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I've read the usenix paper, and I'm reasonably sure I'm right. Care to elaborate on what I'm mistaken about? I (and a few other people I've talked to about schemes isomorphic to this about) may learn something new :-D

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OK, so you can chain bcrypt in theory, but if you look at the API and how it stores rounds/salt, I don't think you'll enjoy implementing the code to do so. You can't just store hash_b, you'll need to store hash_a too. It's not worth it.

Basically, the assumption that bcrypt is just a function that returns a hash oversimplifies the detail of how salt is stored in practice.

I wrote a comment to the effect of "you can just chain bcrypt" but deleted it because I didn't want to wade into exactly this tarpit. If a client asked me how to upgrade the cost of their password hashes, I'd probably tell them to validate incoming plaintext and then create a fresh hash.

I agree: this is not a big deal in practice. bcrypt with a cost factor of 10 is so much slower than SHA1 that this is just not going to be an issue.

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