Operator-interactive verification of a card-based transaction involves reading a magnetic stripe (24) on the card (C) to derive test data (T) representing an arithmetical combination of a uniqueness characteristic of the card (derived from its translucency at defined locations D1-D6) and the assigned- users personal-identification data (PIN). The test data (T) is arithmetically-combined with the personal-identification data (PIN) entered by the user, to produce a decoded representation of the uniqueness characteristic (D') which is compared with a fresh sensing (D) of that characteristic as a basis for verification of both the card (C) and the user-entered data (PIN). The user can choose to alter his operative personal-identification data (PIN), and does this, after first achieving verification using the existing data, by entering the new, chosen data for re-calculation with the decoded uniqueness characteristic (D') to produce new test data (T) which is then recorded on the card (C) in place of the old (T).