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Disabling scanning of PPMd blocks in RAR archives

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Summary

When recovering the password for a large RAR archive, Accent RAR Password Recovery can reduce the high load on the computer’s CPU and RAM and reduce idling by high-performance GPUs, thus speeding up password brute force attempts.

This effect is achieved by disabling resource-intensive scanning of PPMd blocks in the archive.

IMPORTANT: If scanning of PPMd blocks is disabled, RAR archives that have been created with the mandatory Text Compression option enabled will not be found.

Problem

This is what happens when recovering a RAR password from the standpoint of the GPU, CPU, and RAM of a computer:

  1. The CPU prepares a batch of passwords to check and sends them to the GPU
  2. The GPU transforms the password batch into encryption keys of a fixed length and returns them to the CPU
  3. The CPU tries in sequence to decrypt the RAR archive with each of these keys, unpacking the archive into RAM and checking the validity of the files in the archive that are obtained by applying a given key

When handling large archives on a computer with a fast GPU and a slow CPU and RAM, it happens that the GPU is ready to process the next set of passwords but the CPU is still busy checking the encryption keys that the GPU sent for the last round and is unable to prepare the next set for the GPU.

In a case like this, reducing the CPU load when checking encryption keys can increase responsiveness to GPU queries and reduce – or even eliminate – waiting time.

Solution with AccentRPR

When a RAR archive is selected in Accent RAR Password Recovery, the task creation wizard opens, in which you select whether to check PPMd blocks in the archive

Include or disable the RAR PPMd blocks validationDisabling PPMd scanning slashes the CPU and RAM load when working with large files.

How PPMd blocks appear in a RAR archive

PPMd blocks in a RAR archive are the results of the PPM algorithm.

In WinRar, PPM is used to compress text; it is disabled by default. Therefore most RAR archives do not contain PPMd blocks – meaning that many password crackers do not support scanning these files and cannot find their passwords.

You can enable text compression in the archive settings, during archive creation:

WinRAR->Add->Advanced->Compression->Force Text CopressionConclusion

Unlike many competing products, Accent RAR Password Recovery checks the whole gamut, from LZ blocks to PPMd blocks of RAR archives – scanning of these can be safely skipped when handling large, potentially system-choking RAR files.

 
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