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:[email protected]] Sent: Saturday, January 14, 2012 14:28 To: [email protected] 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:[email protected]] > Sent: Saturday, January 14, 2012 01:18 > To: [email protected] > 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 [email protected] -- For unsubscribe instructions e-mail to: [email protected] Problems? http://www.libreoffice.org/get-help/mailing-lists/how-to-unsubscribe/ Posting guidelines + more: http://wiki.documentfoundation.org/Netiquette List archive: http://listarchives.libreoffice.org/global/users/ All messages sent to this list will be publicly archived and cannot be deleted -- For unsubscribe instructions e-mail to: [email protected] Problems? http://www.libreoffice.org/get-help/mailing-lists/how-to-unsubscribe/ Posting guidelines + more: http://wiki.documentfoundation.org/Netiquette List archive: http://listarchives.libreoffice.org/global/users/ All messages sent to this list will be publicly archived and cannot be deleted
