Name

RAND_set_rand_method, RAND_get_rand_method
RAND_SSLeay

select RAND method

Library

libcrypto.lib

Synopsis

#include <openssl/rand.h>
void RAND_set_rand_method(const RAND_METHOD *meth);
const RAND_METHOD *RAND_get_rand_method(void);
RAND_METHOD *RAND_SSLeay(void);

Detailed Description

A RAND_METHOD specifies the functions that OpenSSL uses for random number generation. By modifying the method, alternative implementations such as hardware RNGs may be used. IMPORTANT: See the NOTES section for important information about how these RAND API functions are affected by the use of ENGINE API calls.

Initially, the default RAND_METHOD is the OpenSSL internal implementation, as returned by RAND_SSLeay().

RAND_set_default_method() makes meth the method for PRNG use. NB: This is true only whilst no ENGINE has been set as a default for RAND, so this function is no longer recommended.

RAND_get_default_method() returns a pointer to the current RAND_METHOD. However, the meaningfulness of this result is dependant on whether the ENGINE API is being used, so this function is no longer recommended.

The RAND_METHOD Structure

 typedef struct rand_meth_st
 {
        void (*seed)(const void *buf, int num);
        int (*bytes)(unsigned char *buf, int num);
        void (*cleanup)(void);
        void (*add)(const void *buf, int num, int entropy);
        int (*pseudorand)(unsigned char *buf, int num);
        int (*status)(void);
 } RAND_METHOD;

The components point to the implementation of RAND_seed(), RAND_bytes(), RAND_cleanup(), RAND_add(), RAND_pseudo_rand() and RAND_status(). Each component may be NULL if the function is not implemented.

Return Values

RAND_set_rand_method() returns no value. RAND_get_rand_method() and RAND_SSLeay() return pointers to the respective methods.

Notes

As of version 0.9.7, RAND_METHOD implementations are grouped together with other algorithmic APIs (eg. RSA_METHOD, EVP_CIPHER, etc) in ENGINE modules. If a default ENGINE is specified for RAND functionality using an ENGINE API function, that will override any RAND defaults set using the RAND API (i.e. RAND_set_rand_method()). For this reason, the ENGINE API is the recommended way to control default implementations for use in RAND and other cryptographic algorithms.

See Also

rand(), engine()

History

RAND_set_rand_method(), RAND_get_rand_method() and RAND_SSLeay() are available in all versions of OpenSSL.

In the engine version of version 0.9.6, RAND_set_rand_method() was altered to take an ENGINE pointer as its argument. As of version 0.9.7, that has been reverted as the ENGINE API transparently overrides RAND defaults if used, otherwise RAND API functions work as before. RAND_set_rand_engine() was also introduced in version 0.9.7.

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