qssh

A post-quantum secure shell in pure Rust, with its own protocol.

qssh is a secure shell, file copy and key agent written from scratch in pure Rust around post-quantum primitives: ML-KEM-1024 key exchange, Falcon and SPHINCS+ authentication. It replaces the SSH toolchain when both ends run qssh: it speaks its own protocol by design, shares no code with OpenSSH or OpenSSL, and does not interoperate with OpenSSH peers.

Security notice (2026-09-13)

Every release before 0.4.2 defaults to a key exchange that provides no confidentiality. The FalconSignedShares exchange authenticates the peers, but derives the session key from values sent in cleartext, so a passive observer can reconstruct it. An ML-KEM exchange existed in the code, but the client hardcoded the default algorithm and never selected it. Found 2026-07-15, fixed on main 2026-07-16 (PR #5), released as 0.4.2 on 2026-09-13: ML-KEM-1024 is the default and the configured algorithm is honoured. Earlier crates.io versions are yanked; the v0.4.1 release, the Homebrew formula and the APT package built from the June tag are superseded. Upgrade, and treat any session made under an earlier default configuration as having had no confidentiality against a passive attacker. Details in CHANGELOG.md and SECURITY.md.

0.5.0 (protocol 0.2) rewrote the in-band rekey. Before it, the hourly rotation raced the data reader and switched both keys at once, which killed busy tunnels at every rotation, and derived the new keys from cleartext shares. Rotation now uses a fresh ML-KEM-1024 encapsulation, a two-phase key switch and a single reader; both ends must run 0.5.0.

What It Does

Where qssh Stands

OpenSSH ships a hybrid post-quantum key exchange by default: sntrup761x25519 since 9.0 (2022) and mlkem768x25519 since 10.0 (2025). qssh differs from it in three ways:

• Post-quantum authentication. Falcon-1024 and SPHINCS+ host and user keys, which OpenSSH does not ship yet.
• A single pure Rust codebase. No OpenSSH or OpenSSL code, no C bindings, no GSSAPI. Vulnerability classes that live in those code paths (for example the OpenSSH GSSAPI pre-authentication crash CVE-2026-3497) do not exist here by construction. That says nothing about qssh's own bugs: see the notice above for the one we found and fixed.
• Its own protocol, by design. Both ends run qssh. There is no interoperability with OpenSSH peers and none was ever intended.

What qssh does not have yet is an external security audit.

What Is Verified, and What Is Not

The Lean development is on Zenodo: DOI 10.5281/zenodo.18663125

Related Work

qssh is part of the Paraxiom post-quantum infrastructure stack:

The counts above are what lake build reproduces today. Earlier published portfolio totals were never reproduced by a build and have been withdrawn.

Install

macOS / Linux (Homebrew)

brew tap paraxiom/tap
brew trust paraxiom/tap     # one-time: clears Homebrew's third-party-tap check
brew install qssh

Builds from source (Homebrew pulls in the Rust toolchain automatically). Works on macOS and on Linux via Homebrew — including ARM, where the .deb below is not yet published.

Debian / Ubuntu (APT)

curl -fsSL https://paraxiom.github.io/apt/paraxiom.gpg | sudo tee /usr/share/keyrings/paraxiom.gpg >/dev/null
echo "deb [signed-by=/usr/share/keyrings/paraxiom.gpg] https://paraxiom.github.io/apt stable main" \
  | sudo tee /etc/apt/sources.list.d/paraxiom.list
sudo apt-get update
sudo apt-get install qssh

This installs the qssh client, the qsshd daemon and the supporting tools, plus a systemd service for qsshd:

sudoedit /etc/qssh/qsshd.env     # set listen address / port
sudo systemctl start qsshd       # generates a Falcon-512 host key on first start

Standalone .deb

Download the .deb from Releases and:

sudo apt install ./qssh_*.deb

Building the package yourself and the APT-repo setup are documented in docs/PACKAGING.md and docs/APT-REPO.md.

Releases

Pre-built binaries are available under Releases.

Licence

Two regimes, at the recipient's choice: GPL-3.0-only (the default, reciprocal) or a Paraxiom commercial licence for products that cannot carry the GPL. See LICENSE.md.

We choose collaboration over extraction. If you're working on post-quantum infrastructure — whether in research, government, defence, or industry — write to sylvain@paraxiom.org with a brief description of your work. That door opens when you knock.

Citation

@misc{cormier2025qssh,
  author    = {Cormier, Sylvain},
  title     = {qssh: Post-Quantum SSH with 3-Tier Formal Verification},
  year      = {2025},
  publisher = {Paraxiom Technologies Inc.},
  url       = {https://github.com/Paraxiom/qssh}
}

Contact

Sylvain Cormier Paraxiom Technologies Inc. — Montreal sylvain@paraxiom.org | paraxiom.org