Method of one time authentication response to a session-specific challenge indicating a random subset of password or PIN character positions

A transaction server, comprising:

• a computer database storing a client identifier for a client and an identification code for the client, the identification code comprising a set of password or PiN characters with each consecutive character position number in the set associated with a particular character;
• a computer system coupled to the computer database, the computer system including
• a module to supply to the client, a prompt for entry of a client identifier;
• a module to accept data indicating the client identifier from the client via a data communication medium;
• a module to verify that the data indicating the client identifier matches the stored client identifier for the client;
• a module to supply to the client a session random subset of character position numbers from said identification code for the client particular to a session, wherein the random subset of character position numbers does not include all of the character position numbers from the identification code;
• a module to accept input data from the client, the input data including a random subset of characters from the identification code for the client, wherein the random subset of characters does not include all of the identification code;
• and
• a module to allow processing of a transaction to continue if the input data matches characters from the stored identification code for the client in the session random subset of character positions, and to interrupt the given session if the input data does not match.