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@Gen_ruikt
The BOSS Training Division page/Resources section has pretty much everything I can recommend as well!
Other than that, you might find it useful to write up some python (or your preferred programming language) scripts to decipher the most frequently appeared challenges by looking at past years archive. Always keep an eye out for modifications/variations to known ciphers in the later challenges. But most importantly, enjoy the process of codebreaking and the story! Hope that helps!