# Tag Info

8

Enigma is not a Feistel cipher. A "Feistel cipher" is a block cipher with a specific structure, namely the whole business with the two halves, the combination of one half with a (one-way) function of the other half and a reversible operation (e.g. XOR), and the swap. See the Wikipedia page which has nice schematics. So considering Enigma as a kind of ...

5

The question has morphed over time. I am answering the following. So to be sure, with DES, only when you encrypt something twice with a weak key. You get the back the original plaintext? That is correct as that is the definition of a DES weak key, a key for which encryption and decryption have the same effect. So when using DES in OFB mode with a ...

4

This will probably be OK. It does have some non-trivial side effects/caveats: The effective key length is reduced to 86 bits. Only the low 22 bits of each of the 4 key words will matter, so only 88 bits of the key material are relevant. Then, there are known equivalent-key properties of TEA that further reduce the effective key length to 86 bits. A ...

3

No, it's a rotor machine and more importantly, a stream cipher that operates on a character-by-character basis. Block ciphers operate on a chunk at a time. Feistel ciphers are a way to construct block ciphers. We could talk more about Feistel ciphers or more basically block ciphers, but that's not your question. At its most basic, Enigma is a stream cipher ...

2

There seem to be some errors or inconsistencies in the question. If $P \oplus P' = [0000\delta 000]$, and we use the 2-round structure shown in the picture, then the corresponding ciphertext pairs should satisfy $C \oplus C' = [xyzt\delta000]$. This is different from what you wrote (did you omit the final swap shown in the picture above?). If we let ...

1

A quick follow up, there is a problem with using DES in OFB mode when you are not using the full feedback register. The generated keystream will become cyclic with on average a period of the order $2^{32}$ instead of $2^{64}$. See (R.R. Jueneman, “Analysis of certain aspects of Output Feedback Mode,” Advances in Cryptology, Proceedings Crypto’82, D. ...

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