/**
 * Shared ed25519 key helpers.
 *
 * The host pins public keys in the exact string form `ed25519:<base64>`, where
 * the base64 payload is the RAW 32-byte ed25519 public key -- NOT a DER/SPKI
 * blob and NOT a PEM. node:crypto only exports SPKI, so we slice the raw key
 * out of it (and splice it back in on the way in).
 *
 * An ed25519 SPKI DER is always exactly 44 bytes:
 *   30 2a 30 05 06 03 2b 65 70 03 21 00 || <32-byte raw key>
 * so the prefix is a fixed 12-byte constant.
 */
import { createPublicKey, type KeyObject } from "node:crypto";

/** Fixed 12-byte DER prefix of an ed25519 SubjectPublicKeyInfo. */
const SPKI_PREFIX = Buffer.from("302a300506032b6570032100", "hex");

export const PUBKEY_PREFIX = "ed25519:";

/**
 * The public key currently compiled into the Punktfunk host (slot 1).
 * `verify` defaults to this so a signed index can be checked with no arguments.
 */
export const DEFAULT_PUBLIC_KEY =
  "ed25519:qK18TE2qygIyObtMlHxUI/G1gKby8tPxuieAfnEKgYE=";

/** Encode a node KeyObject public key as the host's `ed25519:<base64>` form. */
export function encodePublicKey(key: KeyObject): string {
  const spki = key.export({ type: "spki", format: "der" });
  if (spki.length !== 44 || !spki.subarray(0, 12).equals(SPKI_PREFIX)) {
    throw new Error(
      `not an ed25519 public key (unexpected SPKI: ${spki.length} bytes)`,
    );
  }
  return PUBKEY_PREFIX + spki.subarray(12).toString("base64");
}

/** Parse the host's `ed25519:<base64>` form into a usable KeyObject. */
export function decodePublicKey(text: string): KeyObject {
  const trimmed = text.trim();
  if (!trimmed.startsWith(PUBKEY_PREFIX)) {
    throw new Error(`public key must start with "${PUBKEY_PREFIX}"`);
  }
  const b64 = trimmed.slice(PUBKEY_PREFIX.length);
  if (!/^[A-Za-z0-9+/]{43}=$/.test(b64)) {
    throw new Error(
      "public key payload must be base64 of exactly 32 raw bytes",
    );
  }
  const raw = Buffer.from(b64, "base64");
  if (raw.length !== 32) {
    throw new Error(`public key must be 32 raw bytes, got ${raw.length}`);
  }
  return createPublicKey({
    key: Buffer.concat([SPKI_PREFIX, raw]),
    format: "der",
    type: "spki",
  });
}