phone_a_friend

Serverless, end-to-end-encrypted chat for people who share access to the same Linux directory (an NFS mount, a group-writable /srv/chat, a shared home server, ...). There is no server process: every client reads and writes plain files in the shared directory and does all encryption, decryption, sending and receiving itself. A curses TUI runs in any bash terminal.

┌─ INVITES ────────────┬─ chat with bob ────────────────────────────┐
│ ✉ carol (chat)       │ 09:12 bob:   lunch?                        │
│ CHATS                │ 09:13 alice: sure - where?                 │
│  bob                 │ 09:14 bob:   the usual                     │
│  dave ●2             │                                            │
│ GROUPS               │                                            │
│  #book-club ●1       ├────────────────────────────────────────────┤
│ USERS                │ > see you at noo▊                          │
│  + erin              │                                            │
└──────────────────────┴────────────────────────────────────────────┘

Features

• Register with a username + passphrase; an X25519 identity keypair is generated for you.
• Invite people to chat — key exchange is push-based: the invite pushes your public key to them; accepting pushes their public key back to you. Without an accepted exchange, messages cannot be sent or decrypted.
• Create groups and invite people — each group has a random symmetric key; inviting someone pushes the group key to them (sealed to their public key). Any member can invite others.
• See who's around — registered users you have not connected with yet are listed under USERS in the left pane; select one to send a chat invite.
• Send/receive messages to users or groups. The left-hand pane lists invites, chats and groups; click (mouse supported) or use ↑/↓ + Enter to open one and read its messages. New messages in the open conversation appear immediately; elsewhere an unread badge (●3) lights up next to the chat or group.
• Encrypted per-user config — each client maintains its user's config file (private key, contact keys, group keys, read state) in the shared directory, encrypted with a key derived from the passphrase (scrypt + ChaCha20-Poly1305).

Security model

• Access control is layered: the filesystem permissions of the shared directory decide who can see the files at all; encryption decides who can read the messages.
• Direct messages are encrypted with a key derived from the static-static X25519 Diffie-Hellman secret of the two participants — readable only by someone holding one of the two private keys and the peer's public key.
• Group messages are encrypted with the group's symmetric key, held only by members who accepted an invite.
• Invites and invite replies are "sealed" (ephemeral X25519 → HKDF → ChaCha20-Poly1305) to the recipient's public identity key.
• All ciphertexts are authenticated (AEAD); tampered files are ignored.
• Out of scope for v1: forward secrecy/key rotation, sender authentication beyond conversation-key possession, traffic analysis (filenames reveal timing; directory names reveal who talks to whom), and revoking group keys.

Install

Requires Python ≥ 3.9 (Linux). From PyPI:

pip install phoneafriend     # installs the `paf` command

From a checkout:

pip install .             # or `pip install -e .` for development
# or, without installing:
pip install -r requirements.txt
alias paf='python -m phone_a_friend'

Set up a shared directory

Any directory all participants can read and write works. Typical setup with a shared POSIX group:

sudo mkdir -p /srv/paf
sudo chgrp chatters /srv/paf
sudo chmod 2770 /srv/paf     # rwx for the group, setgid, nothing for others

The client creates its own subdirectories (drop-boxes get the sticky bit so users cannot delete each other's files).

Use

paf --dir /srv/paf                 # launch the TUI (register on first run)
PAF_DIR=/srv/paf paf               # same, via environment variable

In the TUI:

Selecting an incoming invite prompts you to accept (y) or decline (n); selecting a name under USERS prompts to send them a chat invite.

Every action is also available as a slash command typed into the input line — these work in any terminal, including ones whose host application intercepts Ctrl or function keys (the VS Code integrated terminal binds Ctrl-N/Ctrl-G/Ctrl-O itself):

/invite USER     invite a user to chat
/group NAME      create a group
/ginvite USER    invite a user to the open (or selected) group
/quit            exit

Scripting / headless use

Every operation is also available as a subcommand, which is handy for testing and automation (--passphrase / $PAF_PASSPHRASE avoid the prompt):

paf -d /srv/paf -u alice register
paf -d /srv/paf -u alice invite bob            # chat invite
paf -d /srv/paf -u bob   invites               # list incoming
paf -d /srv/paf -u bob   accept --from alice
paf -d /srv/paf -u alice send --to bob "hello"
paf -d /srv/paf -u bob   read --to alice
paf -d /srv/paf -u alice create-group "book club"
paf -d /srv/paf -u alice invite bob --group "book club"
paf -d /srv/paf -u alice send --group "book club" "welcome"
paf -d /srv/paf -u bob   status                # contacts, groups, unread

Test

python -m unittest discover -s tests -v

Coverage is 100% and enforced in CI (fail_under = 100 in pyproject.toml):

pip install coverage
coverage run -m unittest discover -s tests && coverage report

Releasing to PyPI

Publishing is automated via .github/workflows/publish.yml using PyPI trusted publishing — no API token is stored in the repo. One-time setup on pypi.org: add a GitHub publisher for MartinGallagher-code/phone_a_friend, workflow publish.yml, environment pypi. Then creating a GitHub release (e.g. tag v0.1.0) builds, checks, and uploads the sdist and wheel.

Licensing

Licensed under GPL-3.0-or-later. The repository is compliant with the REUSE Specification: every file carries SPDX copyright and license information, and license texts live in LICENSES/. Compliance is checked in CI with reuse lint.

Shared-directory layout

<shared>/
  users/<name>/identity.json     public identity (name + public key)
  users/<name>/config.enc        that user's client config, encrypted
  invites/<name>/<id>.json       sealed invites pushed TO <name>
  replies/<name>/<id>.json       sealed invite replies pushed TO <name>
  dm/<a>__<b>/<ts>-<rand>.json   direct messages, pair-key encrypted
  groups/<gid>/meta.json         public group metadata
  groups/<gid>/msgs/<...>.json   group messages, group-key encrypted