/**
 * @class Lightweight stream-cipher–style encryption utility using `HMAC-SHA256` for keystream generation and authentication.
 *      - The class derives separate encryption and MAC keys from the provided secret.
 *
 * @remarks
 * - **The encryption scheme is:**
 *   - keystream = `HMAC(encKey, iv || counter)`
 *   - ciphertext = `plaintext XOR keystream`
 *   - tag = `HMAC(macKey, iv || ciphertext)`
 * - This is a custom construction and should not be used for production-grade cryptographic security.
 * - `Cipher` class is a pure JS implementation. It does not rely on `crypto` or Web APIs.
 */
export declare class Cipher {
    #private;
    /**
     * * Creates a new `Cipher` instance using the provided secret.
     *
     * @param secret - The secret string used to derive encryption and MAC keys.
     * 				   Must be a non-empty string.
     */
    constructor(secret: string);
    /**
     * * Encrypts a UTF-8 string.
     *   - The output format is: `base64( iv || ciphertext || tag )`
     *
     * @param text - The plaintext string to encrypt.
     * @returns A base64-encoded encrypted token.
     */
    encrypt(text: string): string;
    /**
     * * Checks if a token is structurally valid and contains a matching MAC using the same secret.
     *
     * @param token - The base64-encoded encrypted blob to validate.
     * @returns `true` if the MAC is valid, `false` otherwise.
     */
    isValid(token: string): boolean;
    /**
     * * Decrypts a previously encrypted token.
     *   - Throws an error if the tag does not match or the token is malformed.
     *
     * @param token - The base64-encoded token produced by `encrypt`.
     * @returns The decrypted plaintext string.
     */
    decrypt(token: string): string;
}