Recover Protected Office Documents

Posted on January 8, 2009 by Daniel Petri in Security with 0 Comments

How can I open protected MS Excel files if I lost (or never knew) the password for opening them?

Loosing a password for an important document could become a real problem. At best, it means that you loose some time recovering the password or retyping all the data. Current password recovery solutions offer brute-force tools which try about 300 000 passwords per second. Sounds optimistic? This number means that recovering a password that consists of 8 upper or lowercase letters and digits will take more than few weeks. 12 characters password could take centuries to recover.

Hopefully there is a way to restore data without recovering password. And almost instantly. How is that possible

Protected MS Word/Excel documents are encrypted with the help of RC4 algorithm, on a key, computed by a password. For the standard encryption MS Office uses relatively short 40 bit key. This allows the sorting out of all 2^40 key values, to find a true value of the key, and to decrypt the document. Such an approach requires considerable (several days) computer time but always leads to a success. Pre-computing encryption keys and using knowledge of MS Office document’s structure makes recovery process almost instant.

Still sounds odd? Visit This is the first instant recovery service for the password protected MS Word and Excel files. The site claims to have the following functionality:

  • All Word/Excel 2003/2002/2000/97 files protected with original 40 bit encryption are supported

  • Instant decryption – under 3 minutes per file, regardless of file password

  • No software to download and install – service is web-only

  • Free online preview of encrypted file content

  • Secure service – all file submissions are encrypted by SSL protocol

Lets go through the recovery process with the test file called decrtyptum-test.xls. The file is protected with the password "DecryptumPWD", but we will pretend we do not know that.

  1. After clicking "Start Decryption" you will be prompted to agree with "Service And Confidentiality Agreement".


  1. On next screen you should enter PIN number. Lets leave PIN number entry field empty to run the service in demo mode.

  1. Step three – uploading the file through SSL encrypted channel.

  1. Now we should wait for the file to be decrypted. It takes about 30-40 seconds. Not so long.

  1. And here we have the preview of the file. Now you can consider if the file is worthwhile paying for decryption.

Nice, huh? Although there are other offline methods of cracking open a protected MS Office file, this method has proven to be one of the most cost effective methods that I know of.

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