[cifs-protocol] of SupportedEncTypes and msDS-SupportedEncryptionTypes

Tom Jebo tomjebo at microsoft.com
Sun Dec 20 10:23:08 MST 2009

Hi Matthieu, 

Thanks for the follow-up questions regarding the supported encryption types blog entry from September.  One of the Open Specification Documentation support team will be in touch with you shortly.

Best regards,
Tom Jebo
Microsoft Open Specification Documentation Support

-----Original Message-----
From: Matthieu Patou [mailto:mat+Informatique.Samba at matws.net] 
Sent: Sunday, December 20, 2009 7:01 AM
To: cifs-protocol at samba.org; Interoperability Documentation Help; pfif at tridgell.net
Subject: of SupportedEncTypes and msDS-SupportedEncryptionTypes


Back in august and september we discuss in case SRX090808600017 about supportedEncTypes and I was quite happy with your answer.

I'm coming back on this subject for two details:

* this blog entry (of msdn)
says :" Although this attribute is present in all the computer objects of the domain regardless of the version of the OS the physical machines have installed, not all of them are aware of its existence hence, older versions (2003 and earlier) do not populate it at any time."  It means that when I join a w2k8 domain with a XP workstation that the DC will create a computer object for this XP workstation and set the msDS-SupportedEncryptionTypes attribute ? if so to which value ? On my tests when I join a w2k3 server to a w2k8 domain the attribute SupportedEncryptionTypes is not set. Can this point be clarified and if possible written in a formal document

This entry also state: "

When the KDC checks the attribute to decide what encryption algorithm to use in order to encrypt the ticket, it could find basically two scenarios:

1)      The attribute is populated

2)      The attribute is empty

If the attribute is populated, then the deal is done since the KDC can determine the best common algorithm to encrypt the ticket with the value present.

However, if the attribute is empty then the KDC will have to work harder being the next step to check another attribute. This attribute is defined in MS-ADA3 (section 2.341) and described in MS-ADTS (section
2.2.15) and it's called userAccountControl. This attribute is also a 4 byte Bit Mask that defines many aspects of the account but the only one the KDC is interested in is the DK (ADS_UF_USE_DES_KEY_ONLY ) bit.

This bit defines what legacy encryption method will be used:

1)      If the bit is set, then only DES will be used

2)      If the bit is NOT set, then DES and RC4 can be used

This check is especially relevant in domains that have Win7 and Windows Server 2008 R2 machines joined because those two newer OSs disable their bit by default so older DES is not an option for them."

* What is the exact meaning of ADS_UF_USE_DES_KEY_ONLY ? if a w2k8 server acting as a domain member within a w2k8 domain has this DK bit set, will the DC not use AES but only DES with it ?

* "This check is especially relevant in domains that have Win7 and Windows Server 2008 R2 machines joined because those two newer OSs disable their bit by default so older DES is not an option for them.", well it seems that a w2k3 server member of w2k8 domain do not have this bit set also (userAccountControl=4096 => only WT flag set).

* Also neither MS-LSAD nor MS-NRPC talk about the link between the attribute msDS-SupportedEncryptionTypes stored in the AD and the fact that it's returned as SupportedEncTypes in NETLOGON_DOMAIN_INFO  call.
I can understand that it can be called "secret" of implementation but when after a workstation tries to update this attribute to let the DC know what are the supported encoding it's better to clarify the link.

Thank you for help with clarifying those points.


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