init

internal/mtproxy/header.go

@@ -0,0 +1,103 @@
+package mtproxy
+
+import (
+	"bytes"
+	"crypto/aes"
+	"crypto/cipher"
+	crand "crypto/rand"
+	"crypto/sha256"
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+// ObfuscateTagRandomizedIntermediate matches Telethon RandomizedIntermediatePacketCodec.
+var ObfuscateTagRandomizedIntermediate = []byte{0xdd, 0xdd, 0xdd, 0xdd}
+
+// randSource is injectable for tests.
+var randSource io.Reader = crand.Reader
+
+// InitHeader returns the 64-byte first payload (MTProxyIO.init_header) and CTR streams.
+func InitHeader(secret16 []byte, dcID int16) (header64 []byte, enc cipher.Stream, dec cipher.Stream, err error) {
+	if len(secret16) != 16 {
+		return nil, nil, nil, fmt.Errorf("mtproxy secret must be 16 bytes")
+	}
+	sec := secret16
+
+	keywords := [][]byte{
+		[]byte("PVrG"),
+		[]byte("GET "),
+		[]byte("POST"),
+		{0xee, 0xee, 0xee, 0xee},
+	}
+	var rnd [64]byte
+	for {
+		if _, err := io.ReadFull(randSource, rnd[:]); err != nil {
+			return nil, nil, nil, err
+		}
+		if rnd[0] == 0xef {
+			continue
+		}
+		bad := false
+		for _, k := range keywords {
+			if len(k) <= 4 && bytes.Equal(rnd[:len(k)], k) {
+				bad = true
+				break
+			}
+		}
+		if bad {
+			continue
+		}
+		if rnd[4] == 0 && rnd[5] == 0 && rnd[6] == 0 && rnd[7] == 0 {
+			continue
+		}
+		break
+	}
+
+	random := rnd[:]
+	rev := make([]byte, 48)
+	for i := 0; i < 48; i++ {
+		rev[i] = random[55-i]
+	}
+
+	encKey := sha256Sum(append(append([]byte{}, random[8:40]...), sec...))
+	encIV := random[40:56]
+	decKey := sha256Sum(append(append([]byte{}, rev[:32]...), sec...))
+	decIV := rev[32:48]
+
+	enc, err = newCTR(encKey, encIV)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+	dec, err = newCTR(decKey, decIV)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	buf := make([]byte, 64)
+	copy(buf, random)
+	copy(buf[56:60], ObfuscateTagRandomizedIntermediate)
+	dcBytes := make([]byte, 2)
+	binary.LittleEndian.PutUint16(dcBytes, uint16(dcID))
+	copy(buf[60:62], dcBytes)
+
+	encChunk := make([]byte, 8)
+	copy(encChunk, buf[56:64])
+	enc.XORKeyStream(encChunk, encChunk)
+	copy(buf[56:64], encChunk)
+
+	return buf, enc, dec, nil
+}
+
+func sha256Sum(b []byte) []byte {
+	h := sha256.Sum256(b)
+	return h[:]
+}
+
+func newCTR(key, iv []byte) (cipher.Stream, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	return cipher.NewCTR(block, iv), nil
+}

internal/mtproxy/header_test.go

@@ -0,0 +1,21 @@
+package mtproxy
+
+import (
+	"testing"
+)
+
+func TestInitHeader_Length(t *testing.T) {
+	key := make([]byte, 16)
+	for i := range key {
+		key[i] = byte(i + 1)
+	}
+	hdr, enc, dec, err := InitHeader(key, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(hdr) != 64 {
+		t.Fatalf("header len %d", len(hdr))
+	}
+	_ = enc
+	_ = dec
+}