[cifs-protocol] [REG:110071868986368] unused bytes after while decoding bkrp requests

Matthieu Patou mat at samba.org
Sun Jul 25 14:55:21 MDT 2010

  Hi Bryan

Any news on this subject ?

In the mean time I worked on the implementation of the protocol for the 
samba project and have more questions now.

So page 31 of MS-BKRP.pdf state that the message format for exchange is :

[in] handle_t h,
[in] GUID* pguidActionAgent,
[in, size_is(cbDataIn)] byte* pDataIn,
[in] DWORD cbDataIn,
[out, size_is(,*pcbDataOut)] byte** ppDataOut,
[out] DWORD* pcbDataOut,
[in] DWORD dwParam
I already asked if there is not some bytes after the dwParam.

After analyzing the out message I have the impression that before the 
ppDataOut there is some kind of integer.
Here is the hex dump of an output message:

00000000  00 00 02 00 44 00 00 00  00 00 00 00 8d 65 cd e4  
00000010  6c 93 62 22 48 e7 04 ff  0c 8f 0e 83 7a e4 dd d4  
00000020  4b d1 8e 74 95 67 4f 85  be a5 9c b7 7f fd 39 2c  
00000030  54 bc a7 60 e4 e0 13 26  49 6f ca 35 ee bb 23 24  
00000040  51 d4 4e c9 37 1d f0 9e  83 69 bd 10 44 00 00 00  
00000050  00 00 00 00                                       |....|

so from byte 4 (0x44 ) we have clearly (at least to me) the ppDataOut 
variable that is NDR encoded (meaning that the size is specified before 
on the wire) up to byte 4B then we have the size (pcbDataOut) (0x44 0x00 
0x00 0x00) and then the return code.

I attach the out message extracted from the trace I sent last time.  
With the following samba idl:

         [public,nopush,nopull,noprint] WERROR bkrp_BackupKey (
                 [in,ref]  GUID *guidActionAgent,
                 [in,ref]  [subcontext(4)] uint8 *data_in,
                 [in]  uint32 data_in_len,
                 [in]  uint32 param,
                 [out] uint32 misc,
                 [out] DATA_BLOB secret,
                 [out] uint32 data_out_len

We have the following result while using our ndrdump tool:

pull returned NT_STATUS_OK
     bkrp_BackupKey: struct bkrp_BackupKey
         out: struct bkrp_BackupKey
             misc                     : 0x00020000 (131072)
             secret                   : DATA_BLOB length=68
             data_out_len             : 0x00000044 (68)
             result                   : WERR_OK
dump OK

As I have also managed to get a correct 
BACKUPKEY_RETRIEVE_BACKUP_KEY_GUID exchange I also witnessed  that on 
the out message there is also "something" (that I named misc in our idl) 
(see the get_key_out file which is the extraction of out message on a 
BACKUPKEY_RETRIEVE_BACKUP_KEY_GUID request). As the certificate that I 
extracted seems to be correct I pretty encline to think I am right as 
first we are able to parse the NDR encoded data, and that the result 
seems sensible.
Can you see if my analysis is correct and if so can you give us the 
explanation of this "misc" parameter. If not, well please tell me the 
correct way to parse the message.

Also I've got questions of what is explained in the document.

First  in paragraph, it is stated

"The server MUST ignore the cbDataIn and pDataIn parameters. It MUST 
return the RSA public key
from the ClientWrap key pair, in the format specified in section 2.2.1. 
If no such key can be found or
created, the server MUST return an error."

The client is supposed to send a 2.2.2 Client-Side-Wrapped Secret struct 
in the pDataIn variable, this struct contains also a version field and a 
guid field.
Nothing is said about this fields, how should they be populated, can you 
explain this ?

Second in paragraph 1.3.1 Call Flows, it is stated
"For the ClientWrap subprotocol, the Microsoft implementation of the 
BackupKey Remote Protocol
server stores the following LSA global secret objects (note that the LSA 
global secret names are
Unicode strings):
1. G$BCKUPKEY_PREFERRED: This contains the 16-byte GUID ([MS-DTYP] 
section of the
RSA key pair currently used for client-side secret wrapping.
2. G$BCKUPKEY_guid: Here, guid is the string GUID that identifies the 
wrapping key, formatted as a
GUIDString ([MS-DTYP] section The value of the secret object 
is the server's ClientWrap
key pair, formatted as specified in section 2.2.5"

Should I conclude that in a given domain there is only "active" rsa key 
on all the server or said in another way no matter which server is asked 
at a given moment we will always receive the same GUID for the key ?

Also just to be sure this will be stored in the currentValue attribute 
but it will be only accessible through a lsaQuerySecret call right ?

Matthieu .

On 19/07/2010 03:06, Bryan Burgin wrote:
> [Mark and dochelp to bcc]
> [Adding case number&  CaseMail]
> Hi Matthieu,
> I began looking at this for you to identify the trailing bytes you specified and to investigate tools to decode/parse the Backup Key request.  I'll reply with more information as soon as I have more information to share.
> Bryan
> -----Original Message-----
> From: Mark Miller (MBD)
> Sent: Sunday, July 18, 2010 11:50 AM
> To: mat at samba.org; Interoperability Documentation Help; pfif at tridgell.net; cifs-protocol at samba.org
> Subject: RE: unused bytes after while decoding bkrp requests
> Hi Matthieu,
> Thank you for your question.  A colleague will contact you to investigate this issue.
> Regards,
> Mark Miller
> Escalation Engineer
> -----Original Message-----
> From: Matthieu Patou [mailto:mat at samba.org]
> Sent: Sunday, July 18, 2010 2:27 PM
> To: Interoperability Documentation Help; pfif at tridgell.net; cifs-protocol at samba.org
> Subject: unused bytes after while decoding bkrp requests
>    Dear dochelp team,
> I started to implement the backup key remote protocol for samba.
> Right now I'm a bit suspicious I got the data structure ok as when I parse some bytes with ndrdump I have ~52 bytes unused.
>   From the attached capture called protected_storage.pcap I managed to extract and decrypt the payload (452 bytes + 12 bytes of padding) at packet 485.
> The payload is also attached to this email as protected_xtr.
> Here are the result of ndrdump
> mat at ares:/usr/local/src/samba4/source4$ ./bin/ndrdump protected_storage bkrp_BakuprKey  in ~/protected_xtr pull returned NT_STATUS_OK WARNING! 52 unread bytes
> [0000] 8A E3 13 71 02 F4 36 71   02 40 28 00 30 7C DE 3D   ...q..6q .@(.0|.=
> [0010] 5D 16 D1 11 AB 8F 00 80   5F 14 DB 40 01 00 00 00   ]....... _.. at ....
> [0020] 04 5D 88 8A EB 1C C9 11   9F E8 08 00 2B 10 48 60   .]...... ....+.H`
> [0030] 02 00 00 00                                       ....
>       bkrp_BakuprKey: struct bkrp_BakuprKey
>           in: struct bkrp_BakuprKey
>               guidActionAgent          : *
>                   guidActionAgent          :
> 47270c64-2fc7-499b-ac5b-0e37cdce899a
>               data_in: struct bkrp_client_side_wrapped
>                   version                  : 0x00000002 (2)
>                   encrypted_secret_len     : 0x00000100 (256)
>                   access_check_len         : 0x00000058 (88)
>                   guid                     :
> a1dc8bbd-743f-473e-8d00-0a4742df76bd
>                   encrypted_secret         : DATA_BLOB length=256
>                   access_check             : DATA_BLOB length=88
>               data_in_len              : 0x00000174 (372)
>               param                    : 0x00000000 (0)
> dump OK
> To me the result looks sensible I'm just concerned that it seems to have some garbage at the end.
> I tried to analyze the frames with netmon 3.4 but it says that it's encrypted (and I didn't find a way to tell him to decrypt ...).
> So here is my question: is it normal that I found some trailing bytes ?
> do you have the capacity to parse the protected_xtr file and give us the result of the parsing with your tools ?
> Cheers, Matthieu.

Matthieu Patou
Samba Team        http://samba.org

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