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: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5068)

crypto/err/openssl.txt

-# Copyright 1999-2018 The OpenSSL Project Authors. All Rights Reserved.
+# Copyright 1999-2017 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
@@ -1321,6 +1321,7 @@ SSL_F_TLS_PARSE_CTOS_RENEGOTIATE:464:tls_parse_ctos_renegotiate
 SSL_F_TLS_PARSE_CTOS_SERVER_NAME:573:tls_parse_ctos_server_name
 SSL_F_TLS_PARSE_CTOS_SESSION_TICKET:574:tls_parse_ctos_session_ticket
 SSL_F_TLS_PARSE_CTOS_SIG_ALGS:575:tls_parse_ctos_sig_algs
+SSL_F_TLS_PARSE_CTOS_SIG_ALGS_CERT:615:tls_parse_ctos_sig_algs_cert
 SSL_F_TLS_PARSE_CTOS_SRP:576:tls_parse_ctos_srp
 SSL_F_TLS_PARSE_CTOS_STATUS_REQUEST:577:tls_parse_ctos_status_request
 SSL_F_TLS_PARSE_CTOS_SUPPORTED_GROUPS:578:tls_parse_ctos_supported_groups

include/openssl/sslerr.h

 /*
  * Generated by util/mkerr.pl DO NOT EDIT
- * Copyright 1995-2017 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 1995-2018 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
@@ -364,6 +364,7 @@ int ERR_load_SSL_strings(void);
 # define SSL_F_TLS_PARSE_CTOS_SERVER_NAME                 573
 # define SSL_F_TLS_PARSE_CTOS_SESSION_TICKET              574
 # define SSL_F_TLS_PARSE_CTOS_SIG_ALGS                    575
+# define SSL_F_TLS_PARSE_CTOS_SIG_ALGS_CERT               615
 # define SSL_F_TLS_PARSE_CTOS_SRP                         576
 # define SSL_F_TLS_PARSE_CTOS_STATUS_REQUEST              577
 # define SSL_F_TLS_PARSE_CTOS_SUPPORTED_GROUPS            578

include/openssl/tls1.h

@@ -146,6 +146,7 @@ extern "C" {
 # define TLSEXT_TYPE_cookie                      44
 # define TLSEXT_TYPE_psk_kex_modes               45
 # define TLSEXT_TYPE_certificate_authorities     47
+# define TLSEXT_TYPE_signature_algorithms_cert   50
 # define TLSEXT_TYPE_key_share                   51
 
 /* Temporary extension type */

ssl/s3_lib.c

@@ -3324,6 +3324,7 @@ void ssl3_free(SSL *s)
     OPENSSL_free(s->s3->tmp.ciphers_raw);
     OPENSSL_clear_free(s->s3->tmp.pms, s->s3->tmp.pmslen);
     OPENSSL_free(s->s3->tmp.peer_sigalgs);
+    OPENSSL_free(s->s3->tmp.peer_cert_sigalgs);
     ssl3_free_digest_list(s);
     OPENSSL_free(s->s3->alpn_selected);
     OPENSSL_free(s->s3->alpn_proposed);
@@ -3343,6 +3344,7 @@ int ssl3_clear(SSL *s)
     OPENSSL_free(s->s3->tmp.ciphers_raw);
     OPENSSL_clear_free(s->s3->tmp.pms, s->s3->tmp.pmslen);
     OPENSSL_free(s->s3->tmp.peer_sigalgs);
+    OPENSSL_free(s->s3->tmp.peer_cert_sigalgs);
 
 #if !defined(OPENSSL_NO_EC) || !defined(OPENSSL_NO_DH)
     EVP_PKEY_free(s->s3->tmp.pkey);

ssl/ssl_err.c

 /*
  * Generated by util/mkerr.pl DO NOT EDIT
- * Copyright 1995-2017 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 1995-2018 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
@@ -568,6 +568,8 @@ static const ERR_STRING_DATA SSL_str_functs[] = {
      "tls_parse_ctos_session_ticket"},
     {ERR_PACK(ERR_LIB_SSL, SSL_F_TLS_PARSE_CTOS_SIG_ALGS, 0),
      "tls_parse_ctos_sig_algs"},
+    {ERR_PACK(ERR_LIB_SSL, SSL_F_TLS_PARSE_CTOS_SIG_ALGS_CERT, 0),
+     "tls_parse_ctos_sig_algs_cert"},
     {ERR_PACK(ERR_LIB_SSL, SSL_F_TLS_PARSE_CTOS_SRP, 0), "tls_parse_ctos_srp"},
     {ERR_PACK(ERR_LIB_SSL, SSL_F_TLS_PARSE_CTOS_STATUS_REQUEST, 0),
      "tls_parse_ctos_status_request"},

ssl/ssl_locl.h

@@ -701,6 +701,7 @@ typedef enum tlsext_index_en {
     TLSEXT_IDX_encrypt_then_mac,
     TLSEXT_IDX_signed_certificate_timestamp,
     TLSEXT_IDX_extended_master_secret,
+    TLSEXT_IDX_signature_algorithms_cert,
     TLSEXT_IDX_signature_algorithms,
     TLSEXT_IDX_supported_versions,
     TLSEXT_IDX_psk_kex_modes,
@@ -1507,10 +1508,13 @@ typedef struct ssl3_state_st {
          * signature algorithms peer reports: e.g. supported signature
          * algorithms extension for server or as part of a certificate
          * request for client.
+         * Keep track of the algorithms for TLS and X.509 usage separately.
          */
         uint16_t *peer_sigalgs;
-        /* Size of above array */
+        uint16_t *peer_cert_sigalgs;
+        /* Size of above arrays */
         size_t peer_sigalgslen;
+        size_t peer_cert_sigalgslen;
         /* Sigalg peer actually uses */
         const SIGALG_LOOKUP *peer_sigalg;
         /*
@@ -2473,7 +2477,7 @@ __owur long ssl_get_algorithm2(SSL *s);
 __owur int tls12_copy_sigalgs(SSL *s, WPACKET *pkt,
                               const uint16_t *psig, size_t psiglen);
 __owur int tls1_save_u16(PACKET *pkt, uint16_t **pdest, size_t *pdestlen);
-__owur int tls1_save_sigalgs(SSL *s, PACKET *pkt);
+__owur int tls1_save_sigalgs(SSL *s, PACKET *pkt, int cert);
 __owur int tls1_process_sigalgs(SSL *s);
 __owur int tls1_set_peer_legacy_sigalg(SSL *s, const EVP_PKEY *pkey);
 __owur int tls1_lookup_md(const SIGALG_LOOKUP *lu, const EVP_MD **pmd);

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,
@@ -280,6 +281,14 @@ static const EXTENSION_DEFINITION ext_defs[] = {
         init_ems, tls_parse_ctos_ems, tls_parse_stoc_ems,
         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,
@@ -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)
 {

ssl/statem/extensions_srvr.c

@@ -276,6 +276,27 @@ int tls_parse_ctos_session_ticket(SSL *s, PACKET *pkt, unsigned int context,
     return 1;
 }
 
+int tls_parse_ctos_sig_algs_cert(SSL *s, PACKET *pkt, unsigned int context,
+                                 X509 *x, size_t chainidx)
+{
+    PACKET supported_sig_algs;
+
+    if (!PACKET_as_length_prefixed_2(pkt, &supported_sig_algs)
+            || PACKET_remaining(&supported_sig_algs) == 0) {
+        SSLfatal(s, SSL_AD_DECODE_ERROR,
+                 SSL_F_TLS_PARSE_CTOS_SIG_ALGS_CERT, SSL_R_BAD_EXTENSION);
+        return 0;
+    }
+
+    if (!s->hit && !tls1_save_sigalgs(s, &supported_sig_algs, 1)) {
+        SSLfatal(s, SSL_AD_DECODE_ERROR,
+                 SSL_F_TLS_PARSE_CTOS_SIG_ALGS_CERT, SSL_R_BAD_EXTENSION);
+        return 0;
+    }
+
+    return 1;
+}
+
 int tls_parse_ctos_sig_algs(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
                             size_t chainidx)
 {
@@ -288,7 +309,7 @@ int tls_parse_ctos_sig_algs(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
         return 0;
     }
 
-    if (!s->hit && !tls1_save_sigalgs(s, &supported_sig_algs)) {
+    if (!s->hit && !tls1_save_sigalgs(s, &supported_sig_algs, 0)) {
         SSLfatal(s, SSL_AD_DECODE_ERROR,
                  SSL_F_TLS_PARSE_CTOS_SIG_ALGS, SSL_R_BAD_EXTENSION);
         return 0;

ssl/statem/statem_clnt.c

@@ -2458,7 +2458,11 @@ MSG_PROCESS_RETURN tls_process_certificate_request(SSL *s, PACKET *pkt)
                 return MSG_PROCESS_ERROR;
             }
 
-            if (!tls1_save_sigalgs(s, &sigalgs)) {
+            /*
+             * Despite this being for certificates, preserve compatibility
+             * with pre-TLS 1.3 and use the regular sigalgs field.
+             */
+            if (!tls1_save_sigalgs(s, &sigalgs, 0)) {
                 SSLfatal(s, SSL_AD_INTERNAL_ERROR,
                          SSL_F_TLS_PROCESS_CERTIFICATE_REQUEST,
                          SSL_R_SIGNATURE_ALGORITHMS_ERROR);

ssl/statem/statem_locl.h

@@ -205,6 +205,8 @@ int tls_parse_ctos_supported_groups(SSL *s, PACKET *pkt, unsigned int context,
 #endif
 int tls_parse_ctos_session_ticket(SSL *s, PACKET *pkt, unsigned int context,
                                   X509 *x, size_t chainidx);
+int tls_parse_ctos_sig_algs_cert(SSL *s, PACKET *pkt, unsigned int context,
+                                 X509 *x, size_t chainidx);
 int tls_parse_ctos_sig_algs(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
                             size_t chainidx);
 #ifndef OPENSSL_NO_OCSP

ssl/t1_lib.c

@@ -1131,7 +1131,8 @@ int tls1_set_server_sigalgs(SSL *s)
      * If peer sent no signature algorithms check to see if we support
      * the default algorithm for each certificate type
      */
-    if (s->s3->tmp.peer_sigalgs == NULL) {
+    if (s->s3->tmp.peer_cert_sigalgs == NULL
+            && s->s3->tmp.peer_sigalgs == NULL) {
         const uint16_t *sent_sigs;
         size_t sent_sigslen = tls12_get_psigalgs(s, 1, &sent_sigs);
 
@@ -1596,7 +1597,7 @@ int tls1_save_u16(PACKET *pkt, uint16_t **pdest, size_t *pdestlen)
     return 1;
 }
 
-int tls1_save_sigalgs(SSL *s, PACKET *pkt)
+int tls1_save_sigalgs(SSL *s, PACKET *pkt, int cert)
 {
     /* Extension ignored for inappropriate versions */
     if (!SSL_USE_SIGALGS(s))
@@ -1605,10 +1606,13 @@ int tls1_save_sigalgs(SSL *s, PACKET *pkt)
     if (s->cert == NULL)
         return 0;
 
-    return tls1_save_u16(pkt, &s->s3->tmp.peer_sigalgs,
-                         &s->s3->tmp.peer_sigalgslen);
+    if (cert)
+        return tls1_save_u16(pkt, &s->s3->tmp.peer_cert_sigalgs,
+                             &s->s3->tmp.peer_cert_sigalgslen);
+    else
+        return tls1_save_u16(pkt, &s->s3->tmp.peer_sigalgs,
+                             &s->s3->tmp.peer_sigalgslen);
 
-    return 1;
 }
 
 /* Set preferred digest for each key type */
@@ -1974,10 +1978,11 @@ int tls1_check_chain(SSL *s, X509 *x, EVP_PKEY *pk, STACK_OF(X509) *chain,
     if (TLS1_get_version(s) >= TLS1_2_VERSION && strict_mode) {
         int default_nid;
         int rsign = 0;
-        if (s->s3->tmp.peer_sigalgs)
+        if (s->s3->tmp.peer_cert_sigalgs != NULL
+                || s->s3->tmp.peer_sigalgs != NULL) {
             default_nid = 0;
         /* If no sigalgs extension use defaults from RFC5246 */
-        else {
+        } else {
             switch (idx) {
             case SSL_PKEY_RSA:
                 rsign = EVP_PKEY_RSA;
@@ -2312,13 +2317,48 @@ static int tls12_get_cert_sigalg_idx(const SSL *s, const SIGALG_LOOKUP *lu)
     if (clu == NULL || !(clu->amask & s->s3->tmp.new_cipher->algorithm_auth))
         return -1;
 
-    /* If PSS and we have no PSS cert use RSA */
-    if (sig_idx == SSL_PKEY_RSA_PSS_SIGN && !ssl_has_cert(s, sig_idx))
-        sig_idx = SSL_PKEY_RSA;
-
     return s->s3->tmp.valid_flags[sig_idx] & CERT_PKEY_VALID ? sig_idx : -1;
 }
 
+/*
+ * Returns true if |s| has a usable certificate configured for use
+ * with signature scheme |sig|.
+ * "Usable" includes a check for presence as well as applying
+ * the signature_algorithm_cert restrictions sent by the peer (if any).
+ * Returns false if no usable certificate is found.
+ */
+static int has_usable_cert(SSL *s, const SIGALG_LOOKUP *sig, int idx)
+{
+    const SIGALG_LOOKUP *lu;
+    int mdnid, pknid;
+    size_t i;
+
+    /* TLS 1.2 callers can override lu->sig_idx, but not TLS 1.3 callers. */
+    if (idx == -1)
+        idx = sig->sig_idx;
+    if (!ssl_has_cert(s, idx))
+        return 0;
+    if (s->s3->tmp.peer_cert_sigalgs != NULL) {
+        for (i = 0; i < s->s3->tmp.peer_cert_sigalgslen; i++) {
+            lu = tls1_lookup_sigalg(s->s3->tmp.peer_cert_sigalgs[i]);
+            if (lu == NULL
+                || !X509_get_signature_info(s->cert->pkeys[idx].x509, &mdnid,
+                                            &pknid, NULL, NULL))
+                continue;
+            /*
+             * TODO this does not differentiate between the
+             * rsa_pss_pss_* and rsa_pss_rsae_* schemes since we do not
+             * have a chain here that lets us look at the key OID in the
+             * signing certificate.
+             */
+            if (mdnid == lu->hash && pknid == lu->sig)
+                return 1;
+        }
+        return 0;
+    }
+    return 1;
+}
+
 /*
  * Choose an appropriate signature algorithm based on available certificates
  * Sets chosen certificate and signature algorithm.
@@ -2355,14 +2395,9 @@ int tls_choose_sigalg(SSL *s, int fatalerrs)
                 || lu->sig == EVP_PKEY_DSA
                 || lu->sig == EVP_PKEY_RSA)
                 continue;
-            if (!tls1_lookup_md(lu, NULL))
+            /* Check that we have a cert, and signature_algorithms_cert */
+            if (!tls1_lookup_md(lu, NULL) || !has_usable_cert(s, lu, -1))
                 continue;
-            if (!ssl_has_cert(s, lu->sig_idx)) {
-                if (lu->sig_idx != SSL_PKEY_RSA_PSS_SIGN
-                        || !ssl_has_cert(s, SSL_PKEY_RSA))
-                    continue;
-                sig_idx = SSL_PKEY_RSA;
-            }
             if (lu->sig == EVP_PKEY_EC) {
 #ifndef OPENSSL_NO_EC
                 if (curve == -1) {
@@ -2381,21 +2416,8 @@ int tls_choose_sigalg(SSL *s, int fatalerrs)
             } else if (lu->sig == EVP_PKEY_RSA_PSS) {
                 /* validate that key is large enough for the signature algorithm */
                 EVP_PKEY *pkey;
-                int pkey_id;
 
-                if (sig_idx == -1)
-                    pkey = s->cert->pkeys[lu->sig_idx].privatekey;
-                else
-                    pkey = s->cert->pkeys[sig_idx].privatekey;
-                pkey_id = EVP_PKEY_id(pkey);
-                if (pkey_id != EVP_PKEY_RSA_PSS
-                    && pkey_id != EVP_PKEY_RSA)
-                    continue;
-                /*
-                 * The pkey type is EVP_PKEY_RSA_PSS or EVP_PKEY_RSA
-                 * EVP_PKEY_get0_RSA returns NULL if the type is not EVP_PKEY_RSA
-                 * so use EVP_PKEY_get0 instead
-                 */
+                pkey = s->cert->pkeys[lu->sig_idx].privatekey;
                 if (!rsa_pss_check_min_key_size(EVP_PKEY_get0(pkey), lu))
                     continue;
             }
@@ -2416,8 +2438,8 @@ int tls_choose_sigalg(SSL *s, int fatalerrs)
                 return 1;
 
         if (SSL_USE_SIGALGS(s)) {
+            size_t i;
             if (s->s3->tmp.peer_sigalgs != NULL) {
-                size_t i;
 #ifndef OPENSSL_NO_EC
                 int curve;
 
@@ -2444,21 +2466,16 @@ int tls_choose_sigalg(SSL *s, int fatalerrs)
                         int cc_idx = s->cert->key - s->cert->pkeys;
 
                         sig_idx = lu->sig_idx;
-                        if (cc_idx != sig_idx) {
-                            if (sig_idx != SSL_PKEY_RSA_PSS_SIGN
-                                || cc_idx != SSL_PKEY_RSA)
-                                continue;
-                            sig_idx = SSL_PKEY_RSA;
-                        }
+                        if (cc_idx != sig_idx)
+                            continue;
                     }
+                    /* Check that we have a cert, and sig_algs_cert */
+                    if (!has_usable_cert(s, lu, sig_idx))
+                        continue;
                     if (lu->sig == EVP_PKEY_RSA_PSS) {
                         /* validate that key is large enough for the signature algorithm */
                         EVP_PKEY *pkey = s->cert->pkeys[sig_idx].privatekey;
-                        int pkey_id = EVP_PKEY_id(pkey);
 
-                        if (pkey_id != EVP_PKEY_RSA_PSS
-                            && pkey_id != EVP_PKEY_RSA)
-                            continue;
                         if (!rsa_pss_check_min_key_size(EVP_PKEY_get0(pkey), lu))
                             continue;
                     }
@@ -2479,7 +2496,7 @@ int tls_choose_sigalg(SSL *s, int fatalerrs)
                  * If we have no sigalg use defaults
                  */
                 const uint16_t *sent_sigs;
-                size_t sent_sigslen, i;
+                size_t sent_sigslen;
 
                 if ((lu = tls1_get_legacy_sigalg(s, -1)) == NULL) {
                     if (!fatalerrs)
@@ -2492,7 +2509,8 @@ int tls_choose_sigalg(SSL *s, int fatalerrs)
                 /* Check signature matches a type we sent */
                 sent_sigslen = tls12_get_psigalgs(s, 1, &sent_sigs);
                 for (i = 0; i < sent_sigslen; i++, sent_sigs++) {
-                    if (lu->sigalg == *sent_sigs)
+                    if (lu->sigalg == *sent_sigs
+                            && has_usable_cert(s, lu, lu->sig_idx))
                         break;
                 }
                 if (i == sent_sigslen) {