=pod

=head1 NAME

EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover_init_ex,
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_init_ex(EVP_PKEY_CTX *ctx,
                                     const OSSL_PARAM params[]);
 int EVP_PKEY_verify_recover(EVP_PKEY_CTX *ctx,
                             unsigned char *rout, size_t *routlen,
                             const unsigned char *sig, size_t siglen);

=head1 DESCRIPTION

EVP_PKEY_verify_recover_init() initializes a public key algorithm context
I<ctx> for signing using the algorithm given when the context was created
using L<EVP_PKEY_CTX_new(3)> or variants thereof.  The algorithm is used to
fetch a B<EVP_SIGNATURE> method implicitly, see L<provider(7)/Implicit fetch>
for more information about implicit fetches.

EVP_PKEY_verify_recover_init_ex() is the same as
EVP_PKEY_verify_recover_init() but additionally sets the passed parameters
I<params> on the context before returning.

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

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

The EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover()
functions were added in OpenSSL 1.0.0.

The EVP_PKEY_verify_recover_init_ex() function was added in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2013-2021 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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