[cifs-protocol] [Pfif] MS-NRPC: AES Schannel problems
Stefan (metze) Metzmacher
metze at samba.org
Mon Sep 14 18:20:12 MDT 2009
>> We confirmed that AesCrypt follows the normative reference of [FIPS197] (http://www.csrc.nist.gov/publications/fips/fips197/fips-197.pdf). As far as the statement about AES128 encryption CFB mode, we also confirmed that we do use 0 as Initialize Vector(IV), so in this case all you have to do is set the IV to the 128-bit quantity consisting of all zeros. The reference we are using for CFB mode is [SP800-38A] ( http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf ) which states that CFB mode requires a valid and unpredictable IV (Section 6.3). Zero is a valid IV, certainly not unpredictable. However, the unpredictability is required only to guard against specific types of attacks, which become possible when a single key is used to encrypt a large number of related plaintexts. Predictable IVs could be used in applications where this is not a concern.
> thanks I'll try that.
> AES128 is also used in section 188.8.131.52.1 "Generating an Initial Netlogon
> Signature Token" under 8., is that the same AesCrypt function (also
> using CFB mode) with a just IV being contructed by using the sequence
> number twice?
I've tried to get that working, but it doesn't work:-(
I've setup a trust between two w2k8r2 domains and captured the
ServerReqChallenge and ServerAuthenticate3. And they're using Netlogon
Schannel with AES. (They also use NDR64, wireshark doesn't handle this
There're 5 ServerAuthenticate3 exchanges in the capture
and I put the data into a simple standalone crypto challenge
So all we need is to find the algorithm to recalculate the examples,
changing the mxnrpc.c file.
>> We will update the document with the correct references to the related statements in the MS-NRPC document.
> It would be really nice if you could also add some more example values
> in secion 4.2 Cryptographic Values for Session Key Validation.
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