Abstract: A document or message is protected against forgery or repudiation by processing a selected part or parts of the text of the document or message to form a hash, usually of fewer characters than the selected part or parts of the text. The processing comprises retrieving numerical values which define the respective characters of the selected part or parts of the text and making a calculation using the numerical values of the successive characters. Preferably the hash is added to the text.

Abstract: A bank card transaction system comprises a first apparatus (10) for use by a card holder and a second apparatus (18) for use by the card issuer. One of these apparatus (e.g. the card holder's apparatus) is arranged for creating an encrypted number, corresponding to at least part of a card number, and the other apparatus is arranged for decrypting the encrypted number upon receipt thereof. The encrypted number is given to the merchant by the card holder, e.g. over the Internet, and is then passed by the merchant to the card issuer for authorisation in the usual way: identifying information is passed over a separate communication link established direct from the card holder to the issuer, to enable the card issuer to decrypt the encrypted card number upon receipt.

Abstract: A secure network comprises a plurality of terminals connected to a common file server. Each terminal holds in its memory an encrypted unique variable and unique first and second conjugates for each user authorized to use that terminal. The unique variable is generated, at the time of registration of a user, from master keys stored on a separate disc or other memory medium, the identity number of the terminal, and the identity number of the user to be registered. A password is randomly generated and used to encrypt the unique variable. The first conjugate is a randomly generated message, which is encrypted using the password to generate the second conjugate. In order to log on at a given terminal, the user must enter his password, and the terminal then uses this password as the key to encrypt the first conjugate, and compares it with the stored second conjugate. If the first and second conjugate agree, the terminal is enabled for the person to use. A high level of security is therefore attained.

Abstract: An encryption/decryption apparatus enables encrypted communication between two stations each incorporating such an apparatus. The apparatus is arranged to generate a set of look-up tables in accordance with a session key and temporarily store these tables in memory, and to convert each successive element, such as a character, of a message to a code through use of the look-up tables. The session key can be changed as often as desired but the fresh set of look-up are created quickly each time; then the conversion process for each element of the message is carried out quickly yet maintaining a high level of security.

Abstract: An encryption/description apparatus enables encrypted communication between two stations each incorporating such an apparatus. The apparatus is arranged when acting as sender to create (at 71) a mutual primitive from stored items of data, to generate a random session key and encrypt the random session key (at 73) in accordance with the mutual primitive for transmission of the encrypted session key to the recipient station. The sender apparatus further encrypts the main message (at 72) in accordance with the random session key for transmission of the encrypted message to the recipient station. The sender apparatus also stores a registration code and transmits this to the recipient station, where it is decoded (at 74) to recreate the mutual primitive from items of data stored at the recipient station. The recipient apparatus decrypts the encrypted session key (at 75), using the recreated mutual primitive, and then decrypts the main message (at 76) using the recreated random session key.