Python Paste SSL server with TLSv1.2 and Forward Secrecy

Question

for a Python app on Mac OS X, I need to setup an HTTPS server with TLSv1.2, Forward Secrecy, and without RC4 ciphers. Using Paste and pyOpenSSL I have write the following code:

from paste import httpserver
from OpenSSL import SSL

context = SSL.Context(SSL.SSLv23_METHOD)
context.use_privatekey_file("/Path/to/my/private.key")
context.use_certificate_chain_file("/Path/to/my/chain-cert.pem")
context.set_options(SSL.OP_NO_SSLv2)
context.set_options(SSL.OP_NO_SSLv3)
context.set_options(SSL.OP_SINGLE_DH_USE)
context.set_cipher_list("EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:EDH+aRSA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4")
logger.debug("OPENSSL version: %s" % SSL.SSLeay_version(SSL.SSLEAY_VERSION))

httpserver.serve(app_logged, host=http_host, port=http_port, server_version=server_version, ssl_context=context, use_threadpool=True, threadpool_workers=15, request_queue_size=5)

But Safari, Chrome and OpenSSL client failed to connect to my server with the error "no shared cipher". So, what did I do wrong?

Note: Default OS X versions of Python (2.7.6) and OpenSSL (0.9.8) are not compatible with TLSv1.2, so I had to compile OpenSSL 1.0.2 and Python 2.7.10 from sources.

If I check my OpenSSL ciphers list, it give me this:

$ /usr/local/bin/openssl ciphers -V 'EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:EDH+aRSA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4'
0xC0,0x2C - ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256) Mac=AEAD
0xC0,0x2B - ECDHE-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(128) Mac=AEAD
0xC0,0x30 - ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(256) Mac=AEAD
0xC0,0x2F - ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(128) Mac=AEAD
0xC0,0x24 - ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA384
0xC0,0x23 - ECDHE-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(128)  Mac=SHA256
0xC0,0x28 - ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA384
0xC0,0x27 - ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA256
0xC0,0x14 - ECDHE-RSA-AES256-SHA    SSLv3 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA1
0xC0,0x0A - ECDHE-ECDSA-AES256-SHA  SSLv3 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA1
0xC0,0x13 - ECDHE-RSA-AES128-SHA    SSLv3 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA1
0xC0,0x09 - ECDHE-ECDSA-AES128-SHA  SSLv3 Kx=ECDH     Au=ECDSA Enc=AES(128)  Mac=SHA1
0x00,0x9F - DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(256) Mac=AEAD
0x00,0x6B - DHE-RSA-AES256-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA256
0x00,0x39 - DHE-RSA-AES256-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA1
0x00,0x88 - DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH       Au=RSA  Enc=Camellia(256) Mac=SHA1
0x00,0x9E - DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(128) Mac=AEAD
0x00,0x67 - DHE-RSA-AES128-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA256
0x00,0x33 - DHE-RSA-AES128-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA1
0x00,0x9A - DHE-RSA-SEED-SHA        SSLv3 Kx=DH       Au=RSA  Enc=SEED(128) Mac=SHA1
0x00,0x45 - DHE-RSA-CAMELLIA128-SHA SSLv3 Kx=DH       Au=RSA  Enc=Camellia(128) Mac=SHA1

and I know that Safari 8 is compatible with cipher "ECDHE-RSA-AES128-SHA256" for example, so why I have a 'no shared cipher' error?

$ /usr/local/bin/openssl s_client -connect 192.168.0.17:4443 -tls1_2
CONNECTED(00000003)
140735274361680:error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:s3_pkt.c:1472:SSL alert number 40
140735274361680:error:1409E0E5:SSL routines:ssl3_write_bytes:ssl handshake failure:s3_pkt.c:656:
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 7 bytes and written 0 bytes
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol  : TLSv1.2
Cipher    : 0000
Session-ID:
Session-ID-ctx:
Master-Key:
Key-Arg   : None
PSK identity: None
PSK identity hint: None
SRP username: None
Start Time: 1439975452
Timeout   : 7200 (sec)
Verify return code: 0 (ok)
---

Alternative: If I change my cipher list to

context.set_cipher_list("HIGH:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:@STRENGTH")

It works, browsers and openssl client are able to establish a secured connection with cipher 'AES256-GCM-SHA384' on protocol TLSv1.2, but it is not an ECDHE cipher (therefore no Forward Secrecy).

EDIT correct answer:

For the DH ciphers, you need a DH parameters file in PEM format, you can generate one with the following command:

$ /usr/local/bin/openssl dhparam 2048 -out dhparams.pem

For the ECDHE ciphers, you will need to set an elliptic curve for the SSL context. You can retrieve the available curves on your system with pyOpenSSL:

OpenSSL.crypto.get_elliptic_curves()

Which give us the correct following python code:

from paste import httpserver
from OpenSSL import SSL
from OpenSSL import crypto

context = SSL.Context(SSL.SSLv23_METHOD)
context.use_privatekey_file("/Path/to/my/private.key")
context.use_certificate_chain_file("/Path/to/my/chain-cert.pem")
context.load_tmp_dh("/Path/to/my/dhparams.pem")
context.set_tmp_ecdh(crypto.get_elliptic_curve("prime256v1"))
context.set_options(SSL.OP_NO_SSLv2)
context.set_options(SSL.OP_NO_SSLv3)
context.set_options(SSL.OP_SINGLE_DH_USE)
context.set_cipher_list("EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:EDH+aRSA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4")
logger.debug("OPENSSL version: %s" % SSL.SSLeay_version(SSL.SSLEAY_VERSION))

httpserver.serve(app_logged, host=http_host, port=http_port, server_version=server_version, ssl_context=context, use_threadpool=True, threadpool_workers=15, request_queue_size=5)

Answer 1

I'll leave this here, since this works with built in libraries as well.

import ssl
from socket import *

context = ssl.create_default_context()
context.set_ciphers('ECDHE+AESGCM:!ECDSA')
context.load_dh_params("dhparams.pem")
context.set_ecdh_curve("prime256v1")

sock = socket()
sock.connect(('hostname', 443))
sock = context.wrap_socket(sock, server_side=False, server_hostname='hostname')

Since the above answer and question is regarding pyOpenSSL, I know this might not be 100% in the right ball park.

But after a bit of searching around why I get ssl.SSLError: [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:1045) without any details to as why this is happening, this post saved my day.. But I also figured it was possible to do natively via ssl (which I'm sure, probably uses OpenSSL but still thought this might be useful here).

Answer 2

To have support for DH ciphers (DHE-RSA-...) you need to have a DH params file and specify it:

 context.load_tmp_dh("dhparams.pem")

To create such file you might use OpenSSL

 openssl dhparam -out dhparams.pem 2048

To support ECDH ciphers you need to specify which curve should be used. I don't have a pyOpenSSL version which supports it but according to the documentation it should be

context.set_tmp_ecdh( OpenSSL.crypto.get_elliptic_curve( "prime256v1" ))