What is the client's primary security responsibility regarding the *storageof OTP secret keys when integrating a software token implementation?

The client's primary security responsibility regarding the storage of OTP secret keys when integrating a software token implementation is to ensure the confidentiality and integrity of the OTP secret key while it is stored at rest on the client device. An OTP (One-Time Password) is a temporary, unique code used for authentication, and a software token is an application that generates these OTPs. The generation of a valid OTP relies on a cryptographic OTP secret key that is known only to the software token and the authenticating server. This secret key is the most critical component for the security of the multi-factor authentication system. If the secret key is compromised through unauthorized access or modification while stored on the client's device, an attacker could generate valid OTPs, thereby bypassing the intended security. Therefore, the client must implement robust secure storage practices, which fundamentally involve encrypting the secret key at rest and utilizing platform-specific secure storage mechanisms. These mechanisms, such as hardware-backed keystores, trusted execution environments, or operating system-protected keychains (e.g., iOS Keychain, Android Keystore), are designed to isolate and protect sensitive cryptographic material from other applications and potential malware, preventing its unauthorized disclosure or alteration.