The throws signature is redundant since method is not re-throwing the BadCodeException.

Futhermore, to avoid NPE, use the expected String as first argument: "needle".equals(haystack[i])

Encryption part was easy, but then I realized that decryption is pretty much simmilar process, so I tried to contitue encrypting till I get the original string.
The problem is that for different inputs, the number of cycles to get original varies and I can't figure out the dependency. For some inputs every 4th iteration equals to input, for other 5, 11, 18, etc.
Could someone advice if I'm going in a right direction or should I just write the decrypt function doing the same in reverse order?

Nice one-liner, but the complexity is crazy because you call IntStream.of(b) a.length times.