Hi, my name is Alan Myrvold, and I am a security tester on the Office Trustworthy Computing Team (TWC). This post introduces the new password rules feature in Office 2010.
Word, Excel, and PowerPoint have been able to password protect documents for several versions by setting the “password to open”. What we felt could be improved was the ability to enforce password strength rules, similar to what may be required when logging into your computer at work.
In Office 2010, the encryption password can be set using the Office Backstage View:
This password can also be set on the General Options dialog from the Save As dialog, as the “Password to open”, just like in previous versions of Office.
Why is password complexity important?
Although historically Word and Excel used 40-bit RC4 encryption, faster computers mean that 40-bit keys are now considered weak. The Office Open XML format (*.docx, *.xlsx, *.pptx) introduced in Office 2007 provided an opportunity for us to improve our mechanism and algorithms used for password based encryption of documents. The Office Open XML format uses 128-bit AES encryption. We also use a slower key derivation algorithm to make brute force password cracking slower. RC4 is still used when saving in Office 97-2003 binary formats. For encrypted Office Open XML documents, the password is the weakest link. A short or commonly used password makes the document less secure, since it is easier for an attacker to guess it.
If an attacker needed to try all possible passwords of 5 lowercase letters from a-z, there are only 265, or about 11 million total passwords to guess during a brute force search. Searching dictionary words might even more quickly find the password. An 8 character password, chosen from lowercase and uppercase a-z, plus digits 0-9 is a much larger space of passwords to guess by brute force, 628 or about 200 trillion, and is more difficult to find with dictionary attacks too. These are all worst case efforts, and NIST estimates far less entropy in user chosen passwords. Having less entropy means that attackers can use heuristics to search the password space more intelligently than brute force.
Attackers can also harness the parallel processing power of graphics cards to help with their attack.
But, for brute force attacks, assuming 10,000 password attempts per second, the length and character set of the passwords can make a big difference.
Enforcing a minimum password length and character set complexity requirements can make passwords more difficult for attackers to guess.
How do I enable password complexity?
By default, complexity settings are not enforced, and registry settings are used to control this feature. Although I am describing the registry keys here, the Office Customization Tool (OCT) will be the easiest mechanism to deploy these policies within an organization, but these settings aren’t present in the OCT yet.
There are 2 registry settings to control this, PolicyLevel and MinLength.
- Value name: PolicyLevel
- Value type: DWORD
- Value data: [ 0 | 1 | 2 | 3 ]
- Use 0 to for no complexity (default), 1 for minimum length, 2 for minimum length plus requiring 3 of 4 character groups, and 3 for all these checks plus enforcing Windows domain password rules.
- Value name: MinLength
- Value type: DWORD
- Specifies the minimum length of password required.
When the policy level is 2 or 3, then the password must contain characters from at least three of four character sets, lowercase a-z, uppercase A-Z, digits 0-9, or non-alphabetic character. When this complexity is enforced, the minimum password length needs to be at least 6, but can be more depending on the MinLength.
Why not just use the Windows domain password policy?
When the policy level setting is 3, then Office will use the Windows domain policy as well as all the settings at level 2. This allows a custom password filter that is installed for Windows passwords to be used. If you are offline or a domain controller cannot be contacted, then the Windows password settings aren’t used, and only the level 2 settings are used. If you don’t have a custom password filter, then using level 2 saves a trip across the network, and would be the best choice.
What if my password doesn’t meet the complexity requirements?
Depending on whether the password is too short, or not complex enough, an error dialog will appear
and then you can re-enter the password.
What if I forget my password? Or the user leaves the company?
Oh dear. We’ve designed the Office Open XML password encryption to be strong and difficult for attackers to crack, which makes password recovery slow. There is no back door, no key escrow, and the 128-bit AES key makes guessing the password the best option.
Unfortunately Microsoft support cannot assist you, as described in KB article 189126.
Microsoft support engineers cannot help you retrieve passwords of files and features in Microsoft products that are lost or forgotten.
Because a forgotten password might result in the loss of critical business information, it is possible to disable setting new passwords in Word, Excel, and PowerPoint, using the DisablePasswordUI setting.
- Value name: DisablePasswordUI
- Value type: DWORD
- Value data: [ 0 | 1 ]
- Use 0 to enable the password UI, or 1 to disable the password UI.
This setting only prevents new passwords from being set. Existing password protected documents can still be opened. The DisablePasswordUI setting, along with the password complexity settings are designed to help balance the need to secure information with the risk of information loss.
The password rules feature is just one security enhancement in Office 2010, and future blog posts will cover more improvements we’ve made.
Security Tester, Office Trustworthy Computing