What do people mean when they talk about forward secrecy in ciphers? I've heard several people and websites talk about ciphers that have forward secrecy. I know about forward secrecy in key agreement protocols, e.g. RLWE, (EC/SI)DH, QKD, etc. This could be what they're talking about and it's a simple case of misused terminology (I've mostly heard it in articles about cipher suites), but I've also heard this used in the context of hash based signature algorithms so I'm wondering if people are using a special variation of a stream cipher or some other method.
What does 'forward secrecy' mean in the context of hash based signatures?


Well, what that term means is that we have a hash based signature scheme with an evolving key, that is, the private key is constantly being updated, and on a state compromise (at state $N$), you cannot get the private keys used prior to state $N$.

I personally don't see that as a good use of the term (it's feels like 'perfect reverse security' would be more apt terminology, as it's discussing the security of the system prior to compromise).  However, that's what people mean by that terminology.