"Recalculate" SHA512+salt string to BLOWFISH+salt - is this possible?

Question

Maybe this is a stupid question, but I wouldn't be shocked if some excellent brains come around with a proper solution or an idea: Is it possible to recalculate/transcode a salted sha512 string into a salted blowfish string ?

The (imo quite interesting) background is: I have a big database of SHA512+salt strings like that $6$rounds=5000$usesomesillystri$D4IrlXatmP7rx3P3InaxBeoomnAihCKREY4... (118 chars) and want to move to another hash/salt algorithm, generating strings like $2a$07$usesomesillystringfore2uDLvp1Ii2e./U9C8sBjqp8I90dH6hi (60 chars).

I'm intentionally NOT asking this on security.stackexchange.com as this is not a security question. It's about transcoding/recalculation.

Answer 1

Is it possible to recalculate/transcode a salted sha512 string into a salted blowfish string ? Nope.

SHA2-512 is a cryptographic hash. Data goes in, but there's no way to get it back out. Do note that the thing you're using is a proposed but not standardized form of crypt that uses SHA2, and is not a raw SHA2 hash.

bcrypt (which is derived from, but is not Blowfish) is a key derivation function, which while a different thing than a cryptographic hash, still has the same result: data goes in, but there's no way to get it back out.

There is no way to simply convert one of these password hash types to another. This is true of almost every hash type. If you need to change the hash type, do so when the user next logs in.