Verifiable version control on authenticated and/or encrypted electronic documents

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;
• wherein the change-set data structure comprises a hash taken over a signature digest of a last significant change, wherein a significant change comprises:
• a change-set that adds a new file, a change-set that removes a file, a group change that adds a new member, and/or a group change that removes a member;
• and wherein the method further comprises:
• verifying whether a hash taken over a signature digest of a previous significant change matches said hash in the change-set data structure;
• 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.

A computer program comprising instructions for implementing a method comprising:

• encrypting at least a part of 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 a client computer;
• wherein the change-set data structure comprises a hash taken over a signature digest of a last significant change, wherein a significant change comprises:
• a change-set that adds a new file, a change-set that removes a file, a group change that adds a new member, and/or a group change that removes a member.