Cryptographic service provision

A computing apparatus adapted to act as a computing node providing a secure service, the computing apparatus comprising a processor, a memory, and means for communicating over a network, wherein the memory comprises executable instructions to program the processor to perform:

• receiving a service request from a requesting party, wherein the service request comprises a request to generate a credential;
• generating the credential using a cryptographic key and an instance counter, wherein the cryptographic key is determined from a currently active key list of cryptographic keys for use by the computing node, wherein key lists are rotated to provide a new currently active key list at a predetermined rotation frequency, and wherein the instance counter is determined by a number of instances of performing the service within a predetermined time window;
• wherein the credential is provided such that a key list reference and an instance counter value can be determined from the credential without decryption;
• wherein the credential further comprises a key list enhancement, such that the key list enhancement is used to prevent clash between credentials having a common key list reference and a common instance counter value.

A method of providing a secure service at a computing node for a requesting party external to the computing node, wherein the computing node is one of a plurality of computing nodes adapted to provide the secure service, the method comprising at the computing node:

• receiving a service request from a requesting party, wherein the service request comprises a request to validate a credential, wherein the credential has been generated using a cryptographic key and an instance counter by one of the computing nodes of the plurality of computing nodes, wherein the cryptographic key has been determined from a then currently active key list of cryptographic keys for use by the generating computing node, wherein key lists are rotated to provide a new currently active key list at a predetermined rotation frequency, and wherein the instance counter was determined by a number of instances of performing the credential generating service at the generating computing node within a predetermined time window;
• wherein the credential is provided such that a key list reference and an instance counter value can be determined from the credential without decryption, and where the credential further comprises a key list enhancement to prevent clash between credentials having a common key list reference and a common instance counter value;
• determining from at least the key list reference and the key list enhancement whether validation of the credential is legitimate, and if so, attempting to validate the credential.