secp521r1is 521-bit ECC curve, suitable for ECDSA digital signatures and ECDH key agreement. It uses 521-bit private keys (encoded as 65-66 bytes, 130-132 hex digits) and 1042-bit public keys (uncompressed, encoded as 130-131 bytes, 260-261 hex digits). The produced signature is 132 bytes (264 hex digits).
secp521r1). Print as output a single word: "valid' or "invalid".