diff options
author | des <des@FreeBSD.org> | 2002-06-27 22:31:32 +0000 |
---|---|---|
committer | des <des@FreeBSD.org> | 2002-06-27 22:31:32 +0000 |
commit | bb02848f18878bddada7a9f55b110116f63c2f3c (patch) | |
tree | d93e4bc5fc0a9a5e99878bd93a4d51c873c1a43e /crypto/openssh/WARNING.RNG | |
parent | 610201f50fdb0594e9885594b69e4ee69c71dd08 (diff) | |
download | FreeBSD-src-bb02848f18878bddada7a9f55b110116f63c2f3c.zip FreeBSD-src-bb02848f18878bddada7a9f55b110116f63c2f3c.tar.gz |
Vendor import of OpenSSH 3.3p1.
Diffstat (limited to 'crypto/openssh/WARNING.RNG')
-rw-r--r-- | crypto/openssh/WARNING.RNG | 96 |
1 files changed, 96 insertions, 0 deletions
diff --git a/crypto/openssh/WARNING.RNG b/crypto/openssh/WARNING.RNG new file mode 100644 index 0000000..ae43930 --- /dev/null +++ b/crypto/openssh/WARNING.RNG @@ -0,0 +1,96 @@ +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 ($etcdir)/ssh_prng_cmds, reading their output and adding it to the +PRNG supplied by OpenSSL (which is hash-based). It also stirs in the +output of several system calls and timings from the execution of the +programs that it runs. + +The ssh_prng_cmds file also specifies a 'rate' for each program. This +represents the number of bits of randomness per byte of output from +the specified program. + +The random number code will also read and save a seed file to +~/.ssh/prng_seed. This contents of this file are added to the random +number generator at startup. The goal here is to maintain as much +randomness between sessions as possible. + +The default entropy collection code has two main problems: + +1. It is slow. + +Executing each program in the list can take a large amount of time, +especially on slower machines. Additionally some program can take a +disproportionate time to execute. + +Tuning the default entropy collection code is difficult at this point. +It requires doing 'times ./ssh-rand-helper' and modifying the +($etcdir)/ssh_prng_cmds until you have found the issue. In the next +release we will be looking at support '-v' for verbose output to allow +easier debugging. + +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 1.5 2002/04/14 13:16:05 djm Exp $ |