Hello and good day.
Im trying to resolve the ctf challenges in root me website but im confused: any challenge of any section of the challenges section in the web is valid? i can choose any challenge from the web? this include the cryptanalisiys section? thanks.
Yes, any challenge from root-me is valid, as long as it fits the criteria in the documentation.
I couldn’t find the exact reference on the docs, when in the past we make a big clarification on the type of challenge allowed on every site:
if it requires technical understanding, using software tools, etc: is allowed,
if not, is not allowed.
For example, mathematical exercises, riddles, etc, are not allowed. Seems like crypto challenges are perfectly fine, unless you could crack it in your mind 
@infinite-loop, do you know where is the reference in the technical challenge doc?