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, 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 recom-
       mended 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 genera-
       tion 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 random-
       ness.

       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 ran-
           dom strings are required (at a minimum).  If your
           RNG state only has 128 bits, you are obviously lim-
           iting 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 num-
       ber 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 incre-
       mented after each use).  The result of this is kept in
       'md' and also xored into the 'state' at the same loca-
       tions 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)



0.9.7c                     2002-08-05                   rand(3)
