diff options
author | Damien Miller <djm@mindrot.org> | 2000-10-25 10:06:04 +1100 |
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committer | Damien Miller <djm@mindrot.org> | 2000-10-25 10:06:04 +1100 |
commit | 6f9c337401d455b92d85913432f0fc1129812b1f (patch) | |
tree | 891a5a416ee5d9826245701578f79d5bb20dfc81 /WARNING.RNG | |
parent | e5a8525242deb13d5263d2da46365053f05262d4 (diff) | |
download | openssh-git-6f9c337401d455b92d85913432f0fc1129812b1f.tar.gz |
- (djm) Added WARNING.RNG file and modified configure to ask users of the
builtin entropy code to read it.
- (djm) Prefer builtin regex to PCRE.
Diffstat (limited to 'WARNING.RNG')
-rw-r--r-- | WARNING.RNG | 80 |
1 files changed, 80 insertions, 0 deletions
diff --git a/WARNING.RNG b/WARNING.RNG new file mode 100644 index 00000000..5f129f40 --- /dev/null +++ b/WARNING.RNG @@ -0,0 +1,80 @@ +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. + +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 +and consider disabling ssh2 support (by adding "Protocol 1" to +sshd_config and ssh_config). + +Please also request that your OS vendor provides a kernel-based random +number collector (/dev/random) in future versions of your operating +systems. + +On to the description... + +The portable OpenSSH contains random number collection support for +systems which lack a kernel entropy pool (/dev/random). + +This 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. + +This approach presents two 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. + +This can be tuned by the administrator. To debug the entropy +collection is great detail, turn on full debugging ("ssh -v -v -v" or +"sshd -d -d -d"). This will list each program as it is executed, how +long it took to execute, its exit status and whether and how much data +it generated. You can the find the culprit programs which are causing +the real slow-downs. + +The 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). + |