I decoded that as "Hello HI!" using basic cryptanalysis, the assumption that the alphabet would be mostly contiguous, the assumption that capital and lower-case

I decoded that as "Hello HI!" using basic cryptanalysis, the assumption that the alphabet would be mostly contiguous, the assumption that capital and lower-case are separated by a bit, and the knowledge that 0x20 is space and 0x21 is exclamation mark. On a larger text, we wouldn't even need these assumptions: cryptanalysis is sufficiently-powerful, and could even reverse-engineer EBCDIC! (Except, it might be difficult to figure out where the punctuation characters go, without some unambiguous reference such as C source code: commas and question marks are easy, but .![]{} are harder.) Edit: I can't count. H and I are consecutive in the alphabet, and it actually says "Hello HN!". I think my general point is valid, though.