This is a list of resources and challenges to learn and practice cryptography and cryptanalysis for CTF's.
Cryptohack.org (start from Introduction)
Cryptopals.com (start from set 1)
For math based practise, projecteuler.org
Join the official discord channel of cryptohack and picoctf and do not hesitate to ask doubts there or to us
Challs and writeups from picoctf crypto, writeups easily available
beginner friendly challs :
CyberApocalypse 2021 :
PhaseStream 1,2,3 and 4
https://blog.cryptohack.org/cyber-apocalypse-2021