Transposition cipher techniques

1. Prof. Neeraj Bhargava
Mrs. Shubha Chaturvedi
Department of Computer Science, School of Engineering
& System Sciences
MDS University Ajmer, Rajasthan

2. *
*The substitution cipher always replaces a plain text symbol
to obtain a cipher text symbol whereas transposition cipher
does not change the character in the plain text when it
generates the cipher text it just rearrange them like
anagram
*It hides the message by rearranging the letter order
without altering the actual letter used .
*It applies some kind of permutation function to the text to
produce rearrangement , which can be reversed if you
know the secret to permutation .

3. *
*Rail Fence Transposition Cipher
*Block (Single Columnar) Transposition Cipher
*Double Columnar Transposition Cipher

4. *
*The rail fence cipher (also called a zigzag cipher) is a form
of transposition cipher. It derives its name from the way in
which it is encoded.
*In the rail fence cipher, the plain text is written downwards
and diagonally on successive "rails" of an imaginary fence,
then moving up when the bottom rail is reached. When the
top rail is reached, the message is written downwards again
until the whole plaintext is written out.

5. For example, let’s consider the plaintext “This is a
secret message”.
To encode this message we will first write over two
lines (the “rails of the fence”) as follows:
The cipher text is then read off by writing the top
row first, followed by the bottom row:
Cipher text: TIIAERTESGHSSSCEMSAE

6. *
*It is also known as row-columnar transpose cipher . Single
columnar transposition is simplest kind of cipher in which the
message is written in a rectangle row by row and read the
message off , column by column .
*Secret key is either numerical or keyword whose digits or
letters are rearranged in ascending order .

7. Example
The key for the columnar transposition cipher is a keyword e.g. GERMAN.The row length that is
used is the same as the length of the keyword.
To encrypt a piece of text, e.g.defend the east wall of the castle we write it out in a special
way in a number of rows (the keyword here is GERMAN):
the above example, the plaintext has been padded so that it neatly fits in a rectangle. This is
known as a regular columnar transposition. An irregular columnar transposition leaves these
characters blank, though this makes decryption slightly more difficult. The columns are now
reordered such that the letters in the key word are ordered alphabetically.
The cipher text is read off along the columns:
nalcxehwttdttfseeleedsoaxfeahl

8. *
* The Double Columnar Transposition is quite similar to Single columnar transposition
but in this process is repeated twice.
* The Double Columnar Transposition was introduced to make cryptanalysis of
messages encrypted by the Columnar Transposition more difficult. It was supposed to
prevent anagrams of the plaintext words appearing in the analysed ciphertext.
* The main idea behind the Double Columnar Transposition is to encrypt the message
twice, by using the original Columnar Transposition, with identical or different
secret keys. The output from the first encryption would be the input to the second
encryption.

9. Example
The key for the columnar transposition cipher is a keyword
e.g. 31524. The row length that is used is the same as the length
of the keyword.
To encrypt a piece of text, e.g. Computer Science
Step1: Arrange the plain text in columns according to the key
length.
Step2:Arrange 1st key digit in ascending order.
3 1 5 2 4
C O M P U
T E R S C
I E N C E
1 2 3 4 5
O P C U M
E S T C R
E C I E N

10. Step3: Change the order of alphabets according to the reverse of
key.
Step4: Arrange the 2nd numerical key in ascending order.
Step5: Encrypt the message read the column down in the order
that are numbered then ,(1,2,3,4,5):
CipherText: UCE PSC MRN OEE CTI
4 2 3 1 3
O P C V M
E S T C R
E C I E N
1 2 3 4 5
V P M O C
C S R E T
E C N E I

11. Q1. Find out the rail Fence Cipher for the Plaintext message
”Networking” by using 2 line Rail Fence?
Q2. Find out the single Columnar Transposition Cipher for the
plaintext “ COMPUERSCIENCE”, using the keyword
”ORANGE”?
Q3. Find out the single Columnar Transposition Cipher for
the plaintext “ COMPUERSCIENCE”, using the keyword
”PINEAPPLE”?