GSM
Security (P7U3)
Since the air interface is vulnerable to
fraudulent access, it is necessary to employ the authentication before
extending the services to a subscriber. Authentication is built around the
following notions.
- Authentication Key (Ki) resides only in two places, SIM card
and Authentication Center.
- Authentication Key (Ki) is never transmitted over air. It is
virtually impossible for unauthorized individuals to obtain this key to
impersonate a given mobile subscriber.
Authentication Parameters
The MS is authenticated by the VLR with a
process that uses three parameters:
- RAND which is completely random number.
- SRES which is an authentication signed response. It is
generated by applying an authentication algorithm (A3) to RAND and Ki.
- Kc which is cipher key. The Kc parameter generated by applying
the cipher key generation algorithm (A8) to RAND and Ki.
These parameters (named an authentication
triplet) are generated by the AUC at the request of the HLR to which the
subscriber belongs. The algorithms A3 and A8, are defined by the PLMN operator
and are executed by the SIM.
Steps
in Authentication Phase:
- The new VLR sends a request to the HLR/AUC (Authentication
Center) requesting the "authentication triplets" (RAND, SRES,
and Kc) available for the specified IMSI.
- The AUC using the IMSI, extracts the subscribers authentication
key (Ki).The AUC then generates a random number (RAND), applies the Ki and
RAND to both the authentication algorithm (A3) and the cipher key,
generation algorithm (A8) to produce an authentication Signed Response
(SRES) and a Cipher Key (Kc). The AUC then returns an authentication
triplet: RAND, SRES and Kc to the new VLR.
- The MSC/VLR keeps the two parameters Kc and SRES for later use
and then sends a message to the MS. The MS reads its Authentication Key
(Ki) from the SIM, applies the received random number (RAND) and Ki to
both its authentication algorithm (A3) and Cipher key generation Algorithm
(A8) to produce an Authentication Signed Response (SRES) and Cipher key
(Kc). The MS saves Kc for later, and will use Kc when it receives command
to cipher the channel.
- The MS returns the generated SRES to the MSC/VLR. The VLR
compares the SRES returned from the MS with the expected SRES received
earlier from the AUC. If equal, the mobile passes authentication. If
unequal, all signaling activities will be aborted. In this scenario, we
will assume that authentication is passed.
Encryption/Ciphering:
Data is encrypted at the transmitter side
in blocks of 114 bits by taking 114-bit plain text data bursts and performing
an EXOR (Exclusive OR) logical function operation with a 114-bit cipher block.
The decryption function at the receiver
side is performed by taking the encrypted data block of 114 bits and going
through the same "exclusive OR" operation using the same 114-bit
cipher block that was used at the transmitter.
The cipher block used by both ends of
transmission path for a given transmission direction is produced at the BSS and
MS by an encryption algorithm called A5. The A5 algorithm uses a 64-bit cipher
key (Kc), produced during the authentication process during call setup and the
22-bit TDMA frame number (COUNT) which takes decimal values from 0 through 2715647,
and has a repetition time of 3.48 hours (hyper frame interval).The A5 algorithm
actually produce two cipher blocks during each TDMA period. One path for the
uplink path and the other for the downlink path.
Time
Slot Staggering:
Time slot staggering is the principle of
deriving the time slot organization of uplink from the time slot organization
of the downlink. A particular time slot of the uplink is derived from the
downlink by shifting the downlink time slot number by three.
Reason:
By shifting three time slots, the mobile
station avoids the ‘transmit and receive’ processes simultaneously. This allows
an easier implementation of the mobile station; the receiver in the mobile
station does not need to be protected from the transmitter of the same mobile
station. Typically a mobile station will receive during one time slot, and then
shifts in frequency by 45 MHz for GSM-900 or 95 MHz for GSM-1800 to transmit
sometime later. This implies that there is one time base for downlink and one
for uplink.
Timing
Advance:
Timing Advance is the process of
transmitting the burst to the BTS (the timing advance) early, to compensate for
the propagation delay.
Why
is it Needed ?
It is required because of the time division
multiplexing scheme used on the radio path. The BTS receives signals from
different mobile stations very close to each other. However when a mobile
station is far from the BTS, the BTS must deal with the propagation delay. It
is essential that the burst received at the BTS fits correctly into time slot.
Otherwise the bursts from the mobile stations using adjacent time slots could
overlap, resulting in a poor transmission or even in loss of communication.
Once a connection has been established, the
BTS continuously measures the time offset between its own burst schedule and
the reception schedule of the mobile station burst. Based on these
measurements, the BTS is able to provide the mobile station with the required
timing advance via the SACCH. Note that timing advance is derived from the
distance measurement which is also used in the handover process. The BTS sends
a timing advance parameter according to the perceived timing advance to each
mobile station. Each of the mobile station then advances its timing, with the
result that signals from the different mobile stations arriving at BTS, and are
compensated for propagation delay.
Time
Advance Process:
- A 6 bit number indicates how many bits the MS must advance its
transmission. This time advance is TA.
- The 68.25 bit long GP (guard period) of the access burst
provides the required flexibility to advance the transmission time.
- The time advance TA can have a value between 0 and 63 bits
long, which corresponds to a delay of 0 to 233 micro second. For instance
the MS at 10 km away from the BTS must start transmitting 66 micro second
earlier to compensate for the round trip delay.
- The maximum mobile range of 35Km is rather determined by the
timing advance value than by the signal strength.
No comments:
Post a Comment