Non-repudiation isn't a part of cryptography? According to the &quot;Handbook of Applied Cryptography&quot; by Alfred J. Menezes, Paul C. van Oorschot, and Scott A. Vanstone (CRC Press, 1997; p.4):

Cryptography is the study of mathematical techniques related to
aspects of information security such as confidentiality, data
integrity, entity authentication, and data origin authentication.

My questions are: Why doesn't non-repudiation appear in that definition? Is it less important? Isn't non-repudiation one of the aspects of information security?
That is clearly a simplified definition, as cryptography encompasses a large number of ideas. It is likely for the same reason that they do not mention zero-knowledge proofs or proof-of-work systems, despite those very often falling squarely in the relm of cryptography.