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Can we consider the Scytale a block cipher ? If yes why ? Can you tell me ?

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Obviously no. Here are the reasons;

  • The Scytale is a transposition cipher that just changes the order of the plaintext, i.e. a permutation on the positions. One might consider the diameter of the rod as the block size but it is not. The block size can be considered as the length of the message parchment when wrapped around the rod. That is not fixed, it must go as long as the message size and one cannot divide the message into blocks. Whereas, in block ciphers, we have a fixed block size and to encrypt we need mode of operation.

  • Also, all block ciphers operate on bits not in characters. Scytale operates on characters like almost all classical ciphers. One can design a block cipher that operates on characters but that will be ridiculous.

  • The key size (space) of Scytale is limited and related to the message. The possible key sizes cannot exceed the message size - and some rods can have unpractical diameters. In block ciphers, the key is not related to the message space, independent from it. The key selected randomly from the key-space that has no relation to the message space.

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  • $\begingroup$ Mhm, could you give me an informal (but precise) definition of block cipher ? $\endgroup$ – AleQuercia Nov 30 '19 at 19:08
  • $\begingroup$ A block cipher consists of two algorithms Enc and Dec that operate on a fixed block size $n$-bit with a key of size $k$-bit to output $n$-bit size. $Dec = Enc^{01}$ $\endgroup$ – kelalaka Nov 30 '19 at 19:13
  • $\begingroup$ Enc = writing the plaintext in the scytale's leather lace as ciphertext. Dec = wrap the leather lace around wooden stick to get the plaintext. Since these two procedures are applied to a certain string of characters (== a block) why can we say that the scytale is a block cipher ? (the key is a lace with a specific lenght) $\endgroup$ – AleQuercia Nov 30 '19 at 20:26
  • $\begingroup$ Nope, that is not a block. You write the ciphertext as the row that depends on the message size that is not fixed. Block ciphers have fixed sizes. Can you divide a message into blocks so that you can operate block by block? The all message must be wrapped before writing the ciphertext. $\endgroup$ – kelalaka Nov 30 '19 at 20:35

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