package agentd

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
)

// DeriveHy2Password derives a node-agnostic Hysteria2 password from the opaque
// data-plane credential id (dp_uuid). The REALITY inbound uses dp_uuid directly as
// the VLESS user uuid; the Hy2 inbound cannot reuse a UUID as a password verbatim
// (it must look like an opaque secret), so it is derived from the SAME source —
// "password = dp_uuid 同源派生" — via a keyed HMAC.
//
// The derivation is deterministic given (dp_uuid, key): the control plane runs the
// exact same function when it builds the client's connect config (doc/02 §3.1), so
// both sides agree without the password ever crossing the agent contract.
//
// Risk note (carried from task #5): the derivation key is shared material. If a
// node is seized the attacker still only sees dp_uuids and this node's key, which
// lets them recompute Hy2 passwords for dp_uuids THEY ALREADY HOLD — it does not
// reveal other subscribers' credentials or any account identity. Rotating the key
// rotates every Hy2 password. When key == "" the password falls back to the raw
// dp_uuid (acceptable for dev; production cloud-init always injects a key).
func DeriveHy2Password(dpUUID, key string) string {
	if key == "" {
		return dpUUID
	}
	mac := hmac.New(sha256.New, []byte(key))
	mac.Write([]byte(dpUUID))
	sum := mac.Sum(nil)
	// base64url without padding → URL/JSON-safe, 43 chars.
	return base64.RawURLEncoding.EncodeToString(sum)
}