Values exclusions

A computing device 100 comprises a processor which generates a key 104 (for example a one-time signature OTS under a stateful signature scheme) using a value (e.g. a state value determined from data stored in the computing device) as an input to generate the key where a seed may be a further input for the key generation, excludes the value from future use as the input in response to generating the key, and stores an indication 106 of a subsequent value to use as the input to generate a subsequent key. The indication is cryptographically associated with an entity 108 (e.g. an enforcer) to control third-party access to the indication, for example it may be encrypted to the entity. The entity verifies data stored in a first signer based on a cryptographic association between the first signer and the computing device. The data are indicative of a value to use as an input to generate a one-time use key that is based on the value, which has not been excluded from future use as the input by the first signer. The entity authorises a second signer to store the value in response to verifying the data.