What do I need to distribute (keys, certs) for Python w/ SSL-socket connection?

Question

I'm trying to write a generic server-client application that will be able to exchange data amongst servers. I've read over quite a few OpenSSL documents, and I have successfully setup my own CA and created a cert (and private key) for testing purposes.

I'm stuck with Python 2.3, so I can't use the standard "ssl" library. Instead, I'm stuck with PyOpenSSL, which doesn't seem bad, but there aren't many documents out there about it.

My question isn't really about getting it working. I'm more confused about the certificates and where they need to go.

Here are my two programs that do work:

Server:

#!/bin/env python

from OpenSSL import SSL
import socket
import pickle

def verify_cb(conn, cert, errnum, depth, ok):
    print('Got cert: %s' % cert.get_subject())
    return ok


ctx = SSL.Context(SSL.TLSv1_METHOD)
ctx.set_verify(SSL.VERIFY_PEER|SSL.VERIFY_FAIL_IF_NO_PEER_CERT, verify_cb)

# ??????
ctx.use_privatekey_file('./Dmgr-key.pem')
ctx.use_certificate_file('Dmgr-cert.pem')
# ??????
ctx.load_verify_locations('./CAcert.pem')

server = SSL.Connection(ctx, socket.socket(socket.AF_INET, socket.SOCK_STREAM))

server.bind(('', 50000))
server.listen(3)

a, b = server.accept()

c = a.recv(1024)
print(c)

Client:

from OpenSSL import SSL
import socket
import pickle


def verify_cb(conn, cert, errnum, depth, ok):
    print('Got cert: %s' % cert.get_subject())
    return ok

ctx = SSL.Context(SSL.TLSv1_METHOD)
ctx.set_verify(SSL.VERIFY_PEER, verify_cb)

# ??????????
ctx.use_privatekey_file('/home/justin/code/work/CA/private/Dmgr-key.pem')
ctx.use_certificate_file('/home/justin/code/work/CA/Dmgr-cert.pem')
# ?????????
ctx.load_verify_locations('/home/justin/code/work/CA/CAcert.pem')

sock = SSL.Connection(ctx, socket.socket(socket.AF_INET, socket.SOCK_STREAM))
sock.connect(('10.0.0.3', 50000))

a = Tester(2, 2)
b = pickle.dumps(a)
sock.send("Hello, world")

sock.flush()
sock.send(b)
sock.shutdown()
sock.close()

I found this information from ftp://ftp.pbone.net/mirror/ftp.pld-linux.org/dists/2.0/PLD/i586/PLD/RPMS/python-pyOpenSSL-examples-0.6-2.i586.rpm which contains some example scripts.

As you might gather, I don't fully understand the sections between the " # ????????." I don't get why the certificate and private key are needed on both the client and server. I'm not sure where each should go, but shouldn't I only need to distribute one part of the key (probably the public part)? It undermines the purpose of having asymmetric keys if you still need both on each server, right?

I tried alternating removing either the pkey or cert on either box, and I get the following error no matter which I remove:

OpenSSL.SSL.Error: [('SSL routines', 'SSL3_READ_BYTES', 'sslv3 alert handshake failure'), ('SSL routines', 'SSL3_WRITE_BYTES', 'ssl handshake failure')]

Could someone explain if this is the expected behavior for SSL. Do I really need to distribute the private key and public cert to all my clients? I'm trying to avoid any huge security problems, and leaking private keys would tend to be a big one...

Thanks for the help!

==================================================================

Thanks to caf for helping me figure out the problem. Based on his recommendation, I created two new certificate pairs: spaceman and dmgr. I then put both of the "spaceman" parts (key, cert) in the client program, and the same for the "dmgr" keys.

Basically, only the following two lines in Client changed, although there was plently of work with openssl on the side.

ctx.use_privatekey_file('/home/justin/code/work/CA/private/Dmgr-key.pem')
ctx.use_certificate_file('/home/justin/code/work/CA/Dmgr-cert.pem')

Corrected version:

ctx.use_privatekey_file('/home/justin/code/work/CA/private/spaceman-key.pem')
ctx.use_certificate_file('/home/justin/code/work/CA/spacemancert.pem')

Answer 1

In an SSL transaction, each side can present a certificate to verify its identity to the other side. To do this, it needs to have the private key corresponding to that certificate. These are intended to be two different certificates, so each side will have two different private keys.

This certificate/private key pair is the one that you are setting using use_privatekey_file() and use_certificate_file(). This should be a different certificate/key pair on the server and the client.

When verifying the peers certificate, you then need to check:

• That the certificate is valid (that is, signed by a CA that you trust for this application, not expired, not revoked); and
• Corresponds to the peer that you think you're connected to (that is, the certificate matches the identity that the peer claims). This identity is stored within the SubjectName field of the certificate, and it is application-specific how you map this to a peer identity (it might be a user login name, a DNS name, or something else).