/**
 * Keychain Credential Storage
 *
 * Provides secure OS keychain integration for storing the recovery phrase
 * using the `keytar` library. Supports macOS Keychain, Windows Credential Vault,
 * and Linux Secret Service (via libsecret).
 *
 * Security notes:
 * - Recovery phrases are stored in the OS-managed credential store
 * - The server never sees plaintext passwords
 * - Credentials are scoped per userId
 * - No passwords are logged or written to disk outside the OS keychain
 */
/**
 * Check whether the OS keychain backend is available in this environment.
 *
 * @returns true if keytar loaded successfully
 */
export declare function isKeychainAvailable(): boolean;
/**
 * Store a recovery phrase in the OS keychain.
 *
 * @param userId  - The user identifier (used as the "account" in the keychain)
 * @param masterPassword - The plaintext recovery phrase to store
 * @throws Error if the keychain backend is unavailable
 */
export declare function storeCredentials(userId: string, masterPassword: string): Promise<void>;
/**
 * Retrieve a recovery phrase from the OS keychain.
 *
 * @param userId - The user identifier
 * @returns The stored recovery phrase, or null if none exists
 * @throws Error if the keychain backend is unavailable
 */
export declare function getCredentials(userId: string): Promise<string | null>;
/**
 * Delete a stored recovery phrase from the OS keychain.
 *
 * @param userId - The user identifier
 * @returns true if a credential was deleted, false if none existed
 * @throws Error if the keychain backend is unavailable
 */
export declare function deleteCredentials(userId: string): Promise<boolean>;
/**
 * Check whether credentials exist in the OS keychain for a given user.
 *
 * @param userId - The user identifier
 * @returns true if a credential is stored for this userId
 * @throws Error if the keychain backend is unavailable
 */
export declare function hasCredentials(userId: string): Promise<boolean>;
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