**Version History: **

Earlier versions: May 2017: ECCC TR 17-084

Dec. 2017: ECCC TR 17-084 (revised)

Computational analogues of information-theoretic notions have given rise to some of the most interesting phenomena in the theory of computation. For example, *computational indistinguishability*, Goldwasser and Micali [9], which is the computational analogue of statistical distance, enabled the bypassing of Shannon’s impossibility results on perfectly secure encryption, and provided the basis for the computational theory of pseudorandomness. *Pseudoentropy*, Håstad, Impagliazzo, Levin, and Luby [17], a computational analogue of entropy, was the key to the fundamental result establishing the equivalence of pseudorandom generators and one-way functions, and has become a basic concept in complexity theory and cryptography.

This tutorial discusses two rather recent computational notions of entropy, both of which can be easily found in any one-way function, the most basic cryptographic primitive. The first notion is *next-block pseudoentropy*, Haitner, Reingold, and Vadhan [14], a refinement of pseudoentropy that enables simpler and more ecient construction of pseudorandom generators. The second is* inaccessible entropy*, Haitner, Reingold, Vadhan, andWee [11], which relates to *unforgeability* and is used to construct simpler and more efficient universal one-way hash functions and statistically hiding commitments.