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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
answered LFSR Output Sampling for Berlekamp-Massey
Jun
25
answered Encrypt-then-MAC: full random keys or keys derived from master key?
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
answered Is it a good idea to use Lagrange/Newtonian interpolation for encryption?
Jun
25
revised Choose a random number that is different from a bunch of other secret numbers
Link to more on commutative encryption.
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
reviewed Approve suggested edit on Choose a random number that is different from a bunch of other secret numbers
Jun
24
revised Choose a random number that is different from a bunch of other secret numbers
added 409 characters in body
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
answered Choose a random number that is different from a bunch of other secret numbers
Jun
24
revised Choose a random number that is different from a bunch of other secret numbers
Incorporate stuff in the comments.
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
24
answered Choose a random number that is different from a bunch of other secret numbers
Jun
24
revised Choose a random number that is different from a bunch of other secret numbers
Remove incorrect statements, to keep only the stuff that'sactually correct.
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.