'Loads a key in PKCS#1 DER format.
:param keyfile: contents of a DER-encoded file that contains the public
key.
:return: a PublicKey object
First let\'s construct a DER encoded key:
>>> import base64
>>> b64der = \'MAwCBQCNGmYtAgMBAAE=\'
>>> der = base64.standard_b64decode(b64der)
This loads the file:
>>> PublicKey._load_pkcs1_der(der)
PublicKey(2367317549, 65537)'

@classmethod
def _load_pkcs1_der(cls, keyfile):

    from pyasn1.codec.der import decoder
    from rsa.asn1 import AsnPubKey
    (priv, _) = decoder.decode(keyfile, asn1Spec=AsnPubKey())
    return cls(n=int(priv['modulus']), e=int(priv['publicExponent']))
