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There are many things wrong with this article. You can't hash secrets like this because hash functions are iterative. You should be using HMAC. Here's a good explanation http://benlog.com/2008/06/19/dont-hash-secrets/

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The article you're referencing speaks about extension attacks. The attack allows you to extend a plaintext and calculate the appropriate hash if you know the original hash and the last parts of the original plaintext. These properties do not apply to password. You could extend the original password and calculate the appropriate hash in theory, but that doesn't help the attacker since the password hash is known and in the database.

As a matter of fact, the article you reference even says so: " “Don’t Hash Secrets” is not always entirely necessary. In the password example, you can hash a password as long as you salt it correctly".

Using a HMAC may be useful since you can add an extra secret that's only known to a separate server or a HSM. The original article describes that in the section titled "Impossible-to-crack Hashes: Keyed Hashes and Password Hashing Hardware".

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