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seen Jan 17 at 23:30

May
8
comment Size of p in ElGamal cryptosystem?
No, it is theoretically impossible, unless you alter either the modes or the ElGamal encryption algorithm beyond recognition. I am curious about if you had any such alterations in mind, and if you can point to a security analysis of the result.
May
8
comment Size of p in ElGamal cryptosystem?
How, and why, would you possibly use ElGamal encryption in a chaining mode? It is not a block cipher and the cipher text is always greater than the plain text.
Apr
25
comment Is the product of two primes only factorisable by those two primes?
The answer is "Yes".
Apr
21
comment Stateless hash based public key cryptography?
Indeed, thanks. Using a seeded deterministic PRF for the private keys will make it possible to recreate everything. I think it requires careful balancing of the parameters to get acceptable performance, though. Using a ten level fractal of hash trees with $2^8$ leaf nodes seems feasible.
Apr
21
comment Stateless hash based public key cryptography?
Could you please expand a bit on the details? Clearly, with this approach you would still have to save all sub-trees in your fractal, because you can't rule out the possibility you might have to revisit them later. (If you could rule out that possibility, you would only need the top level tree, but then you would still have to save all nodes of that tree.)
Apr
16
comment SSL-like protocol with public-key hard-coded in the client
The trivial answer is to use a CA and hard code the public key of the CA in the client. I presume you thought of that already, so what is wrong with that solution?
Apr
3
comment TLS Key Block calculation - What is a PRF?
Nit-pick: "TLS's PRF" should actually be "TLS's TLS PRF". The construct you describe is called "TLS PRF" and is recommended, but not strictly mandatory for new cipher suites.
Mar
31
comment TLS/SSL's usage of Non-Ephemeral DH vs DHE
A diffie-hellman public key will, when encoded for a X.509 certificate, be coupled with it's domain parameters, which includes the prime modulus $p$, generator $g$, group order $q$ and (optionally) co factor $j = (p-1)/q$. An EcPublicKey is otoh commonly coupled with an object identifier signifying which standard group the key belongs to, such as P-256, P-521 etc.
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 $5/6$.