Actually add support for SSL_get_server/peer_tmp_key (aws#1945)

Ruby has a dependency on `SSL_get_server_tmp_key` which we've
exposed as a no-op in the past. Ruby has a couple of tests that expect
the function to actually return a value however, which means we'll have
to actually support this. The relevant information about the server key
was saved in a `tmp` structure in `SSL3_STATE` (originally
`ssl3_state_st` in OpenSSL). This `tmp` structure has been removed by
upstream, with its contents moved either to `SSL_HANDSHAKE` or the
state machine. `peer_key` seems to contain the relevant information we
want and it's been moved to `SSL_HANDSHAKE` in this case. The issue
is any information in `SSL_HANDSHAKE` is shed immediately after the
connection has been established and the contents of `peer_key` is shed
along with it. We may have to revert parts of a4c8ff0 to move the field
back into SSL3_STATE so we can access the field.
Laster versions of OpenSSL have changed this to an alias to
`SSL_get_peer_tmp_key` which means you can retrieve the client’s key if
you’re the server and vice versa. This causes
`SSL_get_server_tmp_key` to have very confusing behavior where you
actually retrieve the client's key when you're a server and Ruby already 
has tests somewhat dependent on the `SSL_get_peer_tmp_key` behavior.

### Testing:
Test that tries retrieving a X25519 or EC_KEY based on the connection
for each TLS version.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

include/openssl/ssl.h

@@ -2703,6 +2703,17 @@ OPENSSL_EXPORT uint16_t SSL_get_group_id(const SSL *ssl);
 // the given TLS group ID, or NULL if the group is unknown.
 OPENSSL_EXPORT const char *SSL_get_group_name(uint16_t group_id);
 
+// SSL_get_peer_tmp_key sets |*out_key| to the temporary key provided by the
+// peer that was during the key exchange. If |ssl| is the server, the client's
+// temporary key is returned; if |ssl| is the client, the server's temporary key
+// is returned. It returns 1 on success and 0 if otherwise.
+OPENSSL_EXPORT int SSL_get_peer_tmp_key(SSL *ssl, EVP_PKEY **out_key);
+
+// SSL_get_server_tmp_key is a backwards compatible alias to
+// |SSL_get_peer_tmp_key| in OpenSSL. Note that this means the client's
+// temporary key is being set to |*out_key| instead, if |ssl| is the server.
+OPENSSL_EXPORT int SSL_get_server_tmp_key(SSL *ssl, EVP_PKEY **out_key);
+
 // *** EXPERIMENTAL — DO NOT USE WITHOUT CHECKING ***
 //
 // |SSL_to_bytes| and |SSL_from_bytes| are developed to support SSL transfer
@@ -5828,11 +5839,6 @@ DEFINE_STACK_OF(SSL_COMP)
 // AWS-LC does not support the use of FFDH cipher suites in libssl. The
 // following functions are only provided as no-ops for easier compatibility.
 
-// SSL_get_server_tmp_key returns zero. This was deprecated as part of the
-// removal of |EVP_PKEY_DH|.
-OPENSSL_EXPORT OPENSSL_DEPRECATED int SSL_get_server_tmp_key(
-    SSL *ssl, EVP_PKEY **out_key);
-
 // SSL_CTX_set_tmp_dh returns 1.
 //
 // TODO (CryptoAlg-2398): Add |OPENSSL_DEPRECATED|. nginx defines -Werror and

ssl/extensions.cc

@@ -2312,6 +2312,7 @@ static bool ext_key_share_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out,
 
 bool ssl_ext_key_share_parse_serverhello(SSL_HANDSHAKE *hs,
                                          Array<uint8_t> *out_secret,
+                                         Array<uint8_t> *out_peer_key,
                                          uint8_t *out_alert, CBS *contents) {
   CBS peer_key;
   uint16_t group_id;
@@ -2333,7 +2334,9 @@ bool ssl_ext_key_share_parse_serverhello(SSL_HANDSHAKE *hs,
     key_share = hs->key_shares[1].get();
   }
 
-  if (!key_share->Finish(out_secret, out_alert, peer_key)) {
+  if (!key_share->Finish(out_secret, out_alert, peer_key) ||
+      // Save peer's public key for observation with |SSL_get_peer_tmp_key|.
+      !out_peer_key->CopyFrom(peer_key)) {
     *out_alert = SSL_AD_INTERNAL_ERROR;
     return false;
   }

ssl/handshake_client.cc

@@ -1163,7 +1163,7 @@ static enum ssl_hs_wait_t do_read_server_key_exchange(SSL_HANDSHAKE *hs) {
 
     // Save the group and peer public key for later.
     hs->new_session->group_id = group_id;
-    if (!hs->peer_key.CopyFrom(point)) {
+    if (!ssl->s3->peer_key.CopyFrom(point)) {
       return ssl_hs_error;
     }
   } else if (!(alg_k & SSL_kPSK)) {
@@ -1508,16 +1508,17 @@ static enum ssl_hs_wait_t do_send_client_key_exchange(SSL_HANDSHAKE *hs) {
     bssl::UniquePtr<SSLKeyShare> key_share =
         SSLKeyShare::Create(hs->new_session->group_id);
     uint8_t alert = SSL_AD_DECODE_ERROR;
-    if (!key_share || !key_share->Accept(&child, &pms, &alert, hs->peer_key)) {
+    if (!key_share ||
+        !key_share->Accept(&child, &pms, &alert, ssl->s3->peer_key)) {
       ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
       return ssl_hs_error;
     }
     if (!CBB_flush(&body)) {
       return ssl_hs_error;
     }
 
-    // The peer key can now be discarded.
-    hs->peer_key.Reset();
+    // The peer key could be discarded, but we preserve it since OpenSSL
+    // allows the user to observe it with |SSL_get_peer_tmp_key|.
   } else if (alg_k & SSL_kPSK) {
     // For plain PSK, other_secret is a block of 0s with the same length as
     // the pre-shared key.

ssl/handshake_server.cc

@@ -1532,7 +1532,9 @@ static enum ssl_hs_wait_t do_read_client_key_exchange(SSL_HANDSHAKE *hs) {
 
     // Compute the premaster.
     uint8_t alert = SSL_AD_DECODE_ERROR;
-    if (!hs->key_shares[0]->Finish(&premaster_secret, &alert, peer_key)) {
+    if (!hs->key_shares[0]->Finish(&premaster_secret, &alert, peer_key) ||
+        // Save peer's public key for observation with |SSL_get_peer_tmp_key|.
+        !ssl->s3->peer_key.CopyFrom(peer_key)) {
       ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
       return ssl_hs_error;
     }

ssl/internal.h

@@ -1275,6 +1275,11 @@ Span<const uint16_t> PQGroups();
 // false.
 bool ssl_nid_to_group_id(uint16_t *out_group_id, int nid);
 
+// ssl_nid_to_group_id looks up the group corresponding to |group_id|. On
+// success, it sets |*out_nid| to the group's nid and returns true. Otherwise,
+// it returns false.
+bool ssl_group_id_to_nid(uint16_t *out_nid, int group_id);
+
 // ssl_name_to_group_id looks up the group corresponding to the |name| string of
 // length |len|. On success, it sets |*out_group_id| to the group ID and returns
 // true. Otherwise, it returns false.
@@ -2061,9 +2066,6 @@ struct SSL_HANDSHAKE {
   // supports with delegated credentials.
   Array<uint16_t> peer_delegated_credential_sigalgs;
 
-  // peer_key is the peer's ECDH key for a TLS 1.2 client.
-  Array<uint8_t> peer_key;
-
   // extension_permutation is the permutation to apply to ClientHello
   // extensions. It maps indices into the |kExtensions| table into other
   // indices.
@@ -2336,6 +2338,7 @@ bool ssl_setup_key_shares(SSL_HANDSHAKE *hs, uint16_t override_group_id);
 
 bool ssl_ext_key_share_parse_serverhello(SSL_HANDSHAKE *hs,
                                          Array<uint8_t> *out_secret,
+                                         Array<uint8_t> *out_peer_key,
                                          uint8_t *out_alert, CBS *contents);
 bool ssl_ext_key_share_parse_clienthello(SSL_HANDSHAKE *hs, bool *out_found,
                                          Span<const uint8_t> *out_peer_key,
@@ -3033,6 +3036,11 @@ struct SSL3_STATE {
   // one.
   UniquePtr<SSL_HANDSHAKE> hs;
 
+  // peer_key is the peer's ECDH key for both TLS 1.2/1.3. This is only used
+  // for observing with |SSL_get_peer_tmp_key| and is not serialized as part of
+  // the SSL Transfer feature.
+  Array<uint8_t> peer_key;
+
   uint8_t write_traffic_secret[SSL_MAX_MD_SIZE] = {0};
   uint8_t read_traffic_secret[SSL_MAX_MD_SIZE] = {0};
   uint8_t exporter_secret[SSL_MAX_MD_SIZE] = {0};

ssl/ssl_key_share.cc

@@ -799,6 +799,17 @@ bool ssl_nid_to_group_id(uint16_t *out_group_id, int nid) {
   return false;
 }
 
+bool ssl_group_id_to_nid(uint16_t *out_nid, int group_id) {
+  GUARD_PTR(out_nid);
+  for (const auto &group : kNamedGroups) {
+    if (group.group_id == group_id) {
+      *out_nid = group.nid;
+      return true;
+    }
+  }
+  return false;
+}
+
 bool ssl_name_to_group_id(uint16_t *out_group_id, const char *name, size_t len) {
   for (const auto &group : kNamedGroups) {
     if (len == strlen(group.name) &&

ssl/ssl_lib.cc

@@ -2668,7 +2668,51 @@ const COMP_METHOD *SSL_get_current_compression(SSL *ssl) { return NULL; }
 
 const COMP_METHOD *SSL_get_current_expansion(SSL *ssl) { return NULL; }
 
-int SSL_get_server_tmp_key(SSL *ssl, EVP_PKEY **out_key) { return 0; }
+int SSL_get_peer_tmp_key(SSL *ssl, EVP_PKEY **out_key) {
+  GUARD_PTR(ssl);
+  GUARD_PTR(ssl->s3);
+  GUARD_PTR(out_key);
+
+  SSL_SESSION *session = SSL_get_session(ssl);
+  uint16_t nid;
+  if (!session || !ssl_group_id_to_nid(&nid, session->group_id)) {
+    return 0;
+  }
+  bssl::UniquePtr<EVP_PKEY> ret(EVP_PKEY_new());
+  if (!ret) {
+    return 0;
+  }
+
+  // Assign key type based on the session's key exchange |nid|.
+  if (nid == EVP_PKEY_X25519) {
+    if (!EVP_PKEY_set_type(ret.get(), EVP_PKEY_X25519)) {
+      return 0;
+    }
+  } else {
+    EC_KEY *key = EC_KEY_new_by_curve_name(nid);
+    if (!key) {
+      // We only support ECDHE for temporary keys, so fail if an unrecognized
+      // key exchange is used.
+      OPENSSL_PUT_ERROR(SSL, SSL_R_UNKNOWN_KEY_EXCHANGE_TYPE);
+      return 0;
+    }
+    if (!EVP_PKEY_assign_EC_KEY(ret.get(), key)) {
+      return 0;
+    }
+  }
+
+  if (!EVP_PKEY_set1_tls_encodedpoint(ret.get(), ssl->s3->peer_key.data(),
+                                      ssl->s3->peer_key.size())) {
+    return 0;
+  }
+  EVP_PKEY_up_ref(ret.get());
+  *out_key = ret.get();
+  return 1;
+}
+
+int SSL_get_server_tmp_key(SSL *ssl, EVP_PKEY **out_key) {
+  return SSL_get_peer_tmp_key(ssl, out_key);
+}
 
 void SSL_CTX_set_quiet_shutdown(SSL_CTX *ctx, int mode) {
   ctx->quiet_shutdown = (mode != 0);

ssl/ssl_test.cc

@@ -10165,6 +10165,38 @@ TEST_P(SSLVersionTest, TicketSessionIDsMatch) {
   EXPECT_EQ(Bytes(SessionIDOf(client.get())), Bytes(SessionIDOf(server.get())));
 }
 
+TEST_P(SSLVersionTest, PeerTmpKey) {
+  if (getVersionParam().transfer_ssl) {
+    // The peer's temporary key is not within the boundary of the SSL transfer
+    // feature.
+    GTEST_SKIP();
+  }
+
+  // Default should be using X5519 as the key exchange.
+  ASSERT_TRUE(Connect());
+  for (SSL *ssl : {client_.get(), server_.get()}) {
+    SCOPED_TRACE(SSL_is_server(ssl) ? "server" : "client");
+    EVP_PKEY *key = nullptr;
+    EXPECT_TRUE(SSL_get_peer_tmp_key(ssl, &key));
+    EXPECT_EQ(EVP_PKEY_id(key), EVP_PKEY_X25519);
+    bssl::UniquePtr<EVP_PKEY> pkey(key);
+  }
+
+  // Check that EC Groups for the key exchange also work.
+  ASSERT_TRUE(SSL_CTX_set1_groups_list(server_ctx_.get(), "P-384"));
+  ASSERT_TRUE(Connect());
+  for (SSL *ssl : {client_.get(), server_.get()}) {
+    SCOPED_TRACE(SSL_is_server(ssl) ? "server" : "client");
+    EVP_PKEY *key = nullptr;
+    EXPECT_TRUE(SSL_get_peer_tmp_key(ssl, &key));
+    EXPECT_EQ(EVP_PKEY_id(key), EVP_PKEY_EC);
+    EC_KEY *ec_key = EVP_PKEY_get0_EC_KEY(key);
+    EXPECT_TRUE(ec_key);
+    EXPECT_EQ(EC_KEY_get0_group(ec_key), EC_group_p384());
+    bssl::UniquePtr<EVP_PKEY> pkey(key);
+  }
+}
+
 static void WriteHelloRequest(SSL *server) {
   // This function assumes TLS 1.2 with ChaCha20-Poly1305.
   ASSERT_EQ(SSL_version(server), TLS1_2_VERSION);

ssl/tls13_client.cc

@@ -420,8 +420,7 @@ static enum ssl_hs_wait_t do_read_server_hello(SSL_HANDSHAKE *hs) {
   uint16_t version;
   if (!supported_versions.present ||
       !CBS_get_u16(&supported_versions.data, &version) ||
-      CBS_len(&supported_versions.data) != 0 ||
-      version != ssl->version) {
+      CBS_len(&supported_versions.data) != 0 || version != ssl->version) {
     OPENSSL_PUT_ERROR(SSL, SSL_R_SECOND_SERVERHELLO_VERSION_MISMATCH);
     ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
     return ssl_hs_error;
@@ -497,15 +496,14 @@ static enum ssl_hs_wait_t do_read_server_hello(SSL_HANDSHAKE *hs) {
   // Resolve ECDHE and incorporate it into the secret.
   Array<uint8_t> dhe_secret;
   alert = SSL_AD_DECODE_ERROR;
-  if (!ssl_ext_key_share_parse_serverhello(hs, &dhe_secret, &alert,
-                                           &key_share.data)) {
+  if (!ssl_ext_key_share_parse_serverhello(hs, &dhe_secret, &ssl->s3->peer_key,
+                                           &alert, &key_share.data)) {
     ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
     return ssl_hs_error;
   }
 
   if (!tls13_advance_key_schedule(hs, dhe_secret) ||
-      !ssl_hash_message(hs, msg) ||
-      !tls13_derive_handshake_secrets(hs)) {
+      !ssl_hash_message(hs, msg) || !tls13_derive_handshake_secrets(hs)) {
     return ssl_hs_error;
   }
 

ssl/tls13_server.cc

@@ -78,7 +78,9 @@ static bool resolve_ecdhe_secret(SSL_HANDSHAKE *hs,
     if (!key_share ||  //
         !CBB_init(public_key.get(), 32) ||
         !key_share->Accept(public_key.get(), &secret, &alert, peer_key) ||
-        !CBBFinishArray(public_key.get(), &hs->ecdh_public_key)) {
+        !CBBFinishArray(public_key.get(), &hs->ecdh_public_key) ||
+        // Save peer's public key for observation with |SSL_get_peer_tmp_key|.
+        !ssl->s3->peer_key.CopyFrom(peer_key)) {
       ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
       return false;
     }