What is a block transposition cipher?

Ra2orLeaf

11/28/23, 3:09 PM

I was looking at the archives for the British national cipher challenge, and a modified version of an ADFGVX cipher came up twice (2003 2011), with block transposition instead of columnar transposition. The question I has is what is this referring to? I tried finding information about block transposition, but there appears to be many different versions of it as per wikipedia. The cipher challenge is a competition for under 18s, so is unlikely to be anything overly complicated, such as classical ciphers if that context helps.

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block-cipher

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Maarten Bodewes

11/28/23, 4:05 PM

Removed my comment; it seems that it is rearrangement *within* the blocks (as per the answer), which is kind of *similar* to block ciphers in the sense that the transposition is performed per block.

Ra2orLeaf

11/28/23, 7:34 PM

@MaartenBodewes this is a slightly different question - for the ciphertext, all letters are ADFGVX, apart from at the end when there are Zs added for some reason (AAZAZ GZZ). Do you know why this might be??

Maarten Bodewes

11/28/23, 8:58 PM

Usually some kind of padding is used if the plaintext cannot fill an entire block. The padding is at the end of the plaintext and will probably give you some idea of where the padding characters should be (and the other chars aren't). But if this is the case here I don't know.

Ra2orLeaf

11/28/23, 9:02 PM

Yep, I've managed to decrypt the cipher now, you are right they are just padding

Score:3

Crypto

Daniel S

11/28/23, 3:50 PM

A block transposition is different from a block cipher such as AES or DES which are referred to be the wikipedia article to which you link.

Instead, it is a rearrangement of the letters of a message where first the message is divided into blocks of equal size (possibly padding out the last block) and then the letters of each block are rearranged according to the same permutation.

For example, if I wished to apply a block transposition to the message TO BE OR NOT TO BE THAT IS THE QUESTION with the key 30421, I would first divide the message into blocks of 5: TOBEO RNOTT OBETH ATIST HEQUE STION and then rearrange the letters per the key: ETOBO TRTON TOHEB SATIT UHEQE OSNIT. Letters can then be regrouped if desired: ETOBOTRTONTOHEBSATITUHEQEOSNIT.

Decryption is achieved by again separating into blocks and applying the reverse permutation (in this case 14302).

The method is sometimes referred to as a simple transposition or anagram.

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EN: What is a block transposition cipher?

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