Ms office 2010 Encrypted File (Agile Encryption) opening

Question

hi.

I'm working on ms office 2010 opening  encrypted file.

I do not understand the open algorithm clearly, but I will show my procedure and questions.

If somebody know the currect things, please explain me very specific.

 

I want to make encryptedVerifierHashInput value.

 

Here is my sample tags - 

<encryption xmlns="http://schemas.microsoft.com/office/2006/encryption"xmlns:p="http://schemas.microsoft.com/office/2006/keyEncryptor/password">

<keyData saltSize="16" blockSize="16" keyBits="128" hashSize="20" cipherAlgorithm="AES" cipherChaining="ChainingModeCBC"hashAlgorithm="SHA1" saltValue="cIJbDY9n789Oo5XBSU0lxA=="/>

<dataIntegrity encryptedHmacKey="PqcXib+YmedKn67o0D/FeH+d/gWAucPIiRpCagUnRXs="encryptedHmacValue="kl0wXelJ5a3hztABvshT1sahO5K6AbM2muv6Wzw3EUU="/>

 <keyEncryptors>

 <keyEncryptor uri="http://schemas.microsoft.com/office/2006/keyEncryptor/password">

<p:encryptedKey spinCount="100000" saltSize="16" blockSize="16" keyBits="128" hashSize="20" cipherAlgorithm="AES"cipherChaining="ChainingModeCBC"

  hashAlgorithm="SHA1" saltValue="lS+abMy38y58Ktf53+SBCA=="encryptedVerifierHashInput="oSRgauy9ilU9+t+nJD1xjA=="

  encryptedVerifierHashValue="xzY+E5Grj3/sZTAJZdiuE67wWbJRnUV8/TQIterZ7xs=" encryptedKeyValue="54ECzuGmskUQjPwcDyzh8A=="/>

</keyEncryptor>

</keyEncryptors>

</encryption>

 

Office Documents cryptography structure specification(2.3.4.11) says

 H0 = H(salt + Password).

To make the salt value we use base64decoding function.

That means

           Salt = base64decodingfunction(saltValue).

Q1. In the sample tags, there are two saltvalues which are "cIJbDY9n789Oo5XBSU0lxA==" and "lS+abMy38y58Ktf53+SBCA==". Which one is right value to make salt?

 

Q2.  2.3.4.11(encryptions key generation) says

Hfinal = H(Hn + block_key).

 I'm wondering if Hfinal is same with keysalt(2.3.4.12 Initialization vector generation)?

Hfinal = keysalt?

Answer 1

Hi sTppp,

Thank you for your question.  A colleague will contact you to work on this issue.

In the meantime, you may find these references helpful:

http://blogs.msdn.com/b/openspecification/archive/2009/05/08/dominic-salemno.aspx

http://blogs.msdn.com/b/david_leblanc/archive/tags/office+crypto/

 

Regards,
Mark Miller
Escalation Engineer
US-CSS DSC PROTOCOL TEAM

Answer 2

Hi sTppp, please take a look at the following thread and let me know if it answers your question as well.

 

http://social.msdn.microsoft.com/Forums/en-US/os_openXML-ecma/thread/bcac35b3-a305-4e43-8d0b-fad69e2f9fb6

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Josh Curry (jcurry) | Escalation Engineer | Open Specifications Support Team

Answer 3

Hi JCurry,

Thank you for your reply.

I've  already read the thread.

 

Q1

You said 

"You need to decrypt EncryptedVerifierHashInput and feed it through the Hash algorithm. Then decrypt EncryptedVerifierHashValue using steps 1a-c again with the 2nd block key (0xd7, 0xaa, …) and compare that value."

 

That means 

if decrypted EncryptedVerifierHashInput and  decrypted EncryptedVerifierHashValue are same, is the password verified?

 

Q2. 2.3.4.11(encryptions key generation) says

Hfinal = H(Hn + block_key).

 I'm wondering if Hfinal is same with keysalt(2.3.4.12 Initialization vector generation)?

Hfinal = keysalt?

Please, reply my questions very specific with example.

Thank you.

Have a great day.

sTppp

Answer 4

Hi Tppp, the answers to your questions are contained in the aforementioned thread. Please read through it in more detail. You might also find the following resources helpful…

http://blogs.msdn.com/b/david_leblanc/archive/2009/01/06/ms-offcrypto-examples.aspx

http://blogs.msdn.com/b/david_leblanc/archive/2009/01/13/ms-offcrypto-example-update.aspx
http://offcrypto.codeplex.com/
http://blogs.msdn.com/b/openspecification/archive/2009/05/08/dominic-salemno.aspx
http://www.ietf.org/rfc/rfc4634.txt
http://www.rfc-editor.org/rfc/rfc2898.txt


An update to the MS-OFFCRYPTO project on codeplex should be posted within the next few weeks that includes an agile decryption solution. The Open Specifications forums are here to provide support for the protocol documentation set. We do not provide examples, source code, or direct support with the implementation of any protocol. Please review the documentation and let me know if you have any further questions.

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Josh Curry (jcurry) | Escalation Engineer | Open Specifications Support Team