Abstract: The present invention concerns a method for encrypting files performed by a client computer, the method comprising encrypting at least a part of the content of a file into a change-set data structure using a secret key shared among a plurality of users, and signing the change-set data structure using a private signing key of a user of the client computer. The invention also concerns a method for decrypting files performed by a client computer, the method comprising receiving a change-set data structure, verifying a digital signature of the change-set data structure using a public verification key of a user who created the change-set, and decrypting an encrypted data portion of the change-set data structure to obtain the file using a secret key shared among a plurality of users.