/**
 * If the error has occured when the client challenges the server or when the server verifies
 * the client's response (or None for unspecified)
 */
export declare enum ChallengeStage {
    None = 0,
    ClientChallenge = 1,
    ServerChallenge = 2
}
/**
 * An error that happens in the MutualKeyChallengeStrategy. It will be the first parameter for
 * the callback to passport.authenticate unless an unexpected exception is thrown.
 */
export declare class ChallengeError extends Error {
    readonly stage: ChallengeStage;
    constructor(message?: string, stage?: ChallengeStage);
    /**
     * @param value The value to check
     * @return If the input's type is ChallengeError
     */
    static isType(value: unknown): value is ChallengeError;
}
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