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Jul
20
comment Could the Enigma algorithm be classified as a Feistel network?
@John Callas Interesting. You say "Computer stream ciphers are a PRNG that is XORed onto the plaintext to yield ciphertext." and you say Enigma is a SC. But Enigma does not xor keystream with paintext since it does not produce any keystream as any SC would (see eSTREAM for state of the art SC; RC4; A5/1) :D I do not see the operate on characters thing as distinguishing since you can define keyed PRPs on small blocks (see e.g. Perfect Block Ciphers with Small Blocks by Granboulan and Pornin).
Jul
19
answered Predicting PRNG given some of its previous output
Jul
19
awarded  Informed
Jul
18
comment Advantages of making streamcipher stream depend on plaintext?
@e-sushi hence the wording "attack model". Designers assume powerful attackers and aim to counter them. In practice attackers are hopefully less powerful... Again, a designer assumes the attacker has the keystream corresponding to a known IV and some (unknown) key to be recovered and should resist the attacker's attempts (theoretically speaking, the attacker should have a negligible advantage in this setting). However, this obviously does not mean that in a real world use case of the algorithm the attackers will effectively put their hands on the keystream...
Jul
18
comment Advantages of making streamcipher stream depend on plaintext?
@e-sushi For the Fluhrer, Mantin and Shamir attack: "Because the first byte of the plaintext comes from the WEP SNAP header, an attacker can assume he can derive the first byte of the keystream from B ⊕ 0xAA."
Jul
18
comment Advantages of making streamcipher stream depend on plaintext?
@e-sushi In practice, an attacker does not have direct access to the keystream (as this would mean access to the plaintext in the case of a synchronous stream cipher!) So a first step is to recover at least part of the keystream from the ciphertext and this is done by relying upon the structure/redundancies of the underlying plaintext. The A5/1 attack is a perfect example of that. The designers of stream ciphers thus assume that the attacker has the keystream for a given key and any chosen IV at will...
Jul
18
comment Advantages of making streamcipher stream depend on plaintext?
@otus For the auhtentication, exactly what I said in the third paragraph of my answer: "due to the lack of authentication". Also note that asynchronous stream ciphers help concealing the underlying structure even if they do not provide authentication (as I said, not all asynchronous stream ciphers provide authentication, e.g. the self-synchronizing ones are asynchronous but do not provide full authentication such as the one provided by Helix/Phelix).
Jul
18
comment Advantages of making streamcipher stream depend on plaintext?
@e-sushi I interpret the first two paragraphs of your question together with its title as implying that Wikipedia does not mention "streams that depends on the plaintext". As I said, this is just what the asynchronous stream ciphers do. As for second part, see my previous comment.
Jul
18
comment Advantages of making streamcipher stream depend on plaintext?
Nope: if you have ASCII text encrypted with, say, RC4, it is straightforward for an attacker, to e.g. uppercase all lowercase letters. This would not happen with Helix for instance. Also note I never wrote "conceal plaintext" but rather conceal the structure of the underlying plaintext (redundancies or format)... This is heavily relied upon in practice by attackers and is among the reasons why the keystream is assumed to be available to attackers in the attack models for stream ciphers.
Jul
18
answered Advantages of making streamcipher stream depend on plaintext?
Jul
18
revised Polynomial representation of the affine part of the AES S-box
added 5 characters in body
Jul
18
comment Polynomial representation of the affine part of the AES S-box
Nope: that's the whole point of using the interpolation of the affine transform over $\text{GF}(2^8)$, getting rid of the $\psi$ and $\psi^{-1}$, see page 7 of the paper of Sean Murphy and Matt Robshaw.
Jul
18
revised Polynomial representation of the affine part of the AES S-box
from the paper linked to the question, the polynomial representation referrs to the affine part of the S-box
Jul
18
suggested suggested edit on Polynomial representation of the affine part of the AES S-box
Jul
18
answered Polynomial representation of the affine part of the AES S-box
May
5
awarded  Nice Answer
Oct
27
awarded  Revival
Oct
10
awarded  Yearling
Dec
11
awarded  Necromancer
Dec
4
awarded  Quorum