Why we do not decrypt Excel/Word 2007-2016 files
When it is impossible to decrypt an Excel or Word file
The answer lies in the encryption algorithms for different Microsoft Office versions and key length (by default) with which the document data is encrypted:
- Excel/Word 97-2003: RC4 algorithm (stream cipher), 40-bit key length – 240 key options
- Excel/Word XP-2003: RC4 algorithm (stream cipher), 128-bit key length – 2128 key options
- Excel/Word 2007-2010: AES algorithm (block cipher), 128-bit key length – 2128 key options
- Excel/Word 2013-2016: AES algorithm (block cipher), 256-bit key length – 2256 key options
The Microsoft Office 97-2003 stream cipher permits (almost always) extracting unambiguously from any document a common part, and the rather short (40-bit) encryption key permits computing the correspondence “encryption key => common part from the document” for all possible options. This is the first stage of creation of a rainbow table. In essence, after obtaining this computed data once, it is possible to take simply the required correspondence and decrypt any document without a password.
But the size of this data is too large. Therefore, in the second stage, the data is compressed by additional computations to an acceptable size — the rainbow table — an excellent example of a compromise of time and memory.
Now, finding in the table a suitable encryption key, it is possible to decrypt instantly any Excel/Word 97-2003 document (actually not always but in 99.9% of cases, but the error can be neglected). The AccessBack.com and Passcovery Suite (Ultimate License) service operate in this manner:
Everything changes in principle with AES block code which is used in Microsoft Office 2007-2016. All data of the document is encrypted using 16-byte blocks, and it is impossible to extract the common parts necessary for creation of the necessary correspondences. And in these Microsoft Office versions, the encryption key length was increased…
Decrypting Excel/Word files without rainbow tables
In fact, using rainbow tables is not totally 100% successful, but 99.9%. The algorithm for creating rainbow tables permits the absence of a vanishingly small number of “key => data” correspondences. In that case, a “key search”, a consecutive search of encryption keys for Excel/Word 97-2003, helps. AccentOPR and Passcovery Suite provide such a capability:
Key search guarantees decryption of the Excel/Word file and successful 100% removal of the password from the document, since all possible variants of the key – 240 values – are covered.
And what about 128 bit encryption in Excel/Word XP/2003?..
It is also impossible to construct rainbow tables for this, but for other reasons.
Even if we exclude the change in principle of the algorithm for encrypting Microsoft Office XP-2003, which does not permit using rainbow tables, the invention is all the same impracticable – we simply do not have enough time to construct rainbow tables.
Breaking Microsoft Office passwords
Based on everything, the conclusion is that calculating rainbow tables for Microsoft Office 2007-2016 is impossible. Passwords for such files must be restored. And to achieve success, any useful dodges are used:
- Extended mask attack when it is possible to specify admissible symbols for each position of the generated password
- Dictionary attack with mutations when it is possible to set rules to change the words which are read out from dictionaries
- Acceleration of password search on a GPU when the password search speed increases by factors of ten (depending on type and numbers of GPUs)
All these capabilities for Microsoft Office 2007-2016 documents are in Passcovery programs: