Explain with examples, keyed and keyless transposition ciphers.
1 Answer

Transposition Cipher:

  • A transposition cipher does not substitute one symbol for another (as in substitution cipher), but changes the location of these symbols.
  • It reorders (jumbles) the given plain-text to give the cipher-text.
  • They are of two types: Keyed and Keyless Transposition Cipher.

Keyless Transposition Cipher:

  • In this cipher technique, the message is converted to ciphertext by either of two permutation techniques:

    a. Text is written into a table column-by-column and is then transmitted row-by-row.

    b. Text is written into a table row-by-row and is then transmitted column-by-column

  • The first method (a) is also popularly known as Rail-fence cipher
  • E.g. We need to send the message “DEFENDTHEEASTWALL”. Arranging into tables we get :

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$\hspace{1cm}{\text{Now, the message is sent row-by-row. So Ciphertext is “DFNTEATALEEDHESWL”} \\ \text{(Note: the no. of rows is 2 by default, unless specified)}}$

  • Similarly for the (b) method, we can arrange the same above message into tables with four columns.

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$\hspace{2cm}{\text{The Data is then transmitted column-by-column as “DNETLEDEWFTAAEHSL”}}$

Keyed Transposition cipher:

  • In this approach, rather than permuting all the symbols together, we divide the entire plaintext into blocks of predetermined size and then permute each block independently.
  • Suppose A wants to send a message to B “WE HAVE AN ATTACK”. Both A and B agreed to had previously agreed oved the blocks size as 5. So the blocks would be as:

    $\boxed{W E H A V}$ $\boxed{E A N A T}$ $\boxed{T A C K X}$

  • The last character X is a bogus character so as to complete the block size of 5.

  • A and B is using the following key for encryption and decryption:
4 1 3 2 5
1 2 3 4 5
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