Thomas

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	2,589 reputation	bio	website		visits	member for	1 year, 4 months
1320 badges		location	Wellington, New Zealand		seen	4 hours ago

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Mar 21	revised	*How to calculate y value from ((yy) mod prime) efficiently** edited tags
Mar 21	comment	Could a very long password theoretically eliminate the need for a slow hash? Only if each character of the password is randomly (and independently) selected from the relevant alphabet, which is typically not the case for humans. Of course, a 43-character password does provide a huge safety margin, but some people will still use "123123123..."
Mar 20	comment	Sensible usecase for restricting special characters in passwords? I'd say ease of use. A password with a newline or control character in it would be somewhat awkward to type in. Since nobody will ever use those, why not disable them and spare an unlucky user the pain of figuring out why his password is not being accepted the next time he types it in?
Mar 20	comment	How would one crack a weak but unknown encryption protocol? It's not so much direct analysis on the ciphertext, which doesn't work unless the encryption algorithm is truly crap (for instance, there is no easy distinguisher between, say, AES and Blowfish). The real danger is someone disassembling software which uses your "unknown" protocol and figures out how it works, at which point you better have a secure key to fall back on, or you're screwed. A single ciphertext won't tell you much if your target's cryptographic skills are beyond ROT13 level.
Mar 19	reviewed	Approve suggested edit on Key Exchange and anonymity issue
Mar 18	comment	Solving congruences using PARI I don't know much about PARI/GP but I think `Mod` does more than just take the value of `a` modulo `b` (it changes its datatype, too). Can you try just using `a%b` instead? Though I'm not sure how to get the modular inverse in that case, you'll need to look that up. I'm sorry, I'm not of much help - I don't use PARI/GP.
Mar 17	comment	Solving congruences using PARI Did you read the algorithm outlined on the Wikipedia page I linked in your previous question? There's a walkthrough with 2 congruences (pairwise coprime, though) and then for the general case. Then you can implement it using only standard arithmetic and a modular inverse algorithm (which PARI should have). If this is about using a built-in function of PARI/GP to solve general systems of congruences, it's off-topic here.
Mar 17	reviewed	Close Tree hash and multithreading for parallelism
Mar 17	comment	Export from US of crypto software with key-size > 56 bits still needs permission? These export regulations are (thankfully) obsolete, though cryptography is still considered a weapon and some restrictions still apply. See current regulations... this is a good read too.
Mar 17	answered	Low Public Exponent Attack for RSA
Mar 12	comment	Method and explanation for calculating difference in speed between DES and RSA The question makes no sense. You don't encrypt the same type of data with RSA or DES, the comparison is meaningless. Why should you care that RSA is "100 times slower than DES"? You're not going to be encrypting megabytes of data with it. It's a useless metric, things like "encryptions/signatures/verifications per second" are much more sensible.
Mar 11	reviewed	Approve suggested edit on Tree hash and multithreading for parallelism
Mar 10	comment	Where to store the private key and the public key in a communication protocol "And i decided to create my own protocol about that." what motivated this change of mind? Generally, it's very dangerous and irresponsible to invent your own crypto protocol.
Mar 9	comment	Is this method for exchanging private key using RSA sound? let us continue this discussion in chat
Mar 9	comment	Is this method for exchanging private key using RSA sound? The randomness comes not from the group parameters, but from the $a$ and $b$ individual secrets of each party, which are selected randomly. Then the shared secret is $g^{ab}$ which is therefore unique to each session. The Wikipedia article is pretty good, if you haven't read it already (though I recommend not to implement it yourself). I cannot help about the C# implementations, though.
Mar 9	comment	Is this method for exchanging private key using RSA sound? Ah, sorry, I did not see that the keys were encrypted with each other's public keys. Ignore my last message. If you already know each other's public keys, then yes, there should be no problem. But there are a few practical issues (not security, but usage). I'll let others elaborate. But I don't quite understand why you don't want to use DH. The shared secret obtained from DH changes for every session, even if you keep the same group parameters.
Mar 7	awarded	Nice Answer
Mar 7	reviewed	Edit suggested edit on Examples of applications that use the Schnorr digital signature?
Mar 7	revised	Examples of applications that use the Schnorr digital signature? Example application provided - OpenSSL
Mar 7	comment	Strength of Combining Hash functions @user5195 Which one? I enumerated two of many possible properties (see CodesInChaos's comment above mine).