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

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 
> 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.
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 [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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