Expire subset of keys in multiparty encryption scheme
cryptography
Solution
That is a problem studied by a specific branch of cryptography called Broadcast Encryption. In particular, it is also assumed that:
- the overhead for sending a message must be minimized
- the traffic for re-keying purposes is limited or even none
- a broadcast/multicast channel exists
- the storage each user has got is limited and sometimes cannot even be changed
- the protocol is safe even if two or more "expired" users collude together (e.g. by putting together their keys and see if together they can still access the data)
- the users have limited computational power
Several protocols exist, and each achieves a different trade-off for the conditions above. However, I am not aware of any standards around any of them.
Two very nice and seminal documents describing the problem and some solutions are:
- Broadcast Encryption by Fiat and Naor
- Key management for Multicast by Wallner and Harder
On the internet you can find much more material, but in most cases they trace back to such two articles.
Problem
Assume a multi-party encryption scheme, similar to the answer to: Encryption with multiple different keys?. That is. A set of keys `K` can be used to decipher a ciphertext. Is there a way to expire: ``` K'⊆ K ``` such that that `K \ K'` can still decipher the ciphertext, but no longer `K'`? I'm of course assuming that the cipher text has been updated as a consequence of the expiration of `K'`. I'm not really sure how to attack this, or if it's even possible?