### NAME

RSA_generate_key_ex, RSA_generate_key, RSA_generate_multi_prime_key - generate RSA key pair

### SYNOPSIS

```
#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);
```

Deprecated since OpenSSL 0.9.8, can be hidden entirely by defining **OPENSSL_API_COMPAT** with a suitable version value, see openssl_user_macros(7):

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

### DESCRIPTION

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.

### RETURN VALUES

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.

### BUGS

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

### SEE ALSO

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

### HISTORY

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

### COPYRIGHT

Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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 https://www.openssl.org/source/license.html.