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Full Discussion: Generate a random password
Top Forums Shell Programming and Scripting Generate a random password Post 302139501 by Perderabo on Sunday 7th of October 2007 06:53:10 PM
Old 10-07-2007
Quote:
Originally Posted by caiser
Whats wrong with:

[code]cat /dev/urandom| .....
Since you asked, the problems start with your use of /dev/urandom. As one example, see "Analysis of the Linux Random Number Generator", March 6, 2006 by Gutterman, Pinkas, and Reinman.

Quote:
Linux is the most popular open source project. The Linux random number generator is part of the kernel of all Linux distributions and is based on generating randomness from entropy of operating system events. The output of this generator is used for almost every security protocol, including TLS/SSL key generation, choosing TCP sequence
numbers, and file system and email encryption. Although the generator is part of an open source project, its source code (about 2500 lines of code) is poorly documented, and patched with hundreds of code patches. We used dynamic and static reverse engineering to learn the operation of this generator. This paper presents a description of the underlying algorithms and exposes several security vulnerabilities. In particular, we show an attack on the forward security of the generator which enables an adversary who exposes the state of the generator to compute previous states and outputs. In addition we present a few cryptographic flaws in the design of the generator, as well as measurements of the actual entropy collected by it, and a critical analysis of the use of the generator in Linux distributions on disk-less devices.
In addition, your generator will not constrain its output to produce passwords that are easy for a human to remember.

I really encourage anyone who thinks that they have a method of generating random passwords to formally test that assumption. My script's output has passed the Diehard suite. And here is a clue, you need a random number generator that can pass Diehard. I am not aware of any version of unix or linux that comes with any random number generator that can do that. Passing Diehard is tough. No linear congruential generator, such as rand(), will even come close.

Note that even if /dev/urandom is reengineered to output random numbers, the fact that other users on the system can examine the current state of the generator and then compute previous states continues to be a problem. You really need a method that will deliver a stream of random numbers to you and only to you.
 

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makepasswd(1)						       UNIX Reference Manual						     makepasswd(1)

NAME
makepasswd - generate and/or encrypt passwords SYNOPSIS
makepasswd [ --chars N ] [ --clearfrom file ] [ --count N ] [ --crypt | --nocrypt | --crypt-md5 ] [ --cryptsalt N ] [ --help ] [ --maxchars N ] [ --minchars N ] [ --randomseed N ] [ --rerandom N ] [ --repeatpass N ] [ --string string ] [ --verbose | --noverbose ] DESCRIPTION
makepasswd generates true random passwords using /dev/urandom, with the emphasis on security over pronounceability. It can also encrypt plaintext passwords given on the command line. OPTIONS
--chars N Generate passwords with exactly N characters (do not use with options --minchars and --maxchars). --clearfrom FILE Use password from FILE instead of generating passwords. Requires the --crypt or the --crypt-md5 options; may not be used with these options: --chars, --maxchars, --minchars, --count, --string, --nocrypt. Trailing newlines are removed but other white space is not. --count N Produce a total of N passwords (the default is one). --crypt Produce encrypted passwords. --crypt-md5 Produce encrypted passwords using the MD5 digest (hash) algorithm. --cryptsalt N Use crypt() salt N, a positive number <= 4096. If random seeds are desired, specify a zero value (the default). --help Ignore other operands and produce only a help display. --maxchars N Generate passwords with at most N characters (default = 10). --minchars N Generate passwords with at least N characters (default = 8). --nocrypt Do not encrypt the generated password(s) (the default). --noverbose Display no labels on output (the default). --randomseed N Use random number seed N, between 0 and 2^32 inclusive. A zero value results in a real-random seed. This generates much less secure passwords than the default; not only does it generate predictable passwords due to the fixed seed, but the range of available seeds is 32 bits rather than the default of 256 bits, and cannot be changed without breaking expectations of previous users of this option. If possible, do not use this option. --rerandom N Set the random seed value every N values used. Specify zero to use a single seed value (the default). Specify one to get true-ran- dom passwords, though note that doing this too frequently will deplete the supply of entropy available in the kernel's entropy pool. --repeatpass N Use each password N times (4096 maximum, --crypt must be set and --cryptsalt may not be set). --string STRING Use the characters in STRING to generate random passwords. --verbose Display labelling information on output. HISTORY
makepasswd was originally part of the mkircconf program used to centrally administer the Linux Internet Support Cooperative IRC network. It may potentially be of use in any situation where passwords must be secure and need not be memorized by humans. Colin Watson modified it to use OpenSSL's pseudo-random number generator. COPYRIGHT
Copyright (c) 1997-1998 by lilo <lilo@linpeople.org>. All rights are reserved by the author. This program may be used under the terms of version 2 of the GNU Public License. SEE ALSO
passwd(5) Debian Distribution 1998 February 9 makepasswd(1)
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