Foundations, Properties, and Security Applications of Puzzles: A Survey
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Cryptographic algorithms have been used not only to create robust ciphertexts, but also to generate cryptograms that, contrary to the classic goal of cryptography, are meant to be broken. These cryptograms, generally called puzzles, required the use of a certain amount of resources to find the solution, introducing a time delay and, in some cases, an economic cost. These powerful features have made puzzles the core of many security protocols, giving them increasingly importance in the IT security landscape. The concept of puzzle has subsequently been extended to other types of schemes that do not use cryptographic functions, like CAPTCHAs, used to discriminate humans from machines, and has experienced a renewed interest with the advent of Bitcoin. In this paper, we provide a comprehensive study of the most important puzzle construction schemes present in the literature, categorizing them according to several attributes, such as resource type, verification type, and applications. We have redefined the term puzzle by collecting and integrating the scattered notions used in different works, to cover all the existing applications. Finally, we provide an overview of the possible applications, identifying key requirements and different design approaches. Then, we highlight features and limitations of each approach, providing a useful guide for the future development of new puzzle schemes.
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