Why do LLM keys all start with sk?

I assume it's loose convention intended to identify the string as "Secret Key", as a hint to the humans who might be moving the data around, and maybe to make it easier to write precautionary code like "hey it looks like your commit has a secret key in it, are you sure you want to push it?"

I think Stripe started it long ago with "pk_" and "sk_" prefixes for public key and secret key.