Secure File Sharing System (CS 161 Project 2)

This repository contains my implementation of a secure file sharing system. The project builds a client-side encrypted file store on top of an untrusted datastore and a trusted keystore for public keys. Users can store, append to, load, share, and revoke access to files while preserving confidentiality and integrity against a malicious datastore.

Spec link: Project 2 spec

Quick Summary

1. Client-side encryption and integrity for all file and user data.
2. Efficient append via an encrypted linked-list of file nodes.
3. Share and revoke access using signed, encrypted invitations.

How It Works

1. User initialization and login
   • InitUser derives a base key from the password using Argon2 with a random salt, then derives separate encryption, MAC, and root keys.
   • The user struct is serialized, encrypted, MACed, and stored at a deterministic UUID derived from the username.
   • Public keys are stored in the keystore under names like username_pke and username_dsv.
2. File storage
   • Each file has a FileMeta record stored at a deterministic UUID derived from the user’s root key and the filename.
   • FileMeta contains per-file symmetric keys plus pointers to the head/tail of a linked list of FileNode objects.
   • Each FileNode stores encrypted content and a pointer to the next node, allowing O(1) append.
3. Append
   • AppendToFile creates a new encrypted FileNode, updates the previous tail node’s Next pointer, and updates FileMeta.
4. Load
   • Owners decrypt FileMeta and traverse the linked list, verifying a MAC on each node.
   • Shared users decrypt a local “handle,” verify the invitation signature, decrypt the invitation to obtain per-file keys, then traverse nodes.
5. Sharing
   • CreateInvitation packages per-file keys and a base-node pointer into an Invitation, encrypts it to the recipient, and signs it.
   • The recipient stores a local SharedFile handle (encrypted/MACed under their root key) that points to the invitation UUID.
6. Revocation
   • Only the original owner can revoke.
   • RevokeAccess rekeys the file by decrypting each node with old keys and re-encrypting with fresh keys.
   • Invitations for remaining users are reissued with new keys; the revoked user’s invitation is overwritten with a revoked marker.

Public API (package client)

1. InitUser(username, password)
2. GetUser(username, password)
3. StoreFile(filename, content)
4. LoadFile(filename)
5. AppendToFile(filename, content)
6. CreateInvitation(filename, recipientUsername)
7. AcceptInvitation(senderUsername, invitationPtr, filename)
8. RevokeAccess(filename, recipientUsername)

Repository Layout

1. client/client.go – implementation
2. client/client_unittest.go – optional white-box unit tests
3. client_test/client_test.go – black-box integration tests
4. my_tests/my_test.go – additional tests

Running Tests

go test ./...

Notes

1. This is a course project and is not production hardened.
2. The system relies on github.com/cs161-staff/project2-userlib for cryptographic primitives and the datastore/keystore API.