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On 01/14/2012 06:18 PM, Dennis E. Hamilton wrote:
The digests and the block ciphers used in ODF encryptions are not alphabetic transpositions.  They 
work at the binary bit level and are difficult to invert, although some digests may leak some 
modest information.  The encryption of textual content is on its compressed binary form, and that 
by its nature adds some entropy: it is the compressed file that is encrypted.  Consequently, the 
easiest language-based attack is on the password since so many are memorable and may even be 

Brute-force attacks on passwords with known digests just get better all of the time and that is an 
indirect hazard if the same password is used for protection of some files and for encryption of 
others.  (All passwords used in setting protection locks should be assumed to be compromised and 
not used for anything else.)

There is structure in the uncompressed ODF plaintexts (e.g., many of the parts in the Zip are XML 
files with known schemas as well as text content).  That structure and other clues can help discern 
whether a password attack has succeeded, though.  There are also a few known plain-texts and 
predictable plain-text portions that are commonly found compressed the same way in almost all 
current ODF packages.  That provides easier confirmation of a success and possible clues to the 
presence of attack-worthy material as well.
If the information is valuable enough for others to want to know then it sounds like file encryption is possibly a very big speed bump to knowing the contents. There numerous inherent weaknesses that someone who understand cryptography and the mathematics behind it can exploit to their advantage. A weakness is that passwords have a finite, if initially unknown, length. The only issue then is the encryption method strong enough to keep the data protected until it has little value.

The issue is to keep the file secure from unauthorized eyes, because once the file gets into the wild one must assume that someone will decrypt it. And once decrypted it will be posted somewhere, possibly in public.

  - Dennis

-----Original Message-----
From: Jay Lozier []
Sent: Saturday, January 14, 2012 14:28
Subject: Re: [libreoffice-users] Encryption algorithms in Libre Office?

On 01/14/2012 04:28 PM, Dennis E. Hamilton wrote:
[ ... ]
The fundamental weakness of the current approach is the use of human-entered passwords (which tend 
to be memorable and easily attackable), some well-known problems with information leakage from Zip 
files and known-/predictable-plaintext attacks.  There is also a vulnerability if the password used 
is used anywhere else (e.g., for protecting fields in documents) such that its SHA1 digest becomes 
known or suspected.
One problem in cryptography is that fact that all alphabetic languages
and alphabetic transcriptions have definite letter frequency in plain
text. For example in English the letter occurs 7% of the time. This was
first discovered and used by William Friedman in the 1920's. Also,
grammatical construction of a sentence could provide clues for the key.
The word 'the' is very common and often before a noun or at the start of
sentence. The sentence structure will provide clues because every
language has rules about proper word order, etc. This is an often
overlooked problem with cryptography, if I know the original language I
know the probable letter frequency and can look for grammatical patterns
to break the key. This is in addition to any other problems such as weak
password/keys, weaknesses in the encryption algorithm, etc.
   - Dennis

-----Original Message-----
From: Riccardo Bernardini []
Sent: Saturday, January 14, 2012 01:18
Subject: [libreoffice-users] Encryption algorithms in Libre Office?

Dear all,
I apologize in advance if this is a FAQ, but I was not able to find an
answer both in the FAQ page and in the first 4-5 pages of the mail archives
(I searched for "password" and "encryption").

I know that Libre Office allows you to save a "password protected
document," but I would like to know some more details about it. For
example, is the document actually encrypted or simply Libre Office refuses
to open it without the right password? (I expect [and hope] the former).
   If the former hypothesis is correct, which encryption algorithms are used?

Thank you for any help.


Jay Lozier

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