From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 13729 invoked by alias); 13 Jun 2013 08:26:38 -0000 Mailing-List: contact glibc-bugs-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Post: List-Help: , Sender: glibc-bugs-owner@sourceware.org Received: (qmail 12935 invoked by uid 55); 13 Jun 2013 08:26:30 -0000 From: "neleai at seznam dot cz" To: glibc-bugs@sourceware.org Subject: [Bug libc/15615] Poor quality output from rand_r Date: Thu, 13 Jun 2013 08:26:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: glibc X-Bugzilla-Component: libc X-Bugzilla-Version: unspecified X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: neleai at seznam dot cz X-Bugzilla-Status: NEW X-Bugzilla-Priority: P2 X-Bugzilla-Assigned-To: unassigned at sourceware dot org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Bugzilla-URL: http://sourceware.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-SW-Source: 2013-06/txt/msg00098.txt.bz2 http://sourceware.org/bugzilla/show_bug.cgi?id=15615 --- Comment #1 from Ondrej Bilka --- On Wed, Jun 12, 2013 at 11:39:09PM +0000, bugdal at aerifal dot cx wrote: > http://sourceware.org/bugzilla/show_bug.cgi?id=15615 > > Bug ID: 15615 > Summary: Poor quality output from rand_r > Product: glibc > Version: unspecified > Status: NEW > Severity: normal > Priority: P2 > Component: libc > Assignee: unassigned at sourceware dot org > Reporter: bugdal at aerifal dot cx > CC: drepper.fsp at gmail dot com > > Created attachment 7075 > --> http://sourceware.org/bugzilla/attachment.cgi?id=7075&action=edit > test program to generate data for analysis by dieharder > > Implementing a decent rand_r is very tricky because the interface requirement > forces the full PRNG state to fit in 32 bits; this rules out pretty much all > good PRNGs. Nonetheless, glibc's rand_r is much worse than it needs to be. > > glibc's rand_r is based on the LCG published in the C standard: > > next = next * 1103515245 + 12345; > return next / 65536 % 32768; > A problem here is that for many users predictability is much more important than quality. Developer expects that when he uses rand_r with state that he controls will not vary. This can cause extra debbuging hastle when code mysteriously fails on one machine but not other or desync issues. > To fully fix rand_r, the approach of concatenating multiple iterations should > be abandoned in favor of a single-LCG-iteration approach followed by an > invertable transformation on the output. Obviously a 32-bit cryptographic block > cipher would give the best statistical properties, but it would be slow. In This is false, I have a replacement of this with four rounds of AES. On intel using aesenc I performance is better than current, I did not propose that due of problems above. I wrote a RFC for random replacement on libc-alpha, browse archives. -- You are receiving this mail because: You are on the CC list for the bug.