znc-crypt2

A drop-in replacement for ZNC's built-in crypt module, upgraded from Blowfish-CBC to AES-256-GCM with HKDF key derivation and AAD target binding.

Same wire format, same commands, same setup. Just better encryption.

What's different from the original crypt module

	crypt	crypt2
Cipher	Blowfish-CBC	AES-256-GCM
IV/Nonce	time + rand() (weak)	RAND_bytes() (cryptographically random)
Authentication	None (malleable)	GCM auth tag (tamper detection)
Key derivation	Raw key bytes	HKDF-SHA256
Target binding	None	AAD prevents message transplantation across targets
Wire format	+OK *<base64>	+OK *<base64> (identical)
Commands	unchanged	unchanged

Requirements

• ZNC 1.8+
• OpenSSL 3.x
• Both sides must run crypt2 — this is not compatible with the original crypt module or Mircryption

Installation

Building inside a Docker container (linuxserver/znc)

# Copy source into your ZNC data directory
sudo cp crypt2.cpp /srv/znc/data/modules/

# Shell into the container
docker exec -it znc sh

# Inside the container — install build tools (Alpine-based image)
apk add znc-dev build-base

# Build the module
# Note: znc-buildmod writes the .so to the current directory (/ by default)
znc-buildmod /config/modules/crypt2.cpp

# Copy the compiled .so to the modules directory
cp /crypt2.so /config/modules/

# Exit the container
exit

Building on a bare-metal ZNC install

znc-buildmod crypt2.cpp
mv crypt2.so ~/.znc/modules/

Loading the module

From your IRC client:

/msg *status LoadModule crypt2

If you're replacing a previous version of crypt2, unload it first:

/msg *status UnloadModule crypt2
/msg *status LoadModule crypt2

Usage

All commands are sent to *crypt2.

Key exchange (automatic, recommended)

Initiate a DH1080 key exchange with another crypt2 user:

/msg *crypt2 KeyX <nick>

Manual key setup

Generate a key:

openssl rand -base64 32

Set it on both sides:

/msg *crypt2 SetKey <nick|#channel> <key>

Both sides must use the same key. If you previously had crypt2 loaded with an older key, you must set a new key — HKDF derives differently from the same input compared to the old version.

Other commands

/msg *crypt2 ListKeys
/msg *crypt2 DelKey <nick|#channel>
/msg *crypt2 GetNickPrefix
/msg *crypt2 SetNickPrefix <prefix>

Suppress encryption on a single message

Prefix your message with `` (two backticks) to send plaintext even with a key set.

Security notes

• AAD (Associated Authenticated Data): ciphertext is cryptographically bound to the target nick or channel. A message encrypted for #channel cannot be successfully decrypted as a private message or replayed to a different channel, even with the same key.
• HKDF: the same raw key produces a different derived key per target, providing additional key separation beyond AAD.
• Tamper detection: any modification to the ciphertext, nonce, or GCM tag causes decryption to fail with (decryption failed - wrong key or tampered message).
• Keys are stored in plaintext on disk, same as the original crypt module. Use SSL between ZNC and your IRC client.
• DH1080 key exchange is supported but is unauthenticated (no MITM protection). For higher security, use manual SetKey with an out-of-band key exchange.

Verifying encryption

The ciphertext byte length should always equal 12 (nonce) + len(plaintext) + 16 (tag):

echo "<base64 blob>" | base64 -d | wc -c