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Sep
30
comment Are NIST's changes to Keccak/SHA-3 problematic?
@K.G., actually, no, I have it right in the question. Remember, I am summarizing the criticisms/concerns raised in the blog post. I have accurately quoted the concerns listed in that blog post. If you mean to suggest that the blog post has it wrong, feel free to post an answer explaining (indeed, I do suspect some possible confusion in the blog post; that's one of the things I was asking about) -- but it's not an error in the question.
Sep
30
comment What are requirements for this grille?
I don't understand the question. What makes you think there are any requirements on the grille? Where does this talk of rotation come from? As far as I know, there is no rotation: you use the grille you have. It looks to me like the question starts from erroneous premises. In any case, I'm having a hard time understanding what you are asking or what the question is; you might need to edit the question to provide more details and more motivation/justification for your assumptions (e.g., what rotation has to do with anything).
Sep
30
comment how to mathematically prove a key exchange algorithm
Welcome to Crypto.SE! Unfortunately, this question is probably too broad and open-ended to be a good fit for this site. What have you studied so far? What's the real problem you have in front of you? What's the context and motivation for your problem? Providing a more detailed and narrowly focused question makes it more likely that it will be possible to provide a useful answer, within the constraints of a StackExchange site.
Sep
30
comment Friedman Index of Coincidence, pruning multiples of the keylength
OK. I suspect, when the IC was invented, it was typically used together with human judgement, so there might not be any standard rules to address this issue. Anyway, it seems like it might be easy to add a special case for this situation: choose the one with the highest IC, except that if it has a divisor whose IC is within 5% of it, choose the divisor. Another plausible solution would be to try running the rest of your attack on both candidates. I would imagine either of these might work fine. Have you tried either of these approaches?
Sep
30
comment Friedman Index of Coincidence, pruning multiples of the keylength
Do you need a solution that is 100% automated (no human involvement), or are you OK with involving human judgement? If the latter, it seems pretty easy for a human to recognize this situation and choose the smaller keylength.
Sep
30
comment Are NIST's changes to Keccak/SHA-3 problematic?
Personally, I am skeptical that there is anything to worry about here, but I'm interested in a better answer about whether we should be concerned.
Sep
28
comment How to attack a fixed LCG with partial output?
@nightcracker, I think it does mean that. A $2^{16}$ attack means the generator is broken. It means absolutely no one should use the generator. At that point, who cares whether there is a $2^8$ attack? How will that affect your decision-making? If news of a $2^8$ attack will cause you to avoid the generator, when news of a $2^{16}$ attack wouldn't, then there is something wrong with your decision-making. P.S. If your question is about other generators like the Windows rand(), but different (say with larger parameters), then that's a different question: post a new question.
Sep
28
comment How to attack a fixed LCG with partial output?
@nightcracker, in that case, it sounds like your question is not about cryptography per se, so I don't think it is on-topic here. The cryptographic aspect of the question has been answered....
Sep
27
comment How much does a successful Distinguishing Attack “break” the attacked stream cipher?
It sounds like your real question is: "Does a distinguishing attack count as a complete break, from a practical perspective?" If so, you might want to re-write your question from this much more general perspective (whether it's a linear distinguishing attack or some other distinguishing attack is probably of limited relevance).
Sep
27
comment How does Dissent protect against Sybil Attacks?
Have you read the research paper? That would be the best place to start, for this sort of question. (Often presentations don't include all of the detail; a lot of details are left to the paper, so you really need to read the paper if you want to answer detailed questions like this.)
Sep
25
comment Is this a structural weakness of Feistel networks?
pg1989, Ahh, now I see: you are uncertain about the definition of diffusion. The definition of diffusion is that each bit of the input affects all bits of the output. For instance, if you flip one bit in the input to the block cipher, then about half of the bits of the output of the block cipher should change. Notice that this doesn't impose any restrictions on what happens internally, inside the block cipher; it only has to do with how the inputs ultimately affect the final outputs of the cipher. See the strict avalance criterion.
Sep
25
comment How broken is a xor of two LCGs?
Many thanks, @fgrieu! You're right, that wasn't right. It should have been "top 40 bits of $x_0$", not "bottom 20 bits of $x_0$". Fixed. Thank you for spotting that!
Sep
25
comment Is this a structural weakness of Feistel networks?
What makes you think this "could violate the diffusion property of the cipher"? It looks to me like that's where your reasoning went awry, but without further details, I'm not able to diagnose with any more specificity what would help clarify this.
Sep
25
comment Is there any existing analysis for this construction to turn a tweakable blockcipher into a PRF?
Got it. Sounds like you should either omit the nonce from the question, or edit the question to explain this business more clearly. (Incidentally, this construction is going to impose some unfortunate limitations on how people can choose the nonce. If your block cipher admits a 128-bit tweak, and you want to encrypt messages up to $2^{40}$ blocks long, then you're only left with 88 bits for the nonce. In that case, it's not safe to generate nonces randomly: after $2^{44}$ nonces, by the birthday paradox, there's a good chance some nonce will be repeated. A bit painful.)
Sep
25
comment Is there any existing analysis for this construction to turn a tweakable blockcipher into a PRF?
@nightcracker, if you are sharing the same key with some other construction (not presented in the question), then that radically changes the question. You need to mention that! This question asks whether the MAC is secure in isolation, but even if this MAC is secure in isolation (if the key is not used for anything else), that doesn't mean it will be secure when combined with your encryption mode. I think, if you don't already have a proof of security for your combined construction, you are following a dangerous path -- this whole concept does not seem like a good idea to me....
Sep
25
comment Is there any existing analysis for this construction to turn a tweakable blockcipher into a PRF?
@nightcracker, sorry, I probably won't have time to look at the pre-print soon. Apologies about that. The alternatives that I suggest also allow for one-pass processing of a message in parallel with encryption. (On the other hand, if you want to share the same key for both the MAC and the encryption, that's a-whole-nother level of trickiness. At that point you really need an extremely careful security proof, and not just a handwavy sketch, because it is so easy to go awry. I would not trust any construction that reused the same key for both purposes without a super-detailed proof.)
Sep
25
comment Is there any existing analysis for this construction to turn a tweakable blockcipher into a PRF?
@nightcracker, (regarding your first comment:) yup, I understand that. I don't think that contradicts anything in my answer. I think my answer remains valid.... If I've missed some implication or connection or inference, you might need to spell it out, as I'm not seeing it.
Sep
24
comment What stops the Multiply-With-Carry Random Number Generator from being a Cryptographically Secure Pseudo-Random Number Generator?
@e-sushi, "statistical purposes" is not an answer to CodesInChaos's question. There is no statistical simulation ever done that will use $2^{256}$ outputs from such a generator.
Sep
22
comment Is SSH public key authentication weakened by 'none' cipher?
@CatalinPatulea, for SSH, compromise of your session is already devastating (because the attacker can insert a single command that places a backdoor into your account on the server), so leaving the session unprotected would be extremely dangerous.
Sep
22
comment Theoretical pi-based stream cipher
Your assumption that there are no patterns in the digits of pi, surprisingly, turns out to be false. Amazing, but true!