Bugtraq mailing list archives
Flaw in commonly used bash random seed method
From: coderpunk <coderpunk () gmail com>
Date: Sat, 1 Apr 2006 18:12:15 -0800
I think I just discovered a flaw in what appears to be a common way to see bash's $RANDOM function. bash provides a pseudo-random number, from 0-32767, using the $RANDOM function. You can seed this by setting RANDOM=42 or some other number. Otherwise it is seeded by the process id and time. There are a plethora of usenet and web posts that suggest using the following to seed the function: SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }') But look at the output, and the manpage for od. All this is doing is reading 1 line of /dev/urandom (a binary stream), passing it to od which is taking 1 byte and converting it to octal and printing it with awk. What's wrong with this picture? You've just taken a random source and turned it into a number from 0-255! I hope you haven't been generating your password lists with any of the scripts using that seed method. Here is a better way: SEED=$(head -c4 /dev/urandom | od -t u4 | awk '{ print $2 }') This reads 4 bytes from /dev/urandom, passed it to od which converts it into a unsigned 4 byte integer and prints it. This should have a range of 2**32 (4294967296L) which is a bit more secure than 0-255. .cp
Current thread:
- Flaw in commonly used bash random seed method coderpunk (Apr 03)
- Re: Flaw in commonly used bash random seed method Matthijs (Apr 03)
- Re: Flaw in commonly used bash random seed method Dave English (Apr 04)
- Re: Flaw in commonly used bash random seed method Matthijs (Apr 04)
- Re: Flaw in commonly used bash random seed method Matthijs (Apr 04)
- Re: Flaw in commonly used bash random seed method Dave English (Apr 04)
- Re: Flaw in commonly used bash random seed method Dave Korn (Apr 05)
- Re: Flaw in commonly used bash random seed method Steve VanDevender (Apr 10)
- Re: Flaw in commonly used bash random seed method Matthijs (Apr 03)