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Reply to FairPlay/CENC cbcs mode compliance
Thanks for pointing this out. I was trying to implement ISO/IEC 23001-7 as well, and parsing the slice header to figure out where the slice data begins was going to be a big pain, as far as I can tell. I'm guessing Apple decided the same thing and just chose to allow 31 bytes for the header, which I'm assuming is more than enough to be safe.This approach still complies with the ISO spec, because you can specify the length of clear bytes in the sample auxiliary info. That is, you could choose to leave 100 bytes clear if you wanted to, and the decrypter should still know what to do if you update the `senc` accordingly.
May ’20