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, double 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
38 controlling default implementations is by using the ENGINE API
39 functions. The default RAND_METHOD, as set by RAND_set_rand_method()
40 and returned by RAND_get_rand_method(), is only used if no ENGINE has
41 been set as the default "rand" implementation. Hence, these two
42 functions are no longer the recommended 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
51 number generator (PRNG). It is used by other library functions for
52 example to generate random keys, and applications can use it when they
53 need 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
68 documentation:
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
78 generate 4096 bit RSA keys, 2 2048 bit random strings are required
79 (at a minimum). If your RNG state only has 128 bits, you are
80 obviously limiting the search space to 128 bits, not 2048. I'm
81 probably getting a little carried away on this last point but it
82 does indicate that it may not be a bad idea to keep quite a lot of
83 RNG state. It should be easier to break a cipher than guess the
84 RNG seed data.
85 4. Any RNG seed data should influence all subsequent random numbers
87 generated. This implies that any random seed data entered will
88 have an influence on all subsequent random numbers generated.
89 5. When using data to seed the RNG state, the data used should not be
91 extractable from the RNG state. I believe this should be a
92 requirement because one possible source of 'secret' semi random
93 data would be a private key or a password. This data must not be
94 disclosed by either subsequent random numbers or a 'core' dump left
95 by a program crash.
96 6. Given the same initial 'state', 2 systems should deviate in their
98 RNG state (and hence the random numbers generated) over time if at
99 all possible.
100 7. Given the random number output stream, it should not be possible to
102 determine the RNG state or the next random number.
103 The algorithm is as follows.
105 There is global state made up of a 1023 byte buffer (the 'state'), a
107 working hash value ('md'), and a counter ('count').
108 Whenever seed data is added, it is inserted into the 'state' as
110 follows.
111 The input is chopped up into units of 20 bytes (or less for the last
113 block). Each of these blocks is run through the hash function as
114 follows: The data passed to the hash function is the current 'md', the
115 same number of bytes from the 'state' (the location determined by in
116 incremented looping index) as the current 'block', the new key data
117 'block', and 'count' (which is incremented after each use). The result
118 of this is kept in 'md' and also xored into the 'state' at the same
119 locations that were used as input into the hash function. I believe
120 this system addresses points 1 (hash function; currently SHA-1), 3 (the
121 'state'), 4 (via the 'md'), 5 (by the use of a hash function and xor).
122 When bytes are extracted from the RNG, the following process is used.
124 For each group of 10 bytes (or less), we do the following:
125 Input into the hash function the local 'md' (which is initialized from
127 the global 'md' before any bytes are generated), the bytes that are to
128 be overwritten by the random bytes, and bytes from the 'state'
129 (incrementing looping index). From this digest output (which is kept in
130 'md'), the top (up to) 10 bytes are returned to the caller and the
131 bottom 10 bytes are xored into the 'state'.
132 Finally, after we have finished 'num' random bytes for the caller,
134 'count' (which is incremented) and the local and global 'md' are fed
135 into the hash function and the results are kept in the global 'md'.
136 I believe the above addressed points 1 (use of SHA-1), 6 (by hashing
138 into the 'state' the 'old' data from the caller that is about to be
139 overwritten) and 7 (by not using the 10 bytes given to the caller to
140 update the 'state', but they are used to update 'md').
141 So of the points raised, only 2 is not addressed (but see RAND_add(3)).
143
145 BN_rand(3), RAND_add(3), RAND_load_file(3), RAND_egd(3), RAND_bytes(3),
146 RAND_set_rand_method(3), RAND_cleanup(3)
1471.0.2k 2017-01-26 rand(3)