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POPassword salts: prepending vs. appending
 

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    <p>I just looked at the implementation of password hashing in Django and noticed <a href="http://code.djangoproject.com/browser/django/trunk/django/contrib/auth/models.py#L16" rel="noreferrer">that it prepends the salt</a>, so the hash is created like <code>sha1(salt + password)</code>, for example.</p> <p>In my opinion, salts are good for two purposes</p> <ol> <li><p>Preventing rainbow table lookups</p> <p>Alright, prepending/appending the salt doesn't really make a difference for rainbow tables.</p></li> <li><p><strong>Hardening against brute-force/dictionary attacks</strong></p> <p><em>This is what my question is about.</em> If someone wants to attack a single password from a stolen password database, he needs to try a lot of passwords (e.g. dictionary words or [A-Za-z0-9] permutations).</p> <p>Let's assume my password is "abcdef", the salt is "salt" and the attacker tries all [a-z]{6} passwords.</p> <p>With a prepended salt, one must calculate <code>hash("salt")</code>, store the hash algorithm's state and then go on from that point for each permutation. That is, going through all permutations would take 26^6 copy-hash-algorithm's-state-struct operations and 26^6 <code>hash(permutation of [a-z]{6})</code> operations. As copying the hash algorithm's state is freakin fast, the salt hardly adds any complexity here, no matter how long it is.</p> <p>But, with an appended salt, the attacker must calculate <code>hash(permutation of [a-z]{6} + salt)</code> for each permutation, leading to 26^10 hash operations. So obviously, <em>appending</em> salts adds complexity depending on the salt length.</p></li> </ol> <p>I don't believe this is for historical reasons because Django is rather new. So what's the sense in <em>prepending</em> salts?</p>
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