[cifs-protocol] MS-NRPC: AES Schannel problems

[Hongwei Sun]

Metze,



    The SharedSecret used for AES session key computation, as described in 3.1.4.3 MS-NRPC , should be the NTOWF (MD4(UNICODE(Passwd))) of the plaintext password.   The section 3.1.1 of MS-NRPC explains what a SharedSecret is used for session key calculation in Windows implementations.  The SharedSecret  is stored in UnicodePwd AD attribute.  Please see section 3.1.1 and Windows Behavior notes <66>,<67> of MS-NRPC for details.



     I will continue working on all questions related to AES encryption.



Thanks!


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Hongwei  Sun - Sr. Support Escalation Engineer
DSC Protocol  Team, Microsoft
hongweis at microsoft.com<mailto:hongweis at microsoft.com>
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-----Original Message-----

From: Stefan (metze) Metzmacher [mailto:metze at samba.org]

Sent: Tuesday, August 25, 2009 11:13 AM

To: Interoperability Documentation Help

Cc: pfif at tridgell.net; cifs-protocol at samba.org

Subject: MS-NRPC: AES Schannel problems



Hi,



I'm currently trying to implement the AES based Netlogon Secure Channel in Samba.



But the documentation is not really clear about the used algorithms.



I have started with the implementation here:

http://gitweb.samba.org/?p=metze/samba/wip.git;a=shortlog;h=refs/heads/master4-schannel



And here's the actual commit that tries to add aes support:

http://gitweb.samba.org/?p=metze/samba/wip.git;a=commitdiff;h=50dca9ce0f051c863f00cc949db2c19bf247887b



In Section "3.1.4.3 Session-Key Computation" the hmac-sha256 base computation of the session-key seems to use the plain SharedSecret and not the NT-HASH of it (MD4(UNICODE(ShareSecret))), is that correct?

I thought the plain text is never stored in AD by default...

Where should the netlogon server get the plain text from?

I just tried the NT-HASH see my netlogon_creds_init_hmac_sha256() function.



In Section "3.1.4.4 Netlogon Credential Computation" there's a AesEncrypt function used. Can you please document the exact algorithm that's used there. You say AES128 is used in CFB mode without initialization vector.



http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation

says that all modes except ECB require an IV.



It would also be nice if you could add some more example values in secion 4.2 Cryptographic Values for Session Key Validation.



metze










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