'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.'

@staticmethod
def from_string(key_pem, is_x509_cert):

    if is_x509_cert:
        key_pem = _helpers._to_bytes(key_pem)
        pemLines = key_pem.replace('    ', '').split()
        certDer = _helpers._urlsafe_b64decode(''.join(pemLines[1:(-1)]))
        certSeq = DerSequence()
        certSeq.decode(certDer)
        tbsSeq = DerSequence()
        tbsSeq.decode(certSeq[0])
        pubkey = RSA.importKey(tbsSeq[6])
    else:
        pubkey = RSA.importKey(key_pem)
    return PyCryptoVerifier(pubkey)
