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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)).

1.0.2t               Last change: 2019-09-10                    3

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)

1.0.2t               Last change: 2019-09-10                    4

See also rand(3C)
See also rand(3bsd)

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