There is disclosed a protection system for intelligent cards. Each card has stored in it a code which is the encryption of a concatenation of a user secret password and a common reference text. The encryption is derived by an initialization terminal which uses the private key associated with the public key of a public-key cryptosystem key pair. Each transaction terminal with which a card is used decrypts the stored code in accordance with the public key. A transaction is effected only if the stored code decrypts into the user password which is inputted on a keyboard and the common reference text.