Separate parsing and privilege
Network parsing runs in restricted, unprivileged workers. A privileged broker retains host keys and independently verifies user authorization. A worker cannot authorize itself.
/etc/psh/security
Security & scope_
The required cryptographic profile, process boundaries, and limits of the current implementation.
Experimental. Unaudited. For evaluation.
psh is a new protocol and implementation. Internal tests and reviews do not establish production readiness, an external security audit, or a formal protocol proof. Strong individual primitives do not prove the security of the whole system.
The required profile
One mandatory profile.
There is no classical-only or legacy fallback. Both endpoints must implement psh/3 and use complete paired identities.
TransportDTLS 1.3 over UDP
ALPN psh/3. Tickets, resumption, early data, and compression are disabled.
Key exchangeX25519 + ML-KEM-768
The mandatory hybrid group combines classical and post-quantum key agreement.
Host & user identityEd25519 + ML-DSA-87
Both components are required. This is a connection-bound psh construction, not a native composite TLS signature.
Record protectionChaCha20-Poly1305
The OpenSSL DTLS implementation supplies authenticated encryption and the HKDF-SHA-256 key schedule.
ImplementationPinned OpenSSL 4.1.0
Cryptographic primitives come from the vendored library. psh does not implement its own encryption primitives.
Trust, in order
Verify the host before disclosing identity.
A possession proof identifies a key. Independently verified fingerprints or trusted provisioning establish whether that key belongs to your host.
- 01
Establish transport
Complete the mandatory hybrid DTLS exchange.
- 02
Verify the host pair
Validate the connection-bound host proof and accept the complete-pair pin.
- 03
Authenticate the user
The broker independently authorizes the pair and verifies both user signatures.
- 04
Launch the session
Run the requested shell or command as the authorized non-root account.
System boundaries
Process and resource boundaries.
Validate a changing path
A new client address is a candidate, not an immediate replacement. The worker promotes it only after an encrypted challenge and matching return-path response.
Put limits on state
Datagrams, queues, sessions, authentication attempts, and timers are bounded. The current limits include 64 total connections and 16 pending authentications.
Protect local identities
Paired private files require restrictive ownership and permissions. Interactive client keys can use a local passphrase. Unattended host keys are unencrypted and root-owned.
Compatibility & limitations
What psh does not do.
psh is its own protocol. It has no SSH wire compatibility and does not claim overall security equivalence to OpenSSH or whole-protocol NIST category 5 / CNSA compliance.
- One live session
- No multiplexing, detached reattachment, or server migration. A new client invocation starts a new session.
- Remote shells and commands
- No agent forwarding, port forwarding, file-transfer subsystem, or PAM integration.
- Endpoint trust still matters
- A compromised endpoint is not made trustworthy by the authentication construction. First-use pins need independent verification.
- Linux runtime requirements
- The daemon requires Linux 5.14+, writable cgroup v2, and the process-management capabilities described in the build guide.
Inspect the evidence.
Current test instructions, historical results, protocol details, and implementation rationale are available in the documentation. Dated results apply to their recorded revision.