Alexa request validation in python

Question

I work on a service that will handle Alexa voice intents. I need to verify the signature of each request and I almost succeed. The only part that is not working is the validation of certificates chain.

From the documentation I know that:

This certificate chain is composed of, in order, (1) the Amazon signing certificate and (2) one or more additional certificates that create a chain of trust to a root certificate authority (CA) certificate.

My code looks like this:

certificates = pem.parse_file("chain.pem")
store = crypto.X509Store()
for cert in certificates[:-1]:
    loaded_cert = crypto.load_certificate(crypto.FILETYPE_PEM,
                                          cert.as_bytes())
    store.add_cert(loaded_cert)

intermediate_cert = crypto.load_certificate(
    crypto.FILETYPE_PEM,
    certificates[-1].as_bytes()
)
# Create a certificate context
store_ctx = crypto.X509StoreContext(store, intermediate_cert)

# Verify the certificate
store_ctx.verify_certificate()

I receive the following error:

OpenSSL.crypto.X509StoreContextError: [20, 0, 'unable to get local issuer certificate']

I don't know what I did wrong, maybe there is someone who already implemented this and can drop a hint.

Answer 1

First and once get the CA Issuer for all Certificate in 'chain.pem':

for cert in pem.parse_file("chain.pem"):
    CA_cert = crypto.load_certificate(crypto.FILETYPE_PEM, cert.as_bytes())
    print('CA_cert:\nissuer :{}\nsubject:{}'.
        format(CA_cert.get_subject(), CA_cert.get_issuer()))

Output, for example:

CA_cert:
issuer :<X509Name object '/C=US/O=thawte, Inc./OU=Certification Services Division/OU=(c) 2006 thawte, Inc. - For authorized use only/CN=thawte Primary Root CA'>
subject:<X509Name object '/C=US/O=thawte, Inc./OU=Certification Services Division/OU=(c) 2006 thawte, Inc. - For authorized use only/CN=thawte Primary Root CA'>

This shown example Certificate is a self signed certificate.

---

Add all shown issuer to CA_store, then do .verify_certificate for all Certificate in 'chain.pem'.

CA_store = crypto.X509Store()
for _pem in ['issuer_1.pem', 'issuer_2.pem']:
    for cert in pem.parse_file(_pem):
        CA_store.add_cert(
            crypto.load_certificate(crypto.FILETYPE_PEM, cert.as_bytes())
        )

for cert in pem.parse_file("chain.pem"):
    try:
        crypto.X509StoreContext(CA_store,
                                crypto.load_certificate(crypto.FILETYPE_PEM, cert.as_bytes())
                                ).verify_certificate()
    except X509StoreContextError as exp:
        cert = exp.certificate
        print('X509StoreContextError:{}\ncertificate\n\tissuer :{}\n\tsubject:{}'.
            format(exp.args, cert.get_issuer(), cert.get_subject()))

Tested with Python:3.4.2 - OpenSSL:17.0.0 - cryptography:1.8.2 - cffi:1.10.0