/usr/man/cat.3/rand.3
rand(3) OpenSSL 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, double 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 recommended 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.
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rand(3) OpenSSL rand(3)
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.
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rand(3) OpenSSL rand(3)
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)).
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rand(3) OpenSSL rand(3)
SEE ALSO
BN_rand(3), RAND_add(3), RAND_load_file(3), RAND_egd(3),
RAND_bytes(3), RAND_set_rand_method(3), RAND_cleanup(3)
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See also rand(3C)
See also rand(3bsd)
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