RSA_GENERATE_KEY(3) | Library Functions Manual | RSA_GENERATE_KEY(3)

# NAME

`RSA_generate_key_ex`, `RSA_generate_key` — generate RSA key pair

# SYNOPSIS

library “libcrypto”
`#include <openssl/rsa.h>`

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

Deprecated:

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

# DESCRIPTION

`RSA_generate_key_ex`() generates a key pair and stores it in rsa.

The modulus size will be of length 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 cb is not `NULL`, it will be called as follows using the BN_GENCB_call(3) function:

- 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 with `BN_GENCB_call`(cb, 3, 1).

`RSA_generate_key`() is deprecated. New applications should use `RSA_generate_key_ex`() instead. `RSA_generate_key`() works in the same way as `RSA_generate_key_ex`() except it uses "old style" call backs. See BN_generate_prime(3) for further details.

# RETURN VALUES

`RSA_generate_key_ex`() returns 1 on success or 0 on error. `RSA_generate_key`() returns the key on success or `NULL` on error.

The error codes can be obtained by ERR_get_error(3).

# SEE ALSO

BN_generate_prime(3), RSA_get0_key(3), RSA_meth_set_keygen(3), RSA_new(3)

# HISTORY

`RSA_generate_key`() appeared in SSLeay 0.4 or earlier and had its cb_arg argument added in SSLeay 0.9.0. It has been available since OpenBSD 2.4.

`RSA_generate_key_ex`() first appeared in OpenSSL 0.9.8 and has been available since OpenBSD 4.5.

# BUGS

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

`RSA_generate_key`() goes into an infinite loop for illegal input values.

June 8, 2025 | Debian