RSA_GENERATE_KEY(3ossl)

🌸 RSA_GENERATE_KEY 🌸 (the 3ossl one, from the OpenSSL manual, bestie)

okay so bestie, this page is literally all about making RSA key pairs. The whole girlboss lineup is EVP_RSA_gen, RSA_generate_key_ex, RSA_generate_key, and RSA_generate_multi_prime_key — that's the NAME section, and yes, that's what they do: generate an RSA key pair. 💅

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✨ SYNOPSIS ✨

first up, the ✨main character✨, the one you should actually be using:

#include <openssl/rsa.h>

EVP_PKEY *EVP_RSA_gen(unsigned int bits);

now for the ones that have been deprecated since OpenSSL 3.0 — and ngl you can hide them 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);

and this next one has been deprecated since OpenSSL 0.9.8 (yes, ancient 💀) — you can hide it entirely too 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);

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💖 DESCRIPTION 💖

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

Now listen up: all of the functions described below are deprecated. Like, every single one. Applications should instead use EVP_RSA_gen(), EVP_PKEY_Q_keygen(3), or EVP_PKEY_keygen_init(3) and EVP_PKEY_keygen(3). That's the glow-up, bestie.

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. 🫶 Heads up though: if the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to external circumstances (see RAND(7)), the operation will fail. no cap.

So 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 👑 (do not do this, bestie, it's giving 1998). The exponent is an odd number, typically 3, 17 or 65537.

In order to maintain an 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. 🦋

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🎀 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.

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💀 BUGS 💀

BN_GENCB_call(cb, 2, x) is used with two different meanings. yeah. it's a whole thing. 🙈

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🌈 SEE ALSO 🌈

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

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⭐ 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). slay.

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👑 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>.

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okay that's the whole thing, mwah 💜 this was OpenSSL 3.5.9, dated 2026-09-29, RSA_GENERATE_KEY(3ossl), and that's on periodt. go forth and generate responsibly, bestie 🧚✨