I want to provide a service that encrypts and decrypts documents and I want to provide the users with proof that I haven't stolen their secret key and read their documents.

I know I can prove when a document was last modified with cryptographic timestamping.  Is it possible to prove a document was or was not encrypted/decrypted in a given time period?

Off the top of my head, I'm thinking maybe there's some way to arrange that some kind of proof-of-work would be needed to encrypt/decrypt a document, but I'm not comfortable with inventing my own crypto.


Question: check if a string has four consecutive letters in ascending or descending order
Answer: There is no way to solve this using regexes apart from the "overkill" solution of listing each of the possible sequences you want to match.  Regexes are not expressive enough to offer a better solution.