fix(tls): verify certificates by default

SPEC/RFC5280_CHAIN_VALIDATION.md

@@ -43,6 +43,6 @@ const tls_config = TlsConfig{
 
 ### Caveats
 
-- `skip_cert_verify` defaults to `true` for backward compatibility
+- `skip_cert_verify` defaults to `false`; callers must opt out explicitly for test-only/self-signed scenarios
 - V1 certificates (no extensions) are accepted as CAs when no basicConstraints is present — this matches common practice but is less strict than RFC 5280's recommendation
 - The interop client always uses `skip_cert_verify=true` since interop test peers use various self-signed certs

src/quic/tls13.zig

@@ -508,7 +508,7 @@ pub const TlsConfig = struct {
     private_key_bytes: []const u8, // Raw ECDSA P-256 private key (32 bytes)
     alpn: []const []const u8,
     server_name: ?[]const u8 = null, // SNI (client only)
-    skip_cert_verify: bool = true, // Skip X.509 chain + CertificateVerify validation
+    skip_cert_verify: bool = false, // Skip X.509 chain + CertificateVerify validation
     ca_bundle: ?*Certificate.Bundle = null, // Caller-owned CA bundle for trust anchor verification
     session_ticket: ?*const SessionTicket = null, // Stored ticket from previous connection (client)
     ticket_key: ?[16]u8 = null, // AES-128-GCM key for encrypting/decrypting tickets (server)
@@ -1119,6 +1119,8 @@ pub const Tls13Handshake = struct {
                         @memcpy(self.leaf_pub_key_buf[0..pub_key.len], pub_key);
                         self.leaf_pub_key_len = @intCast(pub_key.len);
                         self.leaf_pub_key_algo = std.meta.activeTag(parsed.pub_key_algo);
+                    } else {
+                        return error.BadCertificate;
                     }
 
                     // Verify hostname if SNI was set
@@ -1150,10 +1152,9 @@ pub const Tls13Handshake = struct {
 
                 // If this is the last cert, verify against CA bundle
                 if (pos >= cert_list_end) {
-                    if (self.config.ca_bundle) |bundle| {
-                        const now_sec = std.time.timestamp();
-                        bundle.verify(parsed, now_sec) catch return error.BadCertificate;
-                    }
+                    const bundle = self.config.ca_bundle orelse return error.BadCertificate;
+                    const now_sec = std.time.timestamp();
+                    bundle.verify(parsed, now_sec) catch return error.BadCertificate;
                 }
 
                 prev_parsed = parsed;
@@ -1172,7 +1173,9 @@ pub const Tls13Handshake = struct {
 
         if (msg[0] != @intFromEnum(MsgType.certificate_verify)) return error.UnexpectedMessage;
 
-        if (!self.config.skip_cert_verify and self.leaf_pub_key_len > 0) {
+        if (!self.config.skip_cert_verify) {
+            if (self.leaf_pub_key_len == 0) return error.BadCertificateVerify;
+
             // Get transcript hash BEFORE updating with CertificateVerify
             const transcript_hash = self.transcript.current();
 
@@ -2794,6 +2797,16 @@ test "buildServerHello: produces valid message" {
     try std.testing.expectEqual(msg.len - 4, body_len);
 }
 
+test "TlsConfig defaults to certificate verification enabled" {
+    const config = TlsConfig{
+        .cert_chain_der = &.{},
+        .private_key_bytes = &.{},
+        .alpn = &.{},
+    };
+
+    try std.testing.expect(!config.skip_cert_verify);
+}
+
 test "loopback handshake: client and server complete" {
     // Generate an ECDSA P-256 key pair for the server
     const server_key_pair = EcdsaP256Sha256.KeyPair.generate();
@@ -2815,6 +2828,7 @@ test "loopback handshake: client and server complete" {
         .private_key_bytes = &.{},
         .alpn = &[_][]const u8{"h3"},
         .server_name = "localhost",
+        .skip_cert_verify = true,
     };
 
     const server_tp = transport_params.TransportParams{
@@ -2961,6 +2975,7 @@ test "loopback PSK resumption: two handshakes with session ticket" {
         .private_key_bytes = &.{},
         .alpn = &[_][]const u8{"h3"},
         .server_name = "localhost",
+        .skip_cert_verify = true,
     };
 
     const tp = transport_params.TransportParams{