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POPasswordDeriveBytes vs Rfc2898DeriveBytes, Obsolete but way faster
 

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    <p>I'm working on a encryption functionality based on classes inherited from SymmetricAlgorithm such as TripleDes, DES, etc.</p> <p>Basically there're two options to generate consistent key and IV for my algorithm class, <code>PasswordDeriveBytes</code> and <code>Rfc2898DeriveBytes</code>, both inherit from DeriveBytes abstract class.</p> <p>The <code>PasswordDeriveBytes.GetBytes()</code> method is marked as obsolete in .NET framework while Rfc2898DeriveBytes.GetBytes() is recommended, as it matches the PBKDF2 standard. However, based on my testing, calling the same <code>GetBytes()</code> method in Rfc2898DeriveBytes class is almost 15 times slower than that in <code>PasswordDeriveBytes</code> class, which leads to unexpected CPU usage (always higher than 50%).</p> <p>Here're some testing data:</p> <ul> <li>Iterations: 100</li> <li>Algorithm type: DES</li> <li>Original Text: "I'm a test key, encrypt me please" </li> <li>Time: <ul> <li>PasswordDeriveBytes: 99ms</li> <li>Rfc2898DeriveBytes: 1,373ms</li> </ul></li> </ul> <p>Based on the testing, the bad performance of <code>Rfc2898DeriveBytes</code> is not acceptable in production environment. </p> <p>Has anyone noticed this problem before? Any solution I can still use a standard one without hitting the performance? Any risk to use an obsolete method (could be removed in future version)?</p> <p>Thanks guys!</p> <p><strong>Edit:</strong></p> <p>Probably I found where the problem is... The default iteration count number for <code>PasswordDeriveBytes</code> is 100, while for <code>Rfc2898DeriveBytes</code> is 1000. After I changed them to the same number as 1000, executing <code>Rfc2898DeriveBytes</code> is only double time.</p>
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    1. COHow often are you going to be deriving keys in a production environment? And, regarding your timing data, when you said "100 iterations" - is that iterations on the onee key, or did you generate 100 keys. Any perf data based on 100 trials is suspect, but I think you actually tested ONE trial. As in all perf analysis cases, it is simly not appropriate to draw conclusions about server performance based on the response time of a single trial.
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    2. CO@Cheeso The test was just a unit test of the performance for these two classes and it was not done in a real app. The "100 iterations" I mentioned was a little bit confusing, that only means I executed each of them 100 times. That's not a real perf testing but just a comparison.
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    3. COI think you may have missed the point `Rfc2898DeriveBytes` is fundamentally _designed_ to be slow so that password hash checks (done per log-on and hence fairly infrequently) don't notice the performance hit while brute force attacks do. If you need to generate loads of hashes `Rfc2898DeriveBytes` isn't for you, but if you need some security from brute force attacks it is.
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