/**
 * Format a Date as Merit's required UTC timestamp: `yyyyMMddHHmmss`.
 * Merit rejects timestamps that are too old or in the future, so callers must
 * pass the current time (and re-generate it on every retry).
 */
export declare function formatTimestamp(date: Date): string;
/**
 * Compute the base64 HMAC-SHA256 signature for a Merit API request.
 *
 * @param apiId     API ID (GUID) from Merit API settings.
 * @param timestamp UTC timestamp string in `yyyyMMddHHmmss` (see formatTimestamp).
 * @param body      The exact request body string sent on the wire ("" for no body).
 * @param apiKey    API key (base64-looking secret) used verbatim as the HMAC key.
 * @returns         Base64-encoded signature. URL-encode before placing in a query string.
 */
export declare function sign(apiId: string, timestamp: string, body: string, apiKey: string): string;