Abstract: An authentication system that uses a multi-use long string authentication key to authenticate client device access to protected resources, is presented. The authentication system is based on a shared knowledge of long string authentication key. The authentication key is used as a platform to derive digital signatures for access to protected resources. One or more authentication parameters can be used in combination with the authentication key to derive or validate digital signatures. The one or more authentication parameters can include a key index parameter, a key offset parameter, and a key length parameter. The digital signatures derived from authentication keys can be used to control access to various types of protected resources such as, digital house, a vehicle fob key, a remote garage door opener, a hotel room card key, credit or debit cards magnetic strip or chip, online financial accounts, computer or control systems, or website authentication.