Can PKCS7 be signed in Google App Engine?

Question

I need to create a PKCS7 signature for some data using my Python app running on the Google App Engine (GAE). In particular, I am trying to create the PKCS7 signature of the Apple Passbook pass manifest; Skipping Passbook requires the PKCS7 signature file to be present in order to be a complete and valid pass.

I spent almost a week searching and trying to no avail.

I can successfully create a signature using the openssl command line on my local PC using

openssl smime -binary -sign -certfile WWDR.pem -signer certificate.pem -inkey key.pem -in manifest.json -out signature -outform DER 

I can also successfully create a signature using the M2Crypto library on my local PC using

from M2Crypto import BIO, SMIME, X509

s = SMIME.SMIME()
s.load_key('identity.pem')         # my certificate and private key

x509 = X509.load_cert('WWDR.pem')  # Apple intermediate certificate
sk = X509.X509_Stack()
sk.push(x509)
s.set_x509_stack(sk)

p7 = s.sign(bio_manifest, SMIME.PKCS7_DETACHED | SMIME.PKCS7_BINARY)
pkcs7_buffer = BIO.MemoryBuffer()
p7.write_der(pkcs7_buffer)

f = open('signature', 'w')
f.write(pkcs7_buffer.read())
f.close()

However, M2Crypto is a shell for OpenSSL, which is not supported in GAE.

GAE supports the pycrypto library, but it seems that this library does not support PKCS7 signature.

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