=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 B<pkey> for a decryption operation.

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

The EVP_PKEY_decrypt() function performs a public key decryption operation
using B<ctx>. The data to be decrypted is specified using the B<in> and
B<inlen> parameters. If B<out> is B<NULL> then the maximum size of the output
buffer is written to the B<outlen> parameter. If B<out> is not B<NULL> then
before the call the B<outlen> parameter should contain the length of the
B<out> buffer, if the call is successful the decrypted data is written to
B<out> and the amount of data written to B<outlen>.

=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 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 2006-2021 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
L<https://www.openssl.org/source/license.html>.

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