Wireshark · Wireshark-users: Re: [Wireshark-users] SSL decrypter pre master secret handling

[Sake Blok <sake@xxxxxxxxxx>]

On 16 aug 2012, at 20:20, Wayne Blacklock wrote:

When everything isn't ok, the SSL decryption fails:

ssl_decrypt_pre_master_secret:RSA_private_decrypt
pcry_private_decrypt: stripping 146 bytes, decr_len 255
decrypted_unstrip_pre_master[255]:
<snip>
ssl_decrypt_pre_master_secret wrong pre_master_secret length (109, expected 48)
dissect_ssl3_handshake can't decrypt pre master secret

Now I am no SSL expert, but I don't think this is right. What I hope to understand and the reason for this post, is how exactly the byte stripping works... where does the SSL decrypter  get 146 from and could this explain our SSL issues? If so, what exactly is going wrong? I've had a bit of a look through the TLS spec but it isn't clear to me and so I would appreciate any and all advice.

The only situations where I have seen the message "wrong pre_master_secret length" were when the public key in the server certificate did not match the provided private key. Can you verify that the *exact* same certificate is used in both sessions?

As for the amount of stripped data, it is calculated as follows:

    /* strip the padding*/

    rc = 0;

    for (i = 1; i < decr_len; i++) {

        if (decr_data_ptr[i] == 0) {

            rc = i+1;

            break;

        }

    }

    ssl_debug_printf("pcry_private_decrypt: stripping %d bytes, decr_len %d\n",

        rc, decr_len);

    ssl_print_data("decrypted_unstrip_pre_master", decr_data_ptr, decr_len);

    g_memmove(decr_data_ptr, &decr_data_ptr[rc], decr_len - rc);

In other words, the decrypted (unstripped) PreMasterSecret is searched for the first occurrence of the value 0. Everything after the first 0 is then used as the PreMasterSecret and it should be exactly 48 bytes long.

Here is an example:

pre master encrypted[128]:

76 1b 1b ea c3 5e 59 de 9a 3b b9 f7 4e bf 91 09 

b7 38 e8 ad 34 6c 3c e8 26 f8 e9 f6 5d 82 a9 a5 

44 c6 0a fe ae 85 6d 01 1f 5d 11 4d 1f 16 d2 4a 

64 13 ad 32 c2 92 1d 54 b8 f2 6b d2 bb a5 7b 55 

73 87 69 ff 33 bb 9f aa 57 27 3a 58 43 61 29 46 

c2 12 6d f3 6d c8 98 4b df 72 7a 7c 67 72 89 5d 

c5 3d 5d 8f c5 d8 43 68 86 f6 70 f0 d9 b7 91 36 

1c 8a a4 bd 64 b0 27 d4 c0 2d 03 dd 92 f7 df 36 

ssl_decrypt_pre_master_secret:RSA_private_decrypt

pcry_private_decrypt: stripping 79 bytes, decr_len 127

decrypted_unstrip_pre_master[127]:

02 54 f5 61 13 68 03 30 97 14 1f e9 cc 7c 2c 3b 

d2 b0 f6 db 18 f7 2b 81 e0 46 ad 51 dd 26 b3 fe 

94 48 ab 8a 52 db 97 e3 df 9e e4 30 64 96 3e d1 

b4 9a 13 8f e4 ad c0 1d 98 10 e8 dc 0d 89 02 85 

5c 03 3b 5b 9e 71 51 3a df 30 20 40 be 9e 00 03 

01 90 36 b9 c6 b5 f1 2d 7d 3b 6b 3c bb 6a 7c ed 

9a 4c 38 1e 07 67 c6 07 fb 51 5a 0c 2e 2b 3b 41 

96 5a 6f fc d2 88 86 59 ff aa 82 7f e5 bb 42 

pre master secret[48]:

03 01 90 36 b9 c6 b5 f1 2d 7d 3b 6b 3c bb 6a 7c 

ed 9a 4c 38 1e 07 67 c6 07 fb 51 5a 0c 2e 2b 3b 

41 96 5a 6f fc d2 88 86 59 ff aa 82 7f e5 bb 42 

Hope this helps!

Cheers,

Sake