SKey
SKey, often written as S/Key, is a one-time password (OTP) authentication scheme that uses a cryptographic hash chain to generate a sequence of passwords, each of which is used only once. The system aims to prevent replay attacks by ensuring that previously used passwords cannot be reused.
How it works: The user and the authentication server share a secret seed and a chosen hash
History and usage: SKey emerged in the 1990s as an early, widely deployed OTP system, notably used
Advantages and limitations: SKey offers resistance to replay attacks and does not require transmitting a secret