1 This document contains a description of portable OpenSSH's random
   2 number collection code. An alternate reading of this text could
   3 well be titled "Why I should pressure my system vendor to supply
   4 /dev/random in their OS".
   5 
   6 Why is this important? OpenSSH depends on good, unpredictable numbers
   7 for generating keys, performing digital signatures and forming
   8 cryptographic challenges. If the random numbers that it uses are
   9 predictable, then the strength of the whole system is compromised.
  10 
  11 A particularly pernicious problem arises with DSA keys (used by the
  12 ssh2 protocol). Performing a DSA signature (which is required for
  13 authentication), entails the use of a 160 bit random number.  If an
  14 attacker can predict this number, then they can deduce your *private*
  15 key and impersonate you or your hosts.
  16 
  17 If you are using the builtin random number support (configure will
  18 tell you if this is the case), then read this document in its entirety.
  19 
  20 Please also request that your OS vendor provides a kernel-based random
  21 number collector (/dev/random) in future versions of your operating
  22 systems by default.
  23 
  24 On to the description...
  25 
  26 The portable OpenSSH contains random number collection support for
  27 systems which lack a kernel entropy pool (/dev/random).
  28 
  29 This collector operates by executing the programs listed in
  30 ($etcdir)/ssh_prng_cmds, reading their output and adding it to the
  31 PRNG supplied by OpenSSL (which is hash-based). It also stirs in the
  32 output of several system calls and timings from the execution of the
  33 programs that it runs.
  34 
  35 The ssh_prng_cmds file also specifies a 'rate' for each program. This
  36 represents the number of bits of randomness per byte of output from
  37 the specified program.
  38 
  39 The random number code will also read and save a seed file to
  40 ~/.ssh/prng_seed. This contents of this file are added to the random
  41 number generator at startup. The goal here is to maintain as much 
  42 randomness between sessions as possible.
  43 
  44 The entropy collection code has two main problems:
  45 
  46 1. It is slow.
  47 
  48 Executing each program in the list can take a large amount of time,   
  49 especially on slower machines. Additionally some program can take a   
  50 disproportionate time to execute.                                     
  51 
  52 This can be tuned by the administrator. To debug the entropy
  53 collection is great detail, turn on full debugging ("ssh -v -v -v" or
  54 "sshd -d -d -d"). This will list each program as it is executed, how
  55 long it took to execute, its exit status and whether and how much data
  56 it generated. You can the find the culprit programs which are causing
  57 the real slow-downs.
  58 
  59 The entropy collector will timeout programs which take too long
  60 to execute, the actual timeout used can be adjusted with the
  61 --with-entropy-timeout configure option. OpenSSH will not try to
  62 re-execute programs which have not been found, have had a non-zero
  63 exit status or have timed out more than a couple of times.
  64 
  65 2. Estimating the real 'rate' of program outputs is non-trivial
  66 
  67 The shear volume of the task is problematic: there are currently
  68 around 50 commands in the ssh_prng_cmds list, portable OpenSSH
  69 supports at least 12 different OSs. That is already 600 sets of data
  70 to be analysed, without taking into account the numerous differences
  71 between versions of each OS.
  72 
  73 On top of this, the different commands can produce varying amounts of
  74 usable data depending on how busy the machine is, how long it has been
  75 up and various other factors.
  76 
  77 To make matters even more complex, some of the commands are reporting
  78 largely the same data as other commands (eg. the various "ps" calls).
  79