1) first let's compare that if all keys stored
in server:
a) full-meshed topology:
It's celar that the RSA public keys is much less than the symmetric
keys
in terms of number and the device/public-key association vs.
link/public-key association.
In addition, for RSA public key model,
the server in each realm can exchange the stored id/public-key info.
without compromise
any privacy of involed devices. It is scalable
that it can incorporate as many as
servers of realms in the internet.
b) star topology:
Appx. same number of keys stored for both method.
However, the star topooloy Is not scable as the
full-meshed topology:
Hub-spoke topology limits the spoke have only 1 secure link.
It is
difficult for a device to join across two different spoke-hub.
Hence, it is only good for a small realm and is not as scable as
a meshed model.
The Hub-spoke is not what internet's (IP) farvorite topology
and don't mention about
secured internet.
2) what if no server is used. (this means no
3rd-party's help and
no out-of-band secure
channel)
For both the pre-share RSA and symmetric key have same issue of
mutual authentication at beginning...
Operational cost is a factor of mathematical effeciency, there are lots of
algorithms/variations
for symmetric and asymmetric keys.
Let's deal with topology first.
--- David
----- Original Message -----
Sent: Monday, December 03, 2001 2:17
PM
Subject: RE: On shared keys (was RE: SOI:
identity protection and DOS)
David,
I
have been trying to convince people that RSA public operation provides no
clear scalability adavantage over symmetric key. So let me do a orange to
orange comparison again using a full mesh and a hub-spoke case. I am only
willing to agree with your saying that RSA is better in scalability
if such comparison proves it.
Assumption:
1)
No CA
2)
RSA key pair generated in device. Symmetric key generated in
server.
3)
Server needs to deliver either public key or symmetric
key.
case
1: Full mesh: N*(N-1)/2
tunnels
RSA
PSK
1)
total number of
keys N
key
pair
N*(N-1)/2 PSK
2)
key generation
cost high
low
3)
number of key delivery
N*(N-1) N*(N-1)
4)
key storage on the box
1 private
key N-1
PSK
N public key
5)
authentication calculation high
low
cost
6)
key on
server
N public
key
N*(N-1)/2 (usually not stored)
case
2 : hub-spoke : N-1 tunnels
RSA
PSK
1)
total number of
keys N
key
pair N-1
PSK
2)
key generation
cost high
low
3)
number of key delivery
(N-1)*2 (N-1)*2
4)
key storage on the box
hub
N public key
N-1 PSK
spoke 2
public
key 1
PSK
5)
authentication calculation high
low
cost
6)
key on
server
N public
key
N-1 (usually not stored)
So
if you are only considering the total number of keys, RSA wins. If you look at
overall operation cost, the comparison speaks itself. Unfortunately,
there is a lack of scalability adavantage for RSA.
>