'Construct a Verified instance from a string.
Args:
key_pem: string, public key in PEM format.
is_x509_cert: bool, True if key_pem is an X509 cert, otherwise it
is expected to be an RSA key in PEM format.
Returns:
Verifier instance.
Raises:
OpenSSL.crypto.Error: if the key_pem can\'t be parsed.'

@staticmethod
def from_string(key_pem, is_x509_cert):

    key_pem = _helpers._to_bytes(key_pem)
    if is_x509_cert:
        pubkey = crypto.load_certificate(crypto.FILETYPE_PEM, key_pem)
    else:
        pubkey = crypto.load_privatekey(crypto.FILETYPE_PEM, key_pem)
    return OpenSSLVerifier(pubkey)
