=pod

=head1 NAME

RSA_private_encrypt, RSA_public_decrypt - low level signature operations

=head1 SYNOPSIS

 #include <openssl/rsa.h>

 int RSA_private_encrypt(int flen, unsigned char *from,
                         unsigned char *to, RSA *rsa, int padding);

 int RSA_public_decrypt(int flen, unsigned char *from,
                        unsigned char *to, RSA *rsa, int padding);

=head1 DESCRIPTION

These functions handle RSA signatures at a low level.

RSA_private_encrypt() signs the B<flen> bytes at B<from> (usually a
message digest with an algorithm identifier) using the private key
B<rsa> and stores the signature in B<to>. B<to> must point to
B<RSA_size(rsa)> bytes of memory.

B<padding> denotes one of the following modes:

=over 4

=item RSA_PKCS1_PADDING

PKCS #1 v1.5 padding. This function does not handle the
B<algorithmIdentifier> specified in PKCS #1. When generating or
verifying PKCS #1 signatures, L<RSA_sign(3)> and L<RSA_verify(3)> should be
used.

=item RSA_NO_PADDING

Raw RSA signature. This mode should I<only> be used to implement
cryptographically sound padding modes in the application code.
Signing user data directly with RSA is insecure.

=back

RSA_public_decrypt() recovers the message digest from the B<flen>
bytes long signature at B<from> using the signer's public key
B<rsa>. B<to> must point to a memory section large enough to hold the
message digest (which is smaller than B<RSA_size(rsa) -
11>). B<padding> is the padding mode that was used to sign the data.

=head1 RETURN VALUES

RSA_private_encrypt() returns the size of the signature (i.e.,
RSA_size(rsa)). RSA_public_decrypt() returns the size of the
recovered message digest.

On error, -1 is returned; the error codes can be
obtained by L<ERR_get_error(3)>.

=head1 SEE ALSO

L<ERR_get_error(3)>,
L<RSA_sign(3)>, L<RSA_verify(3)>

=head1 COPYRIGHT

Copyright 2000-2016 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>.

=cut
