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Challenges
These are some of the free sites to practice different skills. Mostly infosec and programming:
Cryptography
- https://cryptopals.com/
- https://cryptohack.org/
- https://github.com/fancy-cryptography/fancy-cryptography
Exploitation
- http://exploit.education/
- https://github.com/shellphish/how2heap
- https://pwn.college/
- https://guyinatuxedo.github.io/