1package mox

2

3import (

4 "bytes"

5 "crypto/aes"

6 "crypto/cipher"

7 "encoding/base64"

8 "encoding/binary"

9 "fmt"

10)

11

12var idCipher cipher.Block

13var idRand []byte

14

15func init() {

16 // Init for tests. Overwritten in ../serve.go.

17 err := ReceivedIDInit([]byte("0123456701234567"), []byte("01234567"))

18 if err != nil {

19 panic(err)

20 }

21}

22

23// ReceivedIDInit sets an AES key (must be 16 bytes) and random buffer (must be

24// 8 bytes) for use by ReceivedID.

25func ReceivedIDInit(key, rand []byte) error {

26 var err error

27 idCipher, err = aes.NewCipher(key)

28 idRand = rand

29 return err

30}

31

32// ReceivedID returns an ID for use in a message Received header.

33//

34// The ID is based on the cid. The cid itself is a counter and would leak the

35// number of connections in received headers. Instead they are obfuscated by

36// encrypting them with AES with a per-install key and random buffer. This allows

37// recovery of the cid based on the id. See subcommand cid.

38func ReceivedID(cid int64) string {

39 buf := make([]byte, 16)

40 copy(buf, idRand)

41 binary.BigEndian.PutUint64(buf[8:], uint64(cid))

42 idCipher.Encrypt(buf, buf)

43 return base64.RawURLEncoding.EncodeToString(buf)

44}

45

46// ReceivedToCid returns the cid given a ReceivedID.

47func ReceivedToCid(s string) (cid int64, err error) {

48 buf, err := base64.RawURLEncoding.DecodeString(s)

49 if err != nil {

50 return 0, fmt.Errorf("decode base64: %v", err)

51 }

52 if len(buf) != 16 {

53 return 0, fmt.Errorf("bad length, got %d, expect 16", len(buf))

54 }

55 idCipher.Decrypt(buf, buf)

56 if !bytes.Equal(buf[:8], idRand) {

57 return 0, fmt.Errorf("rand mismatch")

58 }

59 cid = int64(binary.BigEndian.Uint64(buf[8:]))

60 return cid, nil

61}

62