From 666aa9cc1660793c97ef29a6cb66dfbb894dde8f Mon Sep 17 00:00:00 2001 From: des Date: Tue, 22 Jul 2008 18:58:19 +0000 Subject: Revert part of 180714 - the intent was to flatten dist, not to nuke it. --- crypto/openssh/WARNING.RNG | 95 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 95 insertions(+) create mode 100644 crypto/openssh/WARNING.RNG (limited to 'crypto/openssh/WARNING.RNG') diff --git a/crypto/openssh/WARNING.RNG b/crypto/openssh/WARNING.RNG new file mode 100644 index 0000000..97da74f --- /dev/null +++ b/crypto/openssh/WARNING.RNG @@ -0,0 +1,95 @@ +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 $ -- cgit v1.1