Crackme
A small program written to be cracked — a legal practice course for anyone learning reverse engineering.
The idea comes out of the 1990s cracking scene: rather than practise on commercial games, crackers wrote puzzles for each other. Typically the program asks for a password or serial number, and the task is to work it out with a disassembler and debugger — or patch the check out entirely. A variant, the keygenme, only counts as solved once you have written your own keygen that produces valid keys.
Difficulty runs from a plain-text comparison to layers of obfuscation, packing and anti-debugging tricks. The collection site crackmes.de closed in the 2010s and was succeeded by crackmes.one. Today crackmes are a staple of CTF competitions and security job interviews — the same skill that once stripped copy protection, now with an employment contract.