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Subject: The problem of visible hashes for protection keys

We know that, using XML documents, it is easy to subvert a lock on text or
spreadsheet documents that involves locking material against changes (but
not reading).  When the lock is itself locked against removal using a
password hash, this exposes the password used for the hash to compromise.

The problem is the desire by folks to reuse familiar, memorable passwords
for this purpose when the memorable  password is also used to protect
something that is of serious high value.  (That is to say, the password is
more valuable than the trivial-to-break lock in the document.)

Some bad news:


Note that memorable passwords are not the hard ones to crack, and increasing
the strength of the hash will not do much to protect memorable passwords
from discovery against this kind of computational power.

Note: This attack does not work against the PBKDF2 methods used for ODF 1.2
encryption, because the start password is never revealed.  On the other
hand, the techniques by which the password-hash attack were accelerated so
much is probably a reason for concern that the various vulnerabilities of
the ODF 1.2 encryption will be too-soon exploitable as a practical matter.
My personal assumption is that no well-informed government body or
commercial entity that is concerned about document confidentiality will
allow use of the ODF 1.x encryption and would require very strong
whole-package encryption techniques, whether defined for ODF documents or
not.  In that regard, the ETSI draft that we have been asked to comment on
has moved ahead of us in the level of confidentiality-by-encryption that it
attains for Zip-packaged documents.

 - Dennis

Dennis E. Hamilton
NuovoDoc: Design for Document System Interoperability 
mailto:Dennis.Hamilton@acm.org | gsm:+1-206.779.9430 
http://NuovoDoc.com http://ODMA.info/dev/ http://nfoWorks.org 

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