[PYTHON-CRYPTO] M2Crypto and CAcert.org certificates

[Jeremy Morel]

Hi all,

I am trying to use M2Crypto to authentify mails signed with a certificate
delivered by CACert.org. I understand they do not use traditional public
key/private key signing, but rather give you a certificate you use to sign
your mails with embed the public key. Toying with M2Crypto, I was able to
extract that public key, but all my attempts at verifying the signature failed.

My code derives from the example available at
http://sandbox.rulemaker.net/ngps/m2/howto.smime.html

Here :
from M2Crypto import SMIME, X509

# Load the data, verify it.
p7, data = SMIME.smime_load_pkcs7('test_sign.eml')

certif = X509.load_cert('root.crt')
stack = X509.X509_Stack()

signers = p7.get0_signers(stack)
for signer in signers:
    pubkey = signer.get_pubkey()
    print signer  

# Instantiate an SMIME object.
s = SMIME.SMIME()

# Load the signer's cert.
x509 = X509.X509()
x509.set_pubkey(pubkey)
sk = X509.X509_Stack()
sk.push(x509)
s.set_x509_stack(sk)

st = X509.X509_Store()
st.load_info('root.crt')
s.set_x509_store(st)

# Load the data, verify it.
v = s.verify(p7)

In this context, I have 2 files :
 * test_sign.eml is a signed email
 * root.crt is the root certificate from CACert.org

data does contain the body of the mail, but it looks like p7 is empty. I get
this error :
<M2Crypto.X509.X509 instance at 0x6c1b98>
Traceback (most recent call last):
  File "sign.py", line 38, in <module>
    v = s.verify(p7)
  File "build/bdist.macosx-10.5-i386/egg/M2Crypto/SMIME.py", line 215, in verify
M2Crypto.SMIME.PKCS7_Error: no content

Does anyone know how to fix this ?

Thanks in advance,

Jérémy