=pod

=head1 NAME

EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover - recover signature using a public key algorithm

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_PKEY_verify_recover_init(EVP_PKEY_CTX *ctx);
 int EVP_PKEY_verify_recover(EVP_PKEY_CTX *ctx,
                             unsigned char *rout, size_t *routlen,
                             const unsigned char *sig, size_t siglen);

=head1 DESCRIPTION

The EVP_PKEY_verify_recover_init() function initializes a public key algorithm
context using key B<pkey> for a verify recover operation.

The EVP_PKEY_verify_recover() function recovers signed data
using B<ctx>. The signature is specified using the B<sig> and
B<siglen> parameters. If B<rout> is B<NULL> then the maximum size of the output
buffer is written to the B<routlen> parameter. If B<rout> is not B<NULL> then
before the call the B<routlen> parameter should contain the length of the
B<rout> buffer, if the call is successful recovered data is written to
B<rout> and the amount of data written to B<routlen>.

=head1 NOTES

Normally an application is only interested in whether a signature verification
operation is successful in those cases the EVP_verify() function should be
used.

Sometimes however it is useful to obtain the data originally signed using a
signing operation. Only certain public key algorithms can recover a signature
in this way (for example RSA in PKCS padding mode).

After the call to EVP_PKEY_verify_recover_init() algorithm specific control
operations can be performed to set any appropriate parameters for the
operation.

The function EVP_PKEY_verify_recover() can be called more than once on the same
context if several operations are performed using the same parameters.

=head1 RETURN VALUES

EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() 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 EXAMPLE

Recover digest originally signed using PKCS#1 and SHA256 digest:

 #include <openssl/evp.h>
 #include <openssl/rsa.h>

 EVP_PKEY_CTX *ctx;
 unsigned char *rout, *sig;
 size_t routlen, siglen;
 EVP_PKEY *verify_key;

 /*
  * NB: assumes verify_key, sig and siglen are already set up
  * and that verify_key is an RSA public key
  */
 ctx = EVP_PKEY_CTX_new(verify_key, NULL /* no engine */);
 if (!ctx)
     /* Error occurred */
 if (EVP_PKEY_verify_recover_init(ctx) <= 0)
     /* Error */
 if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
     /* Error */
 if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) <= 0)
     /* Error */

 /* Determine buffer length */
 if (EVP_PKEY_verify_recover(ctx, NULL, &routlen, sig, siglen) <= 0)
     /* Error */

 rout = OPENSSL_malloc(routlen);

 if (!rout)
     /* malloc failure */

 if (EVP_PKEY_verify_recover(ctx, rout, &routlen, sig, siglen) <= 0)
     /* Error */

 /* Recovered data is routlen bytes written to buffer rout */

=head1 SEE ALSO

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

=head1 HISTORY

These functions were added in OpenSSL 1.0.0.

=head1 COPYRIGHT

Copyright 2013-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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
