Most password hashes have a cost parameter that indicates how long the algorithm should take. Is there an algorithm where you can increase that cost for a particular hash, without access to the plaintext password?
So I have existing hashes in the database with cost=10, and I want to upgrade these to cost=20, without access to the plaintext password.
This isn't possible for PBKDF2 and bcrypt, because these use the password in each iteration. Is there any algorithm that supports this? Is there a specific name for this property?