fix: no padding if raw clienthello is too short (#263)

* Add a function `AlwaysPadToLen` to generate padding styles according to the raw clienthello.

* Add an extra step in `FromRaw` after parsing the extension list to update the padding style.

u_common.go

@@ -545,6 +545,15 @@ func (chs *ClientHelloSpec) FromRaw(raw []byte, ctrlFlags ...bool) error {
 		return err
 	}
 
+	// if extension list includes padding, we update the padding-to-len according to
+	// the raw ClientHello length
+	for _, ext := range chs.Extensions {
+		if _, ok := ext.(*UtlsPaddingExtension); ok {
+			ext.(*UtlsPaddingExtension).GetPaddingLen = AlwaysPadToLen(len(raw) - 5)
+			break
+		}
+	}
+
 	return nil
 }
 

u_tls_extensions.go

@@ -1062,6 +1062,23 @@ func BoringPaddingStyle(unpaddedLen int) (int, bool) {
 	return 0, false
 }
 
+// AlwaysPadToLen could be used for parsed ClientHello, since some fingerprints
+// might not use BoringSSL padding style and we want to pad to a the same length.
+func AlwaysPadToLen(padToLen int) func(int) (int, bool) {
+	return func(unpaddedLen int) (int, bool) {
+		if unpaddedLen < padToLen {
+			paddingLen := padToLen - unpaddedLen
+			if paddingLen >= 4+1 {
+				paddingLen -= 4
+			} else {
+				paddingLen = 1
+			}
+			return paddingLen, true
+		}
+		return 0, false
+	}
+}
+
 // UtlsCompressCertExtension implements compress_certificate (27) and is only implemented client-side
 // for server certificates. Alternate certificate message formats
 // (https://datatracker.ietf.org/doc/html/rfc7250) are not supported.