- Matasano crypto challenges: <a href="http:&#x2F;&#x2F;cryptopals.com&#x2F;" rel="nofollow">http:&#x2F;&#x2F;cryptopals.com&#x2F;</a><p>- <a href="https:&#x2F;&#x2F;www.crypto101.io&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.crypto101.io&#x2F;</a> both the presentation and the book.<p>I think those are good to begin with.