Update documentation

Add details of the use of PSS for signature algorithms.

Document SSL_get_peer_signature_nid() and SSL_get_peer_signature_type_nid().
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2301)

doc/man3/SSL_CTX_set1_sigalgs.pod

@@ -70,11 +70,14 @@ prohibits them (for example SHA1 if the security level is 4 or more).
 
 Currently the NID_md5, NID_sha1, NID_sha224, NID_sha256, NID_sha384 and
 NID_sha512 digest NIDs are supported and the public key algorithm NIDs
-EVP_PKEY_RSA, EVP_PKEY_DSA and EVP_PKEY_EC.
+EVP_PKEY_RSA, EVP_PKEY_RSA_PSS, EVP_PKEY_DSA and EVP_PKEY_EC.
 
 The short or long name values for digests can be used in a string (for
 example "MD5", "SHA1", "SHA224", "SHA256", "SHA384", "SHA512") and
-the public key algorithm strings "RSA", "DSA" or "ECDSA".
+the public key algorithm strings "RSA", "RSA-PSS", "DSA" or "ECDSA".
+
+The TLS 1.3 signature scheme names (such as "rsa_pss_sha256") can also
+be used.
 
 The use of MD5 as a digest is strongly discouraged due to security weaknesses.
 

doc/man3/SSL_get_peer_signature_nid.pod

+=pod
+
+=head1 NAME
+
+SSL_get_peer_signature_nid, SSL_get_peer_signature_type_nid - get TLS
+message signing types
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int SSL_get_peer_signature_nid(SSL *ssl, int *psig_nid);
+ int SSL_get_peer_signature_type_nid(const SSL *ssl, int *psigtype_nid);
+
+=head1 DESCRIPTION
+
+SSL_get_peer_signature_nid() sets B<*psig_nid> to the NID of the digest used
+by the peer to sign TLS messages. It is implemented as a macro.
+
+SSL_get_peer_signature_type_nid() sets B<*psigtype_nid> to the signature
+type used by the peer to sign TLS messages. Currently the signature type
+is the NID of the public key type used for signing except for PSS signing
+where it is B<EVP_PKEY_RSA_PSS>.
+
+=head1 RETURN VALUES
+
+These functions return 1 for success and 0 for failure. There are several
+possible reasons for failure: the ciphersuite has no signature (e.g. it
+uses RSA key exchange or is anonymous), the TLS version is below 1.2 or
+the functions were called before the peer signed a message.
+
+=head1 SEE ALSO
+
+L<ssl(7)>, L<SSL_get_peer_certificate(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>.
+
+=cut