RE: [libreoffice-users] Encryption algorithms in Libre Office?

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 
pronounceable.  

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.

 - Dennis

-----Original Message-----
From: Jay Lozier [mailto:jslozier@gmail.com] 
Sent: Saturday, January 14, 2012 14:28
To: users@global.libreoffice.org
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 [mailto:framefritti@gmail.com]
> Sent: Saturday, January 14, 2012 01:18
> To: users@global.libreoffice.org
> 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.
>
> Riccardo


-- 
Jay Lozier
jslozier@gmail.com


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