RSA_PRIVATE_ENCRYPT(3ossl)

💖 RSA_PRIVATE_ENCRYPT 💖

okay bestie, so this is the RSA_PRIVATE_ENCRYPT(3ossl) page from the OpenSSL manual — yes, that's the manual name, it's giving "low-level signature operations" and honestly? it's a little vintage. we're doing legacy crypto lore today. buckle up. 🎀

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✨ NAME ✨

RSA_private_encrypt, RSA_public_decrypt — low-level signature operations. no cap, that's literally what they do.

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✨ SYNOPSIS ✨

#include <openssl/rsa.h>

so here's the tea, bestie: 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 — go peek at openssl_user_macros(7) for the deets 🫶

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);

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✨ DESCRIPTION ✨

okay so first things first, and I need you to hear me on this one: both of the functions described on this page are deprecated. 💀 like, they're still here, they still work, but they're the group chat member who never replies.

applications should instead use EVP_PKEY_sign_init_ex(3), EVP_PKEY_sign(3), EVP_PKEY_verify_recover_init(3), and EVP_PKEY_verify_recover(3). that's the glow-up. that's the main character era. ✨

so what do these two actually do? they handle RSA signatures at a low-level. very down in the trenches, very "I do my own thing" energy.

🌸 RSA_private_encrypt()

this one signs the flen bytes at from — usually a message digest with an algorithm identifier, iykyk — using the private key rsa, and stores the signature in to.

⚠️ to must point to RSA_size(rsa) bytes of memory. not a byte less, bestie. not one. 🫶

🎀 padding

padding denotes one of the following modes:

• RSA_PKCS1_PADDING — PKCS #1 v1.5 padding. but here's the catch, bestie: this function does not handle the algorithmIdentifier specified in PKCS #1. so when you're generating or verifying PKCS #1 signatures, you should be using 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. and listen — signing user data directly with RSA is insecure. I'm not being dramatic, that's just the facts. no cap. 😭

🦋 RSA_public_decrypt()

this one 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 — which is smaller than RSA_size(rsa) - 11. yes, that exact number. write it down.

and padding here is the padding mode that was used to sign the data. matching set, obviously. 💜

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✨ RETURN VALUES ✨

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

and on error? -1 is returned. sad girl hours. 😭 you can grab the error codes with ERR_get_error(3).

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✨ SEE ALSO ✨

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

go read those, bestie, they're the upgrade path fr fr 🫶

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✨ HISTORY ✨

both of these functions were deprecated in OpenSSL 3.0. that's the whole history section. short and sweet, like a text you leave on read. 💀

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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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and that's the whole page, bestie!! 💅✨ this was 3.5.9, dated 2026-09-29, and it's the RSA_PRIVATE_ENCRYPT(3ossl) manual — go forth and use the EVP functions instead, I'm literally begging you 🫶💖🎀