I have an RSA private key (more specifically, an SSH key in Putty format, 4096bit) and want to use its SHA256 hash as an AES256 key, instead of using an independent newly generated AES key.
Are there any reasons why doing this might be a (very) bad idea?
In terms of entropy etc., it should be still sufficiently secure for real life (I think?). But are there any pitfalls, like certain properties of good RSA keys that would make the AES key much worse / easy to break (than one directly made by a properly used CSPRNG)?
Both keys are meant to protect the same thing, ie. if an attacker can get one, deriving the other doesn't make the situation worse.
(This not not an XY problem. And I know that this is not the usual use case.)