code: def from_string(key_pem, is_x509_cert):
          
          if is_x509_cert:
              key_pem = _helpers._to_bytes(key_pem)
              pemLines = key_pem.replace(b' ', b'').split()
              certDer = _helpers._urlsafe_b64decode(b''.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)
docs: 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.