Re: tcpdump -E doesn't work for 3des-cbc/hmac-md5

[Michael Mueller <m.mueller99 () kay-mueller de>]

Michael,

Are you sure you tested 3des-cbc with hmac-md5 or with some other 
authentication algorithm? I don't doubt that for some other 
authentication algorithms where authlen is set correctly your code works 
fine.

For *-cbc algorithms the problem seems to be that decryption starts at 
the end of the encrypted area and works its way backwards to the start. 
If authlen is wrong everything is decrypted into garbage. This is 
because the encrypted blocks are chained and a block can only be 
decrypted if the previous block (the one behind) was decrypted sucessfully.

Michael

Michael Richardson wrote:
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> > > > > > "Guy" == Guy Harris <guy () alum mit edu> writes:
>
>     >> Are there any positive or negative reactions to this? Will
>     >> somebody fix it?
>
>     Guy> I'd check in the patch if somebody resolved the issue
>
>     Guy> either by saying that 12 is the right authlen for all
>     Guy> encryption algorithms, saying it's not and supplying a way
>     Guy> (including a patch) to figure out what the right authlen is, or
>     Guy> saying it's not, saying you can't determine it from the packet
>     Guy> contents, and supplying a patch to add the authentication
>
>   I was puzzled by the report, since I wrote the code and use the code
> in a zillion test cases, but willing to accept it that maybe I never 
> cared if the end of the packet was correctly determined.
>   Well, actually, you can't find the next-header value if you don't
> remove the authentication data.
>
>   The test case tests/esp1.sh does:
>
> tcpdump -t -n -E "0x12345678@192.1.2.45 3des-cbc-hmac96:0x4043434545464649494a4a4c4c4f4f515152525454575758" -r 02-sunrise-sunset-esp.pcap 
>
>   I'm confused about the statement that the authlen isn't set.
>   Perhaps it is really that the algorithm has not been set correct by th
> reporters.
>
> - --
> ]     "Elmo went to the wrong fundraiser" - The Simpson         |  firewalls  [
> ]   Michael Richardson,    Xelerance Corporation, Ottawa, ON    |net architect[
> ] mcr () xelerance com      http://www.sandelman.ottawa.on.ca/mcr/ |device driver[
> ] panic("Just another Debian GNU/Linux using, kernel hacking, security guy"); [
>   
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