Method and system using one-time pad data to evidence the possession of a particular attribute

A method and system are provided for evidencing to a first entity, such as a visa-checking authority, that a second entity, such as a traveller, possesses a particular attribute, such as right of entry to a specific country. The method involves a preliminary phase of verifying that the second entity has the attribute of interest; associating that attribute with a first one-time pad held by the first entity; and arranging for the second entity to possess a second one-time pad that is at least a subset of the first one-time pad. Thereafter, when the second entity wishes to prove to the first entity that it has the attribute of interest, it passes evidence data derived from the second one-time pad to the first entity. The first entity can then check for the presence, in the first one-time pad, of data matching, or usable to produce, the evidence data.