path: root/WARNING.RNG
diff options
authorDamien Miller <>2000-10-25 10:06:04 +1100
committerDamien Miller <>2000-10-25 10:06:04 +1100
commit6f9c337401d455b92d85913432f0fc1129812b1f (patch)
tree891a5a416ee5d9826245701578f79d5bb20dfc81 /WARNING.RNG
parente5a8525242deb13d5263d2da46365053f05262d4 (diff)
- (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.
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+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
+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).