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copied!<p>A cool way to secure all the existing passwords: use the existing hash as the input for the new, and better, password hash.</p> <p>So if your existing hashes are straight MD5s, and you plan on moving to some form of <a href="http://en.wikipedia.org/wiki/PBKDF2" rel="nofollow noreferrer">PBKDF2</a> (or <a href="http://en.wikipedia.org/wiki/Crypt_(Unix)#Blowfish-based_scheme" rel="nofollow noreferrer">bcrypt</a>, or <a href="http://www.tarsnap.com/scrypt.html" rel="nofollow noreferrer">scrypt</a>), then change your password hash to:</p> <pre><code>PBKDF2( MD5( password ) ) </code></pre> <p>You already have the MD5 in your database so all you do is apply PBKDF2 to it.</p> <p>The reason this works well is that the weaknesses of MD5 vs other hashes (e.g. SHA-*) don't affect password use. For example, its collision vulnerabilities are devastating for digital signatures but they <a href="http://en.wikipedia.org/wiki/Chosen-prefix_collision_attack#Attack_scenarios" rel="nofollow noreferrer">don't affect password hashes</a>. Compared to longer hashes MD5 reduces the hash search-space somewhat with its 128-bit output, but this is insignificant compared to the password search space itself which is much much smaller.</p> <p>What makes a password hash strong is slowing down (achieved in PBKDF2 by iterations) and a random, long-enough salt - the initial MD5 doesn't adversely affect either of them.</p> <p>And while you're at it, add a version field to the passwords too.</p> <p><strong>EDIT</strong>: The cryptography StackExchange has an <a href="https://crypto.stackexchange.com/questions/2945/is-this-password-migration-strategy-secure">interesting discussion</a> on this method.</p>

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