rand(3) OpenSSL rand(3)NAMErand - pseudo-random number generator
SYNOPSIS
#include <openssl/rand.h>
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);
void RAND_screen(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(RAND_METHOD *meth);
RAND_METHOD *RAND_get_rand_method(void);
RAND_METHOD *RAND_SSLeay(void);
void RAND_cleanup(void);
DESCRIPTION
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
convert 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
random numbers generated. This implies that any
random seed data entered will have an influence on all
subsequent 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
possible source of 'secret' semi random data would be
a private key or a password. This data must not be
disclosed by either subsequent random numbers or a
'core' dump left by a program crash.
6 Given the same initial 'state', 2 systems should
deviate 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
incremented 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
process is used. For each group of 10 bytes (or less), we
do the following:
Input into the hash function the local 'md' (which is
initialized from the global 'md' before any bytes are
generated), 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)).
SEE ALSOBN_rand(3), RAND_add(3), RAND_load_file(3), RAND_egd(3),
RAND_bytes(3), RAND_set_rand_method(3), RAND_cleanup(3)9/Jul/2001 0.9.6j rand(3)
