Unix/Linux Go Back    


OpenDarwin 7.2.1 - man page for rand (opendarwin section 3)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)


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 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 pro-
       vided by an ENGINE module), then it is entirely responsible for the generation and manage-
       ment 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)

0.9.7d					    2003-11-20					  rand(3)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums


All times are GMT -4. The time now is 05:45 AM.