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Date: Tue, 19 Feb 2013 07:36:21 -0700
From: havoc <havoc@...use.ca>
To: discussions@...sword-hashing.net
Subject: Required Output Key

It is important for the submissions to return two values:

(1) A standard hash for verification.
(2) A variable-length encryption key.

If an attacker has access to the hash list, they'll probably have access
to other data in the database (user's name, credit card number,
messages, etc.), and assuming a full compromise, the only way to secure
them is by encryption with a key derived from the user's password.

It's easy to create two different hashes of the single output and use
one as the hash and the other as the key. But if what's standardized
only defines a verification hash output, coders are going to have to
implement that themselves, and they're going to invent all sorts of
wacky and insecure stuff.

Some mistakes I can see happening:

1. Hashing the hash to get the key.
2. Using SHA256(password) as the key.
3. Re-running a different stretching algorithm on the password.
4. Using the hash as the key and a hash of the hash as the hash.
5. Deriving a 32-byte key from the 16-byte output.

... and so on ...

These are obvious mistakes to us, but we are cryptographers, and the
majority of coders using PHC are not. So either PHC should require
separate key/hash output parameters, or we should, in parallel, decide
upon the best scheme for deriving a (key,hash) pair from the single output.

What do you think?

Regards,
-- 
Havoc
(sorry for the nym -- I'll use my real name someday)

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