7-zip files on Linux with AES-256 encryption

I have a requirement to zip up some files in Linux with 7zip using AES-256 encryption. The end user will be opening in Windows with 7zip. I'm using the following command:

7za a myfile.zip test.txt -tzip -mem=AES256 -mx9

The problem is that when I open the zip archive in Windows, the Method shows "Deflate" and does not say anything about AES-256.

How can I (and more importantly the end user) be sure that its using AES-256 as the encryption method?

Here is the doc on how to set the encryption method, which I believe I've set correctly.

Linux 7za version:

me@myhost~> 7za -version
7-Zip (A) [64] 9.20 Copyright (c) 1999-2010 Igor Pavlov 2010-11-18
p7zip Version 9.20 (locale=en_US.UTF-8,Utf16=on,HugeFiles=on,4 CPUs)

The Linux host is SLES 11 SP2, and for Windows I'm on Windows 7.

2 Answers

7-Zip will refuse to encrypt the data unless you provide a password, since it won't know what key you want to encrypt it with:

7za a myfile.zip test.txt -tzip -mem=AES256 -mx9 -pmypassword

How can I (and more importantly the end user) be sure that its using AES-256 as the encryption method?

You can use 7z l -slt myfile.zip, it should output AES-256 (amongst other things).

If you choose the 7z format (e.g., with 7za a -t7z myfile.7z test.txt -tzip -mem=AES256 -mx9 -pmypassword), 7z l -slt myfile.7z should output 7zAES:19 (which is the same as AES-256).

:

Igor Pavlov: 7zAES:19 means AES-256 + (2^19) SHA-256 iterations in password to key function.