U.S. patent application number 11/781360 was filed with the patent office on 2008-01-17 for method and apparatus for storing and distributing encryption keys.
This patent application is currently assigned to MOTOROLA, INC.. Invention is credited to WALTER F. ANDERSON, DAVID J. CHATER-LEA, JASON J. JOHUR, RANDY KREMSKE, DANIEL J. MCDONALD, DENNIS NEWKIRK, SCOTT J. PAPPAS, HANS CHRISTOPHER SOWA, GLENN BRIAN WALTON.
Application Number | 20080013736 11/781360 |
Document ID | / |
Family ID | 25136810 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080013736 |
Kind Code |
A1 |
SOWA; HANS CHRISTOPHER ; et
al. |
January 17, 2008 |
METHOD AND APPARATUS FOR STORING AND DISTRIBUTING ENCRYPTION
KEYS
Abstract
A plurality of infrastructure system devices other than a mobile
station is divided into a plurality of pools. An intrakey is
utilized to encrypt messages passed between infrastructure system
devices in the same pool, and an interkey is utilized to encrypt
messages passed between infrastructure system devices of different
pools.
Inventors: |
SOWA; HANS CHRISTOPHER;
(SCHAUMBURG, IL) ; MCDONALD; DANIEL J.; (CARY,
IL) ; CHATER-LEA; DAVID J.; (CROWTHORNE, GB) ;
PAPPAS; SCOTT J.; (LAKE ZURICH, IL) ; JOHUR; JASON
J.; (MAIDENHEAD, GB) ; NEWKIRK; DENNIS; (WEST
CHICAGO, IL) ; KREMSKE; RANDY; (WOODSTOCK, IL)
; ANDERSON; WALTER F.; (ALGONQUIN, IL) ; WALTON;
GLENN BRIAN; (BRAMLEY, GB) |
Correspondence
Address: |
MOTOROLA, INC.
1303 EAST ALGONQUIN ROAD
IL01/3RD
SCHAUMBURG
IL
60196
US
|
Assignee: |
MOTOROLA, INC.
1303 E. ALGONQUIN ROAD LAW DEPARTMENT
SCHAUMBURG
IL
60196
|
Family ID: |
25136810 |
Appl. No.: |
11/781360 |
Filed: |
July 23, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09785849 |
Feb 16, 2001 |
7266687 |
|
|
11781360 |
Jul 23, 2007 |
|
|
|
Current U.S.
Class: |
380/278 |
Current CPC
Class: |
H04W 84/08 20130101;
H04W 12/04 20130101; H04L 9/3271 20130101; H04L 63/0853 20130101;
H04L 2209/80 20130101; H04W 12/06 20130101; H04L 9/0894 20130101;
H04L 63/0428 20130101; H04W 12/02 20130101; H04L 9/083 20130101;
H04W 12/0431 20210101; H04L 63/062 20130101 |
Class at
Publication: |
380/278 |
International
Class: |
H04L 9/14 20060101
H04L009/14 |
Claims
1. A method comprising the steps of: dividing a plurality of
infrastructure system devices other than a mobile station into a
plurality of pools; utilizing an intrakey to encrypt messages
passed between infrastructure system devices in the same pool; and
utilizing an interkey to encrypt messages passed between
infrastructure system devices of different pools.
2. The method of claim 1, wherein each of the plurality of pools
comprises a mutually exclusive subset of the plurality of
infrastructure system devices.
3. The method of claim 1, wherein the messages comprise at least
one encryption key.
4. The method of claim 1, wherein the messages comprise session
authentication information.
5. The method of claim 1, wherein each different pool utilizes a
different intrakey.
6. The method of claim 1, wherein only one infrastructure system
device from each pool utilizes the interkey.
7. The method of claim 1, wherein the plurality of infrastructure
system devices are part of a communication system infrastructure
that provides encrypted communications.
8. The method of claim 1, wherein at least one of the plurality of
infrastructure system devices has its own protection key, which
protection key is utilized to encrypt and decrypt any of the
intrakey and the interkey for transport to any of the other
infrastructure system devices.
9. The method of claim 1, wherein each pool of the plurality of
pools is comprised of one or more infrastructure system devices
that reside in a single zone of a plurality of zones in a
communication system.
10. The method of claim 9, wherein the one or more infrastructure
system devices that reside in a single zone are comprised of at
least one of a base station, a base site, a TETRA site controller,
and a zone controller.
11. The method of claim 9, wherein only a zone controller within
each of the plurality of zones stores the interkey.
12. The method of claim 1, wherein the interkey is utilized to
encrypt messages passed between an infrastructure system device and
a key management facility.
13. The method of claim 1, wherein a message is encrypted by one of
the intrakey and the interkey based on an infrastructure system
device to which the message is forwarded.
14. The method of claim 1 further comprising the steps of: storing
a protection key for each of the plurality of infrastructure system
devices; and when transporting key material to an infrastructure
system device of the plurality of infrastructure system devices,
encrypting the key material with the protection key associated with
the infrastructure system device.
15. The method of claim 14, wherein the key material is a key
encryption key.
16. The method of claim 14, wherein each of the plurality of
infrastructure system devices has its own unique protection key.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of prior application Ser.
No. 09/785,849, filed Feb. 16, 2001.
FIELD OF THE INVENTION
[0002] This invention relates to encrypted communications,
including but not limited to air interface communication within
secure communication systems.
BACKGROUND OF THE INVENTION
[0003] Encrypted voice and data systems are well known. Many of
these systems provide secure communication between two or more
users by sharing one piece of information between the users, which
permits only those users knowing it to properly decrypt the
message. This piece of information is known as the encryption key
variable, or key for short. Loading this key into the actual
encryption device in the secure communication unit is a basic
requirement that allows secure communication to occur. To retain
security over a long period of time, the keys are changed
periodically, typically weekly or monthly.
[0004] Encryption is known to be performed on an end-to-end basis
within a communication system, i.e., encrypting a message at the
originating communication unit (also known as a mobile station),
passing it transparently (i.e., without decryption) through any
number of channels and/or pieces of infrastructure to the end
user's communication unit, which decrypts the message.
[0005] The Terrestrial Trunked Radio (TETRA) communication standard
is presently utilized in Europe (hereinafter TETRA Standard), with
potential for expansion elsewhere. The TETRA Standard calls for air
interface, also known as air traffic or over-the-air, encryption.
Air interface encryption protects information on the air interface
between the infrastructure and the mobile subscriber. The TETRA
standard calls for an authentication center, also known as a key
management facility or key management center, to generate,
distribute, and authenticate encryption keys and users. The TETRA
standard does not, however, specify how to implement an
authentication center, nor how to generate, distribute, and
authenticate key material to system devices or mobile stations for
information traversing through the infrastructure or SwMI
(Switching and Management Infrastructure), as it is referred to in
the TETRA Standard.
[0006] The TETRA standard fails to provide definition to minimize
burden to call processing and bandwidth, provide encryption and
authentication in a manner tolerant to equipment faults, support
wide-area communications, and to store keys for all communication
units without undue storage burden at local sites.
[0007] Accordingly, there is a need for a method and apparatus for
providing a secure infrastructure for a communication system that
utilizes air interface encryption and generates, distributes, and
authenticates encryption keys and users without causing undue
burden to call processing, bandwidth, security, and storage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a block diagram of a secure communication system
in accordance with the invention.
[0009] FIG. 2 is a block diagram showing key distribution pools in
accordance with the invention.
[0010] FIG. 3 and FIG. 4 are block diagrams showing key storage
within a communication system in accordance with the invention.
[0011] FIG. 5 is a diagram showing key storage and authentication
information distribution within a communication system in
accordance with the invention.
[0012] FIG. 6 is a diagram showing authentication information
storage and authentication decision making within a communication
system in accordance with the invention.
[0013] FIG. 7 is a diagram showing authentication of a mobile
station by an authentication center in accordance with the TETRA
Standard.
[0014] FIG. 8 is a diagram showing authentication of an
authentication center by a mobile station in accordance with the
TETRA Standard.
[0015] FIG. 9 is a diagram showing key storage and authentication
information distribution between a communication system and a
mobile station in accordance with the invention.
[0016] FIG. 10 is a diagram showing a key pull within a
communication system in accordance with the invention.
[0017] FIG. 11 is a diagram showing a key push within a
communication system in accordance with the invention.
[0018] FIG. 12 is a diagram showing distribution of a static cipher
key to a base station within a communication system in accordance
with the invention.
[0019] FIG. 13 is a diagram showing distribution of a static cipher
key to a mobile station within a communication system in accordance
with the invention.
[0020] FIG. 14 is a diagram showing distribution of a common cipher
key to a mobile station and a base station within a communication
system in accordance with the invention.
[0021] FIG. 15 is a diagram showing distribution of a group cipher
key to a base station within a communication system in accordance
with the invention.
[0022] FIG. 16 is a diagram showing distribution of a group cipher
key to a mobile station within a communication system in accordance
with the invention.
[0023] FIG. 17 is a flowchart showing a method of key persistence
at a site in a communication system in accordance with the
invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
[0024] The following describes an apparatus for and method of
providing a secure infrastructure for a communication system that
utilizes air interface encryption and generates, distributes, and
authenticates encryption keys and users without causing undue
burden to call processing, bandwidth, security, and storage. System
devices are divided into groups or pools and encryption keys are
defined to provide secure transfer of key material among the system
devices.
[0025] A block diagram of a secure communication system that is
comprised of a plurality of zones is shown in FIG. 1. The secure
communication system is comprised of a plurality of system devices
that comprise the infrastructure of the system. A Key Management
Facility (KMF) 101 transfers security data, such as session
authentication information and encryption keys, to a User
Configuration Server (UCS) 103, that forwards the information and
data to the appropriate zone based on configuration data within the
UCS 103. Communications for a first zone are provided by a
plurality of system devices including a Zone Manager (ZM) 105, a
Zone Controller 107 that includes a Home Location Register (HLR)
109 and a Visited (also known as a Visitor or Visitors') Location
Register (VLR) 11 1, an air traffic router (ATR) 113, and a
plurality of base stations (BSs) 115 and 117 located at a plurality
of communication sites within the first zone. Communications for a
second zone are provided by a plurality of system devices including
a ZM 119, a ZC 121 that includes an HLR 123 and a VLR 125, an ATR
127, and a plurality of BSs 129 and 131 located at a plurality of
communication sites within the second zone. The BSs 1 15, 117, 129,
and 131 communicate with a plurality of mobile stations (see FIG.
4). The ZCs 107 and 121 communicate via a network 133, such as a
local area network or a wide area network such as an IP (internet
protocol) network. Only two zones and their associated system
devices are shown for the sake of simplicity, although any number
of zones may be successfully incorporated in the secure
communication system.
[0026] For the sake of simplicity, not all system devices will be
shown in each Figure, but rather a representative set of system
devices that illustrates a particular concept will be provided.
Similarly, not all key material is shown stored in each system
device for the sake of space. Each message containing a key, key
material, configuration, or other information is transferred with
an related identity (ID) such as ITSI or GTSI, although the ID is
generally not shown in the drawings for space considerations.
[0027] The KMF 101 is a secure entity that stores the
authentication key (K) for each mobile station (MS) or
communication unit, such as a portable or mobile two-way radio,
Direct Mode Operation (DMO) gateway, receiver, scanner, or
transmitter (for example, see devices 401, 403, and 405 in FIG. 4).
The KMF 101 provides a random seed (RS) and associated session
authentication keys (KS and KS') for each mobile station associated
with the secure communication system. The KMF 101 also
imports/generates various air interface keys, such as Static Cipher
Key (SCK), Group Cipher Key (GCK), and Common Cipher Key (CCK), for
distribution in the system. The KMF 101 functions as the
authentication center (AuC), as referred to in the TETRA
communication standard, in the system. Typically, there is one KMF
server per system, although there may be one or more KMF clients
per system.
[0028] The UCS 103 is a single point of entry for configuration
data in the system. In the preferred embodiment, the UCS 103 stores
and distributes session authentication information, such as RS, KS,
and KS', to the appropriate home zone in the system. The UCS 103
functions as a non-real time distribution point for session
authentication information in the system.
[0029] The ZM 105 or 119 is a management database for a zone. In
the preferred embodiment, the ZM 105 or 119 stores session
authentication information, such as RS, KS, and KS', for the zone
managed by the particular ZM 105 or 119. The ZM functions as a
non-real time storage facility for authentication information in
the zone.
[0030] The ZC 107 or 121 performs real time authentication for the
mobile stations in its zone. The ZC uses the session authentication
information, such as RS, KS, and KS', to perform the real-time
authentication. The HLR 109 or 123 stores session authentication
information for each MS that has the HLR 109 or 123 as its home.
The VLR 111 or 125 stores session authentication information for
each MS visiting the VLR's 111 or 125 zone. The ZC 107 or 121
performs real-time distribution of its home mobile stations'
session authentication information when the MS roams outside its
home zone. In the preferred embodiment, an HLR 109 or 123 and VLR
111 or 125 are part of each zone controller and perform on behalf
of the same zone for which the zone controller is associated. The
HLR 109 or 123 and VLR 111 or 125 may be part of other system
devices or may be stand alone devices. The derived cipher key (DCK)
is generated during authentication. The ZC 107 or 121 generates and
distributes the DCK for the MS to the BSs 115, 117, 129, and 131
that require the DCK for secure communications.
[0031] The ATR 113 or 127 is the conduit used by the KMF 101 to
send rekey messages or key updates to an MS, such as SCK and GCK.
The KMF 101 sends key updates for mobile stations to the home zone
ATR 113 or 127 for dissemination. All rekey acknowledgments (ACKs),
whether infrastructure or MS originated, pass through the ATR 113
or 127 to the KMF 101.
[0032] Each BS 115, 117, 129, and 131 receives and transmits
authentication messages over the air interface. Each BS 115, 117,
129, and 131 acts as a transmitter for its associated ZC 107 or 121
and as a receiver for the MS in the system. The BS 115, 117, 129,
or 131 uses DCK for air interface encryption with the MS. The BSs
115, 117, 129, and 131 are responsible for sending key material to
the MSs 401, 403, 405, and 407. The result of some of these
operations (SCK, GCK) is sent back to the KMF 101. Because each
base site is comprised substantially of one or more base stations,
the terms base site (or site) and base station are used
interchangeably herein, both sharing the acronym BS. In the
preferred embodiment, a TETRA site Controller (TSC) connects all
the base stations at a site, stores key material, and distributes
key material to the base stations as needed, thereby making keys
available to all base stations at a site. Thus, when a key is said
to be stored at a base station or a base site, in the preferred
embodiment, the TSC actually provides storage for the base station
for key material. Because key storage and distribution and other
key-related functions may be performed by a base site, base
station, or TSC, these terms are considered interchangeable for the
purposes of this document.
[0033] The Mobile Station (MS) authenticates the system and/or is
authenticated by the system using a challenge-response protocol.
Each MS has its own key, K, for use during authentication. Each MS
is assigned to one HLR, which typically remains the same. Each MS
is also associated with only one VLR in the zone in which the MS is
presently located. An MS is not registered on a system until the MS
is active and has passed authentication.
[0034] FIG. 2 is a block diagram showing key distribution pools.
Using a single key encryption key (KEK) to encrypt keys for
distribution system wide is a convenient choice, although a single
KEK would result in degraded security due to the higher likelihood
that the KEK would be compromised and the resultant compromise
would affect the whole system. Using a different KEK for each
system device would be more secure, but would burden storage within
system devices and add unnecessary delays to call processing. FIG.
2 shows a system for using KEKs that is more secure than a single
system-wide key, yet not as burdensome as a different KEK for each
system device. Two types of KEKs are assigned to confidentially
distribute key material (such as air interface keys, session
authentication information, data utilized to generate encryption
keys, and other key-related material) to the system devices of the
infrastructure of a system: intrakeys and interkeys. KEKs are 80
bits in the preferred embodiment.
[0035] The first type of KEK is an intrakey, also referred to as an
intrapool key or intra-zone key, KEK.sub.Z. The system devices are
divided into pools or groups 201, 203, 205, and 207. Each pool is
assigned its own unique intrakey, KEK.sub.Z. In the preferred
embodiment, each pool of devices corresponds to a zone in the
communication system, and each pool has a mutually exclusive
collection of system devices, i.e., each system device only belongs
to one pool. The first pool 201 utilizes KEK.sub.Z1 to encrypt key
material, such as encryption keys and/or session authentication
information, for transfer within the first pool (or zone in the
preferred embodiment) and comprises the first zone controller ZC1
107 and its associated BSs 115, 117, and 211. The second pool 203
utilizes KEK.sub.Z2 to encrypt key material for transfer within the
second pool (or zone in the preferred embodiment) and comprises the
second zone controller ZC2 121 and its associated BSs 129, 131, and
213. The third pool 205 utilizes KEK.sub.Z3 to encrypt key material
for transfer within the third pool (or zone in the preferred
embodiment) and comprises the third zone controller ZC3 223 and its
associated BSs 225, 227, and 229. The fourth pool 207 utilizes
KEK.sub.Z4 to encrypt key material for transfer within the fourth
pool (or zone in the preferred embodiment) and comprises the fourth
zone controller ZC4 215 and its associated BSs 217, 219, and 221.
In the preferred embodiment, the intrakey is used by a zone
controller to distribute key material to base sites/base stations
within its zone. KEK.sub.Z is also used by the KMF 101 to
distribute SCK.
[0036] The second type of KEK is an interkey, KEK.sub.M, also
referred to as an interpool key or inter-zone key. The interkey is
used to encrypt key material sent between pools or zones in the
preferred embodiment, or within a certain group 209 of system
devices, particularly from the KMF 101. In the preferred
embodiment, the interkey is used by the KMF 101 to distribute GCK
and individual authentication information to the infrastructure. In
the preferred embodiment, the interkey is stored in one system
device in each zone, in each zone controller 107 and 121, and is
also stored in the KMF 101. The connections shown between the KMF
101 and the zone controllers 107, 121, 215, and 223 are virtual
connections in the preferred embodiment, in that other devices,
such as the UCS 103 and ZMs 105 and 119, are physically located
between the KMF 101 and zone controllers 107, 121, 215, and 223.
The UCS 103 and ZMs 105 and 119 pass encrypted key information in a
transparent manner between the KMF 101 and zone controllers 107,
121, 215, and 223, i.e., the UCS 103 and ZMs 105 and 119 do not
decrypt or encrypt the information, thus no storage of a KEK is
required at the UCS 103 and ZMs 105 and 119, although key material
may be stored in encrypted form at the UCS 103 and ZMs 105 and
119.
[0037] Preferably, a message is encrypted by one of an intrakey and
an interkey, typically using TA31 (decrypted using TA32), based on
a system device to which the message is forwarded. For example,
when the message is intended for a system device in a zone other
than the zone containing the sending device, the interkey is used.
When the message is intended for a system device in the same zone
as the zone containing the sending device, the intrakey is used. In
the preferred embodiment, when the KMF 101 encrypts key material,
such as SCK, CCK, SAI, and GCK, with either the interkey or
intrakey, the KMF 101 uses TA31.
[0038] For example, from time to time, key material is distributed
from the HLR to a VLR and then to the base sites within the zone of
the VLR. In this case, the key material is encrypted by KEK.sub.M
and passed transparently from HLR to VLR. The target VLR decrypts
the key material using its KEK.sub.M and re-encrypts it with the
KEK.sub.Z of the zone for distribution to sites within the
zone.
[0039] Each system device that contains an infrastructure KEK has
its own unique infrastructure or protection key, KI, in the
preferred embodiment. The protection key is only utilized to
decrypt/encrypt KEKs sent by the KMF 101 to the infrastructure
system devices. Preferably, the KI is only able to be loaded by a
key variable loader and is not able to be updated with an OTAR
(over-the-air rekey) operation. In addition to distribution by the
KMF 101, the KEKs may also be manually provided with a Key Variable
Loader. KI is 128 bits long in the preferred embodiment.
[0040] As shown in Table 1 below, KEK.sub.M is only stored by the
zone controllers 107 and 121 and the KMF 101. The intrakey
KEK.sub.Z is held only by the KMF 101, base stations/sites, and
zone controller 107 and 121 within each zone. Each zone has a
unique KEK.sub.Z. Each system device has its own KI. TABLE-US-00001
TABLE 1 Distribution of Key Encryption Key Types Infrastructure
Element Zone 1 Zone 2 Zone Controller (HLR, VLR) KI.sub.1,
KEK.sub.M, KEK.sub.Z1 KI.sub.2, KEK.sub.M, KEK.sub.Z2 Base Sites
KI.sub.3, KEK.sub.Z1 KI.sub.4, KEK.sub.Z2
[0041] The use of intrakeys and interkeys strikes a unique tradeoff
between security and key management complexity as well as speed of
call processing. The KMF 101 need only maintain one interkey plus
one intrakey for each pool or zone in the system. If a KEK.sub.Z is
compromised, the affect and response is localized to that zone,
rather than the whole system, and KI remains intact to redistribute
a new KEK.sub.Z to that zone. KEK.sub.M is stored only at the KMF
101 and the HLR 109 and 123 and VLR 111 and 125 in each zone, which
devices are typically more physically protected from an attack. If
KEK.sub.M is compromised, the KMF 101 changes KEK.sub.M in the ZCs
107 and 121, leaving the sites unaffected.
[0042] Five basic types of air interface keys are used to encrypt
air interface traffic in the secure communication system: a Static
Cipher Key (SCK), a Common Cipher Key (CCK), a Group Cipher Key
(GCK), a Derived Cipher Key (DCK), and a Modified Group Cipher Key
(MGCK). Three basic types of keys are used between the system
devices: an Infrastructure Key (KI) also known as a protection key,
an inter-zone or inter-pool key encryption key also known as an
interkey (KEK.sub.M), and an intra-zone or intra-pool key
encryption key also known as an intrakey (KEK.sub.Z).
[0043] The Static Cipher Key (SCK) is the most basic of the air
interface keys and is used to encrypt inbound (MS to
infrastructure) and outbound (infrastructure to MS) information
when authentication and/or dynamic air interface encryption is not
available. Thus, the generation and distribution of this key has no
relation to authentication.
[0044] The Derived Cipher Key (DCK) is a session key derived within
the authentication procedure. The DCK changes each time an
authentication is performed with the MS and the infrastructure,
also called the SwMI in the TETRA Standard. The DCK is used for
inbound traffic encryption. The DCK is also used for outbound
individually addressed traffic to the MS. DCK is used when using
dynamic air interface encryption operating in TETRA Standard
security class 3.
[0045] This Common Cipher Key (CCK) is a group key in the sense
that multiple MSs have the same CCK. Unlike the GCK, however, the
CCK has no relation to a particular talkgroup (TG). The CCK is
geographically specific, i.e., the CCK serves all units within a
given location area. The location area as defined in the TETRA
standard may be as small as a site or a big as an entire system.
Each unit within a location area uses the same CCK. Group
communications in the outbound direction use CCK when there is no
GCK/MGCK available for that group call. CCK is used for the
encryption of outbound group traffic and identities only. Inbound
identities are encrypted with CCK when DCK is in use.
[0046] Indirectly, the Group Cipher Key (GCK) is used to encrypt
outbound talkgroup calls. In the preferred embodiment, a GCK is
defined for each talkgroup in the system. Actually, the GCK is only
indirectly used for the encryption of traffic information; the
modified group cipher key (MGCK), which is a derivative of the GCK,
is directly used for traffic encryption. GCK is never used for the
actual encryption of traffic as it is considered a long term
key.
[0047] The Modified Group Cipher Key (MGCK) is used to encrypt
outbound talkgroup call traffic. MGCK is formed by the combination
of GCK and CCK. Each GCK has a corresponding MGCK defined in for a
location area.
[0048] Each infrastructure element has an infrastructure or
protection key, KI, that is used as the encryption key for any
infrastructure key encryption key updates. KI is similar in
function to the authentication key, K, in a mobile station. In the
preferred embodiment, KI is updated only by a provisioning device
such as a key variable loader. In the preferred embodiment,
infrastructure key encryption key (KEK) updates cannot be performed
without this key.
[0049] Each zone controller has an interkey, KEK.sub.M, also
referred to as an inter-zone or inter-pool key, which is used to
encrypt all key traffic passed between the KMF and each zone.
KEK.sub.M is also used by the zone controller to pass GCK, CCK, and
DCK, as well as session authentication information, between zones.
In the preferred embodiment, one KEK.sub.M is present in the KMF
and each of the zone controllers in each system.
[0050] Each zone has its own intrakey, KEK.sub.Z, also referred to
as an intra-zone or intra-pool key. The intrakey is used to encrypt
all key traffic within the zone, between the zone controller and
each of the sites within the zones. Each base site and zone
controller has the same KEK.sub.Z in a zone. The KMF stores the
KEK.sub.Z for each zone in the system.
[0051] A method of the present invention establishes an expected
lifetime, or rekey interval, for an encryption key. Table 2 below
shows example rekey intervals for each key stored in the secure
communication system. When the expected lifetime for an encryption
key expires, i.e., when the rekey interval occurs, the encryption
key is replaced.
[0052] A number of storage locations for each type of system device
within a communication system is determined. For example, one KMF
101, one UCS 103, one ZM 105 or 119 per zone, one zone controller
107 or 121 per zone, one HLR 109 or 123 per zone, one VLR 111 or
125 per zone, and a number of sites and corresponding base stations
per site depending on the coverage requirements for each zone.
Based on the expected lifetime for each encryption key and the
number of storage locations for each system device, a type of
system device is assigned to store each encryption key, and the
encryption keys are stored at the system device of the assigned
type. For example, derived cipher keys are stored at base stations
and in the HLR/VLR, common cipher keys are stored at base stations,
modified group cipher keys are stored at base stations, and group
cipher keys that are stored at HLRs and VLRs.
[0053] Table 2 shows the target (user) of each key and the rekey
interval, i.e., time between changes or updates of the specific key
in a preferred embodiment. For example, the MGCK, which is a
combination of CCK and GCK, is updated whenever CCK is changes and
whenever GCK is changed. Table 2 may be changed by the KMF
operator. TABLE-US-00002 TABLE 2 KEY TARGET REKEY INTERVAL SCK All
MS, all BS 1 year/or if compromised DCK MS, BS, HLR, VLR <24
hrs, whenever unit authenticates CCK group (TG HLR), all MS, 24 hrs
all BS GCK group (TG HLR) 6 months MGCK group(BS, MS) 24 hrs -
Minimum of CCK, GCK interval KI All devices using KEK.sub.Z Never
changes or KEK.sub.M (BS, ZC KEK.sub.Z zone 6 months/or if
compromised KEK.sub.M system 6 months/or if compromised
[0054] PC (personal computer) based software programs exist that
provision both mobile stations and infrastructure system devices
with keys. A more secure method utilizes the capabilities of the
Key Variable Loader (KVL), or key loader for short, to load keys
into the infrastructure devices as well as the MS. The key loader
has a hardware based encryption device for the securing of keys
stored within the device. The KVL may obtain keys directly from the
KMF acting as a store and forward agent in order to disseminate the
key encryption keys to the various devices.
[0055] Although a KVL is a very secure way to provide keys, it is a
very time consuming process to use one or more KVLs to provide keys
at each system device and mobile station. A method of key
management is needed to store and distribute the KEKs and other key
material to system devices such as zone controllers and base
sites.
[0056] The KMF 101 is responsible for the generation, key
distribution, and tracking of most of the air interface keys (not
DCK or MGCK) in the system. The base sites 115 and 117 and each
zone controller 107 serve as a proxy to the KMF 101 for key
distribution. The KMF 101 distributes key material to the zones
through the UCS 103, ZMs 105 and 119, and/or ATRs 113 and 127
depending on the key being distributed. The KMF 101 processes
acknowledgement information from the ATR 113 and 127 to maintain
currency of the system devices and MSs 401, 403, 405, and 407. FIG.
3 and FIG. 4 show key material storage within the communication
system.
[0057] As shown in FIG. 3, the KMF 101 stores a protection key and
associated KEK(s) for each system device. The KMF 101 stores a
protection (infrastructure) key, an interkey, and an intrakey for
each zone controller. For example, the first zone controller 107 is
associated with the keys KI.sub.ZC1, KEK.sub.M, and KEK.sub.Z1. The
KMF 101 stores these keys encrypted by a hardware key and the first
zone controller 107 stores KI.sub.ZC1 and the encrypted KEK.sub.M
and KEK.sub.Z1. The KMF 101 stores a protection key and intrakey,
both protected by a hardware key, for each BS. For example, the KMF
101 and the first BS 115 both store the protection key KI.sub.BS1
and the intrakey KEK.sub.Z1. In the preferred embodiment, the KMF
101 stores keys encrypted/protected by a hardware key.
[0058] Prior to distribution of a KEK in the preferred embodiment,
the KMF 101 encrypts KEKs with the protection key, KI, and the use
of encryption algorithms TA41 and TA51, similar to that shown in
FIG. 10 titled "Distribution of SCK to an individual by an
authentication centre" and its associated text in the Terrestrial
Trunked Radio (TETRA); Voice plus Data (V+D); Part 7: Security, EN
300 392-7 V2.1.1, 2000-12 (herein referred to as "TETRA Standard"),
which is incorporated in its entirety herein by reference. The KMF
101 stores an encryption process 301 that combines RSO and the
appropriate KEK, KEKN, and KEK-VN utilizing encryption algorithms
TA41 303 and TA51 305, yielding SKEK, which is a sealed version of
the KEK. RSO, SKEK, KEKN, and KEK-VN are forwarded to the target
system device. Curly brackets { } followed by a key name indicate
that the material within the curly brackets was created using TA41
and TA51 and the key name after the brackets.
[0059] For example, KEK.sub.Z1 is intended to be transferred to the
first zone controller 107 and BS1 115. RSO, KEK.sub.Z1,
KEK.sub.Z1-VN, and KEK.sub.Z1N, and KEK.sub.ZC1 are combined
utilizing encryption algorithms TA41 and TA51, yielding
SKEK.sub.Z1. Key material RSO, SKEK.sub.Z1, KEK.sub.Z1-VN, and
KEK.sub.Z1N are forwarded transparently through ZM1 105 to the
first zone controller 107, which combines this key material with
KI.sub.ZC1 using TA41 and TA52 (as described in the TETRA
Standard), yielding KEK.sub.Z1, which is stored at ZC1 107. RSO,
KEK.sub.Z1, KEK.sub.Z1-VN, and KEK.sub.Z1N, and KI.sub.BS1 are
combined utilizing encryption algorithms TA41 and TA51, yielding
SKEK.sub.Z1. Key material RSO, SKEK.sub.Z1, KEK.sub.Z1-VN, and
KEK.sub.Z1N are forwarded transparently through ZM1 105 to BS1 115,
which combines this key material with KI.sub.BS1 using TA41 and
TA52, yielding KEK.sub.Z1, which is stored at BS 1115. In the
preferred embodiment, an unencrypted acknowledgment of successful
receipt of each key is returned to the KMF 101 via the ATR 113.
[0060] A block diagram showing key storage within a communication
system is shown in FIG. 4. In particular, storage of session
authentication information throughout the communication system is
shown. In the preferred embodiment, session authentication
information includes a random seed, RS, and two session keys, KS
for authentication of an MS and KS' for authentication of the
infrastructure, for each mobile station 401, 403, and 405 (only
three are shown due to space constraints, although numerous MSs are
part of the system). The session authentication information (SAI)
is used to generate a derived cipher key (DCK) for each MS 401.
[0061] For each MS 401, 403, and 405, the KMF 101 stores an
Individual TETRA Subscriber Identity (ITSI), TETRA Equipment
Identity (TEI), and an MS authentication key ("MS key") that is
unique to and stored within each MS 401, 403, and 405. In the
preferred embodiment, the air interface keys and the MS keys are
stored in hardware encrypted fashion using a hardware key K.sub.H
within the KMF 101. The DVI-XL algorithm, available from Motorola,
Inc., is used to encrypt the keys for storage in the KMF 101 in the
preferred embodiment. Square brackets [ ] followed by a key name
indicate that the material within the square brackets is encrypted
by that key.
[0062] The KMF 101 generates session authentication information for
each MS 401, 403, and 405, which SAI is at least partially
encrypted and forwarded in non-real time to the UCS 103 for
storage. For each MS 401, 403, and 405, the UCS 103 stores the
ITSI, TEI, and ID of the HLR associated with each MS, as well as
the SAI. In the preferred embodiment, KS and KS' are stored
encrypted by the interkey (as received from the KMF 101) at the UCS
103 for fast and easy transport, and RS is stored unencrypted. The
UCS 103 is a transparent device in the preferred embodiment, thus
it performs no encryption or decryption functions. In order to
eliminate potential double entry of information, the KMF 101
receives configuration information from the UCS 103. Examples of
configuration information are: Individual TETRA Subscriber Identity
(ITSI), Group TETRA Subscriber Identity (GTSI), home zone, and zone
managers. The KMF uses a table lookup, such as a DNS (Domain Name
Server) lookup table, to obtain the ATR 113 and 127 addresses. The
distribution of each of the different key types has different
configuration requirements, as described herein.
[0063] The UCS 103 forwards the appropriate SAI to each ZM 105 in
non-real time, based on the HLR ID associated with each MS 401. The
ZM 105, like the UCS 103, is a transparent device and performs no
encryption or decryption functions. The ZM 105 stores, for each MS
having the HLR 109 as its home location, an ITSI, TEI, and SAI. In
the preferred embodiment, KS and KS' are stored encrypted by the
interkey (as received from the UCS 103) at the ZM 105 or 119 for
fast and easy transport, and RS is stored unencrypted.
[0064] The ZM 105 forwards the SAI to the HLR 109 in non-real time.
The HLR 109 stores an ITSI and the SAI for each MS 401, 403, and
405. In the preferred embodiment, KS and KS' are stored encrypted
by the interkey (as received from the ZM 103) at the HLR 109, and
RS is stored unencrypted. In the preferred embodiment, RS, KS, and
KS' are stored unencrypted at the VLR 111 for faster
authentication. In an alternative embodiment, KS and KS' may be
stored unencrypted at the HLR 109 for faster authentication.
[0065] When an MS 401 is authenticated at the zone, a new DCK for
the MS 401 is generated by the VLR 111 at the zone controller 107
from the SAI in real time, after any encrypted SAI is decrypted due
to transfer of the SAI from the HLR 109. (The ITSI, SAI, and
previous DCK associated with that MS 401 are forwarded to the VLR
111 in real time before the new DCK is created.) The ITSI, SAI, and
new DCK are forwarded to the HLR 109 in real time for storage. In
the preferred embodiment, the ITSI, SAI, and DCK comes from the HLR
for the MS 401, thus this information may come from a different
zone if the MS 401 does not use the HLR 109 for its home. When the
SAI/DCK comes from a different zone, that zone decrypts/encrypts
the information, as necessary, with the interkey for transport to
the appropriate zone, which also provides appropriate
decryption/encryption within the zone. DCK is stored encrypted by
the intrakey KEK.sub.Z for the zone in which it is stored, for easy
and fast transport to the local BS 115 or 117. In the example shown
in FIG. 4, each DCK is stored encrypted by KEK.sub.Z1. In the
preferred embodiment, KS and KS' are always encrypted with the
interkey KEK.sub.M, for fast and easy transport during the
authentication process, even when transfer is within the same
zone.
[0066] During the authentication process, the BS 115 communicating
with the MS 401 receives, from ZC1 107 in real time, the MS's 401
DCK, encrypted by the intrakey KEK.sub.Z1. The BS 115 stores the
ITSI and DCK unencrypted for immediate use while the MS 401 is at
the coverage area of the BS 115. See FIG. 17 and its associated
text for information regarding key persistence at each site.
[0067] Each MS 401, 403, and 405 stores its own ITSI, TEI, and DCK
in unencrypted form, and K is stored in scrambled or encrypted
form. Each MS 401, 403, and 405 also stores in unencrypted form
relevant CCKs, GCKs, MGCKs, and SCKs as they are received. These
keys may be stored encrypted in the infrastructure in an
alternative embodiment.
[0068] The zone controller 107 is responsible for the real time
distribution of keys and mobility management thereof. It maintains
keys that may need to be distributed in a real-time manner
necessary when roaming, for example. The group cipher key is an
element in each talkgroup record and is kept in the talkgroup HLR.
The common cipher key is a zone or site specific key and is
maintained in the zone controller as well. The ZC is responsible
for the creation of the MGCK (based upon the GCK and CCK) and the
distribution to the sites.
[0069] Because keys reside in the talkgroup and individual HLR 109,
the zone controller 107 is not transparent with respect to the
encryption of key material. The ZC 107 maintains a protection key,
KI, and two infrastructure key encryption keys, interkey KEK.sub.M
and intrakey KEK.sub.Z, for the distribution of key material. KI is
used to seal (encrypt) KEK.sub.M and KEK.sub.Z when they are sent
from the KMF 101. Most key information is encrypted by the KMF 101
with the interkey, KEK.sub.M. The zone controller 107 decrypts the
key material using KEK.sub.M and re-encrypts the same information
using KEK.sub.Z when sending the information to a site within the
zone. Thus, the zone controller 107 has the TETRA algorithms used
for the encryption/decryption of infrastructure keys (such as TA41
and TA52 and TA31 and TA32), as described herein.
[0070] The zone controller sends ACKs from infrastructure re-keying
operations to the KMF 101 via the ATR 113. When a ZC 107 or HLR 109
receives a key update, the device first decrypts key update and
checks for corruption by verifying the integrity of the data and
sends the result of this operation to the KMF 101 via the ATR 113
in the form of an ACK.
[0071] The site is one endpoint for air interface encryption. Audio
on the air interface between the BS 115 and MS 401 is encrypted.
Audio within the infrastructure is not encrypted. Outbound traffic
is encrypted with algorithms using MGCK, CCK, and SCK, or DCK for
individual calls. All inbound traffic is encrypted with algorithms
using DCK or SCK. The site maintains the traffic algorithms and key
storage for SCK, CCK, and MGCK, as well as DCK. Because the base
site has traffic key storage, the base site is not transparent with
respect to the encryption of key material. All key material
distributed to the base site is encrypted by the intrakey,
KEK.sub.Z. Thus, the base site maintains a protection key, KI, and
an interkey, KEK.sub.Z. Thus, the base sites have the TETRA
algorithms used for the encryption/decryption of infrastructure
keys (such as TA41 and TA52 and TA31 and TA32), as described
herein. The MS is the other endpoint point for air interface
encryption. Outbound traffic is encrypted with algorithms using
MGCK, CCK, and SCK, or the DCK if individually addressed. All
inbound traffic is encrypted with algorithms using DCK or SCK, and
identities may be encrypted with SCK or CCK. The MS maintains the
traffic algorithms and key storage for SCK, CCK, GCK, and MGCK as
well DCK.
[0072] The following figures provides examples of the role of the
zone controller 107 or 121 in some of its key generation, key
distribution, and authentication functions, as well as the base
site/base station and MS operations in the key generation, key
distribution, and authentication processes.
[0073] A diagram showing an example of key storage and
authentication information distribution within a communication
system is shown in FIG. 5. Session authentication information (RS,
KS, and KS') is needed to facilitate real-time authentication of
the MS 401 by the ZC 107 and real-time authentication of the system
by the MS, as well as mutual authentication. Triggers for the
transfer of SAI may be a manual initiation by the KMF operator, an
automatic fraud trigger from the system, or a periodic changing of
the SAI by the KMF 101.
[0074] FIG. 5 shows the transfer of SAI for two mobile stations,
ITSI1 401 and ITSI2 403 (both not shown). The KMF 101 encrypts at
least a part of the SAI (e.g., KS and KS') with the interkey
KEK.sub.M for the system, and forwards ITS1, ITSI2, RS, and KS and
KS' encrypted by KEK.sub.M to the UCS 103. The UCS 103 stores a
copy and forwards it to the home ZM 105 or 119 for each ITSI.
Dashed lines within a system device indicate transparent passage of
information through the system device. The ZM 105 or 119 also
stores a copy and forward it to its ZC 107 or 121, in particular,
the HLR 107 or 123. The ZC 107 or 121 stores KS and KS' encrypted
along with RS in the HLR 107 or 123. Once the HLR 109 or 123
receives the SAI, an unencrypted acknowledgement (ACK) is sent,
when decryption using KEK.sub.M fails, back to the KMF 101 via the
ATR 113 or 127 from the zone in which the HLR 109 or 123 resides.
If a VLR 111 for the MS 403 exists, such as ITSI2, the ZC 121 sends
KS and KS' encrypted with the interkey KEK.sub.M to the VLR 111.
Coordination between a previous authentication session information
and a new authentication session information is not needed. The HLR
109 or 123 only needs one copy of SAI per ITSI registered. The UCS
103 and ZM 105 or 119 store copies of authentication session
information to provide recovery from system maintenance or
failures.
[0075] By providing storage and forwarding of session
authentication information and keys in non-real time (i.e., without
time constraint) between first-level system devices and in real
time (i.e., on demand) between second-level system devices as
described above, the authentication system provides a fault
tolerant system that allows for quick fault recovery as well. If
the KMF 101, UCS 103, and/or ZMs 105 and 119 fail or are separated
from the rest of the system, full authentication may still be
performed without interruption on a real-time basis with the
session authentication information, for example for MS2 403, stored
at the HLR 123 and VLR 111. A failure at any of these devices 101,
103, 105, and 119 is not catastrophic, in that the data stored may
be downloaded from any of the other devices that stores the
information. If a zone controller 107, HLR 109, and/or VLR 111
experience a fault or failure, the SAI may be immediately
downloaded from the ZM 105 at the zone. By eliminating the need for
the KMF 101 to participate in real time in the authentication
process, there is less burden on the KMF 101 and less traffic in
general on the communication links between the system devices of
the infrastructure.
[0076] A diagram showing authentication information storage and
authentication decision making within a communication system is
shown in FIG. 6. Four mobile stations are shown within a system
where three mobile stations 401, 403, and 405 use HLR1 109 of the
first zone controller 107, one mobile station 407 uses HLR2 123 of
the second zone controller 121, two mobile stations 401 and 403 use
VLR1 111, and two mobile stations 405 and 407 use VLR2 125. Storage
of SAI is shown throughout the system devices. Also shown are base
station decisions whether or not to authenticate a mobile at a
particular trigger. For example, power-up messages, whether
encrypted or not, require authentication. Any message sent in the
clear (i.e., unencrypted) requires authentication. Encrypted roam
messages may be implicitly authenticated, i.e., the challenge and
response mechanism may be bypassed if the encrypted roam message is
successfully decrypted by the BS 131. Power-up messages, roam
messages, location updates, and other types of messages are
considered requests to communicate within the communication system.
When authentication is required, the BS 115, 117, 129, or 131 sends
a request to authenticate the MS to the infrastructure (to a zone
controller in the preferred embodiment). In the event that the
infrastructure device to which authentication requests are sent
becomes unavailable, e.g., the device fails, is down for
maintenance, or the communication link to the device is not
operable, the BS stores authentication requests during the time
period when the infrastructure device is not available. When the
infrastructure device becomes available, e.g., the device is
returned to service after a failure or maintenance or when the
communication link comes up, the BS forwards the stored
authentication requests to the infrastructure device.
[0077] In one situation shown in FIG. 6, a first MS 401 sends a
clear (unencrypted) power-up message to the first BS 115. In the
preferred embodiment, authentication of the MS 401 in this
situation is required. Because the MS 401 uses HLR 109 in the zone
where the BS 115 is located, the session authentication information
SAI1 for the MS 401 is forwarded from the HLR 109 to the VLR 111 at
the zone for completion of the authentication process.
[0078] The second MS 403 roams from BS1 115 to BS2 117 and sends a
clear (unencrypted) roam message to the second BS 117. In the
preferred embodiment, authentication of the MS 403 in this
situation is required. Because the MS 403 uses the HLR 109 in the
zone where the BS 115 is located, and because the MS 403 roamed
from a site serviced by the same VLR as the new site, the session
authentication information SAI2 for the MS 403 is already located
in the VLR 111 at the zone for completion of the authentication
process.
[0079] The third MS 405 sends an encrypted power-up message to the
third BS 129. In the preferred embodiment, authentication of the MS
405 in this situation is required. Because the MS 405 uses the HLR
123 in the zone where the BS 129 is located, the session
authentication information SAI3 for the MS 405 is forwarded from
the HLR 123 to the VLR 125 at the zone for completion of the
authentication process.
[0080] The fourth MS 407 roams from BS2 117 to BS4 131 and sends an
encrypted roam message to the fourth BS 131. In the preferred
embodiment, (full) authentication of the MS 403 in this situation
is not required. Instead, the MS 407 is implicitly authenticated,
i.e., the challenge and response mechanism is bypassed if the
encrypted roam message is successfully decrypted by the BS 131.
Because the MS 407 uses the HLR 109 in the zone other than the zone
where the BS 131 is located, the encryption key (and if necessary,
the session authentication information SAI4) for the MS 407 must be
forwarded from that HLR 109 to the VLR 125 where the MS 407 has
roamed for completion of the authentication process. Typically, at
least a part of the SAI is encrypted by the interkey prior to
transfer to another zone. If implicit authentication fails, full
authentication of the MS 407 is then performed.
[0081] A diagram showing the challenge-and-response process to
authenticate a mobile station by an authentication center in
accordance with the TETRA Standard is shown in FIG. 7. When
authenticating an MS 707, an authentication center 701, such as a
KMF 101, combines the mobile authentication key, K, with RS
utilizing the encryption algorithm TA11, as defined in the TETRA
Standard. The output of the TA11 process 703 is KS, which is input
with RAND1 (a random number) to the encryption algorithm TA12, as
defined in the TETRA Standard. The TA12 process 705 outputs XRES1,
an expected response, and DCK1, a derived cipher key for the
mobile. RAND1 and RS are provided to the MS 707. The MS 707 goes
through a similar process, by combining its mobile authentication
key, K, with RS received from the AuC 701 utilizing the TA11
process 703. The TA11 process 703 outputs KS, which is input with
RAND1 to the TA12 process 705. The TA12 process 705 in the MS 707
outputs RES1, a response to the challenge, and DCK1, the derived
cipher key for the mobile. The MS 707 forwards RES1 to the AuC 701.
If XRES1 and RES1 match, the AuC 701 sends an authentication pass
message to the MS 707, and communication over the air interface
with the newly created DCK1 may commence. If XRES and RES do not
match, the AuC 701 sends an authentication fail message to the MS
707, and communication over the air interface with the newly
created DCK1 is prohibited, although the old DCK1 may be used upon
authentication failure.
[0082] A diagram showing the challenge-and-response process to
authenticate an authentication center by a mobile station in
accordance with the TETRA Standard is shown in FIG. 8. When
authenticating an AuC 701, such as a KMF 101, an MS 707 combines
the mobile authentication key, K, with RS utilizing the encryption
algorithm TA21, as defined in the TETRA Standard. The TA21 process
801 outputs KS', which is input with RAND2 (a random number) to the
encryption algorithm TA22, as defined in the TETRA Standard. The
TA22 process 803 outputs XRES2, an expected response, and DCK2, a
derived cipher key for the mobile 707. RAND2 is provided to the AuC
701. The AuC 701 goes through a similar process, by combining the
mobile authentication key, K, for the MS 707 with RS utilizing the
TA21 process 801. The TA21 process 801 of the AuC 701 outputs KS',
which is input with RAND2 to the TA22 process 803. The output of
the TA22 process 803 in the AuC 701 is RES2, a response to the
challenge, and DCK1, the derived cipher key for the mobile. The AuC
701 forwards RES and RS to the MS 707. If XRES and RES match, the
MS 707 sends an authentication pass message to the AuC 701, and
communication over the air interface with the newly created DCK1
may commence. If XRES and RES do not match, the MS 707 sends an
authentication fail message to the AuC 701, and communication over
the air interface with the newly created DCK1 does not take
place.
[0083] A diagram showing SAI distribution and the authentication
process between a communication system and a mobile station in real
time in accordance with the invention is shown in FIG. 9. FIG. 9
shows an implementation of the authentication process of the TETRA
Standard including how various system devices within the
infrastructure perform within the authentication process. FIG. 9
shows how the ZC 107, including the HLR 109 and VLR 111, and BS 115
act as proxies, or authentication agents, for the KMF 101 in the
authentication process. In non-real time, KS and KS' encrypted by
the interkey, and RS are passed along from the KMF 101 to the UCS
103, to the first ZM 105, and to the HLR 109 of the first zone
controller 107.
[0084] After the BS 115 sends a request for authentication of the
MS 401 to the ZC 107, the VLR 111 generates RAND1 and uses KS and
RAND1 with the TA12 process to generate XRES1 and DCK1, in accord
with FIG. 7 herein, and forwards RAND1 and RS to the BS 115, which
forwards RAND1 and RS over the air to the MS 401. The MS 401
combines its own K and RS with the TA11 process to generate KS,
then combines RAND1 and KS in accord with FIG. 7 herein, yielding
RES1 and DCK1, and forwards RES1 to the BS 115, which forwards RES1
to the VLR 111 at the ZC 107. The VLR 111 compares RES1 and XRES1,
and the result is R1. When RES1 and XRES1 match, DCK1 and the SAI
for the MS 401 are stored in the VLR 111 and HLR 109 and DCK1
(encrypted by the interkey). In the preferred embodiment, DCK1 is
encrypted with the intrakey for the first zone prior to being sent
to the BS 115. R1 is forwarded to the BS 115 in acknowledgment that
authentication passed, and the BS 115 stores DCK1 and sends R1 to
the MS 401 indicating authentication has passed. When RES1 and
XRES1 do not match, the VLR 111 discards the newly created DCK1
without storing or forwarding to the BS 115 and forwards R1, a
negative acknowledgment of the authentication process, to the BS
115, and the BS 115 sends R1 to the MS 401 indicating
authentication has failed.
[0085] To request authentication of the infrastructure, the MS 403
sends RAND2 to the BS 129, which forwards RAND2 to the VLR 125 in
the ZC 121. The VLR 125 looks up RS and KS' and generates RES2 and
DCK2 using the TA22 process in accord with FIG. 8 herein, and
forwards RES2 and RS to the BS 129, which forwards RES2 and RS over
the air to the MS 403. The MS 403 combines RS and its own K with
process TA21, yielding KS', which is then combined with RAND2 in
the TA22 process in accord with FIG. 8 herein, yielding XRES2 and
DCK2. The MS 403 compares RES2 and XRES2. When RES2 and XRES2
match, the MS 403 sends message R2 to the BS 129 in acknowledgment
that authentication passed, the BS 129 sends R2 to the ZC 121, and
the VLR 125 causes DCK2 and the SAI for the mobile 403 to be stored
in the VLR 125 and the HLR 123 for the MS 403 and forwards DCK2 to
the BS 129, which stores DCK2. In the preferred embodiment, DCK2 is
encrypted with the intrakey for the second zone prior to being sent
to the BS 129. When RES2 and XRES2 do not match, the MS 403 sends
message R2 to the BS 129 indicating that authentication failed, the
BS 129 sends R2 to the ZC 121, and the VLR 125 discards the newly
created DCK2 without sending it to the BS 129.
[0086] In either authentication process, if the VLR 111 in the zone
where the MS 401 or 403 is presently located does not have SAI
stored for the MS 401 or 403, the VLR 111 obtains the SAI from the
HLR for the MS 401 or 403. When the HLR 109 for the MS 401 or 403
is in the same zone, the SAI is simply passed within the ZC 107 to
the VLR 111. When the HLR 109 for the MS 401 or 403 is in a
different zone, the zone for the home HLR is determined from a home
zone mapping table that maps ITSI to its Home Zone, and the SAI is
forwarded to the ZC 107 to the VLR 111. In the preferred
embodiment, when the key material is forwarded from the HLR for the
MS 401 or 403 to the VLR 111, at least some of the SAI, in
particular KS and KS', are encrypted with the interkey. When DCK is
transferred within a zone, DCK is encrypted with KEK.sub.Z.
Similarly, if the zone where authentication takes place is not the
home zone for the MS 401 or 403, updated SAI and DCK information
will be inter key encrypted, at least in part, and forwarded to the
appropriate VLR. As keys are passed between devices that require a
different encryption key, one device receives a message, decrypts
it with one key, and re-encrypts the result with another key for
the next device. Mutual authentication, when the MS and
infrastructure mutually authenticate each other, is described with
respect to FIG. 3 titled "Mutual authentication initiated by SwMI"
and FIG. 4 titled "Mutual authentication initiated by MS" and their
associated text of the TETRA Standard. The resultant DCKs (DCK1 and
DCK2) of each process are combined using the TB4 encryption
algorithm, and the resulting DCK is used to communicate.
[0087] A diagram showing a key pull within a communication system
is shown in FIG. 10. The key pull procedure is used to forward an
air interface key, typically the DCK, although the process may also
be used for GCK/MGCK, into a BS that does not have the DCK for a
mobile station. This situation may occur when an MS switches sites
while idle or a failure arises. FIG. 10 shows MS1 401 switching
from site 1 to site 2 within zone 1 and MS2 403 roaming from zone 2
to zone 1. Although KS, KS', and DCK are stored encrypted at the
HLR, and DCK is stored encrypted at the HLR and VLR in the
preferred embodiment, they are shown unencrypted in FIG. 10 for the
sake of simplicity.
[0088] MS1 401 has roamed from site 1 to site 2 in zone 1. The pull
procedure is initiated by the BS 117 when it recognizes that it
does not have the DCK for the MS 401 that has sent an encrypted
message, for example, a DCK-encrypted location update message. The
BS 117 may optionally forward an acknowledgment of receipt of the
encrypted message to the mobile station 401. The identity, ITSI1,
of the MS 401 is encrypted with CCK, so the BS 117 is able to
determine which MS has sent the message, even though it does not
have DCK1 for the MS 401. The BS 117 requests the DCK1 from the ZC
107. The ZC 107 determines if it needs to request DCK1 from a
different zone. In this case, because MS1 401 is roaming within the
same zone, DCK1 is found in the VLR 111, and the ZC 107 sends DCK1
to the BS 117 encrypted with the intrakey KEK.sub.Z1. The BS 117
uses DCK1 to decrypt the location update message for MS1 401, and
any subsequent message(s) from the MS 401, and forwards the
location update to the ZC 107. In the preferred embodiment, the VLR
111 for the MS 401 is not updated with the MS location until the MS
implicitly authenticates or performs a full authentication. Receipt
of a properly decrypted location update message is considered an
implicit authentication, at which time the VLR 111 would be
updated.
[0089] MS2 403 has roamed from zone 2 to zone 1. The pull procedure
is initiated by the BS 115 when it recognizes that it does not have
the DCK for the MS 403 that has sent an encrypted message, for
example, a DCK-encrypted location update message. The BS 115 may
optionally forward an acknowledgment of receipt of the encrypted
message to the mobile station 403. The identity, ITSI2, of the MS
403 is encrypted with CCK, so the BS 115 is able to determine which
MS has sent the message, even though it does not have DCK2 for the
MS 403. The BS 115 requests the DCK2 from the ZC 107. The ZC 107
determines if it needs to request DCK2 from a different zone, which
is required in this case, because MS2 403 is roaming from a
different zone, zone 2, and the HLR 123 for the MS 403 is in zone
2. The ZC 107 determines which zone has the needed key material and
sends a request to that target zone for the key material. In the
example, DCK2 is found in the HLR 123 for zone 2, which is the
target zone, and DCK2 is sent to the ZC 107 from that zone's HLR
123 after being encrypted with interkey, KEK.sub.M. The ZC 107
sends DCK2 to the BS 115 encrypted with the intrakey KEK.sub.Z1.
The BS 115 uses DCK2 to decrypt the location update message for MS2
403, and any subsequent message(s) from the MS 403, and forwards
the location update to the ZC 107. RS, KS, KS' are requested at a
later time from the HLR 123 so that a full authentication may be
performed as necessary. In the preferred embodiment, the VLR 111
for the MS 403 is not updated with the MS location until the MS
implicitly authenticates or performs a full authentication. Receipt
of a properly decrypted location update message is considered an
implicit authentication, at which time the VLR 111 would be
updated.
[0090] In the situation where it may be desired to pull a GCK/MGCK,
the process is the same as described above with respect to the DCK,
except that the VLR 111 obtains the GCK, combines it with a CCK, as
described below in FIG. 15 and its associated text, and forwards
the resultant MGCK, encrypted with the intrakey KEK.sub.Z1,to the
BS 115 or 117.
[0091] A diagram illustrating a key push within a communication
system is shown in FIG. 11. The key push procedure is used to
forward a key, such as the DCK or GCK/MGCK, to a forwarding site
when an MS switches sites from its current site to the forwarding
site. This process thus provides a mechanism for a key to be
forwarded to a site prior to the arrival of the MS 401 or 403, so
that seamless encrypted handoffs and roaming may occur. FIG. 11
shows an example of a transfer of DCK2 between zones and a transfer
of DCK1 within a zone. The MS initiates the procedure. Although KS,
KS', and DCK are stored encrypted at the HLR, and DCK is stored
encrypted at the HLR and VLR in the preferred embodiment, they are
shown unencrypted in FIG. 11 for the sake of simplicity. MS1 401
begins the process of roaming from BS1 115, having Location Area
Identification 1 (LAID1), at site 1 to BS2 117, having Location
Area Identification 2 (LAID2) at site 2 at zone 1. The MS 401 sends
to BS1 115 a message indicating that MS 1 will roam to site 2. In
the preferred embodiment, this message is an OTAR Prepare message.
The BS 115 relays this message to the ZC 107. The ZC 107 determines
if the DCK needs to be transferred to another zone or not by
determining whether or not the site to which the MS 401 is roaming
is in its zone or not. In this example, site 2 is also serviced by
the ZC 107, thus there is no need to transfer the DCK to another
zone. Because the DCK is transferred within the zone, the ZC 107
responds to the BS 115 with a use short delay message. In this
case, the BS 115 holds off the MS 401 from switching to site 2 by a
delay equivalent to the short delay, which delay approximates the
time it will take to forward DCK to the next site from the VLR 111
in the same zone. In the preferred embodiment, the short delay is
less than 50 ms. The MS 401 waits for an ok from the BS 115 before
operating at the new site, e.g., roaming, switching sites, or
communicating, and the BS 115 sends the ok after the short delay
period expires. During the delay period, the VLR 111 at ZC1 107
encrypts DCK1 with the intrakey and forwards it to BS2 117 at site
2, where the MS 401 and BS2 117 will be able to exchange encrypted
messages using DCK1. In the preferred embodiment, the VLR 111 for
the MS 401 is not updated with the MS location until the MS 401
implicitly authenticates or performs a full authentication.
[0092] MS2 403 begins the process of roaming from BS3 129, having
Location Area Identification 3 (LAID3) at site 3 at zone 2 to BS1
115, having Location Area Identification 1 (LAID1) at site 1 at
zone 1. The MS 403 sends to BS3 129 a message indicating that MS2
will roam to site 1. In the preferred embodiment, this message is
an OTAR Prepare message. The BS 129 relays this message to the ZC
121. The ZC 121 determines if the DCK needs to be transferred to
another zone or not by determining whether or not the site to which
the MS 401 is roaming is in its zone or not. In this example, site
1 is not serviced by the ZC 121, thus there is a need to transfer
the DCK to another zone. Because the DCK is transferred to another
zone, the ZC 121 responds to the BS 129 with a use long delay
message. In this case, the BS 129 holds off the MS 403 from
switching to site 1 by a delay equivalent to the long delay, which
delay approximates the time it will take to forward DCK from the
VLR 111 to the site in the next zone. In the preferred embodiment,
the long delay is greater than or equal to 50 ms. The MS 403 waits
for an ok from the BS 129 before switching sites, and the BS 129
sends the ok after the long delay period expires. During the delay
period, the VLR 125 at ZC1 121 encrypts DCK2 with the interkey and
forwards it to ZC1 107, which decrypts it with the interkey,
encrypts it with the intrakey KEK.sub.Z1, and forwards the result
to BS1 115 at site 1, where the MS 403 and BS2 115 will be able to
exchange encrypted messages using DCK2. In the preferred
embodiment, the VLR 111 for the MS 403 is not updated with the MS
location until the MS 403 implicitly authenticates or performs a
full authentication, at which time the VLR 125 for MS2 in ZC2 121
is eliminated. RS, KS, KS' are requested at a later time from the
HLR at ZC3 223 (the home zone HLR for the MS 403) so that a full
authentication may be performed as necessary.
[0093] FIG. 12 is a diagram showing distribution of a static cipher
key to a BS within a communication system. The SCK is a system wide
voice traffic key that is used to encrypt voice, data, ESI
(encrypted short identity), and signaling traffic when
authentication is not available. SCKs are identified by SCKN and
SCK-VN, and are stored in the KMF 101 encrypted by a hardware key
and in the ZMs 105 and 119 encrypted by TA31. In the preferred
embodiment, there may be up to 32 distinct SCKs in the entire
system. Each BS stores one SCK, identified by SCK number (SCKN),
each of which has an SCK version number (SCK-VN), although SCK may
have multiple versions that are or were used in the system. Each
SCKN has a version number SCK-VN, and in the preferred embodiment,
two version numbers, i.e., two keys, are stored for each SCKN. The
MS must be able to store 32 SCKs for one SCK-VN, in addition to 32
SCKs for another SCK-VN. The 31 additional SCKs in the MS are
defined for direct operation between mobile stations. A new SCK
replaces the oldest SCK-VN. The SCK may be provided to BSs and
mobile stations in several ways, including via a Key Variable
Loader (KVL), via computer software such as RSS Software available
from Motorola, Inc., and via OTAR (Over-the-Air Rekeying) via the
home zone ATR of the MS. Although not shown in the drawing because
of space constraints, SCKN and SCK-VN are sent along with SCK for
identification purposes.
[0094] A process to transfer an SCK to each BS in the system is
shown in FIG. 12. When the KMF 101 determines that an SCK update is
due, the KMF 101 generates a new SCK. In order to determine the
home zone of a BS, in the preferred embodiment, the KMF 101 uses
the BS to home ZC map from the UCS 103 and a table lookup based on
the zone to obtain the address for the ATR in the zone. The KMF 101
encrypts the SCK with the intrakey, KEK.sub.Z, for the zone in
which the BS is located, and sends the encrypted key to the ZM for
that BS. The ZM stores a copy and forwards it to the intended BS.
An unencrypted ACK is sent from the BS to the ZC and to the KMF 101
via the ATR in the zone where the BS resides. The ACK represents
that the SCK was received correctly in the BS.
[0095] A specific example of an SCK transfer to BS1 115 includes a
transfer of site information, including an BS to home zone
controller map, from the UCS 103 to the KMF 101. The KMF 101 uses
the map to determine that BS1 115 is located in zone 1. The KMF 101
generates the SCK and encrypts it with the intrakey, KEK.sub.Z1,
for zone 1 where BS1 is located. The KMF 101 forwards the encrypted
SCK to the ZM 105 for zone 1. ZM1 105 stores a copy of the
encrypted SCK and forwards it to BS1 115 via a wireline link. BS1
115 decrypts the encrypted SCK using KEK.sub.Z1 and stores the SCK
unencrypted. When the SCK is received correctly by BS1, BS1 115
sends an unencrypted ACK to the KMF 101 via ZC1 107 and the ATR 113
in zone 1. Transfers of SCK to BS3 and BS4 are similarly
performed.
[0096] A diagram showing distribution of a static cipher key to a
mobile station within a communication system is shown in FIG. 13.
When the KMF 101 determines that an SCK update for an MS 401 is
due, the KMF 101 generates a new SCK key material for the MS 401
according to FIG. 10 titled "Distribution of SCK to an individual
by an authentication center" and its associated text in the TETRA
Standard. The SCK generation process yields the key material SSCK
(a sealed SCK), SCKN (SCK number), SCK-VN (SCK version number), and
RSO (the random seed used in the process). In order to determine
the ATR for the home zone of the MS 401, in the preferred
embodiment, the KMF 101 uses the ITSI to home ZC map from the UCS
103 and a table lookup based on the zone to obtain the address of
the ATR for the home zone. In the example of FIG. 13, the home zone
for MS1 401 is zone 2. The KMF 101 forwards SSCK, SCKN, SCK-VN, and
RSO to the ATR 127 of the home zone (2) for the MS 401. If the MS
401 is not on the system, the ATR 127 sends a NACK back to the KMF
101. If the MS 401 is on the system, the SCK is delivered to the MS
401 via the zone in which the MS 401 is currently located. In the
preferred embodiment, the SCK key material (e.g., SSCK, SCKN,
SCK-VN, and RSO) are not encrypted for transfer among system
devices. The SCK key material may optionally be encrypted for
transfer among system devices.
[0097] When the MS 401 is not located in its home zone, the home
zone controller 121 of zone 2 determines which zone the MS 401 is
currently located in (zone 1 in FIG. 12) by looking it up in the
HLR 123 of zone 2. ZC2 121 forwards SSCK, SCKN, SCK-VN, and RSO to
the zone controller 107 of the zone where the MS 401 is presently
located. ZC1 107 forwards SSCK, SCKN, SCK-VN, and RSO to the BS 115
where the MS 401 is located. The BS 115 decrypts the SSCK, SCK-VN,
and RSO with the intrakey, KEK.sub.Z1, and forwards the result to
the MS 401. An unencrypted ACK is sent from the MS 401 to the BS
115 to the ZC 107 and to the KMF 101 via the ATR 113 in the zone
where the BS 115 resides. The ACK represents that the SCK was
received and unsealed correctly in the MS (the unsealing process is
described in the TETRA Standard).
[0098] When the MS 401 is located in its home zone (not shown, but
assumed to be at BS3 129 for the sake of this example), the VLR of
the home zone controller 121 forwards SSCK, SCKN, SCK-VN, and RSO
to the BS 129 where the MS 401 is located (not shown but assumed
for this example). The BS 129 forwards SSCK, SCKN, SCK-VN, and RSO
to the MS 401. An unencrypted ACK is sent from the MS 401 to the BS
129 to the ZC 121 and to the KMF 101 via the ATR 127 in the zone
where the BS 115 resides. The ACK represents that the SCK was
received and unsealed correctly in the MS (the unsealing process is
described in the TETRA Standard).
[0099] FIG. 14 is a diagram showing distribution of a common cipher
key to a mobile station and a BS within a communication system. The
CCK is a location area based traffic key that is used to encrypt
voice, data, and signaling within a location area (LA) and is only
used for outbound communications. The CCK is meant for use with the
encryption of group call traffic in the TETRA Standard. The CCK is
also used to encrypt the subscriber identity creating the encrypted
short identity (ESI). Group call traffic within the LA uses the CCK
when there is no GCK available or it is disabled. There is one CCK
per location area. A location area may be a small as a site, thus
there could be as many as CCKs as sites in the system. It is
possible for more than one location area to have the same CCK. CCK
is identified by CCK-ID (e.g., CCK1, CCK2, and so forth) and LAID
(location area identification). Two copies of each CCK (the latest
two CCK-IDs) are in the ZC and the BS to enable a gradual rekeying
of the MS in the system. While one CCK is in use, the next one is
distributed to the MS. In the preferred embodiment, each site
maintains a CCK for each site adjacent to the site for seamless
handoffs between sites and to facilitate consistent mobility
management. When an adjacent CCK is given to an MS, the latest two
CCKs are transferred to the MS. A new CCK replaces the oldest
CCK-ID. Long term storage of CCKs occurs in the ZMs 105 and 119.
The TETRA Standard supports several methods to provision CCK
over-the-air, and the same request/provide methodology used for
each of the air interface keys, and also allows key request upon
registration and cell change by the mobile station.
[0100] The CCK to BS procedure illustrated in FIG. 14 is used to
transfer a CCK from the KMF 101 to a BS (site) 115. The KMF 101
determines that it is time for the CCK of a BS 115 to be updated
and generates appropriate CCK(s). In the preferred embodiment, each
BS is a Location Area (LA) and has its own Location Area
Identification (LAID). FIG. 14 shows the transfer of CCK1 and CCK2
to zone 1 and the transfer of CCK3 to zone 2. The CCKs are
encrypted with the intrakey, KEK.sub.Z, for the zone where the LA
is located. The UCS 103 provides a site-to-zone map and an
ZM-to-zone map to the KMF 101. The KMF 101 uses these maps to send
the keys directly to the appropriate ZM 105 or 119, which stores
CCK and forwards CCK to the zone controller 107 or 121. The UCS 103
obtains the site parameters from the ZMs 105 and 119 to create the
adjacent site list that is sent to the KMF 101 and forwarded the
ZMs 105 and 119 to be forwarded to the zone controllers 107 and 121
for use. If an adjacent site is in a different zone, the key is
transferred between the involved ZCs. The ZC encrypts the CCK with
the interkey, KEK.sub.M, for transfer between zone controllers.
Using the adjacent site list, the zone controllers 107 and 121 send
the adjacent site CCKs to the appropriate sites. Thus, each site on
the adjacent site list will have the CCKs for sites adjacent to
that site. The adjacent CCKs are used so that the MS may request
the CCK for the adjacent site before the MS switches sites. The BS
115 may also forward CCKs to MSs as new CCKs are received at the BS
115. CCKs are encrypted with DCK for the particular MS 401 prior to
transmitting the encrypted CCK to the MS 401. ACKs are sent by the
BS to ZC and are returned to the KMF 101 via the ATR (where the BS
resides). Because the KMF 101 is unaware of adjacency, it does not
need ACKs from adjacent distributions of CCK. Because the KMF 101
tracks which BS is given a CCK, the BS tracks the currency of the
CCKs, i.e., which MS has a CCK for a given Location Area, and
forwards ACKs once the CCK is current.
[0101] Because MGCK is a combination of CCK and GCK, the zone
controller will create four MGCKs using the latest two CCK-IDs and
the latest two GCK-VNs and distribute them accordingly (see FIG. 15
and FIG. 16).
[0102] The CCK is a zone specific parameter so there is no need to
go through the UCS 103. Thus, the KMF 101 sends the CCK information
directly to the appropriate zone manager 105 or 119, which is
different than the re-keying methodology of other air interface
keys. The UCS 103 obtains the site information from the zone
managers 105 or 119 to create the adjacent site list. By placing
CCKs at adjacent sites, real-time processing of CCKs is reduced,
i.e., the BS does not need to query the zone controller for the CCK
for an adjacent BS when an MS requests a CCK for a neighboring
site, thus the MS need not process a CCK when the MS switches
sites.
[0103] FIG. 15 is a diagram showing distribution of a group cipher
key to a BS within a communication system. GCK is identified by
GTSI (Group TETRA Subscriber ID as referred to in the TETRA
standard) and GCK-VN. In the preferred embodiment, GCKN is
logically equivalent to GTSI from a key management perspective.
Long term storage of GCK occurs in the UCS and ZM. MGCK, which is a
combination of GCK and CCK, is identified by GTSI (or GCKN), CCK-ID
(with LAID), and GCK-VN. Four MGCKs per talkgroup (GTSI) are
identified by the latest two CCK-Ids and the latest two GCK-VNs.
MGCKs are not stored in a ZC 107 or 121, but are created by a ZC
107 or 121 and sent to the BS 115 provided that an MS affiliated
with that GTSI is at the site of the BS 115, which does not receive
the GCK because it is a long-term key. Although not shown in the
drawing because of space constraints, GCK-VN is sent along with GCK
and MGCK for identification purposes.
[0104] The procedure to update a GCK for a talkgroup record has two
parts. The first part includes updating the actual GCK in the for
the talkgroup, the second part includes generating the resultant
MGCK as a result of the update and distributing the MGCK to the
sites.
[0105] The procedure of FIG. 15 transfers a GCK from the KMF 101 to
the talkgroup HLR in the zone controller at the home zone for the
talkgroup. When the KMF 101 determines that it is time for the GCK
to be updated, the KMF 101 generates a GCK for each talkgroup and
maintains a GTSI-GCK table. The GCKs are stored hardware encrypted
at the KMF 101. The KMF 101 does not know which ZC has the HLR for
the GTSI, so the KMF 101 sends the GCK encrypted with the interkey,
KEK.sub.M, to the UCS 103. The UCS 103 stores the key material and
forwards it to the home ZM 105 or 119 for the talkgroup (GTSI)
associated with the GCK. The ZM 105 or 119 forwards the key
material to its ZC 107 or 121, which stores the key material in the
group HLR for GTSI encrypted by KEK.sub.M. The ZC 107 verifies that
the key material can be decrypted correctly and sends an ACK back
to the KMF 101 via the ATR 113 where the group HLR 109 for GTSI
resides. The ACK reflects that the HLR 109 contains a correct
encrypted copy of the GCK. The ZC 107 decrypts the key material
with KEK.sub.M and re-encrypts it with the intrakey, KEK.sub.Z, for
storage in the VLR 111. Any other VLRs, such as VLR2 125, outside
of the home zone associated with the GTSI will have GCK encrypted
with KEK.sub.M forwarded to them. FIG. 15 shows both the inter-zone
and intra-zone cases.
[0106] Because MGCK is a combination of GCK and CCK generated by a
ZC using the TA71 algorithm 1501, 1503, or 1505, when GCK changes
or CCK changes, the MGCK must also change accordingly. The four
MGCKs are sent to all sites having a talkgroup affiliation matching
the GTSI for GCK. Because the latest 2 CCK-IDs and latest 2 GCK-VNs
are stored, four versions of the MGCK need to be sent to the
BS.
[0107] As in other cases, when sending MGCK to a site, it needs to
be encrypted using the intrakey, KEK.sub.Z The GCK is obtained from
the VLR talkgroup record and decrypted with the intrakey,
KEK.sub.Z, and combined with CCK to create MGCK. The resultant MGCK
is encrypted using the intrakey, KEK.sub.Z, and sent to the
appropriate sites.
[0108] Transfer of an MGCK to a BS may be triggered by a number of
events. Examples of triggers include a mobile station associated
with the GCK for the MGCK residing at the BS when the either the
GCK or CCK is generated; a mobile station arriving at the BS when
no previous talkgroup affiliation at that BS had occurred; and a
mobile station changing talkgroup affiliation, while residing at
the BS, to a talkgroup not previously associated with the BS.
[0109] A diagram showing distribution of a group cipher key to a
mobile station within a communication system is shown in FIG. 16.
When the KMF 101 determines that an GCK update for an MS 401 is
due, the KMF 101 generates a new GCK key material for the MS 401
according to FIG. 8 titled "Distribution of a group cipher key to
an individual" and its associated text in the TETRA Standard. The
GCK generation process yields the key material SGCK (a sealed GCK),
GCKN (GCK Number), GCK-VN (GCK version number), and RSO (the random
seed used in the process). In order to determine the ATR for the
home zone of the MS 401, in the preferred embodiment, the KMF 101
uses the ITSI to home ZC map from the UCS 103 and a table lookup
based on the zone to obtain the address of the ATR for the home
zone. In the example of FIG. 16, the home zone for MS1 401 is zone
2. The KMF 101 forwards SGCK, GCKN, GCK-VN, and RSO to the ATR 127
of the home zone (2) for the MS 401. If the MS 401 is not on the
system, the ATR 127 sends a NACK back to the KMF 101. If the MS 401
is on the system, the GCK is delivered to the MS 401 via the zone
in which the MS 401 is currently located. In the preferred
embodiment, the GCK key material (e.g., SGCK, GCKN, GCK-VN, and
RSO) are not encrypted for transfer among system devices. The GCK
key material may optionally be encrypted for transfer among system
devices.
[0110] When the MS 401 is not located in its home zone, the home
zone controller 121 of zone 2 determines which zone the MS 401 is
currently located in (zone 1 in FIG. 16) by looking it up in the
HLR 123 of zone 2. ZC2 121 forwards SGCK, GCKN, GCK-VN, and RSO to
the zone controller 107 of the zone where the MS 401 is presently
located. ZC1 107 forwards SGCK, GCKN, GCK-VN, and RSO to the BS 115
where the MS 401 is located. The BS 115 forwards SGCK, GCKN,
GCK-VN, and RSO the MS 401. An unencrypted ACK is sent from the MS
401 to the BS 115 to the ZC 107 and to the KMF 101 via the ATR 113
in the zone where the BS 115 resides. The ACK represents that the
GCK was received and unsealed correctly in the MS (the unsealing
process is described in the TETRA Standard).
[0111] When the MS 401 is located in its home zone (not shown, but
assumed to be at BS3 129 for the sake of this example), the home
zone controller 121 forwards SGCK, GCKN, GCK-VN, and RSO to the BS
129 where the MS 401 is located (not shown but assumed for this
example). The BS 129 forwards SGCK, GCKN, GCK-VN, and RSO to the MS
401. An unencrypted ACK is sent from the MS 401 to the BS 129 to
the ZC 121 and to the KMF 101 via the ATR 127 in the zone where the
BS 115 resides. The ACK represents that the GCK was received and
unsealed correctly in the MS (the unsealing process is described in
the TETRA Standard).
[0112] FIG. 17 is a flowchart showing a method of key persistence
at a site in a communication system in accordance with the
invention. Key persistence refers to the time a key remains stored
at any system device or MS. If an air interface traffic key is
deleted from a site when the MS leaves the site, and the key is
removed too quickly, the MS may return to the site requiring the
key to be set up again. If the MS is traveling between zone borders
or site boundaries for a period of time, the key material for the
MS may need to be constantly set up if the key material is deleted
from a site too quickly after the MS leaves the site. If the key
material is left at a site for too long, duplicate keys may be set
up, creating ambiguity and the likelihood of authentication
failures, particularly for implicit authentication. Thus, the key
persistence for each key needs to be set adequately to prevent such
problems. In the preferred embodiment, the persistence time is
based on an expected average authentication rate in the
communication system, and preferably the persistence time is less
than the expected average authentication rate in the communication
system. The expected average authentication rate is based on an
average number of times a mobile station authenticates within a
time period.
[0113] At step 1701, when a MS arrives at a site, key(s) and/or key
material associated with the MS 401 are stored at the site. If at
step 1703 it is determined that the mobile has left the site, a
persistence timer is set at step 1705, unless it had already been
set or reset, in which case the process simply continues with step
1709. When the timer expires at step 1707, the process continues
with step 1709 where the key(s) and/or key material associated with
the mobile 401 are deleted from the site, and the process ends. If
the mobile 401 has not left the site at step 1703, and it is time
to replace the mobile's key(s) and/or key material at step 1711,
the key(s) and/or key material are replaced at step 1713 and the
process continues with step 1703. Step 1709 may also be reached
(not shown) if a system device, such as a zone controller, directs
the site to delete certain key(s) and/or key material for any
reason. The zone controller typically determines when the mobile
leaves a site based on HLR and VLR updates.
[0114] The present invention may be embodied in other specific
forms without departing from its spirit or essential
characteristics. The described embodiments are to be considered in
all respects only as illustrative and not restrictive. The scope of
the invention is, therefore, indicated by the appended claims
rather than by the foregoing description. All changes that come
within the meaning and range of equivalency of the claims are to be
embraced within their scope.
* * * * *