Stash is an iOS application fulfilling the client-side responsibilities in Steve Gibson’s SQRL (Secure, Quick, Reliable Login) authentication model.
SQRL is an authentication model which can be substituted for (or used alongside) the traditional username and password authentication paradigm. In the traditional model, a user will provide their username and password to a service where it is then validated against a copy stored by the service. SQRL removes the requirement for a service to hold a copy of a user's secret (their password); responsibility for keeping that secret safe is now solely the responsibility of the user. The service, in the SQRL model, only needs to store a 256-bit unique identifier - which does not compromise the user's security if the service’s security is breached. A thorough and more articulate description, including a cryptographic architecture and protocol specification can be found at https://www.grc.com/sqrl/sqrl.htm.