Date: Mon, 3 Feb 2003 22:30:50 -0800 From: Eric Hodel <drbrain@segment7.net> To: David Schultz <dschultz@uclink.Berkeley.EDU> Cc: freebsd-current@freebsd.org Subject: Re: rand() is broken Message-ID: <20030204063049.GU46498@segment7.net> In-Reply-To: <20030204054020.GA2447@HAL9000.homeunix.com> References: <20030202070644.GA9987@rot13.obsecurity.org> <20030202090422.GA59750@nagual.pp.ru> <20030203002639.GB44914@HAL9000.homeunix.com> <20030203100002.GA73386@nagual.pp.ru> <20030204054020.GA2447@HAL9000.homeunix.com>
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--yIMHf/Pa6CzSkARF Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable David Schultz (dschultz@uclink.Berkeley.EDU) wrote: > Rather than me showing you more semi-meaningful numbers from > Marsaglia's tests, why don't you look at the following sequence, > which I get by taking the lowest four bits of the 201st number in > the rand() sequence for seeds of (0, 1, 2, ...). >=20 f c 9 6 2 f c 8 5 2 e b 8 4 1 e b 7 4 1 d a 7 3 0 d 9 6 3 f c 9 6 2 f c 8 5 2 e b 8 4 1 e b 7 4 1 d a 7 3 0 d 9 6 3 f c 9 6 2 f c 8 5 2 e b 8 4 1 e a 7 4 1 d a 7 3 0 d 9 6 3 f c 9 6 2 f c 8 5 2 e b 8 4 1 e a 7 4 1 d a 7 3 0 d 9 6 3 f c 9 5 2 f c 8 5 2 e b 8 4 1 e a 7 4 1 d a 7 3 0 d 9 6 3 f c 9 5 2 f c 8 5 2 e b 8 4 1 e a 7 4 0 d a 7 3 0 d 9 6 3 f c 9 5 2 f c 8 5 2 e b 8 4 1 e a 7 4 0 d a 7 3 0 d 9 6 3 f c 9 5 2 f b 8 5 2 e b 8 4 1 e a 7 4 0 d a 7 3 0 d 9 6 3 f c 9 5 2 f b 8 5 2 e b 8 4 1 e a 7 4 0 d a 7 3 0 d 9 6 3 f c 9 5 2 f b 8 5 2 e b 8 4 1 e a 7 4 0 d a 6 3 0 d 9 6 3 f c 9 5 2 f b 8 5 2 e b 8 4 1 e a 7 4 0 d a 6 3 0 d 9 6 3 f=20 > Notice that 'f c 9' repeats in regular intervals and is always > followed by a 5 or 6. There is a similar pattern for 'e a 7'. I > think this pretty much demonstrates that the algorithm isn't good > enough to generate high-quality randomness with respect to > different seed values. I'm not suggesting that it absolutely must > be replaced, since most rand() implementations aren't very good in > the first place, but I'm pointing out that to do a good job of > fixing it once and for all is harder than you might think. A littele modification shows just how similar these sequences are :) --=20 Eric Hodel - drbrain@segment7.net - http://segment7.net All messages signed with fingerprint: FEC2 57F1 D465 EB15 5D6E 7C11 332A 551C 796C 9F04 --yIMHf/Pa6CzSkARF Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.0 (FreeBSD) iD8DBQE+P14ZMypVHHlsnwQRAqMPAKCl+/RGVU9+Yh1hyQ+JrJ0jUfZRkwCg2NtW vYkdEOBzJBd5Uy6TXretqoA= =JtWN -----END PGP SIGNATURE----- --yIMHf/Pa6CzSkARF-- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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