=pod

=head1 NAME

RSA-PSS - EVP_PKEY RSA-PSS algorithm support

=head1 SYNOPSIS

 #include <openssl/rsa.h>

 int EVP_PKEY_CTX_set_rsa_pss_keygen_md(EVP_PKEY_CTX *pctx,
                                        const EVP_MD *md);
 int EVP_PKEY_CTX_set_rsa_pss_keygen_mgf1_md(EVP_PKEY_CTX *pctx,
                                             const EVP_MD *md);
 int EVP_PKEY_CTX_set_rsa_pss_keygen_saltlen(EVP_PKEY_CTX *pctx,
                                             int saltlen);

=head1 DESCRIPTION

The B<RSA-PSS> EVP_PKEY implementation is a restricted version of the RSA
algorithm which only supports signing, verification and key generation
using PSS padding modes with optional parameter restrictions.

It has associated private key and public key formats.

This algorithm shares several control operations with the B<RSA> algorithm
but with some restrictions described below.

=head1 SIGNING AND VERIFICATION

Signing and verification is similar to the B<RSA> algorithm except the
padding mode is always PSS. If the key in use has parameter restrictions then
the corresponding signature parameters are set to the restrictions:
for example, if the key can only be used with digest SHA256, MGF1 SHA256
and minimum salt length 32 then the digest, MGF1 digest and salt length
will be set to SHA256, SHA256 and 32 respectively.

The macro EVP_PKEY_CTX_set_rsa_padding() is supported but an error is
returned if an attempt is made to set the padding mode to anything other
than B<PSS>. It is otherwise similar to the B<RSA> version.

The EVP_PKEY_CTX_set_rsa_pss_saltlen() macro is used to set the salt length.
If the key has usage restrictions then an error is returned if an attempt is
made to set the salt length below the minimum value. It is otherwise similar
to the B<RSA> operation except detection of the salt length (using
RSA_PSS_SALTLEN_AUTO is not supported for verification if the key has
usage restrictions.

The EVP_PKEY_CTX_set_signature_md() and EVP_PKEY_CTX_set_rsa_mgf1_md() macros
are used to set the digest and MGF1 algorithms respectively. If the key has
usage restrictions then an error is returned if an attempt is made to set the
digest to anything other than the restricted value. Otherwise these are
similar to the B<RSA> versions.

=head1 KEY GENERATION

As with RSA key generation the EVP_PKEY_CTX_set_rsa_rsa_keygen_bits()
and EVP_PKEY_CTX_set_rsa_keygen_pubexp() macros are supported for RSA-PSS:
they have exactly the same meaning as for the RSA algorithm.

Optional parameter restrictions can be specified when generating a PSS key. By
default no parameter restrictions are placed on the generated key. If any
restrictions are set (using the macros described below) then B<all> parameters
are restricted. For example, setting a minimum salt length also restricts the
digest and MGF1 algorithms. If any restrictions are in place then they are
reflected in the corresponding parameters of the public key when (for example)
a certificate request is signed.

EVP_PKEY_CTX_set_rsa_pss_keygen_md() restricts the digest algorithm the
generated key can use to B<md>.

EVP_PKEY_CTX_set_rsa_pss_keygen_mgf1_md() restricts the MGF1 algorithm the
generated key can use to B<md>.

EVP_PKEY_CTX_set_rsa_pss_keygen_saltlen() restricts the minimum salt length
to B<saltlen>.

=head1 NOTES

A context for the B<RSA-PSS> algorithm can be obtained by calling:

 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_RSA_PSS, NULL);

The public key format is documented in RFC4055.

The PKCS#8 private key format used for RSA-PSS keys is similar to the RSA
format except it uses the B<id-RSASSA-PSS> OID and the parameters field, if
present, restricts the key parameters in the same way as the public key.

=head1 RETURN VALUES

All these functions 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 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_CTX_ctrl_str(3)>,
L<EVP_PKEY_derive(3)>

=head1 COPYRIGHT

Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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>.

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