EVP_PKEY_DECRYPT(3ossl) OpenSSL EVP_PKEY_DECRYPT(3ossl)
NAME
EVP_PKEY_decrypt_init, EVP_PKEY_decrypt_init_ex, EVP_PKEY_decrypt -
decrypt using a public key algorithm
SYNOPSIS
#include <openssl/evp.h>
int EVP_PKEY_decrypt_init(EVP_PKEY_CTX *ctx);
int EVP_PKEY_decrypt_init_ex(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
int EVP_PKEY_decrypt(EVP_PKEY_CTX *ctx,
unsigned char *out, size_t *outlen,
const unsigned char *in, size_t inlen);
DESCRIPTION
The EVP_PKEY_decrypt_init() function initializes a public key algorithm
context using key pkey for a decryption operation.
The EVP_PKEY_decrypt_init_ex() function initializes a public key
algorithm context using key pkey for a decryption operation and sets
the algorithm specific params.
The EVP_PKEY_decrypt(3) function performs a public key decryption
operation using ctx. The data to be decrypted is specified using the in
and inlen parameters. If out is NULL then the minimum required size of
the output buffer is written to the *outlen parameter.
If out is not NULL then before the call the *outlen parameter must
contain the length of the out buffer. If the call is successful the
decrypted data is written to out and the amount of the decrypted data
written to *outlen, otherwise an error is returned.
NOTES
After the call to EVP_PKEY_decrypt_init() algorithm specific control
operations can be performed to set any appropriate parameters for the
operation. These operations can be included in the
EVP_PKEY_decrypt_init_ex() call.
The function EVP_PKEY_decrypt(3) can be called more than once on the
same context if several operations are performed using the same
parameters.
RETURN VALUES
EVP_PKEY_decrypt_init(), EVP_PKEY_decrypt_init_ex() and
EVP_PKEY_decrypt(3) return 1 for success and 0 or a negative value for
failure. In particular a return value of -2 indicates the operation is
not supported by the public key algorithm.
WARNINGS
In OpenSSL versions before 3.2.0, when used in PKCS#1 v1.5 padding,
both the return value from the EVP_PKEY_decrypt(3) and the outlen
provided information useful in mounting a Bleichenbacher attack against
the used private key. They had to be processed in a side-channel free
way.
Since version 3.2.0, the EVP_PKEY_decrypt(3) method when used with
PKCS#1 v1.5 padding as implemented in the default provider implements
the implicit rejection mechanism (see
OSSL_ASYM_CIPHER_PARAM_IMPLICIT_REJECTION in provider-asym_cipher(7)).
That means it doesn't return an error when it detects an error in
padding, instead it returns a pseudo-randomly generated message,
removing the need of side-channel secure code from applications using
OpenSSL. If OpenSSL is configured to use a provider that doesn't
implement implicit rejection, the code still needs 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.
EXAMPLES
Decrypt data using OAEP (for RSA keys):
#include <openssl/evp.h>
#include <openssl/rsa.h>
EVP_PKEY_CTX *ctx;
ENGINE *eng;
unsigned char *out, *in;
size_t outlen, inlen;
EVP_PKEY *key;
/*
* NB: assumes key, eng, in, inlen are already set up
* and that key is an RSA private key
*/
ctx = EVP_PKEY_CTX_new(key, eng);
if (!ctx)
/* Error occurred */
if (EVP_PKEY_decrypt_init(ctx) <= 0)
/* Error */
if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_OAEP_PADDING) <= 0)
/* Error */
/* Determine buffer length */
if (EVP_PKEY_decrypt(ctx, NULL, &outlen, in, inlen) <= 0)
/* Error */
out = OPENSSL_malloc(outlen);
if (!out)
/* malloc failure */
if (EVP_PKEY_decrypt(ctx, out, &outlen, in, inlen) <= 0)
/* Error */
/* Decrypted data is outlen bytes written to buffer out */
SEE ALSO
EVP_PKEY_CTX_new(3), EVP_PKEY_encrypt(3), EVP_PKEY_sign(3),
EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3), EVP_PKEY_derive(3)
HISTORY
These functions were added in OpenSSL 1.0.0.
COPYRIGHT
Copyright 2006-2025 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>.
3.4.1 2025-02-12 EVP_PKEY_DECRYPT(3ossl)
openssl 3.4.1 - Generated Thu Feb 20 14:26:50 CST 2025