What specific type of cryptographic material or secret should *neverbe present in client-facing OTP integration documentation, even in redacted or example forms?

The specific type of cryptographic material or secret that should never be present in client-facing One-Time Password (OTP) integration documentation, even in redacted or example forms, is the cryptographic shared secret key. This key, also commonly referred to as the seed or master key, is a unique, high-entropy, randomly generated sequence of bytes or characters that forms the core of an OTP system's security, especially for algorithms like HMAC-based One-Time Passwords (HOTP) and Time-based One-Time Passwords (TOTP). An OTP is a temporary password valid for a single use, designed to enhance security. In the context of HOTP and TOTP, this shared secret key is strictly confidential and known only to the server (the entity validating the OTP) and the legitimate client application or hardware device (the authenticator generating the OTP). For instance, when a user initializes a TOTP authenticator application, the server generates a unique shared secret key for that user and securely transmits it to the authenticator, often via a QR code or a manual entry string. This key is symmetric, meaning the exact same key is employed by the client to generate the OTP and by the server to verify it. The authenticator combines this secret key with a dynamic factor (such as a counter for HOTP or the current time truncated to a specific interval for TOTP) and feeds these into a cryptographic hash function to compute the short, numerical OTP displayed to the user. Exposure of this cryptographic shared secret key in any client-facing documentation would create an immediate and catastrophic security vulnerability. Any unauthorized party obtaining this key could independently generate valid OTPs without needing the legitimate user's device, thereby completely circumventing the multi-factor authentication mechanism. This compromise would allow an attacker to impersonate the user and gain unauthorized access to their account, rendering the entire OTP system useless. Therefore, its complete and absolute exclusion from any form of client-facing material is a paramount security requirement.