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The double transposition cipher can be treated as a single transposition with a key as long as the product of the lengths of the two keys. [ 6 ] In late 2013, a double transposition challenge, regarded by its author as undecipherable, was solved by George Lasry using a divide-and-conquer approach where each transposition was attacked individually.
The resulting message, 3113212731223655 has to be secured by other means if the straddling checkerboard table is not scrambled. By passing digits through an additional transposition or substitution cipher stage can be used to secure message -- to whatever extent transposition or substitution can be considered secure.
The cipher's key is , the number of rails. If N {\displaystyle N} is known, the ciphertext can be decrypted by using the above algorithm. Values of N {\displaystyle N} equal to or greater than L {\displaystyle L} , the length of the ciphertext, are not usable, since then the ciphertext is the same as the plaintext.
The history of cryptology in Japan shows two things. First, the fact that substitution ciphers existed makes the failure of the Japanese to improve on the substitution cipher or to invent the transposition cipher much harder to explain. Second, the lack of a strong cryptographic tradition suggests – almost requires – a correspondingly weak ...
The first letter of each ciphertext pair is the row, and the second ciphertext letter is the column, of the plaintext letter in the grid (e.g., "AF" means "row A, column F, in the grid"). Next, the fractionated message is subject to a columnar transposition. The message is written in rows under a transposition key (here "CARGO"):
Each row in the right columns set contains a scrambled alphabet, with two letters in each cell. (There are 13 columns, so two letters per cell exhausts the alphabet.) A traffic key consist of a digit (from 2 to 7) and a letter. The operator selects one of the first 6 columns using the key digit and then finds the row in which the key letter occurs.
This results in a transposition key of 15 8 4, 19 1 3 5, 16 11 18 6 13, 17 20 2 14, 9 12 10 7. This defines a permutation which is used for encryption. First, the plaintext message is written in the rows of a grid that has as many columns as the transposition key is long. Then the columns are read out in the order given by the transposition key.
If LEMON is the keyword, each letter of the repeated keyword will tell what cipher (what row) to use for each letter of the message to be coded. The cipher alphabet on the second row uses B for A and C for B etc. That is cipher alphabet 'B'. Each cipher alphabet is named by the first letter in it.