Trust Model

DeadLetter is built around cryptographic trust pinning instead of central user accounts.
No part of the system ever requires trusting the backend with plaintext data.

Root of Trust

The system has a single Root Signing Key.

This key:

• Lives only on the registry server
• Signs all user certificates
• Is fetched once by clients and ESP devices
• Is pinned locally and never auto‑updated

If the root key changes, all clients must explicitly clear trust.

This makes silent compromise of the registry impossible.

Certificate Structure

Each user has a certificate:

{
  "handle": "alice",
  "encPub": "<base64 X25519 public key>",
  "keyId": "<random>",
  "expiresAt": 1735689600
}

This certificate is signed by the root key using Ed25519.

Certificate Verification Flow

When a client or ESP wants to send or read messages:

1. Fetch /keys/ from registry
2. Receive:
   • root_pub_b64
   • signing algorithm
3. Pin the root key locally
4. Fetch /keys/<handle>
5. Verify signature using pinned root key
6. Reject any unsigned or invalid certificate

ESP Trust Pinning

Each ESP device stores:

This prevents:

• Evil‑twin registry attacks
• Gateway redirection
• Silent man‑in‑the‑middle attacks

Fingerprint Verification

On first boot the ESP prints:

Root Trust Fingerprint:
aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99

This value is unique per installation.
Never trust the example above – always compare the fingerprint shown on your own ESP device with the one printed by your CLI.

This must be compared with the fingerprint shown by the CLI.

Only if both match is trust established.

Where this happens

This process is fully described in:

→ Device Setup → CLI Usage → Trust Commands

The sender ESP device is always the authority for fingerprint verification.

Trust Revocation

Trust is only reset when:

• The user runs the setup CLI again
• Or manually clears NVS
• Or reflashes firmware

There is no remote trust reset endpoint.

Why This Matters

This model guarantees:

Trust Summary

DeadLetter does not trust infrastructure.

It only trusts:

• Locally pinned keys
• Cryptographic verification
• Physical access when provisioning