1. A method of passing validated information along a series of entities, the series of entities including a source entity, a series of at least one intermediate entity, and a target entity, wherein each of the entities shares a validation parameter with its immediately neighbouring entity or entities in the series, the method comprising the steps, commencing in the source entity, of: (a) in the current entity, generating a validation code for the information, the validation code being based on the validation parameter shared between the current entity and the next entity in the series; (b) outputting the validation code; (c) receiving the validation code in the next entity in the series and making that entity the current entity; (d) verifying the information via the validation code in the current entity using the validation parameter required to verify it; (e) repeating steps (a) to (d) until the last intermediate entity in the series has output the validation code it generated; (f) receiving the validation code in the target entity and verifying the information via the validation code and the validation parameter required to verify it.