=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.

=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 be less than RSA_size(B<rsa>) - 11 for the PKCS #1 v1.5
based padding modes, less than RSA_size(B<rsa>) - 41 for
RSA_PKCS1_OAEP_PADDING and exactly RSA_size(B<rsa>) for RSA_NO_PADDING.
The random number generator must be seeded prior to calling
RSA_public_encrypt().

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<to> must point
to a memory section large enough to hold the decrypted data (which is
smaller than RSA_size(B<rsa>)). B<padding> is the padding mode that
was used to encrypt the data.

=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.

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

=head1 WARNING

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)|ERR_get_error(3)>, L<rand(3)|rand(3)>, L<rsa(3)|rsa(3)>,
L<RSA_size(3)|RSA_size(3)>

=head1 HISTORY

The B<padding> argument was added in SSLeay 0.8. RSA_NO_PADDING is
available since SSLeay 0.9.0, OAEP was added in OpenSSL 0.9.2b.

=cut
