Move clienhello to faketls

mtglib/internal/faketls/clienthello.go

@@ -0,0 +1,67 @@
+package faketls
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/binary"
+	"fmt"
+	"time"
+
+	"github.com/9seconds/mtg/v2/mtglib/internal/faketls/record"
+)
+
+type ClientHello struct {
+	Time      time.Time
+	Digest    [RandomLen]byte
+	SessionID []byte
+}
+
+func ParseClientHello(secret, handshake []byte) (ClientHello, error) {
+	hello := ClientHello{}
+
+	if len(handshake) < ClientHelloMinLen {
+		return hello, fmt.Errorf("lengh of handshake is too small: %d", len(handshake))
+	}
+
+	if handshake[0] != HandshakeTypeClient {
+		return hello, fmt.Errorf("unknown handshake type %#x", handshake[0])
+	}
+
+	copy(hello.Digest[:], handshake[ClientHelloRandomOffset:])
+
+	for i := ClientHelloRandomOffset; i < ClientHelloRandomOffset+RandomLen; i++ {
+		handshake[i] = 0
+	}
+
+	rec := record.AcquireRecord()
+	defer record.ReleaseRecord(rec)
+
+	rec.Type = record.TypeHandshake
+	rec.Version = record.Version10
+	rec.Payload.Write(handshake)
+
+	// mac is calculated for the whole record, not only
+	// for the payload part
+	mac := hmac.New(sha256.New, secret)
+	rec.Dump(mac)
+
+	computedDigest := mac.Sum(nil)
+
+	for i := 0; i < RandomLen; i++ {
+		computedDigest[i] ^= hello.Digest[i]
+	}
+
+	for i := 0; i < RandomLen-4; i++ {
+		if computedDigest[i] != 0 {
+			return hello, ErrBadDigest
+		}
+	}
+
+	timestamp := int64(binary.LittleEndian.Uint32(computedDigest[RandomLen-4:]))
+	hello.Time = time.Unix(timestamp, 0)
+
+	hello.SessionID = make([]byte, handshake[ClientHelloSessionIDOffset])
+	copy(hello.SessionID, handshake[ClientHelloSessionIDOffset+1:])
+
+	return hello, nil
+}