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"He did the mash. He did the monster mash."

Encryption that doesn't bite. Monstermash wraps the battle-tested NaCl (libsodium) cryptography in a friendly CLI, a clean Python API, and an MCP server — so developers, engineers, and now AI agents can encrypt things correctly without touching a single cryptographic knob.

1️⃣ version: 2.1.0  ·  ✍️ author: Mitchell Lisle

PyPI Python

Why Monstermash?

  • 🔒 Safe by default — built on NaCl's Box (Curve25519 + XSalsa20-Poly1305). No hand-rolled crypto, no caller-managed nonces, no footguns.
  • 🪄 Decrypt with just your private key — the sender's public key rides inside the ciphertext (the "Mashed Envelope"), so the recipient needs nothing extra.
  • 🧰 One tool, three surfaces — the same guarantees from the CLI, the Python library, and an optional MCP server for LLMs.
  • 🗝️ Profiles, not key-juggling — store keypairs once (mode 0600, just like your SSH keys) and reference them by name.
  • 🚨 Tamper-evident — authenticated encryption fails loudly; a corrupted or wrong-key ciphertext raises, it never returns garbage.

Install

pip install monstermash

Want the MCP server for AI agents? Grab the optional extra:

pip install "monstermash[mcp]"

Quickstart

1. Generate a keypair

monstermash generate

-----------------
Private Key (keep it secret, keep it safe)
a715a3d11d0f9c13de3bd6d390e36ba4e3322f4f2e4f1a13a54ba85be606de87
Public Key (you can share this one)
01765c67f451f3175f53bbe11d69d73a36d45074da935271473b4a1c460e3d79
-----------------

2. Save it as a profile (optional, but nice)

Profiles live in ~/.monstermashcfg and are written with owner-only (0600) permissions.

monstermash configure \
  --profile default \
  --private-key a715a3d11d0f9c13de3bd6d390e36ba4e3322f4f2e4f1a13a54ba85be606de87 \
  --public-key 01765c67f451f3175f53bbe11d69d73a36d45074da935271473b4a1c460e3d79

3. Encrypt

monstermash encrypt --profile default --data "hello world"

Or pass keys explicitly, or encrypt a whole file:

monstermash encrypt \
  --private-key a715a3d11d0f9c13de3bd6d390e36ba4e3322f4f2e4f1a13a54ba85be606de87 \
  --public-key 03ab4b8a77456729678a8022c2bfe22f64ed2db72692903e5f69e4a92649e646 \
  --file ./secret-plans.txt

4. Decrypt

monstermash decrypt \
  --private-key 91c7b2534454587a3330537bce60056e9da9a9bf75d32507152f49e85514970d \
  --data 01765c67f451f3175f53bbe11d69d73a36d45074da935271473b4a1c460e3d797bee92fa7ff1216eb5324b247fd41cce283adbcc4df92baacfea27765360a7c0feb226cccc1538c0397783003d0283d2841d2a

The --profile flag works here too — no public key needed, it's baked into the ciphertext.

How it works

Monstermash owns no cryptographic primitives — it leans entirely on NaCl. What it adds is ergonomics:

plaintext ──▶ NaCl Box (Curve25519 + XSalsa20-Poly1305) ──▶ [ sender public key │ nonce │ MAC │ ciphertext ] ──▶ hex
                                                                └──────────── the "Mashed Envelope" ────────────┘

Because the sender's public key travels inside the envelope, the recipient decrypts with only their private key. See docs/domain/cryptography.md for the full domain model and design decisions.

MCP Server

Monstermash ships an optional Model Context Protocol server so an LLM can do real encryption natively — without ever handling raw private keys.

Security model: tool arguments flow into the model's context, so private keys never cross the boundary. The model works by profile name (à la ssh-agent); the server reads the key from disk locally. Set a default with MONSTERMASH_MCP_DEFAULT_PROFILE.

Run it over stdio:

monstermash-mcp

Register it with an MCP client (e.g. Claude Desktop):

{
  "mcpServers": {
    "monstermash": {
      "command": "monstermash-mcp"
    }
  }
}

Tools

Tool Description
generate_keypair Generate a keypair, store it under a profile, and return only the public key.
encrypt Encrypt text using a profile's keys. Recipient defaults to the profile; target a stored contact with recipient, or pass a raw public_key.
decrypt Decrypt a ciphertext using a profile's private key.
configure Import an existing keypair under a named profile.
add_contact Store a shared public key under a name (a contact); usable as an encrypt recipient, never able to decrypt.
list_profiles List profile names and public keys — private keys are never returned.

License

See the repository for license details.