Cryptography and SSL/TLS Toolkit



RSA_generate_key_ex, RSA_generate_key, RSA_generate_multi_prime_key - generate RSA key pair


 #include <openssl/rsa.h>

 int RSA_generate_key_ex(RSA *rsa, int bits, BIGNUM *e, BN_GENCB *cb);
 int RSA_generate_multi_prime_key(RSA *rsa, int bits, int primes, BIGNUM *e, BN_GENCB *cb);


 #if OPENSSL_API_COMPAT < 0x00908000L
 RSA *RSA_generate_key(int bits, unsigned long e,
                       void (*callback)(int, int, void *), void *cb_arg);


RSA_generate_key_ex() generates a 2-prime RSA key pair and stores it in the RSA structure provided in rsa. The pseudo-random number generator must be seeded prior to calling RSA_generate_key_ex().

RSA_generate_multi_prime_key() generates a multi-prime RSA key pair and stores it in the RSA structure provided in rsa. The number of primes is given by the primes parameter. The random number generator must be seeded when calling RSA_generate_multi_prime_key(). If the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to external circumstances (see RAND(7)), the operation will fail.

The modulus size will be of length bits, the number of primes to form the modulus will be primes, and the public exponent will be e. Key sizes with num < 1024 should be considered insecure. The exponent is an odd number, typically 3, 17 or 65537.

In order to maintain adequate security level, the maximum number of permitted primes depends on modulus bit length:

   <1024 | >=1024 | >=4096 | >=8192
     2   |   3    |   4    |   5

A callback function may be used to provide feedback about the progress of the key generation. If cb is not NULL, it will be called as follows using the BN_GENCB_call() function described on the BN_generate_prime(3) page.

RSA_generate_key() is similar to RSA_generate_key_ex() but expects an old-style callback function; see BN_generate_prime(3) for information on the old-style callback.

  • While a random prime number is generated, it is called as described in BN_generate_prime(3).

  • When the n-th randomly generated prime is rejected as not suitable for the key, BN_GENCB_call(cb, 2, n) is called.

  • When a random p has been found with p-1 relatively prime to e, it is called as BN_GENCB_call(cb, 3, 0).

The process is then repeated for prime q and other primes (if any) with BN_GENCB_call(cb, 3, i) where i indicates the i-th prime.


RSA_generate_multi_prime_key() returns 1 on success or 0 on error. RSA_generate_key_ex() returns 1 on success or 0 on error. The error codes can be obtained by ERR_get_error(3).

RSA_generate_key() returns a pointer to the RSA structure or NULL if the key generation fails.


BN_GENCB_call(cb, 2, x) is used with two different meanings.


ERR_get_error(3), RAND_bytes(3), BN_generate_prime(3), RAND(7)


RSA_generate_key() was deprecated in OpenSSL 0.9.8; use RSA_generate_key_ex() instead.

Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at