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AES EncryptionMethod

Oct 28 '15 at 17:50


I can't reconcile the note at with what I'm seeing. I've encrypted a zip file with WinZip 19.5 and 256-bit AES. Then I open the zip with Chilkat Zip and see the first entry to have a CompressionMethod of 8 in the debugger rather than 99 as described in the linked documentation. It works the same for 128-bit AES.

What am I missing?


You are seeing the actual compression method 8 (the deflate algorithm). When a zip entry is AES encrypted, the AES encryption is indicated by compression method 99 in the local file header (and central directory entry), and the actual compression method is found in an "extra data" field that follows each header. The 8 that you see is the actual compression method from the "extra data" field.