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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.
Jun
2
comment Security of a security protocol for key exchange, using symmetric-key cryptography
Have you tried using an automated protocol analyzer, like AVISPA, ProVerif, CryptoVerif, Scyther, Tamarin, EasyCrypt, CertiCrypt, Maude-NPA, etc.?
Jun
2
comment Why is plain-hash-then-encrypt not a secure MAC?
@IlmariKaronen, Good point! This is worth considering and discussing. Looks like there are many possibilities. Another possibility that might make even more sense would be to close this question as a dup of crypto.stackexchange.com/q/6069/351, since they are both specifically about stream ciphers. (I do not suggest closing Gilles' older question as a dup of this one; I think Gilles' older question is better-written and better-researched, whereas I find this question a bit confusing and hard to follow.)
May
31
comment Security of the iterated Hill Cipher
@LinearAlgebra, OK, I've edited my answer to show how to break the revised scheme, given your updated question.
May
31
comment Creating a license system based on asymmetric encryption (RSA or ECDSA)
@CodeX, sure. I expanded my answer to elaborate further on revocation (your question #3) and stolen private keys (your question #4).
May
31
comment Why is plain-hash-then-encrypt not a secure MAC?
@e-sushi, this question asks whether $\text{Encrypt}(M || H(m))$ is secure, where $H$ is a hash (not a MAC). The question I linked to asks whether $E_k(M || H(M))$ is secure. It's the same question. OK, the latter question focuses on CBC encryption, but the answers there show an attack when $\text{Encrypt}$ is CBC mode encryption, which already answers this question and shows that this question's scheme is not secure in general. And the case of stream ciphers is already handled by crypto.stackexchange.com/q/6069/351. So this question is completely subsumed by existing questions.
May
31
comment Why is plain-hash-then-encrypt not a secure MAC?
possible duplicate of Does CBC encryption of a hash provide authenticity?
May
31
comment The improvement of the private key exponent in the M.Weiner Attack
I'd expect you to do more research on your own before asking here (on this site we expect you to do a significant amount of research before asking). In the future, one way to answer this sort of thing on your own is to do a literature search.
May
22
comment how to use common modulus attack?
This is not a homework-answering or exercise-solving service. We want to help you, but we can't do that if you haven't shown us what you have tried. So, I suggest you edit the question to show us what you have tried and where you got stuck. Finally, make sure you give proper attribution to the source where you copied this from.
May
22
comment Which tamper-protection algorithm provides the shortest output?
@Gilles, as you say, there are two separate issues: (1) uses out of context, and (2) replays. In my opinion, a good solution needs to solve both ("you may want to" is not enough). In my opinion, this is something that is not optional; it's mandatory, if we want to deploy this in practice and be secure. So, I think this answer would benefit from more on how to handle the practical security challenges (the hard parts of this problem are not the crypto algorithms but how to ensure they'll be used appropriately in practice). Optional security will often fall short, because it be left disabled.
May
22
comment Security assessment between $g^{a_ix_i+r_i}$ and $g^{x_i+r_i}$
@curious, that belongs in your question. Don't ask "chameleon questions", where new requirements get revealed or drip-fed to us only after someone posts a valid answer to the original question. Think through your requirements and all relevant information, and make sure they are present in the question from the start.
May
22
comment Security assessment between $g^{a_ix_i+r_i}$ and $g^{x_i+r_i}$
Why are you trying to invent some new mechanism? Why aren't you using standard "semantically secure" public-key encryption? (You can use additively homomorphic public-key encryption, if that's what you need.) Also, I agree with the comments that the requirements don't seem clear. Finally: please edit the question to include all relevant information in the question itself. Don't just drop them in the comment thread. Comments exist only to help you improve the question, and the question needs to stand on its own (without having to read the comments).
May
22
comment Which tamper-protection algorithm provides the shortest output?
This answer is not sufficient, because it doesn't provide freshness (it doesn't prevent replaying of old values). The fact that your scheme provides both confidentiality and integrity/authenticity for all values is a good thing, though.