Client Puzzle Protocol (CPP) is a proof-of-concept, in the late 1990s and early 2000s, to defend against denial-of-service (DoS) attacks.
Clients connecting to a server need to solve a mathematical puzzle correctly before a connection is established. Although the puzzle requires negligible computation cost from the client end, it is able to slow down simultaneously establishment of a large number of connections.
A simple but brilliant idea it may seem, it is not widely adopted as it requires the modification of the underlying transport protocol (i.e. TCP).
References
Client Puzzle Protocol, Wikipedia.
Client Puzzle: A Cryptographic Countermeasure Against Connection Depletion Attacks, Ari Juels and John Brainard.
RSA Laboratories Unveils Innovative Countermeasure To Recent “Denial of Service” Hacker Attacks, RSA Lab.
Shared by Emil Tan, Skipper & Co-Founder of Div0.