Redefence Cipher

The Redefence cipher is similar to the Rail-Fence transposition cipher except that it uses a key that defines both the number of levels in the zig zag and the order in which the lines are read.

Example: Consider the message DCODEZIGZAG and the key ZIG (of size N=3 letters).

The message is encrypted by Redefence (as with Rail Fence) by writing it in zig-zag on N levels.

Example: (1-Z): D---E---Z--
(2-I): -C-D-Z-G-A-
(3-G): --O---I---G

The characteristic of Redefence is to have a key that defines the reading order of the lines.

Example: The letters of the key ZIG are then arranged in alphabetical order G,I,Z which defines the reading order of the lines 3,2,1.

Example: The encrypted message is therefore composed of line 3 OIG, line 2 CDZGA and line 1 DEZ so the final message is OIGCDZGADEZ

Redefence decryption requires knowing the encryption key because it conditions the number of levels N and the order of reading.

Example: Consider the message DNRFEEEEC and the key ZAG (2,3,1) of size N=3.

The expected shape is drawn (depending on the number of levels and the length of the message).

Example: X---X---X
-X-X-X-X-
--X---X--

Then we write the letters of the text starting with line 1 (defined according to the key).

Example: (2): R---F---E
(3): -E-E-E-C-
(1): --D---N--

The plain message is then readable in zig-zag from left to right.

Example: The plain text is REDEFENCE

Redefence is a transposition cipher, so the frequency analysis and the coincidence index of the message are similar to that of the plaintext language.

As for Rail-Fence, a brute-force attack of all levels (2 to N) is possible. Extra work is required to find the order of the lines.

Redefence is a variant of the Rail Fence cipher adding the possibility of modifying the order of reading of the lines forming the zig-zag.