UnsupportedAlgorithm: This backend does not support this key serialization. - Python cryptography load_pem_private_key

Question

I am trying to generate signed urls for AWS Cloudfront based on the example here . On the line

private_key = serialization.load_pem_private_key(
        key_file.read(),
        password=None,
        backend=default_backend()
    )

I get the error UnsupportedAlgorithm: This backend does not support this key serialization. The full trace is as below:

File "command_util.py", line 98, in rsa_signer
    backend=default_backend()  
File "runtime/cryptography/hazmat/primitives/serialization.py", line 20, in load_pem_private_key
    return backend.load_pem_private_key(data, password)
File "runtime/cryptography/hazmat/backends/multibackend.py", line 286, in load_pem_private_key
    _Reasons.UNSUPPORTED_SERIALIZATION
UnsupportedAlgorithm: This backend does not support this key serialization.

On reading the docs it says that the exception occurs because of the following:

cryptography.exceptions.UnsupportedAlgorithm – the serialized key is of a type that is not supported by the backend or if 
the key is encrypted with a symmetric cipher that is not supported by the backend.

The PEM file given starts with -----BEGIN RSA PRIVATE KEY----- and ends with -----END RSA PRIVATE KEY----- .

I am using google appengine sdk while developing this application.

I need help understanding this error message and how to make this work.

Answer 1

Unfortunately the python cryptography library cannot be used with google appengine(GAE) as this library needs to have C extensions and you cannot install C extensions in GAE. You can only use pure python packages.