Just want to be sure about something before I sign off on a method we're about to put into place.
We have a secret, to which a user-defined key is appended. The user can see the SHA256 hash of the secret.
The secret and the user-key is then hashed with SHA512 to obtain results.
Given the ending hash is SHA512, and the user only knows the SHA256, a hash length extension attack would be impossible, correct? As there is no way to convert a 256 to 512, or to backtrack the 256?
From every angle I can see it, it shouldn't be possible. But want to confirm with those more well versed than I. Thanks.