[tptacek]

I'm sorry. You remain wrong. Predictable IVs in CBC mode are a vulnerability. A low-severity vulnerability; so are most CSRFs. We don't generally write dozens of paragraphs about how irresponsible and clueless someone is when they report a sev:lo CSRF. In fact, what a lot of companies do for that is pay a bounty. (I know, not the fact pattern here).

You're also missing pretty much the whole conversation about why predictable IVs in ZIP's CBC is a vulnerability. Yes, as lots of people have pointed out, in the 95% use case of 7z as a simple file format, the IV doesn't matter. You can't use the flaw to decrypt someone else's 7z file.

But 7z is a file format. Applications build on top of those. Plenty of applications generate zip files. If one of them, for whatever reason, chose to generate 7z zip files with 7z's code, this sev:lo vulnerability stops being sev:lo. A predictable IV plus known prefixes turns CBC mode into ECB mode. If this person had reported 7z using ECB mode, nobody would be question the competence of the report. But they reported a more sophisticated flaw. It flies over people's heads. They claim it's bogus. You can see my issue here.

The world does not need more encouragement to write bad crypto code. We're up to our ears in it. But the world definitely needs more people to do the harder job of looking through the bad crypto and mapping out the pitfalls. Thats's what this person did. I'm fine if you don't want to thank him, I'm even fine if you just want to debate whether they reported it the best possible way (I'll disagree but we disagree on lots of things).

What I will not let anyone get away with is claiming that the finding itself is incompetent because predictable IVs are not a bug. That's not just wrong, it's theatrically wrong. I don't care if you apologize about slagging this person for reporting something. But I think you should apologize for claiming, falsely, that they were incompetent. I'm kind of a little surprised you haven't already!