security
AuC from the secret key, followed by a further random seed. It is this random seed that is passed to the radio as part of the transaction. Te authentication protocol allows either party to
authenticate the other. Te strongest solution is to use a mutual authentication protocol where both parties authenticate each other as part of the same transaction. An output of the authentication exchange is a Derived Cipher Key, which is used for encryption of individually addressed transmissions sent between the MS and the network. Tere are several types of encryption key, and the TETRA
standard incorporates Over Te Air Rekeying (OTAR) protocols for transmitting these keys securely over the air interface. Until recently, the security standard concentrated on the
key management aspects of single TETRA systems. However the Inter-System Interface (ISI), which connects multiple TETRA systems together, has increasing focus and the latest work within ETSI TC TETRA WG6 was to expand the key management mechanisms to provide a secure means of sharing key material over the ISI between two trusted networks.
Air Interface Encryption Te second major security element to consider is Air Interface Encryption. Tis protects information sent between the network and MSs to prevent eavesdropping over the radio link. Apart from some basic synchronization information sent by
the network, air interface encryption encrypts all information – user speech and data, signalling and identities – and can also encrypt most of the broadcast information required by an MS, once it has successfully registered and authenticated to the network.
End-to-end voice encryption Te end-to-end encryption process operates both in Trunked Mode and Direct Mode. Te detailed implementation is specified within SFPG Recommendation 02. It provides solutions for both speech encryption and key management, including an Over Te Air Key management (OTAK) solution. Te term OTAK is used for the end-to-end encryption key management solution to distinguish it from the OTAR solution used for air interface encryption. Te end-to-end encryption solution should be transparent
to the TETRA system. Te key management solution has been designed to use the Short Data Service as a bearer for OTAK messages. Tis means that end-to-end encryption can be operated and managed by an end user community without influence by the network operator. Te OTAK system provides key material to MSs, but also provides the necessary security associations to configure the keys used by the MS for calls to different destinations. Secure means of erasing keys and disabling the end-to-end functionality of the MS are also included. SFPG Recommendation 02 is designed to be a framework
specification to allow different algorithms to be used according to the choice of the end user community. Annexes contain fully-worked sample solutions and related test data. Work within the SFPG has extended Recommendation
02 to provide mechanisms that enable an MS to contain multiple, different, end-to-end encryption algorithms to allow
Issue 4 2011 TE TRA TODAY
interoperation between different groups, where those groups have adopted different encryption algorithms for normal use. Tis allows the end-to-end encryption solution to be fully used in international interoperability situations. SFPG Recommendation 08 has been written to describe
an implementation of end-to-end encryption functions using a Smart Card.
End-to-end data encryption Te SFPG has also provided end-to-end encryption solutions for TETRA data. Te first work provided a solution for end-to-end encrypted Short Data, and this is described in SFPG Recommendation 07. Te solution is designed to be compatible with the speech encryption solution described in Recommendation 02, and uses the same key management mechanisms. A solution has also been designed for encrypted IP packet
data, and this is described in SFPG Recommendation 11. It uses techniques from standard IP Security (IPSEC) specifications so that encryption can be applied at any point in a data network, and can remain compatible with standard IP transport. Recommendation 11 also uses the mechanisms described in SFPG Recommendation 02 for key management, together with some extra signalling messages which are required for data security associations.
Direct Mode security Tere is no explicit authentication mechanism in Direct Mode Operation (DMO) because, to achieve this, every DMO MS would need to share some secret with every other MS. Terefore implicit authentication, as well as confidentiality, is achieved by encrypting transmissions with a common Static Cipher Key (SCK). Classes 2A and 2B allow pre-emption of a transmitting
MS by a third party who does not have the key that the transmitting MS is using. Class 2C requires an MS attempting to pre-empt a transmission to hold the same key. Key management for air interface encryption DMO SCKs
can be carried out by the TMO system, so that automatic key management can also apply in DMO. Te key management system has been designed to avoid losing communications when key changes take place. In this case three keys are associated with each DMO communication group, intended for use as a ‘previous key’, ‘current key’ and ‘future key’. An MS always transmits on its ‘current key’, but can receive on any of the three. Terefore each MS can always receive transmissions sent by the other provided all MSs receive the current set of keys before the keys are changed again. End-to-end encryption can also be used in DMO, using the same specifications as TMO.
The Security and Fraud Protection Group (SFPG) is an independent group of the TETRA Association dealing with security and fraud prevention issues. Development of the TETRA security standard is the responsibility of ETSI’s TETRA Working Group 6. The SFPG has defined a framework for end-to-end security that is flexible enough
to address the needs of all users and their security policies. Even in the case of TETRA, which is currently the most secure standard for
mobile communication, new developments can bring new challenges for hackers and defrauders. The composition of the SFPG, which brings together manufacturers, operators and users, provides a platform not only to keep track of current developments, but to stay one step ahead of attackers
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