# RSA_generate_key

## Synopsis

#include <openssl/rsa.h> |

RSA *RSA_generate_key(int num, unsigned long e, void (*callback)(int,int,void *), void *cb_arg); |

## DESCRIPTION

RSA_generate_key() generates a key pair and returns it in a newly
allocated **RSA** structure. The
pseudo-random number generator must be seeded prior to calling
RSA_generate_key().

The modulus size will be **num** bits,
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.

A callback function may be used to provide feedback about the
progress of the key generation. If **callback** is not **NULL**, it will be called as follows:

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,

**callback(2, n, cb_arg)**is called.When a random p has been found with p-1 relatively prime to

**e**, it is called as**callback(3, 0, cb_arg)**.

The process is then repeated for prime q with **callback(3, 1, cb_arg)**.

## RETURN VALUE

If key generation fails, RSA_generate_key() returns **NULL**; the error codes can be obtained by ERR_get_error(3).