Refactor TLS fragmenting

mtglib/internal/tls/fake/utils.go

@@ -0,0 +1,158 @@
+package fake
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+
+	"github.com/9seconds/mtg/v2/mtglib/internal/tls"
+)
+
+const (
+	maxFragmentsCount = 10
+)
+
+var ErrTooManyFragments = errors.New("too many fragments")
+
+// https://datatracker.ietf.org/doc/html/rfc5246#section-6.2.1
+// client hello can be fragmented in a series of packets:
+//
+//	Bytes on the wire:
+//
+// 16 03 01 00 F8 01 00 00 F4 03 03 [32 bytes random] [session_id] [ciphers] [SNI...]
+// ├─────────────┤├──────────────────────────────────────────────────────────────────┤
+//
+//	TLS record       Payload (248 bytes)
+//	header (5B)
+//
+//	16    = Handshake
+//	03 01 = TLS 1.0 (record layer version)
+//	00 F8 = 248 bytes follow
+//
+//	01       = ClientHello (handshake type)
+//	00 00 F4 = 244 bytes of handshake body
+//	03 03    = TLS 1.2 (actual protocol version)
+//	...rest of ClientHello...
+//
+// Fragmented record look like:
+//
+//	Record 1:
+//
+// 16 03 01 00 03 01 00 00
+// ├─────────────┤├──────┤
+//
+//	TLS header    3 bytes of payload
+//
+//	16    = Handshake
+//	03 01 = TLS 1.0
+//	00 03 = only 3 bytes follow
+//
+//	01       = ClientHello type
+//	00 00    = first 2 bytes of the uint24 length (INCOMPLETE!)
+//
+// Record 2:
+// 16 03 01 00 F5 F4 03 03 [32 bytes random] [session_id] [ciphers] [SNI...]
+// ├─────────────┤├────────────────────────────────────────────────────────────┤
+//
+//	TLS header    remaining 245 bytes of payload
+//
+//	16    = Handshake
+//	03 01 = TLS 1.0
+//	00 F5 = 245 bytes follow
+//
+//	F4    = last byte of uint24 length (now complete: 00 00 F4 = 244)
+//	03 03 = TLS 1.2
+//	...rest of ClientHello continues...
+//
+// So it means that there could be a series of handshake packets of different
+// lengths. The goal of this function is to concatenate these fragments.
+type fragmentedHandshakeReader struct {
+	r             io.Reader
+	buf           bytes.Buffer
+	readFragments int
+}
+
+func (f *fragmentedHandshakeReader) Read(p []byte) (int, error) {
+	if n, err := f.buf.Read(p); err == nil {
+		return n, nil
+	}
+
+	f.buf.Reset()
+
+	for f.buf.Len() == 0 {
+		if f.readFragments > maxFragmentsCount {
+			return 0, ErrTooManyFragments
+		}
+
+		if err := f.parseNextFragment(); err != nil {
+			return 0, err
+		}
+
+		f.readFragments++
+	}
+
+	return f.buf.Read(p)
+}
+
+func (f *fragmentedHandshakeReader) parseNextFragment() error {
+	// record_type(1) + version(2) + size(2)
+	//   16 - type is 0x16 (handshake record)
+	//   03 01 - protocol version is "3,1" (also known as TLS 1.0)
+	//   00 f8 - 0xF8 (248) bytes of handshake message follows
+	header := [1 + 2 + 2]byte{}
+
+	if _, err := io.ReadFull(f.r, header[:]); err != nil {
+		return fmt.Errorf("cannot read record header: %w", err)
+	}
+
+	if header[0] != tls.TypeHandshake {
+		return fmt.Errorf("unexpected record type %#x", header[0])
+	}
+
+	if header[1] != 3 || header[2] != 1 {
+		return fmt.Errorf("unexpected protocol version %#x %#x", header[1], header[2])
+	}
+
+	length := int64(binary.BigEndian.Uint16(header[3:]))
+	_, err := io.CopyN(&f.buf, f.r, length)
+
+	return err
+}
+
+func parseClientHello(r io.Reader) (*bytes.Buffer, *bytes.Buffer, error) {
+	r = &fragmentedHandshakeReader{r: r}
+	header := [1 + 3]byte{}
+
+	if _, err := io.ReadFull(r, header[:]); err != nil {
+		return nil, nil, fmt.Errorf("cannot read handshake header: %w", err)
+	}
+
+	if header[0] != TypeHandshakeClient {
+		return nil, nil, fmt.Errorf("incorrect handshake type: %#x", header[0])
+	}
+
+	// unfortunately there is not uint24 in golang, so we just reuse header
+	header[0] = 0
+	length := int64(binary.BigEndian.Uint32(header[:]))
+
+	clientHelloCopy := &bytes.Buffer{}
+	clientHelloCopy.Write([]byte{tls.TypeHandshake, 3, 1})
+	binary.Write( //nolint: errcheck
+		clientHelloCopy,
+		binary.BigEndian,
+		// 1 for handshake type
+		// 3 for handshake length
+		uint16(1+3+length),
+	)
+	clientHelloCopy.WriteByte(TypeHandshakeClient)
+	clientHelloCopy.Write(header[1:])
+
+	handshakeCopy := &bytes.Buffer{}
+	writer := io.MultiWriter(clientHelloCopy, handshakeCopy)
+
+	_, err := io.CopyN(writer, r, length)
+
+	return clientHelloCopy, handshakeCopy, err
+}