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Mar
29
comment What might be assumed about a PRF if the key has been chosen?
@D.W. TLS 1.2 specifies a generic PRF based on HMAC with a configurable hash function. This composition is called TLS PRF. It is recommended with a "SHOULD" for new cipher suites, but might, technically, be replaced in additional cipher suites defined in other RFCs.
Mar
28
comment What might be assumed about a PRF if the key has been chosen?
FTR, I am personally not sure including the handshake hash in the master_secret is the right way to go, in order to prevent the secure-resumption family of attacks, in particular if the adversary might participate in the design of the cipher suites. I might however be over cautious and would appreciate some input.
Mar
28
comment What might be assumed about a PRF if the key has been chosen?
@RickyDemer: So the answer is "no" and "no"?
Mar
27
comment Any field in a PKI certificate where some text info can be stored?
This is off topic, but the most common acceptable way would be for the issuer to include the text in the Subject name of the issued certificate. The "right" way, however, is to add a Certificate Policies extension, and add a link to a web site with the information you want to associate with the certificate.
Mar
17
comment Hash Based Encryption (fast & simple), how well would this compare to AES?
Why did you mark the first entry as a duplicate?
Mar
17
comment Hash Based Encryption (fast & simple), how well would this compare to AES?
@fgrieu: You are right, related key attacks against SHACAL-2 is an independent topic, and it has been studied.
Mar
1
comment Need Help Reversing my Encryption Algorithm
This is off topic, because any answer would be of little or no use to anyone else. If you want to read up on one common way of making encryption algorithms invertible, you could e.g. check out en.wikipedia.org/wiki/Feistel_cipher
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
Perfectly correct, thanks.
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
$n$ that is a product of the smallest primes is a 'worse' case than powers of $2$.
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
If $GCD(x-1,n) \neq 0$, then $f^{-1}(x) = x$. Otherwise, if $GCD(x-2,n) \neq 0$, then $f^{-1}(x) = \{x-1,x\}$ etc. I think this recursion has an estimate worst case running time of $ln(n)$.
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
Also, testing if $x|0$ is just a matter of testing if $GCD(x,n) = 0$.
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
The value of $n - \varphi(n)$ is highest for $n$ that consist of a product of the smallest primes, each with exponent $1$. For $n = 2\times3\times5\times7\times11\times13\times17\times19\times23$ it is approximately $5/6$.
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
If it does invalidate the 'invertible' term, then, by the pigeon hole principle, there are no functions that satisfy the criterion in the question.
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
I think this depends on the definition of $\mathbb Z_n^\times$. If we exclude not only 0 but also all divisors of 0, and $n$ is composite, then there are more bad points, yes. In such case a trivial example is the identity function except $f(x) = 1$ if $x|0$.
Feb
24
comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$?
Of course there are. A trivial example is the identity function except $f(0) = 1$.
Feb
24
comment Get permutations from password
@fgrieu: I am afraid using a slow PBKDF will not matter much unless a salt is used as well. If no salt is used, slowing the PBKDF down will just slow down the precomputations (generation of permutations corresponding to common passwords).
Feb
24
comment Get permutations from password
@fgrieu: Yes, that would be a more efficient algorithm. It requires a deeper understanding of permutation composition for the reader to deduce surjectivity, though, but it should be added to my answer.
Feb
21
comment Why do we use XTS over CTR for disk encryption?
Nice answer, but newer content replaced with older content will be a concern even if you use authenticated encryption. Prevention of replay attacks requires more than just authenticated encryption.
Feb
21
comment Is script execution time a decent source of pseudorandom number generation?
There might be a certain amount of unpredictability in this (presuming the adversary doesn't have full access to the parent system and you don't do this in the context of responding to the request), but you need a high resolution timer (not just microtime) and more than a single timing.
Feb
20
comment What informal indicators exist for estimating the computational infeasibility of cryptographic problems?
Also, it would be possible to formulate a valid hypothesis based on anecdotal experience, either if it demonstrates that no such study could ever eliminate a significant amount of inherently unknown hidden statistics, or, conversely, if it indicates exactly what kind of evidence should be collected to reach a reliable answer.