ESP32 WireGuard Keys: Per-Device or One Baked-In Key?

The sketch puts the WireGuard private key in a character array, and the tunnel works, which is why the question keeps coming up. The decision you actually have to make is not "is this safe" but "which of three conditions applies to this deployment". They are: who can physically hold the device, what a flash read yields on your part, and whether the fleet shares one key or carries one each. Answer those and the design follows, because each one changes the cost of a leaked key rather than the fact that it can leak.

What the key authorizes

WireGuard authenticates peers by public key. The server maps each peer public key to an address and an AllowedIPs list, so the private key is what lets a client source traffic from that address.

The same entry decides which destinations the device may reach, so a leaked key exposes the network behind the tunnel as well as the tunnel address.

Condition one: who can hold the device

If the unit lives in a locked cabinet and nobody outside your team handles it, the exposure is mostly a firmware-distribution question. If it ships to customers, sits in a vehicle, or comes back under warranty, physical possession is the credential and the secrecy of the binary is secondary. A person holding the device does not need to read the key, because the running firmware will open the tunnel for them. That is why the server-side answer has to include revocation and a bounded lifetime, not only a protected image.

Decide this condition before the other two, because it sets the budget for both.

Condition two: what a flash read yields

Anyone who can read the flash image recovers a key stored as a literal, whether through a debug connection, an update file, a backup or a returned unit. Flash encryption raises the cost of a direct dump, and the ESP-IDF documentation describes what the feature covers for a given part and configuration. It does not make the key unextractable under every attack model, and it does nothing for the device-in-hand case. Treat it as one layer that changes cost, not as the answer.

Read the platform documentation for your exact chip and configuration before claiming a property in a design review.

Condition three: reuse across the fleet

One key for a production run means every unit shares one identity. The server cannot tell them apart in logs, cannot revoke one without revoking all, and cannot distinguish a clone from an original. Per-device keys fix all three, at the cost of an enrollment step and a mapping from serial to public key. That mapping is the asset that turns revocation into a one-line change instead of a fleet-wide reflash.

If the fleet is five units on a bench, sharing a key may be a reasonable trade. If it is five hundred in the field, it is not a trade at all.

The enrollment pattern that makes per-device keys practical

Generate the key pair on the device at first boot, keep the private half in protected non-volatile storage, and send only the public key to an enrollment service that assigns an address and writes the peer entry. Authenticate the enrollment response, or an attacker on the local network can register their own key against your account. Never print the private key to the serial console during provisioning. Record the device serial next to the public key, so a lost unit maps to exactly one peer entry.

That is a small service, and it is the difference between a fleet you can audit and a fleet you can only reflash.

Time, endpoints and the decision

The example synchronises time before starting the tunnel, and that matters beyond TLS: WireGuard handshakes carry a timestamp used for replay protection, so a device booting with a wrong clock can have handshakes rejected. The endpoint is a hostname, which puts DNS in the trust path even though the server's public key still authenticates the peer. A wrong address wastes attempts and reveals when the device is online. Handle both before arguing about the key.

Then decide: a shared key only while possession is controlled and the fleet is small, per-device keys as soon as either stops being true.