Michal Stanek over at Twitter did a quick look at the crypto behind #7zip file encryption. Not entirely surprisingly, what he found wasn't pleasant:
He didn't check how they derive the encryption key from your password, so I took a quick look. Apparently, #7zip uses PBKDF2-HMAC-SHA1 with 1000 iterations (hardcoded). In other words, even with the rest of it all implemented flawlessly you better choose a damn strong password if file encryption should be of any use.
Actually, 1000 iterations is exactly the value used by Brian Gladman in http://archive.li/RhBVp which they refer to in the code. A value chosen in 2002. Like, seventeen years ago. Why should anybody adjust it to newer recommendations or hardware advancements?
Ok, apparently I was only looking at WinZip compat code. The code responsible for encrypting 7z archives uses a different approach: SHA-256 hash applied 524,288 times. A lot better but still not exactly an optimal choice either.
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