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Code Conversion Language (2019)
Update: I forgot that I did a brief analysis on this many years ago, using ROT13 as example. Update: Noam Postavsky pointed out on #emacs that CCL is not turing-complete after all as a full simulation (as opposed to just interpreting a single line) requires an Emacs Lisp loop.
A lesser-known fact is that there’s two more byte-code interpreters in its C sources, one for compiled regular expressions and another designed for encoding and decoding text, known as Code Conversion Language (CCL). A text can be encrypted by replacing each character with the result of XORing it against a secret byte, similarly it can be decrypted by applying the same transformation again. This example shows a few more interesting things, generating text of unknown length is rather hard, so I’m using the standard magnification factor of 1 and estimate how big the buffer will be to create an appropriately sized input string.
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