Zstandard inside zip, PKWARE method 20

[klauspost]

Seems like PKWARE decided to use method 20 for zstd inside zip: https://pkware.cachefly.net/webdocs/APPNOTE/APPNOTE-6.3.7.TXT

Because why use the value everyone else use?

Either way. It seems like it should be trivial to add that as a supported decompression method.

Here is a sample zip: zstdzip.zip. file1.txt is WinZip (method 93) and file2.txt is method 20, PKWARE.

I have been unable to download PKWARE zip software, so unfortunately I have been unable to confirm that this is the correct method. This is from my own library.

[mcmilk]

I had written a email to both, and method id93 was the result in the end, see this issue: #132 (comment)

If you take a look into the next pkware versions, the notes to zstd changes: https://pkware.cachefly.net/webdocs/APPNOTE/APPNOTE-6.3.8.TXT

But of cause, we could add method id 20 for decompression only ... will you create an PR or should I do in the next release ?

[klauspost]

@mcmilk Yeah, decompression only would be neat.

I don't have a dev env set up currently, so I can't submit a PR (that I've tested anyway). Sorry for that.

[mcmilk]

I will do it into the next release...

[ghost]

In 21.02 Alpha, Igor has added both IDs (20 and 93).

• 20: ZstdPk
• 93: ZstdWz

This means that 7-Zip 21.02 can recognize ZIP entries which use Zstd, but since upstream 7-Zip does not support Zstd, it cannot decompress them.

[mcmilk]

I have added decompression of id=20 ... testing needs to be done, but I think it will work with 21.02 release.

[mcmilk]

Works