Weechancrypt

Python plugin for Weechat that allows encrypted messages in an IRC channel. Uses a pre-shared key so all channel members with the correct passphrase can recieve and send messages.

Motivation

IRC (Internet Relay Chat) is an inherently unencrypted, unauthenticated communications medium. There have been some attempts to implement encryption on top of IRC (such as FiSH), or implement IRC-like services that have encryption and authentication built-in (such as SILC).

One protocol that has gained some widespread acceptance is OTR (Off-The-Record). Unfortunately, OTR is (by design) a point-to-point system - it can only encrypt messages between two parties. This makes OTR excellent for communicating via private messages, but cannot be used in a group chat system.

Chancrypt is an attempt to implement a protocol that can be used to secure communications in public channels between parties that have exchanged a pre-shared key through out-of-band methods, such as the telephone or in person. Once users have agreed on a pre-shared key, it is then used to encrypt in-channel messages so that only those users can decode them. This system can support a large number of users, since key negotiation happens offline. In theory, there should be no upper limit to the number of parties in the channel.

Chancrypt can be used alongside OTR, and in fact I encourage you to use OTR for securing privmsg chats.

Requirements

Requires pycrypto to be installed. On Debian-like systems, this is usually the python-crypto package, or you can use pip install pycrypto.

Requires WeeChat versions newer than 0.3.0.

Crypto

This plugin uses AES-256 in CTR mode with random per-message IVs. PBKDF2 is used to derive encryption keys from the user-supplied passphrase.

Loading

Clone this repo, then run ln -s weechancrypt/chancrypt.py ~/.weechat/python/autoload/chancrypt.py to enable auto-loading of the script.

Then, in weechat, run /script load chancrypt.py to load the script.

Usage

To enable encrypted messages in a channel, set a passphrase by running:

/set plugins.var.python.chancrypt.passphrase.<server>.<channel> PASSPHRASE

This will enable encrypted message support in that channel. Note that the encryption is opt-in only - if a passphrase is not set, no encryption or decryption will be attempted.

• Valid incoming encrypted messages can be distinguished by the green key icon prefix.
• Invalid messages have a red key icon.
• Unencrypted messages will not have an icon at all.

Users that do not have this plugin will only see a message similar to:

!ENC c29tZSByYW5kb20gSVY6c29tZSBlbmNyeXB0ZWQgZGF0YQo=

Caveats

• This plugin only supports symmetric pre-shared key encryption. An attacker can decrypt messages in the channel if they have access to this pre-shared key.
• This plugin does not force encryption in a channel. Only messages that are prefixed with !ENC will be decrypted, so users that are joined to the channel and conversing in plaintext will still show up in the conversation window.
• If different users are using different passphrases in the same channel, the plugin will complain that the message is invalid for those messages that do not match the encryption key.
• This plugin is hard-coded to use AES-256 in CTR mode with random IVs.
• This plugin does not take IRC server line-length limits into account at the moment. It does compress the ciphertext with zlib before transmitting, but base64 is not an overly efficient encoding method so that offsets the savings from zlib somewhat. In the future I may implement message chunking to avoid line-length limits.

Wishlist

Crypto-related:

• Hashed passphrases so they are not stored in plaintext in Weechat configs
• Don't use CRC32 checksums since they're not a cryptographic hash (@kisom recommends hmac-sha-256 encrypt-then-MAC) by @kisom)
• Switch to either scrypt or the HKDF from the MAC we pick instead of using PBKDF2 (as recommended by @kisom)
• Perfect Forward Secrecy with the pre-shared key so an attacker who discovers the PSK cannot access previous conversations.

UI-related:

• Show a statusbar on the channel's buffer to indicate if encryption for that channel is enabled
• Show a encryption status prefix on outgoing encrypted messages
• Chunk messages that are larger than the IRC line length limit for the channel+server