Elektronika 2009-11.pdf - Instytut Systemów Elektronicznych

The client may be interested on authenticity verification of
received geodata. Server communicates simultaneously with
many clients, and each client generates many requests during
his session. In such situation standard methods of securing
data authenticity are not suitable, digital signing of each
response wouldnt be effective. The proposal of efficient data
authentication method relevant to the described Web GIS architecture
is presented in the next section.
Data authentication schema
The most of communication in described model is confined to
receiving by client geodata or ready map fragments from
server (e.g. by WMS GetMap call), how it was mentioned
above. Presented scheme should enable authenticity verification
of geodata received from server. Whole communication
process between client and server, which is referred to as
a session, consists of geodeta transmission stage, authenticity
proofs publication stage and received geodata authenticity.
We assume that during one session client may ask for and
receive geodata many times, but authenticity verification is
done once at the end of the session. Process of proofs publication
is done in fixe time intervals, independently of running
sessions. Server serves simultaneously many session, potentially
with different clients.
Downloading geodata from server
Course of i-th session between client C and server M is presented
in Fig. 2. For our schema we introduces following symbols:
C - client, M - map server, sid i - identifier of i-th session,
q i,j - j-th question during i-th session, α i,j - j-th geodata answer
during i-th session, st i - time of i-th session begining, et i - time
of i-th session ending, ct i - time of i-th session authenticity
proof publication, p i - authenticity proof.
The session is started by a client, which sends to server
StartSession call. As an answer server sends back StartSessionAcknowledgment
with new session identifier sid i and time
of session beginning st i .
Then client begins sending Request calls to the server
(appropriate for particular server type geodata request e.g.
WMS GetMap). Single Request contains of right question q i,j
and current session indetifier sid i . Server sends back
Response for each Request. Single Response contains demanded
geodata α i,j .
The session is ended by a client. It sends to server End-
Session with session identifier sid i . In answer server sends
back EndSessionAcknowledgment, which contain among others
session end time et i and time ct i of next authenticity proofs
publication. In this publication will be present authenticity information
for the session.
Client sends GetSessionProof just after ct i moment of time
(which was pointed by server in EndSessionAcknowledgment)
and in response it receives SessionProof containing p i data
necessary to verification of geodata transmitted in this session.
Server proof publication
The server serves simultaneously many session and many
different clients. It publishes authenticity proofs in fixed time intervals.
The time period between two following publishing
points will be called a round.
We introduce additional symbols: h - cryptographic hash
function, h 2 - two-argument cryptographic hash function, h 3 -
three argument cryptographic hash function, d i,j - in-round
message digest for pair (q i,j , α i,j ), f i,k - k-th round-ending message
digest for i-th session, f i,k - k-th round-ending message
digest for i-th session, s k - round-ending signature, T k - Merkle
hash tree for k-th round, P i,k - path from leaf to root of T k tree
for i-th session.
At the beginning of each session client and server initialize
theirs local copies of first in-round message digest using session
identifier:
During the session for each (q i,j , α i,j ) request/response pair
client and servers creates their local copies of digests:
Optionally client may store in its local archives all pairs
(q i,j , α i,j ) generated during the session and compute right digest
in a moment of geodata authenticity verification.
At the end of the round server checks which sessions were
active during this time period. Then server creates Merkle
hash tree [7], using as leafs last digest of each active in-round
session. Cryptographic functions h and h 2 are used for tree
creation.
Creation of T k tree for k-th round runs as follows. At the
beginning are created digest for first level nodes (leafs):
(1)
(2)
Digest for second level nodes are computed on the bases of
first level nodes values:
This procedure is repeated for each next level (number of levels
depends on number of first level nodes which equel sessions
number). Proccess ends on the last level, which
contains one and only node, accumulating all previous digest
(tree root):
(3)
(4)
Fig. 2. i-th session between client C and server M
Rys. 2 . i-ta sesja pomiędzy klientem C a serwerem M
(5)
46 ELEKTRONIKA 11/2009