How is Plex hash calculated?

General Discussions
1

Hey,

One of my drives fell and while some data is gone, I could recover most of my data successfully. However the directory structure and filenames were lost. I still have all of them indexed in Plex though, so I thought I could just iterate over the http://localhost:32400/library/metadata/<number>/tree?X-Plex-Token=redacted API and that has a file and a hash value which would be perfect for me.

However I couldn’t figure out how is this hash generated exactly. It looks like SHA1 by the size of it, but it’s probably not for the whole file. I only found this thread that mentions what it is, but if I just read the first 4096 bytes, I get a completely different hash, same like the guy in the original post.

If I could figure, that would help me a lot, I wouldn’t have to manually redo my entire library…

Thanks!

2

Some more research: I added some media to Plex, grabbed the hash and wrote a quick program that reads one byte at a time, then calculates the hash and there was seemingly no match… So it can’t be simply just taking SHA1… :cry:

Then to test the theory found in the other post where they claimed Plex only takes the first 4K block, so I used dd on the original file to produce a file with the same beginning, just cut off after 10 MB and the hash was different… So it can’t be just that. Because then the hash would be the same.

3

Try reading 4 KB from the beginning and 4k from the end. Concatenate these and compute the overall hash.

4

I did that when I wrote the post, but missed it out. Thanks for pointing that out. Unfortunately the hash changes every time.
I even went as far as to read 10 MB from the beginning and the end of the file and put it together, but unfortunately that also produced a different hash. So they must rely on something else too. I assume they hash the duration or the filesize (or both) or whatever as well.

5

There are a few steps:

  • Create a string with the filesize in decimal
  • Take the SHA1 hash of the first 64kB (65536 bytes) and append this to the string in hex.
  • If the file is greater than 64kB, do the same with the last 64kB
  • SHA1 hash this string

As an example, let’s look at a copy of Big Buck Bunny:

  • The file size is 928670754 bytes
  • The first 64kB has a SHA1 hash of 87a82ca143a5d84ba4ba33f421f25fbac9811f89
  • The last 64kB has a SHA1 hash of ce2f3dd83c1cc4ffa4deda5588a9118be004ce09
  • Take the SHA1 hash of 92867075487a82ca143a5d84ba4ba33f421f25fbac9811f89ce2f3dd83c1cc4ffa4deda5588a9118be004ce09
  • This is 782e3038c7290470c29320a840e5f92123912e56 which matches the hash column in the media_parts table if you add this exact file to it.
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6

Fantastic. Words can’t express how thankful I am. Here is a simple Python script to hopefully reproduce the algorithm you mentioned: Computes the file hash just how Plex would do it. All credits for the logic go to: https://forums.plex.tv/t/how-is-plex-hash-calculated/904178/5 · GitHub

I could indeed get the correct hash for multiple files back.

PS: Seems like I was close to the solution. But I tried reading 4K and hashed the file size too… :sweat_smile:

7

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