Questions about trust signatures

[bezna]

Hi,

I think I made a small error in my last post, and I want to alleviate any
confusion. I made the following statment:

"The next hop in the chain will have an appropriate trust value of
"undefined", and the one following it will simply be "unknown"."

This is somewhat incorrect. In the case where Bob issues a tsign with a
depth greater than the one issued to him by Alice, the next link in the
chain (Charlie) will appear as valid to Alice but his trust will be
"undefined". David, who was tsigned by Charlie, will then appear to Alice
with a validity of "undefined" and a trust of "undefined". Finally, Elena,
who was signed normally (tsign of 0) by Charlie, will have a validity of
"unknown" and a trust of "unknown". This all makes sense to me semantically
but I just wanted to clarify so people don't get confused by my last post.

George
 

David Shaw wrote:
> 
> On Mon, Jun 16, 2008 at 01:14:52PM -0700, bezna wrote:
>> 
>> Dear David,
>> 
>> First, thank you for your reply and for the working link to the white
>> paper.
>> You have my enormous gratitude for taking the time to share your
>> knowledge
>> with me. 
>> 
>> 
>> David Shaw wrote:
>> > 
>> >> Furthermore, if Bob tsigned Carmen with a depth of 4 (for his own
>> >> purposes), the chain of trust linking Eve to Alice would be broken
>> >> since GPG computes certificate validity (and trust in the case of
>> >> tsigns) only down paths where each next node in the path was tsigned
>> >> with a lower ???depth??? than the ???depth??? of the tsign on the
>> >> node before it.
>> > 
>> > The chain isn't broken, the depth at each step is just lowered to
>> > match the perspective of the head of the chain (Alice in this case).
>> > If Bob tsigns Carmen, then from Bob's perspective, that chain in the
>> > link had a depth of 4.  Similarly, if Roger signs Bob's key with a
>> > depth of 5, then we have a new chain from Roger's perspective where
>> > the Bob->Carmen link also has a depth of 4.  If Roger signed Bob's key
>> > with a depth of 4, then the Bob->Carmen link has a depth of 3 (as it
>> > is lowered to match the maximum depth granted by Roger).  If Roger
>> > signed Bob's key with a depth of 50, then the Bob->Carmen link has a
>> > depth of 4 (as it can't be larger than what Bob granted).
>> > 
>> 
>> I believe you might be mistaken on this point. I ran some trials and it
>> seems that if the next trust signature in the chain has a higher depth
>> than
>> the one preceding it, it is treated just as a regular signature (depth 0) 
>> and the trust data contained in the signature is discarded, effectively
>> breaking the chain. The next hop in the chain will have an appropriate
>> trust
>> value of "undefined", and the one following it will simply be "unknown".
>> See
>> for yourself:
> 
> Interesting.  I'm going to have to go back to my notes from when I
> wrote that code back in 2002, and see what I was shooting for.  My
> memory is that I wanted the trust depth to automatically degrade as
> the chain continued.  It's possible this is just a bug, or it is
> possible I did it this way on purpose (PGP compatibility, maybe?)
> 
> I'll let you know what I find.
> 
> David
> 
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> 

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