Function signMessage

http/unstable_message_signatures.ts:820–856

(
  options: SignOptions<T>,
)

 * @returns A new message with signature headers appended.
 */
export async function signMessage<T extends Request | Response>(
  options: SignOptions<T>,
): Promise<T> {
  const { message, params, key, request } = options;
  const label = params.label ?? "sig";

  validateSignParams(params);
  assertUniqueLabel(message.headers, label);

  const algorithm = params.algorithm ?? inferAlgorithm(key);

  const baseOpts: CreateSignatureBaseOptions = { message, params };
  if (request) baseOpts.request = request;
  const base = createSignatureBase(baseOpts);
  const baseBytes = UTF8_ENCODER.encode(base);

  const signParams = getSignParams(algorithm);
  const signatureBytes: Uint8Array_ = new Uint8Array(
    await crypto.subtle.sign(signParams, key, baseBytes),
  );

  // Build Signature-Input value
  const components = params.components.map(normalizeIdentifier);
  const sigParamsValue = buildSignatureParamsValue(components, params);

  // Build headers
  const sigInputHeader = `${label}=${sigParamsValue}`;
  const sigHeader = serializeDictionary(
    new Map([[label, sfItem(binary(signatureBytes))]]),
  );

  // clone() preserves the concrete type at runtime
  const clone = message.clone() as T;
  appendHeader(clone.headers, "Signature-Input", sigInputHeader);
  appendHeader(clone.headers, "Signature", sigHeader);
  return clone;
}

function appendHeader(headers: Headers, name: string, value: string): void {
  const existing = headers.get(name);