Do not use default TLS cipher

As per RFC, if TLS server cannot pickup a suitable cipher from a client
list, it has to send handshake_failure alert. For us it means that we
have to route a request to a fronting domain, because we want to have it
exactly like a real webserver does. So, if it misbehaves, so do we.

mtglib/internal/tls/fake/client_side.go

@@ -5,8 +5,8 @@ import (
 	"crypto/hmac"
 	"crypto/sha256"
 	"crypto/subtle"
-	"crypto/tls"
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -25,7 +25,6 @@ const (
 	// https://medium.com/asecuritysite-when-bob-met-alice/in-cybersecurity-what-is-grease-9f8850558dea
 	GreaseMask      = 0x0f0f
 	GreaseValueType = 0x0a0a
-	DefaultCipher   = tls.TLS_AES_128_GCM_SHA256
 
 	sniDNSNamesListType = 0
 )
@@ -33,6 +32,8 @@ const (
 var (
 	emptyRandom = [RandomLen]byte{}
 	extTypeSNI  = [2]byte{}
+
+	ErrCannotFindCipher = errors.New("cannot find a cipher")
 )
 
 type ClientHello struct {
@@ -110,9 +111,7 @@ func parseHandshake(r io.Reader) (*ClientHello, error) {
 		return nil, fmt.Errorf("cannot read client version: %w", err)
 	}
 
-	hello := &ClientHello{
+	hello := &ClientHello{}
-		CipherSuite: DefaultCipher,
-	}
 
 	if _, err := io.ReadFull(r, hello.Random[:]); err != nil {
 		return nil, fmt.Errorf("cannot read client random: %w", err)
@@ -133,7 +132,6 @@ func parseHandshake(r io.Reader) (*ClientHello, error) {
 	}
 
 	cipherSuiteLen := int64(binary.BigEndian.Uint16(header[:]))
-	foundCipher := false
 
 	// Pick the first non-GREASE cipher suite from the list.
 	// Real TLS servers never select GREASE values (RFC 8701, pattern 0x?a?a),
@@ -144,17 +142,20 @@ func parseHandshake(r io.Reader) (*ClientHello, error) {
 			return nil, fmt.Errorf("cannot read cipher suite: %w", err)
 		}
 
-		if foundCipher {
+		if hello.CipherSuite != 0 {
+			// do not forget we have to scan until the end
 			continue
 		}
 
 		if cs := binary.BigEndian.Uint16(header[:]); cs&GreaseMask != GreaseValueType {
 			hello.CipherSuite = cs
-			// do not forget we have to scan until the end
-			foundCipher = true
 		}
 	}
 
+	if hello.CipherSuite == 0 {
+		return nil, ErrCannotFindCipher
+	}
+
 	if _, err := io.ReadFull(r, header[:1]); err != nil {
 		return nil, fmt.Errorf("cannot read compression methods length: %w", err)
 	}

mtglib/internal/tls/fake/client_side_test.go

@@ -2,6 +2,7 @@ package fake_test
 
 import (
 	"bytes"
+	cryptotls "crypto/tls"
 	"encoding/binary"
 	"encoding/json"
 	"io"
@@ -234,9 +235,24 @@ func (suite *ParseClientHelloHandshakeBodyTestSuite) TestCannotSkipRemainingCiph
 	suite.ErrorContains(err, "cannot read cipher suite")
 }
 
+func (suite *ParseClientHelloHandshakeBodyTestSuite) TestCannotFindCipher() {
+	// All cipher suites are GREASE values — must return ErrCannotFindCipher.
+	body := make([]byte, 2+fake.RandomLen+1+2+4+1)
+	binary.BigEndian.PutUint16(body[2+fake.RandomLen+1:], 4)
+	binary.BigEndian.PutUint16(body[2+fake.RandomLen+1+2:], 0x0a0a)
+	binary.BigEndian.PutUint16(body[2+fake.RandomLen+1+2+2:], 0x1a1a)
+	body[2+fake.RandomLen+1+2+4] = 1
+
+	suite.writeBody(body)
+
+	_, err := fake.ReadClientHello(suite.connMock, suite.secret.Key[:], suite.secret.Host, TolerateTime)
+	suite.ErrorIs(err, fake.ErrCannotFindCipher)
+}
+
 func (suite *ParseClientHelloHandshakeBodyTestSuite) TestCannotReadCompressionMethodsLength() {
 	body := make([]byte, 2+fake.RandomLen+1+2+2)
 	binary.BigEndian.PutUint16(body[2+fake.RandomLen+1:], 2)
+	binary.BigEndian.PutUint16(body[2+fake.RandomLen+1+2:], cryptotls.TLS_AES_128_GCM_SHA256)
 
 	suite.writeBody(body)
 
@@ -247,6 +263,7 @@ func (suite *ParseClientHelloHandshakeBodyTestSuite) TestCannotReadCompressionMe
 func (suite *ParseClientHelloHandshakeBodyTestSuite) TestCannotSkipCompressionMethods() {
 	body := make([]byte, 2+fake.RandomLen+1+2+2+1)
 	binary.BigEndian.PutUint16(body[2+fake.RandomLen+1:], 2)
+	binary.BigEndian.PutUint16(body[2+fake.RandomLen+1+2:], cryptotls.TLS_AES_128_GCM_SHA256)
 	body[2+fake.RandomLen+1+2+2] = 1
 
 	suite.writeBody(body)
@@ -282,6 +299,7 @@ func (suite *ParseClientHelloSNITestSuite) writeExtensions(extensions []byte) {
 	// cipherSuite(2) + compressionLen(1) + compression(1) = 41
 	body := make([]byte, 41)
 	binary.BigEndian.PutUint16(body[35:], 2)
+	binary.BigEndian.PutUint16(body[37:], cryptotls.TLS_AES_128_GCM_SHA256)
 	body[39] = 1
 
 	suite.readBuf.Write(body)