On Tue, Sep 20, 2016 at 10:23:20AM -0400, Theodore Ts'o wrote: > On Tue, Sep 20, 2016 at 03:15:19AM -0800, Kent Overstreet wrote: > > Not on the list or I would've replied directly, but on Haswell, ChaCha20 (in > > software) is over 2x as fast as AES (in hardware), at realistic (for a > > filesystem) block sizes: > > On Skylake and Broadwell processors, AES is faster (the posting is > from a ChaCha20 enthusiast): > > https://blog.cloudflare.com/it-takes-two-to-chacha-poly/
The performance delta in his graphs isn't near as big as what I've measured, which makes me suspect OpenSSL's ChaCha20 implementation isn't nearly as fast as the kernel's. > My big worry though is that schemes that require that nonces/IV's must > **never** be reused are fragile. It's for the same reason that DSA > makes my skin crawl. If you ever screw up --- maybe after a crash, or > a file system bug, you end up reusing a nonce, it's game over. > > So if there are hardware solutions which are faster or fast enough > that the crypto is no longer dominant cost, why not use a cipher > scheme which is more robust? Block ciphers have their own downsides though - XTS is really a big pile of hacks and workarounds. On the whole, if you can get nonces right, a stream cipher cryptosystem (and ChaCha20 especially) is on the whole drastically simpler, and thus easier to understand and audit. And if you can do nonces correctly, ChaCha20/Poly1305 is pretty much one of the gold standards - it's secure against pretty much any vaguely realistic threat model. XTS, not so much - it's just the best you can do given the constraints of typical disk crypto. The gold standards of encryption today are the AEADs - and AES/GCM fails badly with nonce reuse too, there aren't any AEADs yet that don't fail badly with nonce reuse. > P.S. We're also both ignoring the cost of whatever changes are needed in > the file system to guarantee that the nonce is never, ever reused... I'm definitely not advocating for hacking stream ciphers into existing filesystems - if you don't have the machinery you need to be 100% rigorous about nonces, then definitely stick with XTS. But I already had most of what I needed in bcachefs, and I can still break the on disk format if I need to (and encryption is a breaking change), so for me ChaCha20/Poly1305 was a no brainer. BTW though, if there do turn out to be platforms where AES is significantly faster than ChaCha20 I can still add AES support pretty easily - I've already got all the relevant switch statements, since encryption is handled as another checksum type. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
