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48e7916f | 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. | |
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 | and consider disabling ssh2 support (by adding "Protocol 1" to | |
20 | sshd_config and ssh_config). | |
21 | ||
22 | Please also request that your OS vendor provides a kernel-based random | |
23 | number collector (/dev/random) in future versions of your operating | |
24 | systems. | |
25 | ||
26 | On to the description... | |
27 | ||
28 | The portable OpenSSH contains random number collection support for | |
29 | systems which lack a kernel entropy pool (/dev/random). | |
30 | ||
31 | This collector operates by executing the programs listed in | |
32 | ($etcdir)/ssh_prng_cmds, reading their output and adding it to the | |
33 | PRNG supplied by OpenSSL (which is hash-based). It also stirs in the | |
34 | output of several system calls and timings from the execution of the | |
35 | programs that it runs. | |
36 | ||
37 | The ssh_prng_cmds file also specifies a 'rate' for each program. This | |
38 | represents the number of bits of randomness per byte of output from | |
39 | the specified program. | |
40 | ||
41 | The random number code will also read and save a seed file to | |
42 | ~/.ssh/prng_seed. This contents of this file are added to the random | |
43 | number generator at startup. | |
44 | ||
45 | This approach presents two problems: | |
46 | ||
47 | 1. It is slow. | |
48 | ||
49 | Executing each program in the list can take a large amount of time, | |
50 | especially on slower machines. Additionally some program can take a | |
51 | disproportionate time to execute. | |
52 | ||
53 | This can be tuned by the administrator. To debug the entropy | |
54 | collection is great detail, turn on full debugging ("ssh -v -v -v" or | |
55 | "sshd -d -d -d"). This will list each program as it is executed, how | |
56 | long it took to execute, its exit status and whether and how much data | |
57 | it generated. You can the find the culprit programs which are causing | |
58 | the real slow-downs. | |
59 | ||
60 | The entropy collector will timeout programs which take too long | |
61 | to execute, the actual timeout used can be adjusted with the | |
62 | --with-entropy-timeout configure option. OpenSSH will not try to | |
63 | re-execute programs which have not been found, have had a non-zero | |
64 | exit status or have timed out more than a couple of times. | |
65 | ||
66 | 2. Estimating the real 'rate' of program outputs is non-trivial | |
67 | ||
68 | The shear volume of the task is problematic: there are currently | |
69 | around 50 commands in the ssh_prng_cmds list, portable OpenSSH | |
70 | supports at least 12 different OSs. That is already 600 sets of data | |
71 | to be analysed, without taking into account the numerous differences | |
72 | between versions of each OS. | |
73 | ||
74 | On top of this, the different commands can produce varying amounts of | |
75 | usable data depending on how busy the machine is, how long it has been | |
76 | up and various other factors. | |
77 | ||
78 | To make matters even more complex, some of the commands are reporting | |
79 | largely the same data as other commands (eg. the various "ps" calls). | |
80 |