[cifs-protocol] SMB2 Session/Signing Key with NTLMv2 + SPNEGO

Brad Fewster brad.fewster725 at gmail.com
Thu Feb 21 11:20:56 MST 2013

To update:

I was able to finally track the code down for session key generation
to auth/ntlmssp/ntlmssp_client.c - cli_credentials_get_ntlm_response
function and realized that the signing key was in fact the random session
key I was generating as part of NTLMv2 with key exchange.

Given that I am able to sign messages now, it also appears the the key in
fact does change between SMB2 sessions wheras SMB1 does not.

I guess that I've answered my own questions.  So unless something I've said
is incorrect, thanks everyone for your time!

On Wed, Feb 20, 2013 at 4:09 PM, Brad Fewster <brad.fewster725 at gmail.com>wrote:

> Hi everyone,
> First off, I'm new to the list so excuse me if my mailing list etiquette
> is not quite up to par.
> I have a few questions regarding SMB2 and signing that I'm hoping someone
> here can help me with.  Note that I'm only concerned with the client, not
> the server, so all of my questions fall within the context of an SMB2
> client.
> Also I've done ALOT of reading of internet sources, Samba code, and MS
> documentation which has left me in a thoroughly confused state as to which
> methodologies apply to SMB, SMB2, NTLMv1, and NTLMv2.
> My apologies if this isn't the right place for NTLM questions, but I am
> not sure where else to ask since this is apparently directly related to the
> SMB2 signing key.
> Question 1: Signing Key Format with NTLMv2
> I'm looking at a connection in which the session setup is occurring (with
> regard to packet layout) using NTLMv2 inside NTLMSSP inside SPNEGO inside
> What I'd like to know is what exactly is used as the "session key" to sign
> SMB2 packets?  I am confused by the signing key information contained
> within [MS-NLMP] with regard to signing and sealing keys, some resources
> seem to indicate that that has no bearing on the SMB signing key, but I'm
> not sure that applies to SMB and/or SMB2 as the lines seem to be a bit
> blurred.
> The current routine I'm using to generate the key used to sign SMB2
> packets per [MS-SMB2] section is:
>   username = ASCIIZ username (i.e. "brad")
>   domain = ASCIIZ target domain (typically the hostname from the
> NTLMSSP_CHALLENGE, i.e. "WIN7-X64-01")
>   password = ASCIIZ password (i.e. "secret")
>   server_challenge = 8-byte challenge received from the server (from
> NTLMSSP_CHALLENGE's NTLM Server Challenge field)
>   client_challenge = random 8-byte nonce (generated by me)
>   target_info = "target info" block from NTLMSSP_CHALLENGE from server
>   v1Hash = MD4(UNICODE_LE(password))  [ as per heim_ntlm_nt_key ] (16
> bytes)
>   v2Hash = HMAC_MD5(UNICODE_LE(username)+UNICODE_LE(domain)) using all
> 16bytes of v1Hash as key
>   ntlmv2_response = all but the first 16 bytes of the response that I am
> SENDING to the client
>   hmac = HMAC_MD5(ntlmv2_response) using all 16 bytes of v2Hash as the key
>   signingKey = HMAC_MD5(v2Hash) using all 16 bytes of 'hmac' as the
> session key
> This isn't working for signing - my Windows 7 x64 Ultimate box rejects
> packets signed using the above signingKey.  Simply an ACCESS_DENIED error
> is returned; if I disable signing then the commands work so it's not a
> legitimate permissions problem.  I've gone over the actual signing code
> many times, which is simple compared to the NTLMv2 negotiation.  I'm
> confident the signing code is OK and the key above is incorrect.
> Authentication for NTLMv2 is woking correctly, so I also have access to
> all of the various artifacts of that process.
> I think I've gotten lost somewhere in the mountain of hashes and different
> signing keys.  Does SMBv2 require some of the "magic signing keys" to be
> used as documented in [MS-NLMP] to derive it's signing keys?  Various
> scraping of information on the interwebs leads me to believe 'no', but I
> can't be 100% certain.
> Various other permutations have been tries using assorted keys from the
> NTLMv2 process, the server and client challenge's, magic signing keys from
> [MS-NLMP], etc to no avail.
> BTW, the flags I'm using in my NTLMSSP_NEGOTIATE are 0x60088215 and the
> ones returned by the server are 0x628a8215.
> Question 2: Signing Key Retention
> Assuming that Q1 is answered and I'm able to obtain the proper signing key
> for SMB2 packets, is the key used to set the 'signature' field in the SMB2
> header retained across SessionSetup/Logoff instances?
> This appears to be the case in SMB1 and looking at [MS-SMB2] in section
> it reads "The client MUST NOT regenerate Session.SessionKey.".  I
> read this to mean that SMB2 behaves like SMB1 in that if a session key is
> established during the initial SessionSetup exchange that it will remain
> unchanged even if a Logoff is processed followed by another SessionSetup.
> Thanks for anyone who can help,
> B
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