=pod

=head1 NAME

EVP_PKEY_decrypt_init, EVP_PKEY_decrypt_init_ex,
EVP_PKEY_decrypt - decrypt using a public key algorithm

=head1 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);

=head1 DESCRIPTION

The EVP_PKEY_decrypt_init() function initializes a public key algorithm
context using key I<pkey> for a decryption operation.

The EVP_PKEY_decrypt_init_ex() function initializes a public key algorithm
context using key I<pkey> for a decryption operation and sets the
algorithm specific I<params>.

The EVP_PKEY_decrypt() function performs a public key decryption operation
using I<ctx>. The data to be decrypted is specified using the I<in> and
I<inlen> parameters. If I<out> is NULL then the minimum required size of
the output buffer is written to the I<*outlen> parameter.

If I<out> is not NULL then before the call the I<*outlen> parameter must
contain the length of the I<out> buffer. If the call is successful the
decrypted data is written to I<out> and the amount of the decrypted data
written to I<*outlen>, otherwise an error is returned.

=head1 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.

=head1 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.

=head1 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 B<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 B<default> provider implements
the implicit rejection mechanism (see
B<OSSL_ASYM_CIPHER_PARAM_IMPLICIT_REJECTION> in L<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 L<ERR_set_mark(3)> and L<ERR_pop_to_mark(3)>
calls or by using the L<ERR_clear_error(3)> call.

=head1 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 */

=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_encrypt(3)>,
L<EVP_PKEY_sign(3)>,
L<EVP_PKEY_verify(3)>,
L<EVP_PKEY_verify_recover(3)>,
L<EVP_PKEY_derive(3)>

=head1 HISTORY

These functions were added in OpenSSL 1.0.0.

=head1 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
L<https://www.openssl.org/source/license.html>.

=cut
