### EVP_RSA_gen

### NAME

EVP_RSA_gen, RSA_generate_key_ex, RSA_generate_key, RSA_generate_multi_prime_key - generate RSA key pair

### SYNOPSIS

```
#include <openssl/rsa.h>
EVP_PKEY *EVP_RSA_gen(unsigned int bits);
```

The following functions have been deprecated since OpenSSL 3.0, and can be hidden entirely by defining **OPENSSL_API_COMPAT** with a suitable version value, see openssl_user_macros(7):

```
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);
```

The following function has been deprecated since OpenSSL 0.9.8, and 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

EVP_RSA_gen() generates a new RSA key pair with modulus size *bits*.

All of the functions described below are deprecated. Applications should instead use EVP_RSA_gen(), EVP_PKEY_Q_keygen(3), or EVP_PKEY_keygen_init(3) and EVP_PKEY_keygen(3).

RSA_generate_key_ex() generates a 2-prime RSA key pair and stores it in the **RSA** structure provided in *rsa*.

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. 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

EVP_RSA_gen() returns an *EVP_PKEY* or NULL on failure.

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

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

### HISTORY

EVP_RSA_gen() was added in OpenSSL 3.0. All other functions described here were deprecated in OpenSSL 3.0. For replacement see EVP_PKEY-RSA(7).

### COPYRIGHT

Copyright 2000-2021 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.