Repairing corrupt .aac (Advanced Audio Coding) files

By | November 24, 2020

Corrupt .aac files

Although no advertised service I received an email from someone asking if I could repair .aac files. Until I try I can not answer yes or no. My interest was further sparked by the fact the extensions for the files changed to .kasp which suggests files were corrupted by ransomware. What helps is having some known good files created by the same device/recorder.

Tinkering with a hex editor

With the help of the HxD hex editor, various descriptions of the AAC file format found on the internet, the reference file I started tinkering and managed to repair one file. I sent the file to client, accompanied by a quote for repair of the rest of the files. Client agreed and all files are now repaired and playable.

repairing .aac files using HxD

What also helped in this case that these were mostly larger files, with audio spanning some times over an hour. As the ransomware in question encrypts part of the data (no decryptor available), this portion of the file is lost. Rough calculation, a 15 MB file was good for 34 minutes of audio, say 440 KB per minute audio. The amount lost due to the ransomware is 150 KB (encrypted data), so only a fraction of all audio is lost.

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