Questions about trust signatures

[bezna]

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:

A nice "continuous" chain of tsigs of descending order:

-------------------------------------------
pub   1024D/B2D7B73D 2008-06-10
uid       [ultimate] Alice
sig 3        B2D7B73D 2008-06-10  Alice
sub   2048g/D4380BAE 2008-06-10
sig          B2D7B73D 2008-06-10  Alice

pub   1024D/87E9321A 2008-06-10
uid       [  full  ] Bobby
sig 3        87E9321A 2008-06-10  Bobby
sig 3      3 B2D7B73D 2008-06-16  Alice
sub   2048g/E88071D6 2008-06-10
sig          87E9321A 2008-06-10  Bobby

pub   1024D/0C73E6E9 2008-06-10
uid       [  full  ] Carlos
sig 3        0C73E6E9 2008-06-10  Carlos
sig 3      2 87E9321A 2008-06-16  Bobby
sub   2048g/4EE6F059 2008-06-10
sig          0C73E6E9 2008-06-10  Carlos

pub   1024D/026D69FA 2008-06-10
uid       [  full  ] David
sig 3        026D69FA 2008-06-10  David
sig 3      1 0C73E6E9 2008-06-16  Carlos
sub   2048g/D9A7D20C 2008-06-10
sig          026D69FA 2008-06-10  David

pub   1024D/1D764C2F 2008-06-10
uid       [  full  ] Elena
sig 3        1D764C2F 2008-06-10  Elena
sig 3        026D69FA 2008-06-16  David
sub   2048g/8829A23F 2008-06-10
sig          1D764C2F 2008-06-10  Elena


And now a chain with a tsig of 4 at the Bobby==>Carlos link, as described in
the earlier post, which apparently generates an interruption in the chain of
trust:

----------------------------------------
pub   1024D/B2D7B73D 2008-06-10
uid       [ultimate] Alice
sig 3        B2D7B73D 2008-06-10  Alice
sub   2048g/D4380BAE 2008-06-10
sig          B2D7B73D 2008-06-10  Alice

pub   1024D/87E9321A 2008-06-10
uid       [  full  ] Bobby
sig 3        87E9321A 2008-06-10  Bobby
sig 3      3 B2D7B73D 2008-06-16  Alice
sub   2048g/E88071D6 2008-06-10
sig          87E9321A 2008-06-10  Bobby

pub   1024D/0C73E6E9 2008-06-10
uid       [  full  ] Carlos
sig 3        0C73E6E9 2008-06-10  Carlos
sig 3      4 87E9321A 2008-06-16  Bobby
sub   2048g/4EE6F059 2008-06-10
sig          0C73E6E9 2008-06-10  Carlos

pub   1024D/026D69FA 2008-06-10
uid       [  undef ] David
sig 3        026D69FA 2008-06-10  David
sig 3      1 0C73E6E9 2008-06-16  Carlos
sub   2048g/D9A7D20C 2008-06-10
sig          026D69FA 2008-06-10  David

pub   1024D/1D764C2F 2008-06-10
uid       [ unknown] Elena
sig 3        1D764C2F 2008-06-10  Elena
sig 3        026D69FA 2008-06-16  David
sub   2048g/8829A23F 2008-06-10
sig          1D764C2F 2008-06-10  Elena


Curious stuff. What do you think?

George

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