RSA_PUBLIC_ENCRYPT(3SSL) OpenSSL RSA_PUBLIC_ENCRYPT(3SSL)
NAME
RSA_public_encrypt, RSA_private_decrypt - RSA public key cryptography
SYNOPSIS
#include <openssl/rsa.h>
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_public_encrypt(int flen, const unsigned char *from,
unsigned char *to, RSA *rsa, int padding);
int RSA_private_decrypt(int flen, const unsigned char *from,
unsigned char *to, RSA *rsa, int padding);
DESCRIPTION
Both of the functions described on this page are deprecated.
Applications should instead use EVP_PKEY_encrypt_init_ex(3),
EVP_PKEY_encrypt(3), EVP_PKEY_decrypt_init_ex(3) and
EVP_PKEY_decrypt(3).
RSA_public_encrypt() encrypts the flen bytes at from (usually a session
key) using the public key rsa and stores the ciphertext in to. to must
point to RSA_size(rsa) bytes of memory.
padding denotes one of the following modes:
RSA_PKCS1_PADDING
PKCS #1 v1.5 padding. This currently is the most widely used mode.
However, it is highly recommended to use RSA_PKCS1_OAEP_PADDING in
new applications. SEE WARNING BELOW.
RSA_PKCS1_OAEP_PADDING
EME-OAEP as defined in PKCS #1 v2.0 with SHA-1, MGF1 and an empty
encoding parameter. This mode is recommended for all new
applications.
RSA_NO_PADDING
Raw RSA encryption. This mode should only be used to implement
cryptographically sound padding modes in the application code.
Encrypting user data directly with RSA is insecure.
When encrypting flen must not be more than RSA_size(rsa) - 11 for the
PKCS #1 v1.5 based padding modes, not more than RSA_size(rsa) - 42 for
RSA_PKCS1_OAEP_PADDING and exactly RSA_size(rsa) for RSA_NO_PADDING.
When a padding mode other than RSA_NO_PADDING is in use, then
RSA_public_encrypt() will include some random bytes into the ciphertext
and therefore the ciphertext will be different each time, even if the
plaintext and the public key are exactly identical. The returned
ciphertext in to will always be zero padded to exactly RSA_size(rsa)
bytes. to and from may overlap.
RSA_private_decrypt() decrypts the flen bytes at from using the private
key rsa and stores the plaintext in to. flen should be equal to
RSA_size(rsa) but may be smaller, when leading zero bytes are in the
ciphertext. Those are not important and may be removed, but
RSA_public_encrypt() does not do that. to must point to a memory section
large enough to hold the maximal possible decrypted data (which is equal
to RSA_size(rsa) for RSA_NO_PADDING, RSA_size(rsa) - 11 for the PKCS #1
v1.5 based padding modes and RSA_size(rsa) - 42 for
RSA_PKCS1_OAEP_PADDING). padding is the padding mode that was used to
encrypt the data. to and from may overlap.
RETURN VALUES
RSA_public_encrypt() returns the size of the encrypted data (i.e.,
RSA_size(rsa)). RSA_private_decrypt() returns the size of the recovered
plaintext. A return value of 0 is not an error and means only that the
plaintext was empty.
On error, -1 is returned; the error codes can be obtained by
ERR_get_error(3).
WARNINGS
Decryption failures in the RSA_PKCS1_PADDING mode leak information which
can potentially be used to mount a Bleichenbacher padding oracle attack.
This is an inherent weakness in the PKCS #1 v1.5 padding design. Prefer
RSA_PKCS1_OAEP_PADDING.
In OpenSSL before version 3.2.0, both the return value and the length of
returned value could be used to mount the Bleichenbacher attack. Since
version 3.2.0, the default provider in OpenSSL does not return an error
when padding checks fail. Instead it generates a random message based on
used private key and provided ciphertext so that application code
doesn't have to implement a side-channel secure error handling.
Applications that want to be secure against side-channel attacks with
providers that don't implement implicit rejection, still need to handle
the returned values using side-channel free code. Side-channel free
handling of the error stack can be performed using either a pair of
unconditional ERR_set_mark(3) and ERR_pop_to_mark(3) calls or by using
the ERR_clear_error(3) call.
CONFORMING TO
SSL, PKCS #1 v2.0
SEE ALSO
ERR_get_error(3), RAND_bytes(3), RSA_size(3), EVP_PKEY_decrypt(3),
EVP_PKEY_encrypt(3)
HISTORY
Both of these functions were deprecated in OpenSSL 3.0.
COPYRIGHT
Copyright 2000-2024 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>.
3.5.7 2026-06-09 RSA_PUBLIC_ENCRYPT(3SSL)
Generated by dwww version 1.16 on Sat Oct 3 06:01:50 CEST 2026.