Add support for the TLS 1.3 signature_algorithms_cert extension

The new extension is like signature_algorithms, but only for the
signature *on* the certificate we will present to the peer (the
old signature_algorithms extension is still used for signatures that
we *generate*, i.e., those over TLS data structures).

We do not need to generate this extension, since we are the same
implementation as our X.509 stack and can handle the same types
of signatures, but we need to be prepared to receive it, and use the received
information when selecting what certificate to present.

There is a lot of interplay between signature_algorithms_cert and
signature_algorithms, since both affect what certificate we can
use, and thus the resulting signature algorithm used for TLS messages.
So, apply signature_algorithms_cert (if present) as a filter on what
certificates we can consider when choosing a certificate+sigalg
pair.

As part of this addition, we also remove the fallback code that let
keys of type EVP_PKEY_RSA be used to generate RSA-PSS signatures -- the
new rsa_pss_pss_* and rsa_pss_rsae_* signature schemes have pulled
the key type into what is covered by the signature algorithm, so
we should not apply this sort of compatibility workaround.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5068)

1 files changed, 18 insertions, 0 deletions

diff --git a/ssl/statem/extensions.c b/ssl/statem/extensions.c
index 5a0fa25571..5ad86f20af 100644
--- a/ssl/statem/extensions.c
+++ b/ssl/statem/extensions.c
@@ -29,6 +29,7 @@ static int init_npn(SSL *s, unsigned int context);
 #endif
 static int init_alpn(SSL *s, unsigned int context);
 static int final_alpn(SSL *s, unsigned int context, int sent);
+static int init_sig_algs_cert(SSL *s, unsigned int context);
 static int init_sig_algs(SSL *s, unsigned int context);
 static int init_certificate_authorities(SSL *s, unsigned int context);
 static EXT_RETURN tls_construct_certificate_authorities(SSL *s, WPACKET *pkt,
@@ -281,6 +282,14 @@ static const EXTENSION_DEFINITION ext_defs[] = {
         tls_construct_stoc_ems, tls_construct_ctos_ems, final_ems
     },
     {
+        TLSEXT_TYPE_signature_algorithms_cert,
+        SSL_EXT_CLIENT_HELLO | SSL_EXT_TLS1_3_CERTIFICATE_REQUEST,
+        init_sig_algs_cert, tls_parse_ctos_sig_algs_cert,
+        tls_parse_ctos_sig_algs_cert,
+        /* We do not generate signature_algorithms_cert at present. */
+        NULL, NULL, NULL
+    },
+    {
         TLSEXT_TYPE_signature_algorithms,
         SSL_EXT_CLIENT_HELLO | SSL_EXT_TLS1_3_CERTIFICATE_REQUEST,
         init_sig_algs, tls_parse_ctos_sig_algs,
@@ -1090,6 +1099,15 @@ static int init_sig_algs(SSL *s, unsigned int context)
     return 1;
 }
 
+static int init_sig_algs_cert(SSL *s, unsigned int context)
+{
+    /* Clear any signature algorithms extension received */
+    OPENSSL_free(s->s3->tmp.peer_cert_sigalgs);
+    s->s3->tmp.peer_cert_sigalgs = NULL;
+
+    return 1;
+}
+
 #ifndef OPENSSL_NO_SRP
 static int init_srp(SSL *s, unsigned int context)
 {