code: 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)
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.
  
          Raises:
              OpenSSL.crypto.Error: if the key_pem can't be parsed.