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Jun
25
comment How come Shamir Secret Sharing uses Lagrange interpolation?
Thanks, I improved it a bit further, to make it a bit a clearer exactly which calculations are made.
Jun
17
comment Is HMAC needed for a SHA-3 based MAC?
@figlesquidge Security is a relative notion. The question you link to in your comment, as well as the accepted answer to that question, explains in detail to what extent HMAC-SHA3 would be more secure than using in SHA3 in its built in MAC mode.
May
30
comment Update to “Cryptographic Right Answers”
@otus: true, using a CA provides no advantage in case you know the server private key is compromised. However, an educated guess is that most of the time a private key is compromised the owner is blissfully unaware of the situation, which is why certificates are normally set to expire. Expiration requires a CA, in order to be handled transparently by the client software.
May
30
comment Update to “Cryptographic Right Answers”
@otus: In order to avoid having to replace all client software due to private key compromise, the client software should include the public key corresponding to the private key that stands the least risk of becoming compromised.
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.