Cryptography Reference
In-Depth Information
1. It sends ID
n
to the
r
previous customers ID
n
−
1
,...,
ID
n
−
r
.
2. It computes
z
n
−
1
=
H
(
L
n
−
1
) where
H
is a hash function and
L
n
−
1
was previ-
ously computed.
3. It sets
L
n
to the sequence ((
t
n
−
r
,
ID
n
−
r
,
y
n
−
r
,
z
n
−
r
)
,...,
(
t
n
−
1
,
ID
n
−
1
,
y
n
−
1
,
z
n
−
1
)).
4. It takes the current date and time
t
n
.
5. It authenticates
8
and sends [
n
,
t
n
,
ID
n
,
y
n
,
L
n
]toID
n
. (Note that ID
n
will later
ID
n
+
r
as well.)
6. Once in a while, e.g. when
n
is a multiple of
k
, it publishes (
n
receive ID
n
+
1
,...,
,
z
n
,
ID
n
)ina
newspaper.
When someone wants to verify a timestamp
y
n
,
t
n
from ID
n
, he asks for
L
n
and
ID
n
+
1
,...,
r
. He can thus de-
duce the
r
depositors before and after
t
n
. By contacting them and collecting answers,
he will be able to check that previous timestamps are embedded by
H
in
L
n
(thus, they
happened before), and that the timestamp of
L
n
is embedded by
H
in later timestamps
(thus happened before as well). He can therefore check the right sequence of events.
Going backwards and forwards a sufficient number of times, he will reach published
timestamps and will be able to check that they are indeed embedded in a social event
like the newspaper. It is actually hard to falsify
a posteriori
a newspaper which has
been widely distributed. It is necessary here to use some data from a social event (see
Ref. [23]).
ID
n
+
r
. He can retrieve ID
n
−
i
from
L
n
for
i
=
1
,...,
5.6
Wireless Communication: Two Case Studies
5.6.1 The GSM Network
GSM (Global System for Mobile communications) is a standard developed by ETSI for
mobile wireless communications. It includes security standards. Security objectives are
quite low in the sense that they only prevent attacks against the radio channel between
the terminal and a fixed base. The main threats are
usage of the GSM network with a fake or stolen identity,
monitoring of private communications through the radio channel,
monitoring of positioning information in the radio channel.
Secure charging of networks use and privacy in the wireless link are thus the main
concerns. Neither confidentiality of transmissions in the wired part nor authentication
of the network to the terminal are provided in GSM. The GSM security goals thus
just consist of protecting confidentiality, anonymity, and authentication of terminals in
the wireless part. Two standard interfaces are defined: the A3/8 authentication algo-
rithm and the A5 encryption algorithm. Interfaces are publicly available. As it will be
8
Authentication is performed by a digital signature, which will be studied in Chapter 10.
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