Verifying Windows 8 inapp-purchase receipt signature in Python

Question 1

The problem seems to be that using mngr.certLoad on a certificate file returns 0, ie. success, when in fact the key manager does not hold any valid keys (bug?). I got the idea from these two lines, which imply that there is no key:

func=xmlSecKeysMngrGetKey:file=keys.c:line=1370:obj=unknown:subj=xmlSecKeysMngrFindKey:error=1:xmlsec library function failed: 
func=xmlSecDSigCtxProcessKeyInfoNode:file=xmldsig.c:line=871:obj=unknown:subj=unknown:error=45:key is not found:

The solution is to convert the certificate file into a public key file using openssl:

openssl x509 -inform pem -in cert -pubkey -noout > pubkey

Then, you can use this key to verify the signature with the python library:

>>> key = xmlsec.cryptoAppKeyLoad('pubkey', xmlsec.KeyDataFormatPem, None, None, None)
>>> dsig_ctx = xmlsec.DSigCtx()
>>> dsig_ctx.signKey = key
>>> dsig_ctx.verify(node)
0
>>> dsig_ctx.status == xmlsec.DSigStatusSucceeded
True

Or, same thing with a key manager:

>>> key = xmlsec.cryptoAppKeyLoad('pubkey', xmlsec.KeyDataFormatPem, None, None, None)
>>> mngr = xmlsec.KeysMngr(); xmlsec.cryptoAppDefaultKeysMngrInit(mngr)
0
>>> xmlsec.cryptoAppDefaultKeysMngrAdoptKey(mngr, key)
0
>>> dsig_ctx = xmlsec.DSigCtx(mngr)
>>> dsig_ctx.verify(node)
0
>>> dsig_ctx.status == xmlsec.DSigStatusSucceeded
True

I stumbled upon an old email thread in which a user of the xmlsec c library describes his trouble with using the certificate file directly, and gives the aforementioned command to convert it to a public key.

He is also able to get the library to perform the conversion a few emails later by making a call to xmlSecOpenSSLAppKeyFromCertLoadBIO. Presumably the xmlsec command line utility does this when given the --pubkey-cert-pem flag. However, I was not able to find a corresponding method in the python library after greping around for a bit. So it looks like at the moment it's not possible.

Edit

It is possible to convert the certificate to a public key using M2Crypto (documentation), a Python wrapper for OpenSSL. I adapted answers from Extracting public key from certificate... and a blog post from Sheogora to work with this particular case.

>>> from M2Crypto import X509
>>> cert = X509.load_cert('cert', X509.FORMAT_PEM)
>>> pubkey = cert.get_pubkey().get_rsa()
>>> pubkey.save_key('pubkey', cipher=None)
1 # Success

The public key will be saved in PEM format with no encryption to file pubkey and can now be loaded using xmlsec.cryptoAppKeyLoad('pubkey', xmlsec.KeyDataFormatPem, None, None, None).

Question 2

This is how the receipt can be validated without external files:


import xmlsec
from lxml import etree
from M2Crypto import X509
import StringIO

def validate_win_signature(receipt, cert):
    xml = etree.fromstring(receipt)
    xmlsec.tree.add_ids(xml, ["ID"])
    signature_node = xmlsec.tree.find_node(xml, xmlsec.Node.SIGNATURE)
    assert signature_node is not None
    assert signature_node.tag.endswith(xmlsec.Node.SIGNATURE)
    ctx = xmlsec.SignatureContext()
    certx509 = X509.load_cert_string(cert)
    pubkey = certx509.get_pubkey().get_rsa().as_pem(cipher=None)
    keystream = StringIO.StringIO(pubkey)
    key = xmlsec.Key.from_memory(keystream, xmlsec.KeyFormat.PEM)
    ctx.key = key
    ctx.verify(signature_node)