=pod

=head1 NAME

EVP_PKEY_verifyrecover_init, EVP_PKEY_verifyrecover - recover signature using a public key algorithm

=head1 SYNOPSIS

 #include <openssl/evp.h>

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

=head1 DESCRIPTION

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

The EVP_PKEY_verifyrecover() 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_verifyrecover_init() algorithm specific control
operations can be performed to set any appropriate parameters for the
operation.

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

=head1 RETURN VALUES

EVP_PKEY_verifyrecover_init() and EVP_PKEY_verifyrecover() 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:

[to be added]

=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 0.9.9.

=cut