1rand(3) OpenSSL rand(3)
2
6 rand - pseudo-random number generator
7
9 #include <openssl/rand.h>
10 int RAND_set_rand_engine(ENGINE *engine);
12 int RAND_bytes(unsigned char *buf, int num);
14 int RAND_pseudo_bytes(unsigned char *buf, int num);
15 void RAND_seed(const void *buf, int num);
17 void RAND_add(const void *buf, int num, int entropy);
18 int RAND_status(void);
19 int RAND_load_file(const char *file, long max_bytes);
21 int RAND_write_file(const char *file);
22 const char *RAND_file_name(char *file, size_t num);
23 int RAND_egd(const char *path);
25 void RAND_set_rand_method(const RAND_METHOD *meth);
27 const RAND_METHOD *RAND_get_rand_method(void);
28 RAND_METHOD *RAND_SSLeay(void);
29 void RAND_cleanup(void);
31 /* For Win32 only */
33 void RAND_screen(void);
34 int RAND_event(UINT, WPARAM, LPARAM);
35
37 Since the introduction of the ENGINE API, the recommended way of con‐
38 trolling default implementations is by using the ENGINE API functions.
39 The default RAND_METHOD, as set by RAND_set_rand_method() and returned
40 by RAND_get_rand_method(), is only used if no ENGINE has been set as
41 the default "rand" implementation. Hence, these two functions are no
42 longer the recommened way to control defaults.
43 If an alternative RAND_METHOD implementation is being used (either set
45 directly or as provided by an ENGINE module), then it is entirely
46 responsible for the generation and management of a cryptographically
47 secure PRNG stream. The mechanisms described below relate solely to the
48 software PRNG implementation built in to OpenSSL and used by default.
49 These functions implement a cryptographically secure pseudo-random num‐
51 ber generator (PRNG). It is used by other library functions for example
52 to generate random keys, and applications can use it when they need
53 randomness.
54 A cryptographic PRNG must be seeded with unpredictable data such as
56 mouse movements or keys pressed at random by the user. This is
57 described in RAND_add(3). Its state can be saved in a seed file (see
58 RAND_load_file(3)) to avoid having to go through the seeding process
59 whenever the application is started.
60 RAND_bytes(3) describes how to obtain random data from the PRNG.
62
64 The RAND_SSLeay() method implements a PRNG based on a cryptographic
65 hash function.
66 The following description of its design is based on the SSLeay documen‐
68 tation:
69 First up I will state the things I believe I need for a good RNG.
71 1 A good hashing algorithm to mix things up and to convert the RNG
73 'state' to random numbers.
74 2 An initial source of random 'state'.
76 3 The state should be very large. If the RNG is being used to gener‐
78 ate 4096 bit RSA keys, 2 2048 bit random strings are required (at a
79 minimum). If your RNG state only has 128 bits, you are obviously
80 limiting the search space to 128 bits, not 2048. I'm probably get‐
81 ting a little carried away on this last point but it does indicate
82 that it may not be a bad idea to keep quite a lot of RNG state. It
83 should be easier to break a cipher than guess the RNG seed data.
84 4 Any RNG seed data should influence all subsequent random numbers
86 generated. This implies that any random seed data entered will
87 have an influence on all subsequent random numbers generated.
88 5 When using data to seed the RNG state, the data used should not be
90 extractable from the RNG state. I believe this should be a
91 requirement because one possible source of 'secret' semi random
92 data would be a private key or a password. This data must not be
93 disclosed by either subsequent random numbers or a 'core' dump left
94 by a program crash.
95 6 Given the same initial 'state', 2 systems should deviate in their
97 RNG state (and hence the random numbers generated) over time if at
98 all possible.
99 7 Given the random number output stream, it should not be possible to
101 determine the RNG state or the next random number.
102 The algorithm is as follows.
104 There is global state made up of a 1023 byte buffer (the 'state'), a
106 working hash value ('md'), and a counter ('count').
107 Whenever seed data is added, it is inserted into the 'state' as fol‐
109 lows.
110 The input is chopped up into units of 20 bytes (or less for the last
112 block). Each of these blocks is run through the hash function as fol‐
113 lows: The data passed to the hash function is the current 'md', the
114 same number of bytes from the 'state' (the location determined by in
115 incremented looping index) as the current 'block', the new key data
116 'block', and 'count' (which is incremented after each use). The result
117 of this is kept in 'md' and also xored into the 'state' at the same
118 locations that were used as input into the hash function. I believe
119 this system addresses points 1 (hash function; currently SHA-1), 3 (the
120 'state'), 4 (via the 'md'), 5 (by the use of a hash function and xor).
121 When bytes are extracted from the RNG, the following process is used.
123 For each group of 10 bytes (or less), we do the following:
124 Input into the hash function the local 'md' (which is initialized from
126 the global 'md' before any bytes are generated), the bytes that are to
127 be overwritten by the random bytes, and bytes from the 'state' (incre‐
128 menting looping index). From this digest output (which is kept in
129 'md'), the top (up to) 10 bytes are returned to the caller and the bot‐
130 tom 10 bytes are xored into the 'state'.
131 Finally, after we have finished 'num' random bytes for the caller,
133 'count' (which is incremented) and the local and global 'md' are fed
134 into the hash function and the results are kept in the global 'md'.
135 I believe the above addressed points 1 (use of SHA-1), 6 (by hashing
137 into the 'state' the 'old' data from the caller that is about to be
138 overwritten) and 7 (by not using the 10 bytes given to the caller to
139 update the 'state', but they are used to update 'md').
140 So of the points raised, only 2 is not addressed (but see RAND_add(3)).
142
144 BN_rand(3), RAND_add(3), RAND_load_file(3), RAND_egd(3), RAND_bytes(3),
145 RAND_set_rand_method(3), RAND_cleanup(3)
1460.9.8b 2002-08-05 rand(3)