'Loads a key in PKCS#1 DER format.
:param keyfile: contents of a DER-encoded file that contains the private
key.
:return: a PrivateKey object
First let\'s construct a DER encoded key:
>>> import base64
>>> b64der = \'MC4CAQACBQDeKYlRAgMBAAECBQDHn4npAgMA/icCAwDfxwIDANcXAgInbwIDAMZt\'
>>> der = base64.standard_b64decode(b64der)
This loads the file:
>>> PrivateKey._load_pkcs1_der(der)
PrivateKey(3727264081, 65537, 3349121513, 65063, 57287)'

@classmethod
def _load_pkcs1_der(cls, keyfile):

    from pyasn1.codec.der import decoder
    (priv, _) = decoder.decode(keyfile)
    if (priv[0] != 0):
        raise ValueError(('Unable    to    read    this    file,    version    %s    !=    0' % priv[0]))
    as_ints = tuple((int(x) for x in priv[1:9]))
    return cls(*as_ints)
