Crypt 21 Cipher

8/3/2019 Crypt 21 Cipher

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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

12/20/20112 A Novel Key-based Transposition Scheme for Text Encryption


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Basic Idea

12/20/2011A Novel Key-based Transposition Scheme for Text Encryption3

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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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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Example (Encryption)

Step 2, Substitution:

Substitution of plaintext using extended key is done

as follows



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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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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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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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Novel Key-based Transposition Scheme for Text Encryption17

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

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Crypt 21 Cipher