[Matroska-devel] Encryption

ivanburnin at hushmail.com ivanburnin at hushmail.com
Sat Nov 19 12:30:23 CET 2005


I've got some spare cycles now so I'm looking into the encryption. 
I've got a few questions though. A link was poste before without 
much information, is there a document with deeper information? Has 
anyone ever implemented encryption for Matroska files? (if so 
everything should be compatible) And lastly, would anyone mind if I 
changed things radically?

At this point I'm considering something along the lines of an 
encryption header that contains
{integer identifier, UTF-8 String name, blob of bits to pass to the 
initializer} this would occur once per encrypter per file and only 
for the encryptor(s) that are used in that file, the name would be 
the name of the method (used for universal identification), 
identifier would be an in file identification number to allow for 
multiple encryptors per file.

Then each encrypted segment (for some paring specific definition of 
segment that does not necessarily have anything to do with segment 
in the video sense) is composed of

{integer identifier, blob of bits to pass to the decryptor}

I would also propose that 0x00000000 be defined as the null 
encryptor, allowing it to be used for the sake of sanity when 
writing certain processors. It is also important to note that the 
identifier for a given encryptor/decryptor can change across files 
and that the per file header is the only dependable source for this 
information. A smaller sized integer would be acceptable, I simply 
chose 32-bits because it is commonly and quickly available.

This moves an enormous quantity of the cryptography decision out of 
the core Matroska document, creating small supplimental documents 
for each type. I would of course be writing a number of these in 
order to build a baseline that people could depend on. 

This format allows me to work very quickly, for other competing 
designers to work quickly, and security. There are a lot of 
implementation details that I haven't covered, for example there 
should be a version number embedded in there, and a definition for 
the signature that is mentioned briefly in the current spec, I 
haven't addressed either of these. Actually binary format is 
relatively unimportant from a security standpoint, and can be 
addressed by someone more familiar with the inner workings than I 
am.



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