Group file sharing as a key-management problem.
HybridCipher treats collaboration as an end-to-end encryption system: devices hold keys, the server coordinates encrypted envelopes, and membership changes are enforced through epoch rotation rather than policy alone.
Architecture Layers
The collaboration design separates identity, key delivery, file encryption, and auditability.
Device-bound membership
Users authenticate without sending passwords to the server. Authorized devices receive wrapped epoch material, not plaintext file keys.
Epoch-based group keys
Membership changes advance the group epoch. New file keys are wrapped for the current epoch, and older material can be rewrapped during background maintenance.
Client-side encryption
File contents are encrypted before they reach storage. The coordination service handles metadata and key-delivery envelopes, not plaintext.
Tamper-evident events
Membership, key-delivery, and rekey events can be committed into hash-chained logs and Merkle roots for later verification.
Revocation Flow
The important distinction is that removal changes cryptographic state. A revoked member should stop receiving future epoch keys, and historical access depends on whether older file keys have been rewrapped under the new epoch.
Read Architecture NotesAdmin removes a member from the group state.
The group advances to a new epoch and stops issuing future keys to the removed identity.
New writes use the current epoch immediately.
Existing file keys are rewrapped or re-encrypted according to coverage policy.
Audit entries record the membership transition and rekey coverage.
Design Tradeoffs
The model favors explicit cryptographic guarantees over vague claims about server-side control.
Cryptographic revocation is not just an ACL
Removing access requires key evolution and, for older files, a rekey operation. The model makes that work explicit instead of pretending a server-side permission change is enough.
Offline work needs cached authority
Authorized devices can continue using cached epoch keys, while new devices or membership changes require coordination when connectivity returns.
Auditability adds state
Transparency logs and coverage proofs improve accountability, but they also introduce verification data that must be stored, synced, and checked.