[cifs-protocol] Session keys are not always 16 bytes long

Stefan (metze) Metzmacher metze at samba.org
Wed Sep 17 15:57:29 GMT 2008

Hongwei Sun schrieb:
> Metze,
>    When you tested SMB signing with 32 byte AES session key,  did you test Vista with Samba server , Samba client with Windows server or both ?

Samba as client and Windows 2008 as server.

I found a few bugs in Samba's smb signing implementation.

Windows only signs the last session setup response
and samba also signs the last session setup request.

I have some hacks which fix samba, but I haven't done a clean
implementation yet.

The key thing is that the client needs to process the last session
setup response first and then verify its signature after when the
correct session key is avaliable.


> -----Original Message-----
> From: Stefan (metze) Metzmacher [mailto:metze at samba.org]
> Sent: Friday, September 05, 2008 3:25 PM
> To: Hongwei Sun
> Cc: Andrew Bartlett; pfif at tridgell.net; cifs-protocol at samba.org
> Subject: Re: [cifs-protocol] Session keys are not always 16 bytes long
> Hongwei Sun schrieb:
>> Metze/Andrew,
>>   The subkey in the EncAPRepPart of the AP-REP should be used as the session key when the mutual authentication is enabled(as described in RFC 4121).    When DES and RC4 are used in Kerberos, the implementation is based on RFC1964 (instead of RFC4121).  According to RFC1964, you can pick the subkey in AP_REQ as the session key as it is the same as the subkey in AP_REP, but this is not true when AES is used because both subkeys are different (again AES means RFC4121 being used, not RFC1964).       This explains what you observed.   We will add the information to [MS-KILE] to describe how the session key is selected.
>>    The session key returned from  Kerberos package can be used for SMB signing as described in the section 4.3 of  [MS-SMB].  You have to truncate the keys to 128 bits in your code  because SMB does the truncation on the windows side.
>>    I ran the similar testing as you did.  I had one Vista machine connected to Windows 2008 DC/KDC and configured AES256-CTS-HMAC-SHA1-96 as Kerberos encryption type with mutual authentication enabled.  I cannot duplicate your SMB signing problem(see the network capture attached). It is working between Windows servers and clients.
> I got it working, the session key isn't truncated for the SMB signing.
> The problem was that we reseted the sequence number when updating the session key from the initiator subkey to the acceptor subkey between the session setup request and response.
> Also windows signs the response with the acceptor subkey, so that the client needs to check the signature after processing the response.
> metze

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