=pod

=head1 NAME

RSA_padding_add_PKCS1_type_1, RSA_padding_check_PKCS1_type_1,
RSA_padding_add_PKCS1_type_2, RSA_padding_check_PKCS1_type_2,
RSA_padding_add_PKCS1_OAEP, RSA_padding_check_PKCS1_OAEP,
RSA_padding_add_SSLv23, RSA_padding_check_SSLv23,
RSA_padding_add_none, RSA_padding_check_none - asymmetric encryption
padding

=head1 SYNOPSIS

 #include <openssl/rsa.h>

 int RSA_padding_add_PKCS1_type_1(unsigned char *to, int tlen,
    unsigned char *f, int fl);

 int RSA_padding_check_PKCS1_type_1(unsigned char *to, int tlen,
    unsigned char *f, int fl, int rsa_len);

 int RSA_padding_add_PKCS1_type_2(unsigned char *to, int tlen,
    unsigned char *f, int fl);

 int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
    unsigned char *f, int fl, int rsa_len);

 int RSA_padding_add_PKCS1_OAEP(unsigned char *to, int tlen,
    unsigned char *f, int fl, unsigned char *p, int pl);

 int RSA_padding_check_PKCS1_OAEP(unsigned char *to, int tlen,
    unsigned char *f, int fl, int rsa_len, unsigned char *p, int pl);

 int RSA_padding_add_SSLv23(unsigned char *to, int tlen,
    unsigned char *f, int fl);

 int RSA_padding_check_SSLv23(unsigned char *to, int tlen,
    unsigned char *f, int fl, int rsa_len);

 int RSA_padding_add_none(unsigned char *to, int tlen,
    unsigned char *f, int fl);

 int RSA_padding_check_none(unsigned char *to, int tlen,
    unsigned char *f, int fl, int rsa_len);

=head1 DESCRIPTION

The RSA_padding_xxx_xxx() functions are called from the RSA encrypt,
decrypt, sign and verify functions. Normally they should not be called
from application programs.

However, they can also be called directly to implement padding for other
asymmetric ciphers. RSA_padding_add_PKCS1_OAEP() and
RSA_padding_check_PKCS1_OAEP() may be used in an application combined
with B<RSA_NO_PADDING> in order to implement OAEP with an encoding
parameter.

RSA_padding_add_xxx() encodes B<fl> bytes from B<f> so as to fit into
B<tlen> bytes and stores the result at B<to>. An error occurs if B<fl>
does not meet the size requirements of the encoding method.

The following encoding methods are implemented:

=over 4

=item PKCS1_type_1

PKCS #1 v2.0 EMSA-PKCS1-v1_5 (PKCS #1 v1.5 block type 1); used for signatures

=item PKCS1_type_2

PKCS #1 v2.0 EME-PKCS1-v1_5 (PKCS #1 v1.5 block type 2)

=item PKCS1_OAEP

PKCS #1 v2.0 EME-OAEP

=item SSLv23

PKCS #1 EME-PKCS1-v1_5 with SSL-specific modification

=item none

simply copy the data

=back

The random number generator must be seeded prior to calling
RSA_padding_add_xxx().

RSA_padding_check_xxx() verifies that the B<fl> bytes at B<f> contain
a valid encoding for a B<rsa_len> byte RSA key in the respective
encoding method and stores the recovered data of at most B<tlen> bytes
(for B<RSA_NO_PADDING>: of size B<tlen>)
at B<to>.

For RSA_padding_xxx_OAEP(), B<p> points to the encoding parameter
of length B<pl>. B<p> may be B<NULL> if B<pl> is 0.

=head1 RETURN VALUES

The RSA_padding_add_xxx() functions return 1 on success, 0 on error.
The RSA_padding_check_xxx() functions return the length of the
recovered data, -1 on error. Error codes can be obtained by calling
L<ERR_get_error(3)|ERR_get_error(3)>.

=head1 WARNING

The RSA_padding_check_PKCS1_type_2() padding check leaks timing
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 PKCS1_OAEP padding. Otherwise it can
be recommended to pass zero-padded B<f>, so that B<fl> equals to
B<rsa_len>, and if fixed by protocol, B<tlen> being set to the
expected length. In such case leakage would be minimal, it would
take attacker's ability to observe memory access pattern with byte
granilarity as it occurs, post-factum timing analysis won't do.

=head1 SEE ALSO

L<RSA_public_encrypt(3)|RSA_public_encrypt(3)>,
L<RSA_private_decrypt(3)|RSA_private_decrypt(3)>,
L<RSA_sign(3)|RSA_sign(3)>, L<RSA_verify(3)|RSA_verify(3)>

=head1 HISTORY

RSA_padding_add_PKCS1_type_1(), RSA_padding_check_PKCS1_type_1(),
RSA_padding_add_PKCS1_type_2(), RSA_padding_check_PKCS1_type_2(),
RSA_padding_add_SSLv23(), RSA_padding_check_SSLv23(),
RSA_padding_add_none() and RSA_padding_check_none() appeared in
SSLeay 0.9.0.

RSA_padding_add_PKCS1_OAEP() and RSA_padding_check_PKCS1_OAEP() were
added in OpenSSL 0.9.2b.

=cut
