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=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);
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)|EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_encrypt(3)|EVP_PKEY_encrypt(3)>,
L<EVP_PKEY_decrypt(3)|EVP_PKEY_decrypt(3)>,
L<EVP_PKEY_sign(3)|EVP_PKEY_sign(3)>,
L<EVP_PKEY_verify(3)|EVP_PKEY_verify(3)>,
L<EVP_PKEY_derive(3)|EVP_PKEY_derive(3)>
=head1 HISTORY
These functions were first added to OpenSSL 1.0.0.
=cut