Cryptography Reference
In-Depth Information
Server
T
oke
n
User
PIN
2
Challenge
Random
number
generator
1
Challenge
3
K
A
K
A
6
4
Response
= ?
5
7
Decision
Figure 8.3.
Example of a dynamic password scheme based on challenge-response
5. The user sends this response back to the server. This step might involve the
user reading the response off the screen of the token and then typing it into a
computer that is being used to access the authentication server.
6. The server checks that the challenge is still valid (recall our discussion in
Section 8.2.3 regarding windows of acceptance for nonce-based freshness
mechanisms). If it is still valid, the server inputs the challenge into the password
function and computes the response, based on the same algorithm
A
and
key
K
.
7. The server compares the response that it computed itself with the response
that was sent by the user. If these are identical then the server authenticates
the user, otherwise the server rejects the user.
ANALYSIS OF DYNAMIC PASSWORD SCHEME
The dynamic password scheme in Figure 8.3 merits a closer look, just to make
sure that we appreciate both what has been gained in comparison to conventional
passwords and the limitations of this idea.
Firstly, we establish the basis for assurance that the user is who they claim to
be (the security 'bottom line'). From the perspective of the server, the only entity
apart from itself that can compute the correct response is the only other entity
in possession of both the algorithm
A
and the key
K
. The only other entity to
know
K
is the token. The only way of accessing the token is to type in the correct
PIN. Knowledge of the PIN is therefore the basis for assurance of authentication.
So long as only the correct user knows the correct PIN, this dynamic password
scheme will successfully provide entity authentication of the user.
