Great, but SynthID watermarking isn't a cryptographically-secure scheme, nor could one be made, because all the things necessary for the scheme to be crypto-secure are also things that go against being human-comprehensible.

The SynthID watermark scheme relies on the LLM's own logits being a map into acceptable substitutions for specific words. For example, if we have the sentence "The quick brown fox jumps over the lazy dog", the LLM might give the last word[0] as 98.5% "dog", 1% "hound", 0.5% "canine". In normal decoding we sample the logits after scaling them against a temperature parameter; in SynthID we additionally bias the logits to encode output from the watermark CSPRNG - e.g. the CSPRNG might decide to boost the third logit of this particular token to 65%.

But wait! The LLM might have selected valid synonyms for the token, but those synonyms will never be correct because this is a very well-known sentence. To humans, it's one sentence-long token, in the sense that most English-speaking humans who encounter this sentence will see the first few words and already know the ending. It thus has one bit of entropy[1]. Adding SynthID watermarks to this sentence necessarily has to change the words and thus adds more information - namely, that some process outside of the LLM itself has modified the text.

We don't need to cryptographically distinguish AES from random if both the AES (SynthID) and random (normal decoding) happens to also be constrained to form a valid English sentence.

[0] I am too lazy to account for multi-token words. Let's just assume the LLM was trained with a pretokenizer that splits exclusively on whitespace and every word in the training set got assigned a token.

[1] Compressible information