On a whim I thought I’d try getting ChatGPT to do a bit of steganography for me. There are a bazillion ways (give or take) for hiding a secret message in unrelated cleartext, where there’s a trade-off of secret bandwidth against cleartext flexibility. I chose Morse code encoded in the last letter of each word, because it’s obvious and easy to express as rules that anybody can follow.

Anybody except for ChatGPT, it turns out.

The rules I gave were simple enough:

  • A word ending with a vowel represents a dot.
  • A word ending with a consonant represents a dash.
  • A word ending with a y represents the gap between letters.
  • Express a message in morse code, using the above substitutions of words for symbols.
  • The words should be chosen to form coherent sentences.

That seemed to leave plenty of freedom for choosing words.

It turns out schemes like these are called acrostics or telestichs (the latter in my case). The extra layer of using morse code and groups of letters helps to make it less obvious than traditional acrostics, but it takes several words to make a letter of the hidden message.

I thought an LLM should be able to churn out a coherent paragraph under those constraints, and I’m sure that it could if it could remember what it was supposed to do, but I had some difficulties.

For the secret message example, ChatGPT gave me:

Alone Henry left a trail then slept quietly, and followed slowly too.

I don’t know what that means or why it wrote it, but it decodes as:

. -.--- -- .

Which comes back out as E�ME. That’s not quite right.

Let’s try fixing it by hand:

Alone, Henry left the tree. Then sleepily he sat. Slowly following on my horse. What could he say? The adverbs are too many to try.

. -..- .- -- .--. .-.. .

There. Fixed. But that wasn’t as much fun as I thought it would be. The adverbs are indeed too many. While there are plenty of words ending in y to choose from, it gets hard to think of things that aren’t adverbs or adjectives. And that need comes up too frequently.

Obviously having only words ending in one letter to choose from for that marker is too restrictive, and y is an especially distracting choice. Some markers could simply be dropped because there are a lot of cases where it’s not ambiguous, but unambiguous cases are at the end of infrequent letters, where it’s frustrating already.

I won’t try to fix this because it’s not my priority. All I wanted was something with the simplicity of a children’s game. Unfortunately I feel like it’s a bit too tedious for many kids to encode their own messages. At least without the support of an editor or thesaurus to offer up practical synonyms.

Or, of course, one could just make an LLM do it, as was the original plan. I’m sure it should have no trouble if it can be put in a suitable wrapper to keep it on track. But I’m too lazy, which is why I went to ChatGPT in the first place.

But there are many related schemes would could be devised, and I think technology is in a place, right now, where it should be trivial to automate. I just can’t be bothered doing that.