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.

Alexa request validation in python

2 Answers