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Jul
22
comment MAC using a modified CBC mode of operation
What have you tried? What are your thoughts? What kinds of attacks have you considered? Have you tried to see if you could prove it secure? Have you looked at the special case where there are exactly 2 plaintext blocks? I don't think copy-pasting an exercise from an earlier course is a great fit for this site. We want to help clear up any confusions you might have, but as for doing an exercise problem for you -- well, maybe not so much. I don't think it benefits the world for us to be a site where people can paste exercises and have someone else produce a complete solution for them.
Jul
20
comment Turning PRPs in PRGs with a counter
You might like to read about the PRP/PRF switching lemma, which helps show that CTR mode is a secure PRG "up to the birthday bound" (i.e., as long as you don't generate too many outputs).
Jul
18
comment Modes of operation that allow padding oracle attacks
Can you define what you mean by padding oracle attacks? If you count switching the order of the blocks around in ECB mode to be a padding oracle attack, then you're using a non-standard meaning of the phrase (since in that case you're not even attacking the padding function nor using it as an oracle). Do you maybe simply mean chosen-ciphertext attacks on confidentiality?
Jul
16
comment Does having a known plaintext prefix weaken AES256?
What research have you done? This is already answered by several questions on this site, e.g., crypto.stackexchange.com/q/1512/351, crypto.stackexchange.com/q/3952/351. (If you'd searched, you should have found them immediately; and they already show up in the related questions list.) We expect you to do a significant amount of research before asking here, including searching on this site for other questions and answers that might shed light on your question. At worst, it will help you frame a better, more focused question; at best, it might answer your question for you.
Jul
10
comment CBC-MAC length extension attack with IV simulation
Please edit the question to make clearer what the problem is. We don't have the second part of 49th problem in Matasano's problem set memorized in our head. Right now, the question does not seem answerable to me. Also, I suggest you edit the question to tell us what research you've done, what attacks on CBC-MAC you are familiar with, and what you've tried.
Jul
10
comment Choosing primes in the Paillier cryptosystem
I can't understand what you are asking. What does "Give me a reference if proof for above equation complex otherwise provide a proof." mean? Also, what research have you done? Did you read the original research paper?
Jul
2
comment Password-based encrypted key storage?
@otus, RFC 5959 only uses AES in ECB mode if the plaintext is exactly 128 bits long. That is not malleable (not in any useful way that would enable related-key attacks).
Jun
26
comment RSA: Common modulus attack problem
CGFox, rather than editing your question here, it would be better to propose an edit to the answer on the other thread that adds a clarification of how to deal with a negative exponent, i.e., what it means to raise to the $s_1$ power if $s_1$ is negative.
Jun
25
comment Choose a random number that is different from a bunch of other secret numbers
@CedricMartin, the standard commutative encryption algorithms (e.g., Pohlig-Hellman) do not require a TTP to hold any keys. The prime $p$ is public and shared by everyone; each participant's exponent is secret and is their key and is generated by them and known only to them. No TTP is needed. If you have questions how to use commutative encryption in practice, I suggest reading the existing questions, then posting a new question if it's not covered by the ones that are already on this site.
Jun
25
comment Choose a random number that is different from a bunch of other secret numbers
@CedricMartin, you are aware that this scheme (from otus) doesn't require a TTP, right?
Jun
25
comment Choose a random number that is different from a bunch of other secret numbers
Very nice! Thank you for elaborating. This looks to me like the best answer I've seen.
Jun
24
comment Choose a random number that is different from a bunch of other secret numbers
(...) in the first place, and could we change that procedure? Also, you might want to post a separate question where you take a step back and look at your application's requirements (what problem you want to use this protocol to solve) and ask about how to solve that problem, without assuming this protocol is the best way to do it. I suspect there might be other very different approaches that are possible, but without knowing the application, it's hard for tell.
Jun
24
comment Choose a random number that is different from a bunch of other secret numbers
I've been thinking about this some more, and I think there are a lot of options here. Cedric, can you tell us more? Will this protocol be repeated many times (with the same secret values for the participants), and if so, what are the requirements (a different random number each time, or it's OK for them to repeat) and how many times (more than $x-n$?)? What security property does your application need for the random numbers? Is there a trusted third party? Is there a trusted dealer (a TTP who can provide initial setup/keys)? How did the participants choose their secret values (...)
Jun
24
comment Choose a random number that is different from a bunch of other secret numbers
This is not a great scheme. The problem is that any time a participant vetos a candidate random number, everyone learns what that participant's secret number was (it must have been the same as the candidate random number we were considering). See my answer for a scheme that is a bit better, because it doesn't reveal which participant vetoed the candidate.
Jun
24
comment Choose a random number that is different from a bunch of other secret numbers
Thanks, nice edit! Here's one more intereseting fact. Proving that two numbers are different can actually be done efficiently, using the proper scheme. See my answer (you use a scheme that is homomorphic for 2-DNF formulas, noticing that $C$ computes a 2-DNF formula).
Jun
23
comment Fastest random number generator
Are you looking for true random numbers, a cryptographic-strength pseudorandom number generator, or a non-cryptographic pseudorandom generator? Please edit the question to state explicitly. Also please edit the question to respond to the other questions in the comments.
Jun
23
comment What would make it impossible to deny that decryption of a package has taken place?
package? What do you mean by that? Do you mean packet?
Jun
23
comment Which concrete applications benefit from Oblivious RAM constructions?
This paper is a great find and very relevant. On the other hand, its threat model makes some assumptions that might not be realistic in practice. It requires that the server know some of the queries (a known-plaintext setting) and have a priori knowledge on some frequency statistics concerning the unencrypted emails. I find it hard to tell whether their attacks will be realistic in the real world or not.
Jun
23
comment how to truncate a PRF on n bits to PRF on t bits where t < n?
Better, in what way? The construction Boneh provides is a perfectly reasonable choice. Unless you can define in what objective metric/criteria you would use to define "better", I don't think this question is well-defined.
Jun
23
comment Cryptography: Oblivious Transfer with at most one transfer?
I am finding this question hard to follow. What do you mean by a "transfer method"? What is $Sc$? Do you mean $s_c$? What counts as bits transferred to Bob? Can they invoke $F$ as many times as they want? Who provides the inputs to $F$ (does $x$ come from Alice and $y$ from Bob)? Are we guaranteed that $p,q$ are chosen randomly subject to the condition that $p \oplus q = x \times y$? Please edit the question to make the problem statement clearer.