To understand what is a private key (service key), you need to know how 2FA codes are created. We divide 2FA tokens into two types: time-based tokens (TOTP) and event-based tokens (HOTP). Regardless of the type, for the codes to be one-time, individual, and private, they must be calculated based on two variables: moving factor and constant factor (also called seed). Moving factor causes differentiation of the generated code each time. Most often, it’s time (in the case of TOTP codes) or an event: code refresh counter (in the case of HOTP codes). Constant factor/seed is a variable responsible for assigning the generated codes only for you. And this variable is your private key (also known as the service key).