/**
 * Connection Challenge Authentication (Client-side)
 *
 * This module provides client-side challenge token generation for secure
 * WebSocket connections. It uses the Web Crypto API (SubtleCrypto) for
 * browser compatibility.
 */
/**
 * Generate a challenge token for the client to send.
 * Uses Web Crypto API for browser compatibility.
 *
 * Format: base64(timestamp:hmac(secret, clientGroupID + timestamp + userID))
 */
export declare function generateChallengeToken(secret: string, clientGroupID: string, userID: string): Promise<string>;
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