RSA_PRIVATE_ENCRYPT(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 RSA_PRIVATE_ENCRYPT(3ossl) — like, the low-key signature ops bestie

okay so this is a page from the 3ossl manual, and honestly? It's giving "vintage crypto, handle with care." Buckle up 🎀

✨ NAME ✨

RSA_private_encrypt, RSA_public_decrypt — low-level signature operations, aka the OG way of doing signatures before EVP swooped in and ate 💅

🌸 SYNOPSIS 🌸

#include <openssl/rsa.h>

so like, plot twist: the following functions have been deprecated since OpenSSL 3.0, and you can hide them entirely by defining OPENSSL_API_COMPAT with a suitable version value — the deets are in openssl_user_macros(7), go read it bestie 🫶

int RSA_private_encrypt(int flen, unsigned char *from,
                        unsigned char *to, RSA *rsa, int padding);

int RSA_public_decrypt(int flen, unsigned char *from,
                       unsigned char *to, RSA *rsa, int padding);

🦋 DESCRIPTION 🦋

⚠️ both of the functions described on this page are deprecated. Like, both of them. Apps should use EVP_PKEY_sign_init_ex(3), EVP_PKEY_sign(3), EVP_PKEY_verify_recover_init(3), and EVP_PKEY_verify_recover(3) instead. That's the main character energy now ✨

These functions handle RSA signatures at a low-level. Raw. Unfiltered. no cap.

💖 RSA_private_encrypt()

Signs the flen bytes at from (usually a message digest with an algorithm identifier) using the private key rsa, and stores the signature in to.

to must point to RSA_size(rsa) bytes of memory. Not vibes, actual bytes. 🧚

🌈 padding — pick your mode bestie

  • RSA_PKCS1_PADDING — PKCS #1 v1.5 padding. Lowkey important: this function does not handle the algorithmIdentifier specified in PKCS #1. So when you're generating or verifying PKCS #1 signatures, use RSA_sign(3) and RSA_verify(3) instead. Periodt.
  • RSA_NO_PADDING — raw RSA signature. This mode should only be used to implement cryptographically sound padding modes in the application code. Signing user data directly with RSA is insecure — like, genuinely, please don't 😭

🌸 RSA_public_decrypt()

Recovers the message digest from the flen bytes long signature at from, using the signer's public key rsa.

to must point to a memory section large enough to hold the message digest — and heads up, that thing is smaller than RSA_size(rsa) - 11 👀

padding is the padding mode that was used to sign the data. Match it up, iykyk.

👑 RETURN VALUES 👑

  • RSA_private_encrypt() returns the size of the signature (i.e., RSA_size(rsa)).
  • RSA_public_decrypt() returns the size of the recovered message digest.

On error, -1 is returned — and you can grab the error codes with ERR_get_error(3) 💀

🔥 SEE ALSO 🔥

ERR_get_error(3), RSA_sign(3), RSA_verify(3), EVP_PKEY_sign(3), EVP_PKEY_verify_recover(3)

^(go read these, they're iconic)

🎀 HISTORY 🎀

Both of these functions were deprecated in OpenSSL 3.0. That's the whole section. Short and slay.

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>.

and that's the whole page, bestie 🫶 this was 3.5.0, dated 2026-09-25, filed under RSA_PRIVATE_ENCRYPT(3ossl) — mwah 💋

📦 where this page comes from