On 01/14/2012 04:28 PM, Dennis E. Hamilton wrote:
Saving a document with password is indeed an encryption. The encryption
methods are specified in the ODF Specification for encrypting the parts of the
Zip package. (There is no ODF-specified encryption for the single- XML-file
form of an ODF document.)
The default method, that works for all ODF 1.0/1.1/1.2 documents in packages
(e.g., ODT, ODP, and ODS files), is by Password Based Key Derivation (PBKDF2)
using HMAC and SHA1 starting with an SHA1 digest of the UTF-8 user-chosen
password. The encryption with the derived key is Blowfish with 8-bit Cipher
Feedback (8-bit CFB). This is done on each file of the Zip package that
carries the parts of the ODF document. (Each part has a different,
randomly-derived initialization vector, but the derived key is the same for all
of them.)
Starting with ODF 1.2, additional encryption methods can be chosen. However,
there are interoperability issues if the document is intended to be opened with
anything but the computer and software that was used to encrypt it (actually a
common use case but not when secure interchange is intended).
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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