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 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 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)
