RSA_PADDING_ADD_PKCS1_TYPE_1(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 RSA_PADDING_ADD_PKCS1_TYPE_1 (3ossl) — the OpenSSL padding girlies 🎀

okay so bestie, this is the manual page for all the RSA_padding_* functions, and it lives in the OpenSSL section 3 manual, iykyk. Buckle up, there's a whole roster of them and they're all lowkey deprecated, but you still need to know them fr fr. 💖

🌸 NAME 🌸

RSA_padding_add_PKCS1_type_1, RSA_padding_check_PKCS1_type_1, RSA_padding_add_PKCS1_type_2, RSA_padding_check_PKCS1_type_2, RSA_padding_add_PKCS1_OAEP, RSA_padding_check_PKCS1_OAEP, RSA_padding_add_PKCS1_OAEP_mgf1, RSA_padding_check_PKCS1_OAEP_mgf1, RSA_padding_add_none, RSA_padding_check_none — literally all of them are about asymmetric encryption padding ✨

🦋 SYNOPSIS 🦋

First thing, the include:

#include <openssl/rsa.h>
🚨 vibe check: 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 — see openssl_user_macros(7). No cap. 💀

Here's the whole squad, verbatim, don't @ me:

int RSA_padding_add_PKCS1_type_1(unsigned char *to, int tlen,
                                 const unsigned char *f, int fl);

int RSA_padding_check_PKCS1_type_1(unsigned char *to, int tlen,
                                   const unsigned char *f, int fl, int rsa_len);

int RSA_padding_add_PKCS1_type_2(unsigned char *to, int tlen,
                                 const unsigned char *f, int fl);

int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
                                   const unsigned char *f, int fl, int rsa_len);

int RSA_padding_add_PKCS1_OAEP(unsigned char *to, int tlen,
                               const unsigned char *f, int fl,
                               const unsigned char *p, int pl);

int RSA_padding_check_PKCS1_OAEP(unsigned char *to, int tlen,
                                 const unsigned char *f, int fl, int rsa_len,
                                 const unsigned char *p, int pl);

int RSA_padding_add_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
                                    const unsigned char *f, int fl,
                                    const unsigned char *p, int pl,
                                    const EVP_MD *md, const EVP_MD *mgf1md);

int RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
                                      const unsigned char *f, int fl, int rsa_len,
                                      const unsigned char *p, int pl,
                                      const EVP_MD *md, const EVP_MD *mgf1md);

int RSA_padding_add_none(unsigned char *to, int tlen,
                         const unsigned char *f, int fl);

int RSA_padding_check_none(unsigned char *to, int tlen,
                           const unsigned char *f, int fl, int rsa_len);

🌈 DESCRIPTION 🌈

Okay so first of all, every single function on this page is deprecated. 💅 Applications should instead use the EVP PKEY APIs. That's the assignment, bestie.

So the RSA_padding_xxx_xxx() functions get called from the RSA encrypt, decrypt, sign and verify functions. Normally you should not be calling them from your application programs — they're backstage main character energy, not the vibe you're supposed to touch directly. 🙈

That said, you can also call them directly to implement padding for other asymmetric ciphers. Specifically, RSA_padding_add_PKCS1_OAEP() and RSA_padding_check_PKCS1_OAEP() may be used in an application combined with RSA_NO_PADDING in order to implement OAEP with an encoding parameter. Slay, flexible. ✨

Now the mechanics, fr fr: RSA_padding_add_xxx() encodes fl bytes from f so as to fit into tlen bytes and stores the result at to. An error occurs if fl does not meet the size requirements of the encoding method. That's the whole deal. 🫶

🎀 the encoding methods, ranked (not really)

  • PKCS1_type_1 — PKCS #1 v2.0 EMSA-PKCS1-v1_5 (PKCS #1 v1.5 block type 1); used for signatures ✍️
  • PKCS1_type_2 — PKCS #1 v2.0 EME-PKCS1-v1_5 (PKCS #1 v1.5 block type 2)
  • PKCS1_OAEP — PKCS #1 v2.0 EME-OAEP 🌸
  • none — simply copy the data. no thoughts, head empty, just copy 🫶
⚠️ important, bestie: the random number generator must be seeded prior to calling RSA_padding_add_xxx(). And if the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to external circumstances (see RAND(7)), the operation will fail. Not a drill. 😭

Then RSA_padding_check_xxx() verifies that the fl bytes at f contain a valid encoding for a rsa_len byte RSA key in the respective encoding method and stores the recovered data of at most tlen bytes (for RSA_NO_PADDING: of size tlen) at to. Cute, right? 💖

For RSA_padding_xxx_OAEP(), p points to the encoding parameter of length pl. And p may be NULL if pl is 0. 🎀

For RSA_padding_xxx_OAEP_mgf1(), md points to the md hash — if md is NULL that means md=sha1 — and mgf1md points to the mgf1 hash, if mgf1md is NULL that means mgf1md=md. Defaults on defaults, iconic. 👑

⭐ RETURN VALUES ⭐

  • The RSA_padding_add_xxx() functions return 1 on success, 0 on error. ✅
  • The RSA_padding_check_xxx() functions return the length of the recovered data, and -1 on error. 🫠
  • Error codes can be obtained by calling ERR_get_error(3).

💀 WARNINGS 💀

okay this is the serious part, so like, pay attention bestie. 😭

The result of RSA_padding_check_PKCS1_type_2() is exactly the information which is used to mount a classical Bleichenbacher padding oracle attack. This is an inherent weakness in the PKCS #1 v1.5 padding design. Prefer PKCS1_OAEP padding. Periodt.

If that is not possible, the result of RSA_padding_check_PKCS1_type_2() should be checked in constant time — like, check if it matches the expected length of the plaintext — and additionally some application specific consistency checks on the plaintext need to be performed in constant time. 🕰️

And if the plaintext is rejected, it must be kept secret which of the checks caused the application to reject the message. Like, do not snitch on which check failed. 🤫

🚨 do NOT remove the zero-padding from the decrypted raw RSA data which was computed by RSA_private_decrypt() with RSA_NO_PADDING, as this would create a small timing side channel which could be used to mount a Bleichenbacher attack against any padding mode including PKCS1_OAEP. That's the whole boss fight, do not skip it. 💀

Also: you should prefer the use of EVP PKEY APIs for PKCS#1 v1.5 decryption as they implement the necessary workarounds internally. Let them handle the lore. 🙏

🔥 SEE ALSO 🔥

RSA_public_encrypt(3), RSA_private_decrypt(3), RSA_sign(3), RSA_verify(3), RAND(7)

🧚 HISTORY 🧚

All of these functions were deprecated in OpenSSL 3.0. That's it, that's the history lesson. 🎀

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 little love letter came to you from OpenSSL 3.5.0, dated 2026-09-2, manual page RSA_PADDING_ADD_PKCS1_TYPE_1(3ossl) — go forth, pad responsibly, and remember: constant time or it didn't happen 💅💖🦋🌸

📦 where this page comes from