7/31/2014

cryptography5

Cryptography 5



Classical Encryption Techniques:

2- Transposition Cipher:

Transposition Cipher is an encryption method which elements are rearranged.
There are many types of Transposition Cipher such as (Rail Fence, Columnar Transposition  and Double Transposition).

Rail Fence:

Rail Fence is the simplest form of Transposition Cipher which the plaintext is written down as a sequence of diagonals and read off as a sequence of rows.

For Example :

The plaintext = WELCOME TO CRYPTOGRAPHY
Then the encryption algorithm will be

W   L   O   E   O   R   P   O   R   P   Y
  E   C   M   T   C   Y   T   G   A   H

So, the ciphertext = WLOEORPORPYECMTCYTGAH

We can make the encryption algorithm 3 rows

W  C  E  C  P  G  P
 E  O  T  R  T  R  H
  L  M  O  Y  O  A  Y

So, the Ciphertext = WCECPGPEOTRTRHLMOYOAH

Columnar Transposition:

In Columnar Transposition the plaintext is written down in rows and the read off by the order of columns.
The order of columns is the secret key to Columnar Transposition algorithm, any spare spaces are filled with null or blank.

For Example :

The plaintext     = WELCOME TO CRYPTOGRAPHY
The secret key    = 615342

Then the encryption algorithm


Note : I filled the spaces here with "ABC".

Now we read columns by it's order (secret key)

The ciphertext    =  ETTHMYACCCGAORRBLOOYWEPP

previous types are easy to break because the ciphertext has the same letters frequency as the plaintext, and cryptanalysis can put the ciphertext in a matrix and playing with column positions.

Double Transposition:

Double Transposition is the same as Columnar Transposition but twice, this one is more secure.

For Example:

As the previous example
The plaintext     = WELCOME TO CRYPTOGRAPHY
The secret key    = 615342
Then the encryption algorithm (first round)




The ciphertext    =  ETTHMYACCCGAORRBLOOYWEPP

And again

The plaintext    =  ETTHMYACCCGAORRBLOOYWEPP
The secret key    = 615342
Then the encryption algorithm (Second round)


The ciphertext   =  TCRYYAOPHCBEMGLPTCRWEAOO

Note : The ciphertext here has also the same letters frequency but playing with column positions now is harder.

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