 is filled column wise into a pattern table of akey and then extracted out reading row wise.
y Final ciphertext received is given below:
JQFDQWBQWPWSVMVCZXCSVRWQVEEAPIKKDLUPBO
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Description of a Single Round
Example (Decryption)Step 1, Reverse Transposition:
Decryption algorithm needs key and a ciphertext
y Ciphertext:JQFDQWBQWPWSVMVCZXCSVRWQVEEAPIKKDLUPBO
y Key: EXCESS
Take a key pattern and ll the ciphertext row wise in the matrices and read it column wise
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Description of a Single Round
Example (Decryption)
Step 2, Extension ofKey:
Follow exactly the same steps in Step1 of encryption to get extended newkey and as a result same extended key will be generated similar to that inencryption.
y NEWKEY: XCESSECESSEXESSEXC
Step 3, De-Substitution:
In this step, plaintext is computed by using new key and
ciphertext by ceaser decryption formulay P = C1 - K mod 26 (4)
y P= Maryhadalittlekeyitisallshecouldexport
P= Maryhad a littlekeyit is allshe couldexport
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Iterative Design
y Whyiterative?
y General phenomenon that more the rounds of cipher, more
secure it is considered.
y The idea of increasing rounds was to add complexityinto
the system so that the overall process is somewhat
computationallyexpensivefor brute force attacks.
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Iterative Designy The cipher is extended up to N
roundsby introducing a Key
schedule function.
y Initial Key is used only once in
Extended Key function, and
further in a pattern in
transposition.
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Strengths and WeaknessesStrengths
Strengths of this cipher include it is based on
classical concepts as well as modern iterative
methods.y The transposition method is newandentirely dependent on key and key length as
compared to previous methods
y Frequency analysis is not easy after successive iterations of cipher.
y It depends upon the length andvalue of key letters in original keywhich
determine theextended key i.e. forevery key, there is a variable lengthextended
key. Which
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Strengths and Weaknesses
Weaknesses
y Most of the previous ciphers have been broken due to weak
key schedule.
y A good key schedule however should have the following
properties:
y Given all but one of the sub-keys, it is impossible to determine the
remaining sub-key.
y Given all of the sub-keys, it is impossible to determine the master key.
y
Each sub-key bit is a random function of all of the bits of the master key.
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Strengths and Weaknesses
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y When the attacker knows number of rounds and the key schedule
function like ROT he is always able to determine
y the number of bits changing after every round which helpshim to
derive original key length.
y Once, the key length is found, the brute force attack complexityreduces by a large number.
y Thus a stronger key schedule may be introduced rather than just a
Rotation Function.
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Conclusion and Future Directionsy A new transposition scheme based block cipher is proposed. It uses
classical substitution whereas proposed transposition scheme which isentirely dependent on key.
y
Strengths include that it is safe from frequency analysisy and dependant on original key in many ways.
y Weaknesses include weak key schedule.
y Improvements that can be made in key schedule
y By generating the hash of extended key or using any one way function which
may not yield any related keys.y Future directions are extending this algorithm to bit level values
y includingASCII characters in ciphertext as well.
12/20/201118A
Novel Key-based Transposition Scheme for Text Encryption