Date: Sat, 29 Aug 2015 20:24:28 +0200 From: magnum <john.magnum@...hmail.com> To: john-dev@...ts.openwall.com Subject: Re: length based branching On 2015-08-29 19:45, Aleksey Cherepanov wrote: > Common case of data based branching is length based branching. I guess > that some raw formats could get 10%+ speed up if they had a limit on > length and threated part of message block as constants. Also we have > some formats with length limits, most notable for me is raw-sha1-ng: > length limit 15 but it gives 30% speed up. Most of our SIMD formats has such a limit, for this reason. The natural limit for one block of MD4/MD5/SHA-1/SHA-256 is 55 bytes. SHA-1-ng's further limiting is another story: It's because set_key() uses SIMD to read the key. On AVX2 that limit of 15 is naturally bumped to 31 (more generally it's sizeof(vector) - 1, so on AVX-512 it's 63 and on AVX-1024 and above it will bump into the global max. of 125). > In case of raw-sha1, there is limit implied by algo: length <= 15 -> > algo from raw-sha1-ng, otherwise -> algo from raw-sha1. Not possible, for the reason I described. The algo is the same, it's set_key() that differs. In a future version of JtR, I'd like length to be supplied as an argument to set_key(). In most (all?) cases, the mode did know the length anyway, and a range of optimizations would become possible. > Also it would be nice to implement "fallbacks" in all algorithms with > small limit on length. (Length limits of dynamics were raised.) Or at > least, warn when algorithm does not support desired length (it is > needed to distinguish truncating formats and just not working > formats). Very few of our formats have a really limiting length as far as I'm aware, but our md5crypt format has. It should have a fallback to scalar multi-block code. Or even better, it should just be improved to handle longer; our sha2crypt formats handles much longer (it sorts the keys). magnum
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