> For the complete documentation index, see [llms.txt](https://docs.gobtcpay.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.gobtcpay.com/api-reference/pos-api/gobtc-pay-pos-api/authentication.md).

# Authentication

Requests are authenticated **per POS terminal**. Each terminal is issued an `apiKey` (a shared secret) when it is created. The secret is **never transmitted** — it only signs requests, which gives integrity and replay-attack protection.

A signed request carries two extra fields inside `params`:

* `ts` — Unix timestamp in **milliseconds** when the request was created.
* `signature` — hex-encoded HMAC of the request payload.

**Computing the signature (client side):**

1. Take the request `params` object and remove the `signature` field.
2. Sort the remaining entries by key (ascending) and serialize with `JSON.stringify` — this is the canonical message.
3. `signature = HMAC-SHA256(key = terminal apiKey, message = canonical JSON)`, hex-encoded.

**Validation (server side):**

1. The server resolves the terminal, recomputes the HMAC and rejects the request unless it equals `signature`.
2. `ts` must be at most 60s in the future and no older than 5 minutes.


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