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Mar
28
comment What might be assumed about a PRF if the key has been chosen?
I'm confused by "what might be assumed about..." -- you can of course assume anything you want. Do you mean, assuming only that $f$ is a PRF, what can we say about security if the key parameter is not chosen randomly? If so, the answer is: there are no guarantees whatsoever. There are bad cases where the function $f$ is no good at all, if the key is predictable or has low entropy. In TLS, the spec calls the function a PRF, but actually the protocol implicitly requires stronger assumptions than just that it's a PRF; it also needs to have some hash-like properties as well.
Mar
28
comment ECDSA signature verifiable 1-way transformations
@ChristopheBiocca, you can turn any interactive ZKPoK into a non-interactive ZKPoK using the Fiat-Shamir heuristic (basically, use a hash function to choose the challenge). That should give you what you want.
Mar
28
comment Distinguishing Attack on CBC-MACs
@user11291, read fgrieu's earlier comment, and do some learning on your own about the birthday paradox.
Mar
28
comment secure integer comparison
@ahmed, alas, I don't know of a reference for this protocol, and unfortunately I don't know anything better -- sorry about that -- but if you want to explore further, it might be worth taking a look at the protocol I mention in the last paragraph.
Mar
22
comment Is PKCS #1 v1.5 RSA encryption padding secure under these conditions?
Your statement "Alice cannot know if Bob accepted the message" does not seem to be well-justified (at least, it does not follow from the rest of your conditions).
Mar
21
comment Parallel-resistant proof-of-work scheme without hidden knowledge
Thanks, that helps! For randomized schemes, can't you always make them deterministic by seeding a deterministic PRNG with some seed and using its output everywhere that the algorithm makes a random choice? Now the problem/puzzle generated will be a deterministic function of the seed: the same seed always yields the same puzzle. Does that achieve what you're looking for or does it fall short in some way?
Mar
21
comment Is this approach to generating a “random” number from a sha512 hash effective?
Good answer! I'd also like to refer the author to crypto.stackexchange.com/q/767/351.
Mar
21
comment Can passwords be stored securely so that a similarity comparison can be made?
There are better answers to this question at security.stackexchange.com/q/3170/971 and security.stackexchange.com/q/36893/971 and security.stackexchange.com/q/53481/971.
Mar
21
comment secure integer comparison
Is it OK to reveal the offer of the winning ISP (the one with the best offer), in identity to the winning ISP? Do you care more about computation time or latency (total time)? Is it OK to do an interactive computation that involves many round of interaction if this keeps the computation time down?
Mar
21
comment Parallel-resistant proof-of-work scheme without hidden knowledge
I confess I don't understand point 3. What would it mean to violate point 3? Also, what do you want the third party to be able to verify, and would it be OK if verification requires cooperation from the creator of the puzzle?
Mar
21
comment Entropy when iterating cryptographic hash functions
Thanks, Stephen! I've edited the question further to ask about the general situation, as @fgrieu suggests.
Mar
20
comment What is the idea behind hashing the QueryString in OAuth?
I'd speculate that they want to protect the querystring parameters from tampering/modification. If they weren't included in the hash/MAC input, an attacker could change them freely and the modification would go undetected.
Mar
19
comment Zero Knowledge Proof for Correctness of the product of additive ElGamal Ciphers
I encourage you to put in a bit more effort on formatting the question to be easily readable. Did you know you can use Latex (Mathjax) on this site? See the help center for more.
Mar
19
comment Entropy when iterating cryptographic hash functions
@fgrieu, great point! For large enough $i$, this formula certainly becomes inaccurate. For instance, when $i \ge 2^{n/2}$, it is likely that the entropy will be about $n/2$ bits, and after a certain point it won't get any smaller no matter how much you increase $i$ (because typically when iterating a large random function, there is a single large cycle of length about $2^{n/2}$ that most inputs feed into). Thank you!
Mar
18
comment Subexponential algorithms for DLP in $\mathbb{Z}_s \times \mathbb{Z}_t$
JasonJones, yup! It does suggest that the DLP is easier to solve than in the corresponding elliptic curve. It does suggest that the DLP can be solved faster than exponential time. The faulty premise is in assuming this means it is claiming that the DLP can be solved in sub-exponential time, e.g., in $L_n[\alpha,c]$ time.
Mar
18
comment Meet in the middle attack - message and key
Read about known-plaintext attacks.
Mar
18
comment Subexponential algorithms for DLP in $\mathbb{Z}_s \times \mathbb{Z}_t$
This question starts from a faulty premise. The accepted answer you link to does not suggest that there are subexponential algorithms for solving DLP in $\mathbb{Z}_s \times \mathbb{Z}_t$. (In fact I don't even see the word subexponential in that answer.)
Mar
18
comment Are there hash algorithms with variable length output?
@curious, again, ask a new, separate question -- and make sure to follow best practices before doing so (e.g., searching to make sure it hasn't been answered before). This is not a discussion forum. One question per question. I'm not going to answer any new questions posed in the comment threads.
Mar
18
comment Are there hash algorithms with variable length output?
@curious, that's a different question, which is probably best answered separatately, but the concise answer is: treat the output of SHA256(m) as a 256-bit integer, reduce it modulo 360, and use the remainder a syour random number.
Mar
16
comment What functions allow for practical indistinguishability obfuscation?
Thanks, @xagawa! On log-size circuits: What is the log of? i.e., log of which parameter? (presumably not the size of the input to the circuit) For normal forms: are there known classes of circuits that have a normal form? I know that BDDs can be put into normal form. That's all I know but I'm guessing probably more is known....