=pod

=head1 NAME

RSA_public_encrypt, RSA_private_decrypt - RSA public key cryptography

=head1 SYNOPSIS

 #include <openssl/rsa.h>

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

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

=head1 DESCRIPTION

RSA_public_encrypt() encrypts the B<flen> bytes at B<from> (usually a
session key) using the public key B<rsa> and stores the ciphertext in
B<to>. B<to> must point to RSA_size(B<rsa>) bytes of memory.

B<padding> denotes one of the following modes:

=over 4

=item RSA_PKCS1_PADDING

PKCS #1 v1.5 padding. This currently is the most widely used mode.
However, it is highly recommended to use RSA_PKCS1_OAEP_PADDING in
new applications. SEE WARNING BELOW.

=item RSA_PKCS1_OAEP_PADDING

EME-OAEP as defined in PKCS #1 v2.0 with SHA-1, MGF1 and an empty
encoding parameter. This mode is recommended for all new applications.

=item RSA_SSLV23_PADDING

PKCS #1 v1.5 padding with an SSL-specific modification that denotes
that the server is SSL3 capable.

=item RSA_NO_PADDING

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

=back

B<flen> must not be more than RSA_size(B<rsa>) - 11 for the PKCS #1 v1.5
based padding modes, not more than RSA_size(B<rsa>) - 42 for
RSA_PKCS1_OAEP_PADDING and exactly RSA_size(B<rsa>) for RSA_NO_PADDING.
When a padding mode other than RSA_NO_PADDING is in use, then
RSA_public_encrypt() will include some random bytes into the ciphertext
and therefore the ciphertext will be different each time, even if the
plaintext and the public key are exactly identical.
The returned ciphertext in B<to> will always be zero padded to exactly
RSA_size(B<rsa>) bytes.
B<to> and B<from> may overlap.

RSA_private_decrypt() decrypts the B<flen> bytes at B<from> using the
private key B<rsa> and stores the plaintext in B<to>. B<flen> should
be equal to RSA_size(B<rsa>) but may be smaller, when leading zero
bytes are in the ciphertext. Those are not important and may be removed,
but RSA_public_encrypt() does not do that. B<to> must point
to a memory section large enough to hold the maximal possible decrypted
data (which is equal to RSA_size(B<rsa>) for RSA_NO_PADDING,
RSA_size(B<rsa>) - 11 for the PKCS #1 v1.5 based padding modes and
RSA_size(B<rsa>) - 42 for RSA_PKCS1_OAEP_PADDING).
B<padding> is the padding mode that was used to encrypt the data.
B<to> and B<from> may overlap.

=head1 RETURN VALUES

RSA_public_encrypt() returns the size of the encrypted data (i.e.,
RSA_size(B<rsa>)). RSA_private_decrypt() returns the size of the
recovered plaintext. A return value of 0 is not an error and
means only that the plaintext was empty.

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

=head1 WARNINGS

Decryption failures in the RSA_PKCS1_PADDING mode leak information
which can potentially be used to mount a Bleichenbacher padding oracle
attack. This is an inherent weakness in the PKCS #1 v1.5 padding
design. Prefer RSA_PKCS1_OAEP_PADDING.

=head1 CONFORMING TO

SSL, PKCS #1 v2.0

=head1 SEE ALSO

L<ERR_get_error(3)>, L<RAND_bytes(3)>,
L<RSA_size(3)>

=head1 COPYRIGHT

Copyright 2000-2019 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