Update fingerprints + add default spec version (#25)

Update fingerprints + add default spec version

 * Adds fingerprints for Chrome 75, iOS 12.1, and Firefox 65(=Firefox 63)
 * If min/max tls versions are not explicitly specified in the ClientHelloSpec,
   uTLS will try to parse versions from SupportedVersions extension,
   and fallback to [TLS 1.0, TLS 1.2] if SupportedVersions is absent.
 * Adds mimicked FakeRecordSizeLimitExtension and FakeCertCompressionAlgsExtension
   to be used instead of GenericExtension{} for clarity and extensibility
   (we are ready to use those with Firefox and Chrome fps with correct values
   whenever actual functionality is implemented)
* SetTLSVers: parse the right extensions + cosmetics

u_tls_extensions.go

@@ -392,32 +392,6 @@ func (e *GenericExtension) Read(b []byte) (int, error) {
 	return e.Len(), io.EOF
 }
 
-/*
-FAKE EXTENSIONS
-*/
-
-type FakeChannelIDExtension struct {
-}
-
-func (e *FakeChannelIDExtension) writeToUConn(uc *UConn) error {
-	return nil
-}
-
-func (e *FakeChannelIDExtension) Len() int {
-	return 4
-}
-
-func (e *FakeChannelIDExtension) Read(b []byte) (int, error) {
-	if len(b) < e.Len() {
-		return 0, io.ErrShortBuffer
-	}
-	// https://tools.ietf.org/html/draft-balfanz-tls-channelid-00
-	b[0] = byte(fakeExtensionChannelID >> 8)
-	b[1] = byte(fakeExtensionChannelID & 0xff)
-	// The length is 0
-	return e.Len(), io.EOF
-}
-
 type UtlsExtendedMasterSecretExtension struct {
 }
 
@@ -712,5 +686,94 @@ func (e *CookieExtension) Read(b []byte) (int, error) {
 	return e.Len(), io.EOF
 }
 
-// TODO: FakeCertificateCompressionAlgorithmsExtension
-// TODO: FakeRecordSizeLimitExtension
+/*
+FAKE EXTENSIONS
+*/
+
+type FakeChannelIDExtension struct {
+}
+
+func (e *FakeChannelIDExtension) writeToUConn(uc *UConn) error {
+	return nil
+}
+
+func (e *FakeChannelIDExtension) Len() int {
+	return 4
+}
+
+func (e *FakeChannelIDExtension) Read(b []byte) (int, error) {
+	if len(b) < e.Len() {
+		return 0, io.ErrShortBuffer
+	}
+	// https://tools.ietf.org/html/draft-balfanz-tls-channelid-00
+	b[0] = byte(fakeExtensionChannelID >> 8)
+	b[1] = byte(fakeExtensionChannelID & 0xff)
+	// The length is 0
+	return e.Len(), io.EOF
+}
+
+type FakeCertCompressionAlgsExtension struct {
+	Methods []CertCompressionAlgo
+}
+
+func (e *FakeCertCompressionAlgsExtension) writeToUConn(uc *UConn) error {
+	return nil
+}
+
+func (e *FakeCertCompressionAlgsExtension) Len() int {
+	return 4 + 1 + (2 * len(e.Methods))
+}
+
+func (e *FakeCertCompressionAlgsExtension) Read(b []byte) (int, error) {
+	if len(b) < e.Len() {
+		return 0, io.ErrShortBuffer
+	}
+	// https://tools.ietf.org/html/draft-balfanz-tls-channelid-00
+	b[0] = byte(fakeCertCompressionAlgs >> 8)
+	b[1] = byte(fakeCertCompressionAlgs & 0xff)
+
+	extLen := 2 * len(e.Methods)
+	if extLen > 255 {
+		return 0, errors.New("too many certificate compression methods")
+	}
+
+	b[2] = byte((extLen + 1) >> 8)
+	b[3] = byte((extLen + 1) & 0xff)
+	b[4] = byte(extLen)
+
+	i := 5
+	for _, compMethod := range e.Methods {
+		b[i] = byte(compMethod >> 8)
+		b[i+1] = byte(compMethod)
+		i += 2
+	}
+	return e.Len(), io.EOF
+}
+
+type FakeRecordSizeLimitExtension struct {
+	Limit uint16
+}
+
+func (e *FakeRecordSizeLimitExtension) writeToUConn(uc *UConn) error {
+	return nil
+}
+
+func (e *FakeRecordSizeLimitExtension) Len() int {
+	return 6
+}
+
+func (e *FakeRecordSizeLimitExtension) Read(b []byte) (int, error) {
+	if len(b) < e.Len() {
+		return 0, io.ErrShortBuffer
+	}
+	// https://tools.ietf.org/html/draft-balfanz-tls-channelid-00
+	b[0] = byte(fakeRecordSizeLimit >> 8)
+	b[1] = byte(fakeRecordSizeLimit & 0xff)
+
+	b[2] = byte(0)
+	b[3] = byte(2)
+
+	b[4] = byte(e.Limit >> 8)
+	b[5] = byte(e.Limit & 0xff)
+	return e.Len(), io.EOF
+}