A portable intelligent shopping device which can be worn as a wristwatch or carried on a belt. It makes paying for purchases more secure. The device stores the wearer's private key and digital certificate. It can also store the public keys of various stores. When the wearer (shopper) buys something from a store, the store can request a payment of the appropriate amount from the wearer's watch. The authorization for payment of the amount specified by the request for payment may be transmitted using IR or RF. The watch can then display the amount and request the shopper to confirm the transaction by entering a PIN number by tapping on the touch screen or using the roller wheel. Once the correct PIN is entered, the watch can send a message to the store authorizing payment of the requested amount. The store can verify the message based on the shopper's encrypted certificate by decrypting it using the shopper's public key. The entire payment authorization signal can be encrypted by the shopper using the store's public key so that only the store can decrypt it with its private key.