Microsoft Office password protection

The Microsoft Office password protection is a security feature to protect Microsoft Office (Word, Excel, PowerPoint) documents with a user-provided password. Word and Excel 95 used a 32-bit key protection algorithm, later enhanced up to 40 bits in Excel 97, and then augmented to 128-bit Advanced Encryption Standard (AES) protection in Office 2007.

Currently, the 40-bit key RC4 protection used in Office 97–2003 can typically be bypassed with password hacking software. The 128-bit key AES protection employed in newer Office 2007–2010 remains secure. In fact, the distributed.net RC5 project has been trying to brute force an RC5 72-bit key since 2002, and has not successfully done so yet.[1] Furthermore, even utilizing all known breaks (that speed up brute force attacks by a factor of about four) it would take a typical computer millions of years or longer to break a 128-bit AES key of sufficient length and complexity.

Types

Microsoft Office applications offer the use of two main groups of passwords that can be set to a document depending on whether they encrypt a password-protected document or not.

Passwords that do not encrypt a password-protected document have different security level features for each of Microsoft Office applications as mentioned below.

Because of the lack of document encryption, all the passwords mentioned above cannot reliably protect a document from a hacker. Most password cracking software can remove such protection from a password-protected document in very little time.

The password that encrypts a document also restricts the user from opening the document. It is possible to set this type of password in all Microsoft Office applications. If a user fails to enter a correct password to the field which appears after an attempt to open a password-protected document, viewing and editing the document will not be possible. Due to the encryption of a document protected by a password to open it, a hacker needs to decrypt the document to get access to its contents. To provide an improved security, Microsoft has been consistently enhancing the Office encryption algorithm strength.

History of Microsoft Encryption password

In Excel and Word 95 and prior editions a weak protection algorithm is used that converts a password to a 16-bit key.[4] Hacking software is now readily available to find a 16-bit key and decrypt the password-protected document instantly.[5]

In Excel and Word 97 and 2000 the key length was increased to 40 bits.[4] This protection algorithm is also currently considered to be weak and presents no difficulties to hacking software.

The default protection in Office XP and 2003 was not changed, but an opportunity to use a custom protection algorithm was added.[4] Choosing a non-standard Cryptographic Service Provider allows increasing a key length so that a key which is used to encrypt a document can’t be found. However, password-cracking programs can enter multiple random passwords with the same speed, so use of CSPs does not slow down password recovery at all. Weak passwords can still be recovered fast enough even if a custom CSP is on.

In Office 2007 (Word, Excel and PowerPoint), protection was significantly enhanced since a modern protection algorithm named Advanced Encryption Standard was used.[4] At present there is no software that can break this encryption. With the help of the SHA-1 hash function, a password is converted into a 128-bit key 50,000 times before opening the document; as a result, password recovery speed is vastly reduced.

Excel and Word 2010 still employ AES and a 128-bit key, but the number of SHA-1 conversions has doubled to 100,000[4] further reducing password recovery speed.

Office 2013 uses 128-bit AES, however hash algorithm has been updated to SHA-2 class, and it is SHA-512 by default.

Password recovery attacks

There are a number of attacks that can be employed to find a password or remove password protection from Excel and Word documents.

Password removal can be done with the help of precomputation tables or a guaranteed decryption attack.

Attacks that target the original password set in Microsoft Excel and Word include dictionary attack, rule-based attack, brute-force attack, mask attack and statistics-based attack.

The efficiency of attacks can be considerably enhanced if one of the following means is applied: multiple CPUs (distributed attack), GPGPU[6] (applicable only to Microsoft Office 2007–2010 documents) and cloud computing. Due to weak passwords, at the moment, cloud computing facilities are capable of unlocking as many as ca. 80% of the files saved in the Office 2007–2010 format. Passwords of sufficient length and complexity typically cannot be brute-forced.

Office 2013 introduces SHA-512 hashes in the encryption algorithm, making brute-force attacks nearly impossible. With SHA-512 hash, it is practically not possible to compute another hash for stored password, e.g. brute-force hash. This trick used to work from VBA in Excel 2003-2010 due to weaker SHA-1 hash..

There is specialized software designed to recover lost Microsoft Office passwords on pre-AES encryption and weak passwords.

References

  1. ↑ "Distributed.net: Project RC5".
  2. 1 2 "Password protect documents, workbooks, and presentations – Support – Office.com". Office.microsoft.com. Retrieved 26 December 2012.
  3. ↑ "How to Password Protect Excel Workbooks". The Desktop School. Retrieved 14 April 2015.
  4. 1 2 3 4 5 "Microsoft Office File Format Documents". Msdn.microsoft.com. Retrieved 26 December 2012.
  5. ↑ "Russian Password Crackers: Password Recovery (Cracking) FAQ". Password-crackers.com. Retrieved 26 December 2012.
  6. ↑ "GPU estimations". Golubev.com. 22 June 2012. Retrieved 26 December 2012.
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