EVP_PKEY_DECRYPT(3)

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() 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() 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() 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() 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() 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>.