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The Misuse of RC4 in Microsoft Word and Excel




There is a serious security flaw in Microsoft Word and Excel. (To the best of 
my knowledge, this flaw has not been reported in public before.) 

The stream cipher RC4 with key length up to 128 bits is used in Microsoft Word 
and Excel to encrypt the documents. But when an encrypted document gets 
modified and saved, the initialization vector of RC4 remains the same. Thus the 
same keystream generated from RC4 is applied to encrypt the different versions 
of that document. The consequence is disastrous since a lot of information of 
that document could be recovered easily from those encrypted different 
versions. 

The details of the flaw are described in the document (PDF):

    http://eprint.iacr.org/2005/007.pdf