onsdag 10 januari 2007


For some time now I've been working on a new subsystem in the KDC for remotely validating digest credentials for a user. Right now it support CHAP (MD5), SASL DIGEST-MD5, HTTP DIGEST and NTLM (version 1 and 2)

It allows a server to validate a users digest without knowing the password directly, it can delegate the authentication to the KDC.

A coworker of mine used to build a proof-of-concept patch till the radius server Radiator that uses the new digest protocol to support the EAP-CHAP mechanism.

The digest protocol also support sending back Kerberos credentials in the protocol, so for example a WEB-DAV server using HTTP digest authentication can get an AFS ticket back that it can use to authenticate to AFS. This way the user limited service ticket (not the full KRBTGT) will get exposed to service, and only services that the user actually authenticated too.

The NTLM authentication is a slight different beast. After the context exchange (The NTLM type 1, 2 and 3 packets) the KDC sends back the session key material (if that was negotiated) to the server. This way the server also can provide session security to the client without dealing with the KDC any more. To test this I wrote a GSS-API mechanism that looks like the NTLMSSP in SSPI, it support both initiators and acceptors, session security. It was really really useful to debug SPNEGO so it have defiantly served its purpose even if it never will get used.

The interface is exposed in the library as the krb5_digest and the krb5_ntlm interfaces, its not a very large interface and it only uses opaque data types to enable us to add new digest types in the future. There is also a binary, <b>kdigest</b> that let the server access this from a shell/Perl script without interfacing with any c-code.

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