AMSCO Cipher

AMSCO Encryption consists in writing a text in a grid according to a cutting sequence then use a permutation key to switch columns.

Example: Encrypt the message DCODEAMSCO, with a cutting sequence 1,2 (alternation of 1 letter then 2 letters in the grid)

Select a permutation key (of length L), and write the message in lines, cut over L columns.

Example: If the key is KEY (equivalent to 2,1,3) of length 3, then write the message over 3 columns :

Read the grid in columns, in the order of the key (this reading serves as permutation).

Example: Column 2 (COM) then 1 (DEAO), and the 3 (DSC). The message is COMDEAODSCC.

AMSCO decryption requires knowledge of the permutation key (of length L) and the cutting sequence (usually 1,2 or 2,1).

Example: Decrypt the message COMDEAODSCC (of 10-characters length) with the key KEY (equivalent to the permutation 2,1,3) of length 3, and cutting sequence 1.2.

Create a table with L columns, in which the number of characters are noted in each cell (in respect to the cutting sequence and limited by the length of the message).

Example: 1,2 for 10 characters corresponds to (1+2+1+2+1+2+1+0+0 = 10) :

Write the message in the table in columns following the order of the columns indicated by the key.

Example: Write CO, then M in column 2, then D,EA,O in column 1, then D, DC in column 3. This leads to the grid:

The plain message is transcribed by reading the table in lines.

Example: Reading each lines gives the original plain text is DCODEAMSCO.

The text must alternate sizes of cuts which then should form diagonal sets:

Example: A grid of width 3, cut by (1,2):

Example: A grid of width 3, cut by (2,1):

Example: A grid of width 4, cut by (1,2):

Example: A grid of width 3, cut by (3,2,1):

The ciphered message is subjected to a single transposition, so it has an index of coincidence similar to the one of the plain text.

It is possible to try to infer the key length by analyzing bigrams obtained after writing in columns.

AMSCO is a variant of the classical transposition cipher. It adds the cut sequence that can be more complex than the usual 1,2.

By A. M. Scott, in the 19th century