Edit File: WARNING.RNG
This document contains a description of portable OpenSSH's random number collection code. An alternate reading of this text could well be titled "Why I should pressure my system vendor to supply /dev/random in their OS". Why is this important? OpenSSH depends on good, unpredictable numbers for generating keys, performing digital signatures and forming cryptographic challenges. If the random numbers that it uses are predictable, then the strength of the whole system is compromised. A particularly pernicious problem arises with DSA keys (used by the ssh2 protocol). Performing a DSA signature (which is required for authentication), entails the use of a 160 bit random number. If an attacker can predict this number, then they can deduce your *private* key and impersonate you or your hosts. If you are using the builtin random number support (configure will tell you if this is the case), then read this document in its entirety. Alternately, you can use Lutz Jaenicke's PRNGd - a small daemon which collects random numbers and makes them available by a socket. Please also request that your OS vendor provides a kernel-based random number collector (/dev/random) in future versions of your operating systems by default. On to the description... The portable OpenSSH contains random number collection support for systems which lack a kernel entropy pool (/dev/random). This collector (as of 3.1 and beyond) comes as an external application that allows the local admin to decide on how to implement entropy collection. The default entropy collector operates by executing the programs listed in ($etcdir)/ssh_prng_cmds, reading their output and adding it to the PRNG supplied by OpenSSL (which is hash-based). It also stirs in the output of several system calls and timings from the execution of the programs that it runs. The ssh_prng_cmds file also specifies a 'rate' for each program. This represents the number of bits of randomness per byte of output from the specified program. The random number code will also read and save a seed file to ~/.ssh/prng_seed. This contents of this file are added to the random number generator at startup. The goal here is to maintain as much randomness between sessions as possible. The default entropy collection code has two main problems: 1. It is slow. Executing each program in the list can take a large amount of time, especially on slower machines. Additionally some program can take a disproportionate time to execute. Tuning the random helper can be done by running ./ssh-random-helper in very verbose mode ("-vvv") and identifying the commands that are taking excessive amounts of time or hanging altogher. Any problem commands can be modified or removed from ssh_prng_cmds. The default entropy collector will timeout programs which take too long to execute, the actual timeout used can be adjusted with the --with-entropy-timeout configure option. OpenSSH will not try to re-execute programs which have not been found, have had a non-zero exit status or have timed out more than a couple of times. 2. Estimating the real 'rate' of program outputs is non-trivial The shear volume of the task is problematic: there are currently around 50 commands in the ssh_prng_cmds list, portable OpenSSH supports at least 12 different OSs. That is already 600 sets of data to be analysed, without taking into account the numerous differences between versions of each OS. On top of this, the different commands can produce varying amounts of usable data depending on how busy the machine is, how long it has been up and various other factors. To make matters even more complex, some of the commands are reporting largely the same data as other commands (eg. the various "ps" calls). How to avoid the default entropy code? The best way is to read the OpenSSL documentation and recompile OpenSSL to use prngd or egd. Some platforms (like earily solaris) have 3rd party /dev/random devices that can be also used for this task. If you are forced to use ssh-rand-helper consider still downloading prngd/egd and configure OpenSSH using --with-prngd-port=xx or --with-prngd-socket=xx (refer to INSTALL for more information). $Id: WARNING.RNG,v 1.8 2005/05/26 01:47:54 djm Exp $
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