From owner-freebsd-bugs@freebsd.org Sun Feb 24 21:11:21 2019 Return-Path: Delivered-To: freebsd-bugs@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 575F5150C907 for ; Sun, 24 Feb 2019 21:11:21 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from mailman.ysv.freebsd.org (mailman.ysv.freebsd.org [IPv6:2001:1900:2254:206a::50:5]) by mx1.freebsd.org (Postfix) with ESMTP id E0F5F88953 for ; Sun, 24 Feb 2019 21:11:20 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: by mailman.ysv.freebsd.org (Postfix) id 9BC83150C906; Sun, 24 Feb 2019 21:11:20 +0000 (UTC) Delivered-To: bugs@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 6120A150C905 for ; Sun, 24 Feb 2019 21:11:20 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from mxrelay.ysv.freebsd.org (mxrelay.ysv.freebsd.org [IPv6:2001:1900:2254:206a::19:3]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) server-signature RSA-PSS (4096 bits) client-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "mxrelay.ysv.freebsd.org", Issuer "Let's Encrypt Authority X3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id F2B2E8894B for ; Sun, 24 Feb 2019 21:11:19 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from kenobi.freebsd.org (kenobi.freebsd.org [IPv6:2001:1900:2254:206a::16:76]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mxrelay.ysv.freebsd.org (Postfix) with ESMTPS id 4B12C7523 for ; Sun, 24 Feb 2019 21:11:19 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from kenobi.freebsd.org ([127.0.1.118]) by kenobi.freebsd.org (8.15.2/8.15.2) with ESMTP id x1OLBJip026162 for ; Sun, 24 Feb 2019 21:11:19 GMT (envelope-from bugzilla-noreply@freebsd.org) Received: (from www@localhost) by kenobi.freebsd.org (8.15.2/8.15.2/Submit) id x1OLBJRn026156 for bugs@FreeBSD.org; Sun, 24 Feb 2019 21:11:19 GMT (envelope-from bugzilla-noreply@freebsd.org) X-Authentication-Warning: kenobi.freebsd.org: www set sender to bugzilla-noreply@freebsd.org using -f From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 236010] rand(3) values strongly correlate with time while this doesn't have to be the case Date: Sun, 24 Feb 2019 21:11:19 +0000 X-Bugzilla-Reason: AssignedTo X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: Base System X-Bugzilla-Component: misc X-Bugzilla-Version: 12.0-STABLE X-Bugzilla-Keywords: X-Bugzilla-Severity: Affects Only Me X-Bugzilla-Who: yuri@freebsd.org X-Bugzilla-Status: New X-Bugzilla-Resolution: X-Bugzilla-Priority: --- X-Bugzilla-Assigned-To: bugs@FreeBSD.org X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version rep_platform op_sys bug_status bug_severity priority component assigned_to reporter Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: https://bugs.freebsd.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: freebsd-bugs@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Bug reports List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 24 Feb 2019 21:11:21 -0000 https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D236010 Bug ID: 236010 Summary: rand(3) values strongly correlate with time while this doesn't have to be the case Product: Base System Version: 12.0-STABLE Hardware: Any OS: Any Status: New Severity: Affects Only Me Priority: --- Component: misc Assignee: bugs@FreeBSD.org Reporter: yuri@freebsd.org ---Problem description--- This simple program > #include > #include > int main() { > srand(time(0)); > printf("%d\n", rand()); > } demonstrates how a lot of software uses srand(3)/rand(3) functions. The first random number seems to strongly correlate with time, here are subsequent run outputs: > $ ./rand=20 > 50768478 > $ ./rand=20 > 50785285 > $ ./rand=20 > 50802092 > $ ./rand=20 > 50818899 > $ ./rand=20 > 50852513 > $ ./rand=20 > 50869320 It's always 50,000,000-something, which is strongly correlated with time. ---Practical problem that this causes--- Please observe this use of rand(3): https://github.com/ccxvii/mujs/blob/master/jsmath.c#L71 When the rand(3) ou= tput is simply mapped to a floating point like this, the resulting floating point values aren't random at all from run to run. Somebody could use such floati= ng point pseudo-random number, truncate the less significant digits, and get a completely predictable values. The consecutive outputs of this routine that= I observed are: 0.03040289788544448, 0.03041072425470755, 0.03041072425470755. ---Proposed solution--- Some 32-bit hash function can be used on the seed value that would map the value to a less seemingly related value. --=20 You are receiving this mail because: You are the assignee for the bug.=