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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 cipher is a transposition based grille cipher, consisting of a grid with 25 columns and 24 rows. Each row contains 10 randomly placed white cells (to be filled with text) and 15 black cells. [1] The columns are labeled with shuffled digraphs and numbers and the rows with digraphs. [3]
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.
Although ciphers can be confusion-only (substitution cipher, one-time pad) or diffusion-only (transposition cipher), any "reasonable" block cipher uses both confusion and diffusion. [2] These concepts are also important in the design of cryptographic hash functions , and pseudorandom number generators , where decorrelation of the generated ...
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.
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.
This count, either as a ratio of the total or normalized by dividing by the expected count for a random source model, is known as the index of coincidence, or IC or IOC [2] or IoC [3] for short. Because letters in a natural language are not distributed evenly, the IC is higher for such texts than it would be for uniformly random text strings.
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.