[gnutls-help] "Error in the push function" using gnutls >= 3.2.1-w32

[Daniel Kahn Gillmor]

On Thu 2013-10-17 11:30:09 -0400, João Távora <joaotavora at gmail.com> wrote:
> I'm on Microsoft Windows XP. Using gnutls 3.1.8-w32, I get in some
> servers, but not others:
>
>    $ ./gnutls-cli -p 443 siscog.campfirenow.com


trying with this server using gnutls 3.2.4 on debian:

0 dkg at alice:~$ gnutls-cli --debug 9999 siscog.campfirenow.com
[...]
|<3>| HSK[0xf5fec0]: SERVER HELLO (2) was received. Length 95[95], frag offset 0, frag length: 95, sequence: 0
|<3>| HSK[0xf5fec0]: Server's version: 3.3
|<3>| HSK[0xf5fec0]: SessionID length: 32
|<3>| HSK[0xf5fec0]: SessionID: 8380f0fd7f664bef694585d3a5ecab256a5b10adf2fa3206c684832c88d20371
|<3>| HSK[0xf5fec0]: Selected cipher suite: ECDHE_RSA_AES_128_CBC_SHA1
|<3>| HSK[0xf5fec0]: Selected compression method: NULL (0)
|<3>| EXT[0xf5fec0]: Parsing extension 'SAFE RENEGOTIATION/65281' (1 bytes)
|<3>| EXT[0xf5fec0]: Parsing extension 'SERVER NAME/0' (0 bytes)
|<3>| EXT[0xf5fec0]: Parsing extension 'SUPPORTED ECC/10' (4 bytes)
|<2>| ASSERT: ecc.c:95
|<2>| ASSERT: gnutls_extensions.c:207
|<2>| ASSERT: gnutls_handshake.c:1801
|<2>| ASSERT: gnutls_handshake.c:2225
|<2>| ASSERT: gnutls_handshake.c:1442
|<2>| ASSERT: gnutls_handshake.c:2701
*** Fatal error: An illegal TLS extension was received.
|<4>| REC: Sending Alert[2|110] - An unsupported extension was sent
|<4>| REC[0xf5fec0]: Preparing Packet Alert(21) with length: 2 and target length: 2
|<9>| ENC[0xf5fec0]: cipher: NULL, MAC: MAC-NULL, Epoch: 0
|<7>| WRITE: enqueued 7 bytes for 0x4. Total 7 bytes.
|<7>| WRITE FLUSH: 7 bytes in buffer.
|<7>| WRITE: wrote 7 bytes, 0 bytes left.
|<4>| REC[0xf5fec0]: Sent Packet[2] Alert(21) in epoch 0 and length: 7
*** Handshake has failed
GnuTLS error: An illegal TLS extension was received.
|<4>| REC[0xf5fec0]: Start of epoch cleanup
|<4>| REC[0xf5fec0]: End of epoch cleanup
|<4>| REC[0xf5fec0]: Epoch #0 freed
|<4>| REC[0xf5fec0]: Epoch #1 freed
1 dkg at alice:~$

So the server is sending the "Supported Elliptic Curves" extension,
which should only be sent by the client:

  https://tools.ietf.org/html/rfc4492#section-5.1

lib/ext/ecc.c:95  shows the assertion:

0 dkg at alice:~/src/gnutls/gnutls$ cat -n lib/ext/ecc.c | head -n 100 | tail -n 26
    75	/* 
    76	 * In case of a server: if a SUPPORTED_ECC extension type is received then it stores
    77	 * into the session security parameters the new value. The server may use gnutls_session_certificate_type_get(),
    78	 * to access it.
    79	 *
    80	 * In case of a client: If a supported_eccs have been specified then we send the extension.
    81	 *
    82	 */
    83	static int
    84	_gnutls_supported_ecc_recv_params (gnutls_session_t session,
    85	                               const uint8_t * data, size_t _data_size)
    86	{
    87	  int new_type = -1, ret, i;
    88	  ssize_t data_size = _data_size;
    89	  uint16_t len;
    90	  const uint8_t* p = data;
    91	
    92	  if (session->security_parameters.entity == GNUTLS_CLIENT)
    93	    {
    94	      /* A client shouldn't receive this extension */
    95	      return gnutls_assert_val(GNUTLS_E_RECEIVED_ILLEGAL_EXTENSION);
    96	    }
    97	  else
    98	    { /* SERVER SIDE - we must check if the sent supported ecc type is the right one 
    99	       */
   100	      if (data_size < 2)
0 dkg at alice:~/src/gnutls/gnutls$ 

Maybe GnuTLS should be more permissive here, and just ignore this
extension (maybe emitting a warning) if it comes from the server?


> I had to switch to gnutls >= 3.2.1 since it is the first one supporting
> the ECC cypher, which appears to be TLS extension required by this
> server (but not by github.com, apparently). However I get an even
> weirder error:

This shouldn't be necessary; testing from another machine, i was able to
connect with gnutls-cli from version 2.12.20, which doesn't support ECC
at all, and i was also able to connect with a priority string that
disables all ECC, which apparently means the buggy server won't try to
send the extension it wasn't supposed to send in the first place:

 gnutls --priority NORMAL:-CURVE-ALL siscog.campfirenow.com



hth,

        --dkg
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