Security can be a tricky business. This is especially true when you want security on an open network where anyone can see or modify your packet in transit. Of course, you can encrypt your message and everything would be fine. Right?

No. The problem is that there are more than one encryption mechanisms and before you can use any of them, the sender and receiver must agree on which one they will be using. But since there is no mechanism selected yet, the negotiation for a mechanism happens in clear text. Hence, the list sent by a client can be modified by an attacker in the middle so that subsequent communications can be watched and possibly be modified by the attacker.

Enter the mechListMIC token. mechListMIC stands for Mechanism List Message Integrity Code. The purpose of a mechListMIC token is to verify the mechanism list that was originally sent in clear text. Exchange of mechListMIC tokens is generally optional (see RFC 4178) and therefore they are not exchanged frequently. Microsoft’s extension to “The simple and Protected Generic Security Service Application Program Interface Negotiation Mechanism” aka SPNEGO makes it mandatory to exchange mechListMIC tokens, if AES Kerberos ciphers are exchanged between the client and server when the client/server operating system is Windows Vista and/or Windows Server 2008. Other conditions are described in RFC 4178 that are not reproduced here for the sake of brevity. One key condition that governs the generation of mechListMIC tokens is that the selected mechanism must support per-message integrity protection. If all conditions are met, until mechListMIC tokens are exchanged, the negotiation for encryption mechanism is not complete.

AES is short for Advance Encryption System and is one of the many Encryption types Kerberos can use. The rest of this document will assume that the security mechanism chosen is Kerberos.

Microsoft implements RFC4537 and negotiation for encryption type (Etype) is performed through it, where the supported etypes are sent as a part of Kerberos AP-REQ message in the authenticator. If the encryption type is not negotiated in AP-REQ (no authenticator is present), the etype used is one in the ticket in AP-REQ

One scenario where mechListMIC token are exchanged goes like this:

1. The client sends a NegTokenInit to the server with a list of Authentication Mechanisms it supports, sorted in order of preference with most preferred being the first one. In the following example, the mechanisms suggested by client are

i. MsKerberos

ii. Kerberos

iii. NT LAN Manager (NTLM)

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2. If the server supports any of the authentication mechanisms, it will choose the most preferred one that it also supports. It will also send a NegTokenResp message to the client. In this example, the server supports MsKerberos:

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3. Now that client and server agree on an authentication mechanism, they will exchange mechanism specific tokens. In this example, they have agreed on Kerberos. So, the client will send a NegTokenResp containing a Kerberos AP-REQ token.

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4. The server will process the token and, if everything goes well, will send a NegTokenResp that will contain a Kerberos AP-REP and a mechListMIC token - if the etype is AES128 or AES256.

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5. At this point, Kerberos authentication is complete but mechListMIC token exchange is not complete. A valid security context will not be established in the client and the server if this exchange fails. The client will send a NegTokenResp with its own generated mechListMIC token and set the NegState to accept-completed.

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6. This will finish the negotiation and authentication phase. The regular message exchange between the client and server can now commence.

The client and server code that generated this message exchange is available at msdn.microsoft.com at the following links:

http://msdn.microsoft.com/en-us/library/aa379449(VS.85).aspx

The code samples at the above mentioned link use SSPI with windows sockets. Generally an application protocol, like SMB2, will send the SSPI security BLOBs as part of its own protocol messages. In case of SMB2, the authentication mechanism negotiation will happen in SMB2 Session Setup messages.