INTRODUCTION TO TETRA SECURITY

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INTRODUCTION TO TETRA SECURITY
Brian Murgatroyd UK Police IT Organization
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Agenda

• Threats to systems
• Network Security
• Overview of standard TETRA security features
• Authentication
• Air interface encryption
• Key Management
• Terminal Disabling
• DMO security
• End to End Encryption

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Security Threats

• What are the main threats to your system?
• Confidentiality?
• Availability?
• Integrity?

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Message and User Related Threats

• Message threats
• Interception
• Eavesdropping
• Masquerading
• Manipulation of data.
• Replay

• User related threats
• traffic analysis
• observability of user behaviour.

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System Related Threats
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Network Security
IT security is vital in TETRA networks Gateways
are particularly vulnerable. Operating staff need
vetting
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TETRA Communications Security

• Security is not just encryption!
• Terminal Authentication
• User logon/Authentication
• Stolen Terminal Disabling
• Key Management with minimum overhead
• All the network must be secure, particularly with
  a managed system

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User authentication (aliasing)

• Second layer of security
• Ensures the user is associated with terminal
• User logon to network aliasing server
• log on with Radio User Identity and PIN
• Very limited functionality allowed prior to log
  on
• Log on/off not associated with terminal
  registration
• Could be used as access control for applications
  as well as to the Radio system

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Authentication

• Used to ensure that terminal is genuine and
  allowed on network.
• Mutual authentication ensures that in addition to
  verifying the terminal, the SwMI can be trusted.
• Authentication requires both SwMI and terminal
  have proof of secret key.
• Successful authentication permits further
  security related functions to be downloaded.

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Authentication
Generate RS
Authentication Centre (AuC)
K known only to AuC and MS
K
RS
TA11
KS
K
RS
Generate RAND1
KS (Session key) RS (Random seed)
TA11
KS
RAND1
RS, RAND1
KS
RAND1
RES1
TA12
DCK
EBTS
TA12
XRES1
DCK1
Compare RES1 and XRES1
RES1
DCK1
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Encryption Process
Traffic Key (X)CK
Key Stream Generator (TEAx)
CN LA CC
Combining algorithm (TB5)
Key Stream Segments
Initialization Vector (IV)
A
B
C
D
E
F
G
H
q
c
I
y
4
M
v

Q
t
Clear data in
Encrypted data out
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Air Interface traffic keys

• Four traffic keys are used in class 3 systems-
• Derived cipher Key (DCK)
• derived from authentication process used for
  protecting uplink, one to one calls
• Common Cipher Key(CCK)
• protects downlink group calls and ITSI on initial
  registration
• Group Cipher Key(GCK)
• Provides crypto separation, combined with CCK
• Static Cipher Key(SCK)
• Used for protecting DMO and TMO fallback mode

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DMO Security
Implicit Authentication Static Cipher keys No
disabling
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TMO SCK OTAR scheme
SwMI
Key Management Centre
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Key Overlap scheme used for DMO SCKs
Transmit
Past
Present
Future
Receive

• The scheme uses Past, Present and Future versions
  of an SCK.
• System Rules
• Terminals may only transmit on their Present
  version of the key.
• Terminals may receive on any of the three
  versions of the key.
• This scheme allows a one key period overlap.

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Disabling of terminals

• Vital to ensure the reduction of risk of threats
  to system by stolen and lost terminals
• Relies on the integrity of the users to report
  losses quickly and accurately.
• Disabling may be either temporary or permanent
• Permanent disabling removes all keys including
  (k)
• Temporary disabling removes all traffic keys but
  allows ambience listening

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End to end encryption

• Protects messages across an untrusted
  infrastructure
• Provides enhanced confidentiality
• Voice and SDS services
• IP data services (soon)

Network
MS
MS
Air interface security between MS and network
End-to-end security between MSs
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Key management for end to end encryption
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Benefits of end to end encryption in combination
with Air Interface encryption

• Air interface (AI) encryption alone and end to
  end encryption alone both have their limitations
• For most users AI security measures are
  completely adequate
• Where either the network is untrusted, or the
  data is extremely sensitive then end to end
  encryption may be used in addition
• Brings the benefit of encrypting addresses and
  signalling as well as user data across the Air
  Interface and confidentiality right across the
  network

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Conclusions

• Security functions built in to TETRA from the
  start!
• User friendly and transparent key management.
• Air interface encryption protects, control
  traffic, IDs as well as voice and user traffic.
• Key management comes without user overhead
  because of OTAR.