Cryptography yes, but are you sure about parsers? As far as I can tell, there's some kind of U-curve there. Beginners code them by hand, intermediate-level programmers and intermediate-scope projects use parser generators, and people maintaining the most sophisticated parsers prefer to code them by hand too. For example, GCC used to have a bison parser, but they switched to a hand-coded recursive descent one because that let them produce more helpful error messages. Clang uses recursive descent too.