Fixee

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Oct 15	comment	Does AES have any fixed-points? Do you consider the proof that all AES permutations are even to be "suspicious" as well then?
Oct 15	comment	Does AES have any fixed-points? @Paŭlo In all fairness, this is probably a research question (ie, it's open and probably hard) and your answer above is quite sound as long as you emphasize that you must make this unproven (but reasonable) assumption about AES's randomness.
Oct 15	comment	Does AES have any fixed-points? But AES is not a random subset of the $2^{128}!$ permutations on 128 bits; for example, it generates only even permutations. How do we know it doesn't select only from the derangements?
Oct 15	comment	Now that quantum computers have been out for a while, has RSA been cracked? "Their claimed speedup over classical algorithms appears to be based on a misunderstanding of a paper my colleagues van Dam, Mosca and I wrote on "The power of adiabatic quantum computing." That speed up unfortunately does not hold in the setting at hand, and therefore D-Wave's "quantum computer" even if it turns out to be a true quantum computer, and even if it can be scaled to thousands of qubits, would likely not be more powerful than a cell phone." -- Umesh Vazirani, UC Berkeley
Oct 15	comment	What are the practical difference between 256-bit, 192-bit, and 128-bit AES encryption? It should also be noted that 128-bit AES has had a lot more scrutiny than 192- and 256-bit AES.
Oct 12	comment	Why has the RSA factoring challenge been withdrawn? Perhaps RSA needed to invest that money into their new SecurID technology! :)
Sep 28	comment	For Diffie-Hellman, must g be a generator? @Thomas: Improved attacks could also kill DH (or require parameter adjustments). I guess I'm trying to advocate that authors of cryptographic content stop saying (for example) "factoring large integers is impossible" and instead say "there is no publicly-known efficient factoring algorithm on conventional computers". Although I suppose it can get tiresome after a while. Nonetheless, I'm very careful to say the latter when teaching.
Sep 28	comment	For Diffie-Hellman, must g be a generator? Implied in all of the above is "according to current cryptanalysis". The parameters above are a function of (1) our current computing model and (2) our current state of knowledge.
Sep 27	comment	Is modern encryption needlessly complicated? @Ivo: DES was regarded as "broken" upon release (due to key length); 3DES is still secure (but painfully slow); if you know an attack, please post it. 3DES has an effective key length of at least 110 bits. When I said that DES has never been broken, I mean the construction, not the parameter set.
Sep 27	comment	In RSA, do I calculate d from e or e from d? In general, I would trust this site for crypto more than wikipedia since things are vetted here pretty much immediately after they're written.
Sep 27	comment	How to fairly select a random number for a game without trusting a third party? Another fix is to require the number of random bits be higher than (say) 256, and disallow repeated digests. Or better, to force the 2nd publisher of a repeated digest to reveal first (better because it allows you conclusively identify cheaters).
Sep 22	comment	Accelerating SHA-1 @fgrieu: Is there a way to see if I'm having cache misses and how much they're costing me?
Sep 22	comment	Accelerating SHA-1 @Thomas: reducing my buffer results in no difference in runtime; I'm not sure why you think L1 cache matters here since we process data exactly once (I'm running SHA1 over an 8GiB file).
Sep 22	comment	Accelerating SHA-1 @jug: I've never heard this before (that Intel is quite a bit faster on a single core)... can you point me at a reference for this? I'd like to see if switching hardware would help.
Sep 21	comment	Accelerating SHA-1 @jug: I did find that link, which is what led me to updating to the latest OpenSSL. Unfortunately, it was no faster.
Sep 21	comment	Accelerating SHA-1 @Samuel: I'm processing a single incoming stream arriving at 5Gbps from the network and I'm currently bottlenecked by SHA-1 at 2.6Gbps; you're right that SIMD is useful: the XMM and YMM registers are part of the SSE(1-4) instructions which are already used in the OpenSSL code.
Sep 20	comment	Accelerating SHA-1 Thomas: the authors of the paper I cite in my answer claim that a GPU does help. But they talk about "SSL flows" so perhaps they are inducing parallelism this way?!
Sep 20	comment	Accelerating SHA-1 @fgrieu: I'm hashing 128KiB at a time via SHA_Update (OpenSSL) in a tight read loop. I need a hash for the entire file, so I cannot run hash invocations in parallel (since SHA1 needs the previous chaining value to start computing the next).
Sep 20	comment	Accelerating SHA-1 I have some latitude with the protocol... you're thinking to hash separately on multiple cores?
Sep 19	comment	Converting a stream cipher into a block cipher I'm asking out of general interest; to me it's interesting if one primitive can be efficiently converted to the other.