Messages in this thread | | | Date | Mon, 27 Dec 2021 10:52:17 -0500 | From | "Theodore Ts'o" <> | Subject | Re: [PATCH v2 2/2] random: use BLAKE2s instead of SHA1 in extraction |
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On Thu, Dec 23, 2021 at 03:11:13PM +0100, Jason A. Donenfeld wrote: > This commit addresses one of the lower hanging fruits of the RNG: its > usage of SHA1. > > BLAKE2s is generally faster, and certainly more secure, than SHA1, which > has [1] been [2] really [3] very [4] broken [5]. Additionally, the > current construction in the RNG doesn't use the full SHA1 function, as > specified, and allows overwriting the IV with RDRAND output in an > undocumented way, even in the case when RDRAND isn't set to "trusted", > which means potential malicious IV choices. And its short length means > that keeping only half of it secret when feeding back into the mixer > gives us only 2^80 bits of forward secrecy. In other words, not only is > the choice of hash function dated, but the use of it isn't really great > either. > > This commit aims to fix both of these issues while also keeping the > general structure and semantics as close to the original as possible. > Specifically: > > a) Rather than overwriting the hash IV with RDRAND, we put it into > BLAKE2's documented "salt" and "personal" fields, which were > specifically created for this type of usage. > b) Since this function feeds the full hash result back into the > entropy collector, we only return from it half the length of the > hash, just as it was done before. This increases the > construction's forward secrecy from 2^80 to a much more > comfortable 2^128. > c) Rather than using the raw "sha1_transform" function alone, we > instead use the full proper BLAKE2s function, with finalization. > > This also has the advantage of supplying 16 bytes at a time rather than > SHA1's 10 bytes, which, in addition to having a faster compression > function to begin with, means faster extraction in general. On an Intel > i7-11850H, this commit makes initial seeding around 131% faster. > > BLAKE2s itself has the nice property of internally being based on the > ChaCha permutation, which the RNG is already using for expansion, so > there shouldn't be any issue with newness, funkiness, or surprising CPU > behavior, since it's based on something already in use. > > [1] https://eprint.iacr.org/2005/010.pdf > [2] https://www.iacr.org/archive/crypto2005/36210017/36210017.pdf > [3] https://eprint.iacr.org/2015/967.pdf > [4] https://shattered.io/static/shattered.pdf > [5] https://www.usenix.org/system/files/sec20-leurent.pdf > > Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > Reviewed-by: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Looks good, thanks for the work!
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
- Ted
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