rand(3)
NAME
rand - pseudo-random number generator
SYNOPSIS
#include <openssl/rand.h> int RAND_set_rand_engine(ENGINE *engine); int RAND_bytes(unsigned char *buf, int num); int RAND_pseudo_bytes(unsigned char *buf, int num); void RAND_seed(const void *buf, int num); void RAND_add(const void *buf, int num, int entropy); int RAND_status(void); int RAND_load_file(const char *file, long max_bytes); int RAND_write_file(const char *file); const char *RAND_file_name(char *file, size_t num); int RAND_egd(const char *path); void RAND_set_rand_method(const RAND_METHOD *meth); const RAND_METHOD *RAND_get_rand_method(void); RAND_METHOD *RAND_SSLeay(void); void RAND_cleanup(void); /* For Win32 only */ void RAND_screen(void); int RAND_event(UINT, WPARAM, LPARAM);
DESCRIPTION
Since the introduction of the ENGINE API, the recommended
way of controlling default implementations is by using the
ENGINE API functions. The default RAND_METHOD, as set by
RAND_set_rand_method() and returned by
RAND_get_rand_method(), is only used if no ENGINE has been
set as the default "rand" implementation. Hence, these two
functions are no longer the recommened way to control
defaults.
If an alternative RAND_METHOD implementation is being used
(either set directly or as provided by an ENGINE module),
then it is entirely responsible for the generation and
management of a cryptographically secure PRNG stream. The
mechanisms described below relate solely to the software
PRNG implementation built in to OpenSSL and used by
default.
These functions implement a cryptographically secure
pseudo-random number generator (PRNG). It is used by other
library functions for example to generate random keys, and
applications can use it when they need randomness.
A cryptographic PRNG must be seeded with unpredictable
data such as mouse movements or keys pressed at random by
the user. This is described in RAND_add(3). Its state can
be saved in a seed file (see RAND_load_file(3)) to avoid
having to go through the seeding process whenever the
application is started.
RAND_bytes(3) describes how to obtain random data from the
PRNG.
INTERNALS
The RAND_SSLeay() method implements a PRNG based on a
cryptographic hash function.
The following description of its design is based on the
SSLeay documentation:
First up I will state the things I believe I need for a
good RNG.
- 1 A good hashing algorithm to mix things up and to con
- vert the RNG 'state' to random numbers.
- 2 An initial source of random 'state'.
- 3 The state should be very large. If the RNG is being
- used to generate 4096 bit RSA keys, 2 2048 bit random
strings are required (at a minimum). If your RNG
state only has 128 bits, you are obviously limiting
the search space to 128 bits, not 2048. I'm probably
getting a little carried away on this last point but
it does indicate that it may not be a bad idea to keep
quite a lot of RNG state. It should be easier to
break a cipher than guess the RNG seed data. - 4 Any RNG seed data should influence all subsequent ran
- dom numbers generated. This implies that any random
seed data entered will have an influence on all subse
quent random numbers generated. - 5 When using data to seed the RNG state, the data used
- should not be extractable from the RNG state. I
believe this should be a requirement because one pos
sible source of 'secret' semi random data would be a
private key or a password. This data must not be dis
closed by either subsequent random numbers or a 'core'
dump left by a program crash. - 6 Given the same initial 'state', 2 systems should devi
- ate in their RNG state (and hence the random numbers
generated) over time if at all possible. - 7 Given the random number output stream, it should not
- be possible to determine the RNG state or the next
random number. - The algorithm is as follows.
- There is global state made up of a 1023 byte buffer (the
'state'), a working hash value ('md'), and a counter
('count'). - Whenever seed data is added, it is inserted into the
'state' as follows. - The input is chopped up into units of 20 bytes (or less
for the last block). Each of these blocks is run through
the hash function as follows: The data passed to the hash
function is the current 'md', the same number of bytes
from the 'state' (the location determined by in incre
mented looping index) as the current 'block', the new key
data 'block', and 'count' (which is incremented after each
use). The result of this is kept in 'md' and also xored
into the 'state' at the same locations that were used as
input into the hash function. I believe this system
addresses points 1 (hash function; currently SHA-1), 3
(the 'state'), 4 (via the 'md'), 5 (by the use of a hash
function and xor). - When bytes are extracted from the RNG, the following pro
cess is used. For each group of 10 bytes (or less), we do
the following: - Input into the hash function the local 'md' (which is ini
tialized from the global 'md' before any bytes are gener
ated), the bytes that are to be overwritten by the random
bytes, and bytes from the 'state' (incrementing looping
index). From this digest output (which is kept in 'md'),
the top (up to) 10 bytes are returned to the caller and
the bottom 10 bytes are xored into the 'state'. - Finally, after we have finished 'num' random bytes for the
caller, 'count' (which is incremented) and the local and
global 'md' are fed into the hash function and the results
are kept in the global 'md'. - I believe the above addressed points 1 (use of SHA-1), 6
(by hashing into the 'state' the 'old' data from the
caller that is about to be overwritten) and 7 (by not
using the 10 bytes given to the caller to update the
'state', but they are used to update 'md'). - So of the points raised, only 2 is not addressed (but see
RAND_add(3)).