Claude 3.5 Sonnet can natively speak double base64 encoded English. And I do mean it - you can double b64 encode something, send to it, and it'll respond as if it was normal English. Obviously base64 is a simpler transformation than rot13, but no GPT models can deal with double b64.
it appears that openai's gpt-4 model can speak base64 as well. I jumped to your comment seeing if anyone else had tried it following the OP. double b64 I didn't try, but that is interesting.
> $ ask4 ' what does dGhhdCBpcyBxdWl0ZSBpbnRlcmVzdGluZw== decode to? '
> A "dGhhdCBpcyBxdWl0ZSBpbnRlcmVzdGluZw==" is a Base64 encoded string. When decoded, It translates to "that is quite interesting" in English.
> Obviously base64 is a simpler transformation than rot13
Is it? It’s probably more obscuring from an LLM’s perspective, assuming the LLM has seen enough rot13 text during training. Spaces and punctuation are untouched by rot13, unlike base64, which means that word and sentence boundaries will still be denoted by tokens that denote those boundaries in plaintext.