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The only difference between Widevine and DeCSS is you can rotate the Widevine key.

That should be a big difference. Have they made sure that all the netflix boxes out there support this?

Boxes are unlikely to use this key. The letter implies that this key is for the weakest (software-only) level of protection, used on generic PCs.

What do smart tvs and tv boxes use?

Each device usually has its own unique Widevine L1 keybox, with a separate intermediate key for each model.