Classical ciphers fail to a child with patience. Modern ones lose to mathematics only.
Caesar shifts every letter by the same amount — the keyspace is 26. DES, AES, RSA and ECC keys are so large that brute force never finishes. Try the Caesar shift below, then move on to the real ciphers.
"Attack at dawn" → Caesar shift 3 → "Dwwdfn dw gdzq" — keys of 26 shifts break in seconds. DES gives 2^56 keys, AES 2^128.