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Jun
20
comment Public and private key lengths
What are you trying to achieve? What problem are you trying to solve? What's wrong with leaving that there? It is part of the standard format: see PGP and OpenPGP.
Jun
18
comment LFSR and Markov chain question
I don't understand in what sense the Berlekamp-Massey algorithm is an algorithm for "this problem" - what problem? Please try to be more precise. Berlekamp-Massey has nothing to do with Markov chains; it has to do with recovering the LFSR initial fill and feedback polynomial, given observations of its output. As far as your last question in your comment: for what task? what Markov chain formulation? It's impossible to say what resources are needed, if we don't know what the task is. Again, LFSRs are a deterministic process, so modelling it as a Markov chain is not very helpful.
Jun
18
comment LFSR and Markov chain question
I find this question unclear. What do you have in mind as the statespace of the Markov chain? If the state is the entire state of the LFSR, then of course there is a trivial Markov chain, since it is a deterministic process, but this does not seem likely to be useful. So what did you have in mind? Let me put it another way. Why do you ask this question? What's the motivation or context where you plan to use this? How will you evaluate or use answers? As it stands the question seems a bit on the open-ended/ill-defined end.
Jun
18
comment Length-preserving all-or-nothing transform
@fgrieu, actually, I think you have it backwards. For large size, it is straightforward (in the random oracle), as explained in the section "Theoretical perspective": use a cryptographic hash function as the F-function of a 6-round Feistel network. I actually think the more challenging case is to do it for small block size, not large block size.
Jun
18
comment Authenticated encryption without padding
@RickyDemer, I've updated my answer to provide a construction that you might like better. If provable security is a goal, a better construction is likely to be $E_{k_2} \circ P \circ E_{k_1}$ where $P$ is a public random permutation and $E$ is a length-preserving encryption scheme. (Then even in the case where $E$ is merely xor with a constant, i.e., $E_k(x)=k \oplus x$, then it can still be proven to provide reasonable security, as this devolves to the Even-Mansour construction, which offers provable security guarantees.)
Jun
18
comment Can the premaster secret generated by SRP be used as a secure private key?
I recommend that you state the question you want answered in the body of the question.
Jun
18
comment SHA1 collision event probability after n iterations
Did you look at crypto.stackexchange.com/q/15068/351 ? (Did you remember to use search before asking?)
Jun
18
comment SHA1 collision event probability after n iterations
What do you think? What have you tried? Where did you get stuck? Where did you run into this question? What is the context/motivation for your question? This is not a site where you copy-paste your exercise and we do your exercise for you.
Jun
18
comment What are good combinations of public key algorithms or primitives for long term security?
I wouldn't recommend lattice-based methods at this point in time, for long-term security; they haven't been studied in as much detail as others. It wouldn't be shocking if someone discovered a new attack 10 years from now that breaks them significantly faster than we currently know how to.
Jun
11
comment Does this protocol provide Perfect Forward Secrecy / are there potential security flaws?
Please make sure to provide a specific technical question in the body of the question. The title is for, well, a title -- and does not replace to need to ask a specific question in the body of the question.
Jun
7
comment Security of a security protocol for key exchange, using symmetric-key cryptography
OK, I see, I apologize for my misunderstanding.
Jun
7
comment Obfuscate an “I'll reveal if you do” function
An interactive commitment protocol would be fine, as long as the verification is non-interactive (given an alleged opening of the commitment from Alice, Bob can check entirely on his own whether it is indeed a valid opening). If this problem gets easier for Naor commitments, that would be fascinating and useful!
Jun
7
comment Obfuscate an “I'll reveal if you do” function
@mikeazo, I don't recall what the standard definition of a commitment scheme is; my recollection is that it doesn't really matter what $V(C(x,r),x,r')$ outputs, if $r \ne r'$ (it could output either $0$ or $1$, and no harm would be done either way).
Jun
7
comment Update to “Cryptographic Right Answers”
@otus, absolutely! You make a very timely and accurate point. The challenge is to find a simpler protocol that one can recommend to developers to provide a secure channel. I'm not aware of one; but maybe you have some suggestions?
Jun
7
comment Obfuscate an “I'll reveal if you do” function
Thank you! Yes, I do have an a-priori bound on the length of $x$ and $y$, so that removes one possible complication. It would be fine to use any commitment scheme that makes the obfuscation task easy, so if Merkle tree based commitments make obfuscation easier, that would be fine.
Jun
6
comment Attack on a key-exchange,symmetric-key cryptography protocol
I don't understand this answer yet. Are you thinking of gathering this information for all possible values of the nonce? Normally, nonces are chosen to be from a large enough space that such an attack will be wildly impossible: e.g., a nonce might be a random 128-bit number. Would you like to try editing your answer to clarify what attack you had in mind and what security property is violated by your attack?
Jun
5
comment Universal Circuits in Indistinguishability Obfuscation Candidate Construction
I don't know. Maybe the security proof requires use of the universal circuit? (e.g., to ensure the branching program will have the same structure regardless of what the original circuit is, or something)
Jun
3
comment Theory for pre-paid debit card: card-to-card transfer
That's one kind of approach. The other kind is to use a coin-based scheme for e-cash, e.g., Chaum-style e-cash (based upon blind signatures).
Jun
3
comment Theory for pre-paid debit card: card-to-card transfer
What research have you done?
Jun
2
comment Which one of the Block Cipher modes is the best?
There is no "best". Best for what purpose? They have different characteristics. Those characteristics are well-documented.