Challenges from https://cryptopals.com/
link: https://cryptopals.com/sets/1/challenges/1
The string:
49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d
Should product:
SSdtIGtpbGxpbmcgeW91ciBicmFpbiBsaWtlIGEgcG9pc29ub3VzIG11c2hyb29t
Always operate on raw bytes, never on encoded strings. Only use hex and base64 for pretty-printing.