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I'm writing a paper for uni about password security, specifically about cracking passwords in the context of a password manager. I've coded a password encryption scheme which uses PBKDF2(SHA512) to hash a master password into a key that AES256 uses to encrypt a password database or vault.

I'm trying to estimate the time to crack a password with this encryption scheme via a brute force attack guessing passwords until the right password is found. Assume the salt is known to the attacker and there is no pepper. As part of my calculation I need the PBKDF2 hashrate for a top of the line GPU like the RTX 4090 with the following paramaters:

PBKDF2:

-Hashing algorithm: SHA512
-Iteration count: 600.000
-Output length: 32 bytes -Salt length : 12 bytes

I'm having trouble finding the hashrate in research papers and don't have an RTX 4090 to benchmark it myself. If anyone knows where I can get this information that would be much appreciated :).

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    $\begingroup$ Googling "Hashcat benchmark RTX 4090" returned this where these is one entry with PBKDF2-SHA512, albeit with a stated 1023 iteration count. Increasing the iteration would decrease rate at most (and perhaps about) linearly. Your Mileage May Vary a lot according to the quality of the implementation, and how the AES part of the test of a candidate password is handled. $\endgroup$
    – fgrieu
    Commented Aug 29, 2023 at 14:05
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    $\begingroup$ @fgrieu Not sure about that part on AES. Generally you can have a pretty good guess if the key is correct by decrypting a few blocks, and then you can decrypt more or the entire package if the key is likely correct. For e.g. AES-GCM you'd just decrypt with AES-CTR of course. $\endgroup$
    – Maarten Bodewes
    Commented Aug 29, 2023 at 15:54
  • $\begingroup$ I've timed the AES(GCM) verification step and I'm treating it as negligible, and the implementation is Microsoft's .NET 7 System.Security.Cryptography.dll. Could you be more specific on how increasing the iteration count slows down the hashrate linearly. $\endgroup$
    – Luka Gecko
    Commented Aug 29, 2023 at 17:09

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From this source I extract

Short Benchmark for the RTX 4090
CUDA API (CUDA 11.8)
* Device #1: NVIDIA GeForce RTX 4090, 23867/24252 MB, 128MCU

Benchmark relevant options:
* --optimized-kernel-enable
* --workload-profile=4

* Hash-Mode 1700 (SHA2-512)
Speed.#1.........:  7425.2 MH/s (288.57ms) @ Accel:64 Loops:1024 Thr:256 Vec:1

* Hash-Mode 7100 (macOS v10.8+ (PBKDF2-SHA512)) [Iterations: 1023]
Speed.#1.........:  2825.7 kH/s (221.31ms) @ Accel:32 Loops:511 Thr:512 Vec:1

The vastly different order of magnitude shows that in 7425.2 MH/sand 2825.7 kH/s, the H stands for many more SHA-512 in the second case.

By definition of PBKDF2, the iteration count is the number of iterations of a Pseudo Random Function that in practice is HMAC with the specified hash, here SHA-512, which (for the sizes considered) uses two SHA-512 round functions. Hence one PBKDF2-SHA512 with $1023$ iterations is expected to perform $2046$ SHA-512 round functions, when one SHA2-512 probably makes $1$. This is roughly consistent with the $2627$ times lower H/s value reported for the second data point.

In the computation of PBKDF2 with more than a few rounds, assuming ideal parallelization, the average time should be of the form $(u\,c+v)n$ where $c$ is the iteration count, $n$ is the (assumed large) number of PBKDF2 evaluations, and $u$, $v$ are some positive constants in seconds. Roughly, $u$ is an overhead per PBKDF2 evaluation, and $v$ is an extra time per PRF evaluation, all assuming ideal parallelization. It follows that there are $1/(u\,c+v)$ PBKDF2 evaluations per second.

We can't tell $u$ or $v$ exactly, but knowing that they are positive is enough to extrapolate a minimum PBKDF2 rate for $c=600000$ iterations from the one for $c=1023$: that should be at least $2825700\times1023/600000=4817$ PBKDF2 per second. In practice, I believe this won't be far off, because hashcat is a reputable program, thus well optimized, thus hopefully, even for $c=1023$, $u\,c\gg v$.

I've timed the AES(GCM) verification step and I'm treating it as negligible, and the implementation is (some library on the PC's CPU)

I trust that when PBKDF2 is on the PC's CPU, the AES-GCM verification step uses a negligible fraction of the CPU in the password cracking effort, assuming the (unspecified) amount of data entering AES-GCM is moderate. However, that might not stand when the PBKDF2 part is GPU-accelerated.

  • If we use the same AES-GCM code, will it be able to perform the (vastly higher) $\approx4817$ AES-GCM tests per second required so that it's not the bottleneck? Also, I can't tell for sure how much having to export the results of PBKDF2 to the PC's memory (rather than checking everything in the GPU as I imagine the benchmarked code does) will slow down the GPU code, and if making the necessary change requires a day or a month of experience writing GPU code.
  • If we move the AES-GCM code to the GPU, that's a significant change in the GPU code, and I can't forecast how efficient that will be, or even if that will be faster than the previous option.
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  • $\begingroup$ Excellently argued and presented, thank you very much. You make a very good point regarding exporting PBKDF2's results to memory being a bottleneck. If I get the time I'll run some tests with this in mind. One love :) $\endgroup$
    – Luka Gecko
    Commented Aug 29, 2023 at 23:15

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