### RSA_generate_key

### 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:

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

### 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 2000-2019 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 https://www.openssl.org/source/license.html.