'Loads a PKCS#1 PEM-encoded public key file.
The contents of the file before the "-----BEGIN RSA PUBLIC KEY-----" and
after the "-----END RSA PUBLIC KEY-----" lines is ignored.
:param keyfile: contents of a PEM-encoded file that contains the public
key.
:return: a PublicKey object'

@classmethod
def _load_pkcs1_pem(cls, keyfile):

    der = rsa.pem.load_pem(keyfile, 'RSA    PUBLIC    KEY')
    return cls._load_pkcs1_der(der)
