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:jsloz...@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:framefri...@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
jsloz...@gmail.com
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