Security vulnerabilities

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An in-depth review published in July 2023 by the company Midnight Blue of the TETRA standard and encryption algorithms, the first made public in the last 20 years, has found multiple security flaws, collectively referred to as TETRA:BURST. A total of 5 flaws were filed to the CVE database:^[5]

• The Air Interface Encryption (AIE) keystream generator is vulnerable to decryption oracle attacks due to the use of publicly-broadcast network time^[5]—keystream reuse can be triggered.^[6]
• TEA1 contains a secret reduction step that effectively downgrades the cryptographic strength from 80 to 32 bits, allowing anyone to break the cipher and subsequently decrypt the signal in as little as one minute using a consumer laptop. "There's no other way in which this can function than that this is an intentional backdoor," "This constitutes a full break of the cipher, allowing for interception or manipulation of radio traffic", according to the news report posted on ComputerWeekly.^[7] The deliberately weakened TEA1 flaw seems to be known in intelligence circles and is referred to in the famous 2006 Wikileaks dump of US diplomatic communications ^{[citation needed]}.
• AIE contains no authentication for the ciphertext, making malleability attacks possible.^[5]
• The cryptographic anonymization scheme is weak and can be partially reversed to track users.^[5]
• The authentication algorithm allows attackers to set the Derived Cypher Key (DCK) to 0,^[5] circumventing session authentication.^[6]

In addition, the Midnight Blue team spots a "peculiarity regarding the TEA3 S-box", but has yet to determine whether it constitutes a weakness.^[6]

These vulnerabilities remained publicly unknown for 28 years after TETRA's publication because TETRA does not make definitions of its cryptographic algorithms public, an example of security through obscurity. The Midnight Blue team gained access to TETRA's cryptographic code by attacking the trusted execution environment on a TETRA-enabled radio. The team points to a list of previously broken cryptographic systems relying on obscurity and argues that the Kerckhoffs's principle should have been followed: the system would have been safer when its structure is publicly known.^[6]

Radio aspects

For its modulation TETRA, uses π⁄4 differential quadrature phase-shift keying. The symbol (baud) rate is 18,000 symbols per second, and each symbol maps to 2 bits, thus resulting in 36,000 bit/s gross.

As a form of phase shift keying is used to transmit data during each burst, it would seem reasonable to expect the transmit power to be constant. However it is not. This is because the sidebands, which are essentially a repetition of the data in the main carrier's modulation, are filtered off with a sharp filter so that unnecessary spectrum is not used up. This results in an amplitude modulation and is why TETRA requires linear amplifiers. The resulting ratio of peak to mean (RMS) power is 3.65 dB. If non-linear (or not-linear enough) amplifiers are used, the sidebands re-appear and cause interference on adjacent channels. Commonly used techniques for achieving the necessary linearity include Cartesian loops, and adaptive predistortion.

The base stations normally transmit continuously and (simultaneously) receive continuously from various mobiles on different carrier frequencies; hence the TETRA system is a frequency-division duplex (FDD) system. TETRA also uses FDMA/TDMA (see above) like GSM. The mobiles normally only transmit on 1 slot/4 and receive on 1 slot/4 (instead of 1 slot/8 for GSM).

Speech signals in TETRA are sampled at 8 kHz and then compressed with a vocoder using algebraic code-excited linear prediction (ACELP). This creates a data stream of 4.567 kbit/s. This data stream is error-protection encoded before transmission to allow correct decoding even in noisy (erroneous) channels. The data rate after coding is 7.2 kbit/s. The capacity of a single traffic slot when used 17/18 frames.

A single slot consists of 255 usable symbols, the remaining time is used up with synchronisation sequences and turning on/off, etc. A single frame consists of 4 slots, and a multiframe (whose duration is 1.02 seconds) consists of 18 frames. Hyperframes also exist, but are mostly used for providing synchronisation to encryption algorithms.

The downlink (i.e., the output of the base station) is normally a continuous transmission consisting of either specific communications with mobile(s), synchronisation or other general broadcasts. All slots are usually filled with a burst even if idle (continuous mode). Although the system uses 18 frames per second only 17 of these are used for traffic channels, with the 18th frame reserved for signalling, Short Data Service messages (like SMS in GSM) or synchronisation. The frame structure in TETRA (17.65 frames per second), consists of 18,000 symbols/s; 255 symbols/slot; 4 slots/frame, and is the cause of the perceived "amplitude modulation" at 17 Hz and is especially apparent in mobiles/portables which only transmit on one slot/4. They use the remaining three slots to switch frequency to receive a burst from the base station two slots later and then return to their transmit frequency (TDMA).

Radio frequencies

Air interface encryption

To provide confidentiality the TETRA air interface is encrypted using one of the TETRA Encryption Algorithm (TEA) ciphers. The encryption provides confidentiality (protect against eavesdropping) as well as protection of signalling.

Currently 4 different ciphers are defined, TEA1 up to TEA4. These TEA ciphers should not be confused with the block cipher Tiny Encryption Algorithm. The TEA ciphers have different availability due to export and use restrictions. Few details are published concerning these proprietary ciphers. Riess^[12] mentions in early TETRA design documents that encryption should be done with a stream cipher, due to the property of not propagating transmission errors. Parkinson^[13] later confirms this and explains that TEA is a stream cipher with 80-bit keys. The algorithms were later reversed and it appeared that TEA1 reduces its key strength to 32 bits.^[14] TEA1 and TEA4 provide basic level security, and are meant for commercial use.^[15] The TEA2 cipher is restricted to European public safety organisations. The TEA3 cipher is for situations where TEA2 is suitable but not available.^[16]

Cell selection

Virtual MMI for terminals

Any given TETRA radio terminal using Java (Java ME/CLDC) based technology, provides the end user with the communication rights necessary to fulfil his or her work role on any short duration assignment.

For dexterity, flexibility, and evolution ability, the public transportation radio engineering department, have chosen to use the open sources, Java language specification administered by Sun and the associated work groups in order to produce a transport application tool kit.

Service acquisition admits different authorised agents to establish communication channels between different services by calling the service identity, and without possessing the complete knowledge of the ISSI, GSSI, or any other TETRA related communication establishment numbering plan. Service acquisition is administered through a communication rights centralised service or roll allocation server, interfaced into the TETRA core network.

In summary, the TETRA MMI aims are to:

• Allow any given agent while in exercise, to exploit any given radio terminal without materiel constraint.
• Provide specific transportation application software to the end-user agents (service acquisition, fraud, and aggression control).

This transport application tool-kit has been produced successfully and with TETRA communication technology and assures for the public transport application requirements for the future mentioned hereafter.

The home (main) menu presents the end user with three possibilities:

1. Service acquisition,
2. Status SDS,
3. End-user parameters.

Service acquisition provides a means of virtually personalising the end user to any given radio terminal and onto TETRA network for the duration the end user conserves the terminal under his possession.

Status SDS provides the end user with a mechanism for generating a 440 Hz repeating tone that signals a fraud occurrence to members within the same (dynamic or static) Group Short Subscriber Identity (GSSI) or to a specific Individual Short Subscriber Identity (ISSI) for the duration of the assignment (an hour, a morning patrol or a given short period allocated to the assignment). The advantage being that each of the end users may attach themselves to any given terminal, and group for short durations without requiring any major reconfiguration by means of radio software programming tools. Similarly, the aggression feature functions, but with a higher tone frequency (880 Hz), and with a quicker repetitious nature, so to highlight the urgency of the alert.

The parameters tab provides an essential means to the terminal end-user allowing them to pre-configure the target (preprogrammed ISSI or GSSI ) destination communication number. With this pre-programmed destination number, the end-user shall liaise with the destination radio terminal or roll allocation server, and may communicate, in the group, or into a dedicated server to which the service acquisition requests are received, preprocessed, and ultimately dispatched though the TETRA core network. This simplifies the reconfiguration or recycling configuration process allowing flexibility on short assignments.

The parameters tab also provides a means of choosing between preselected tones to match the work group requirements for the purposes of fraud and aggression alerts. A possibility of selecting any given key available from the keypad to serve as an aggression or fraud quick key is also made possible though the transport application software tool kit. It is recommend to use the asterisk and the hash keys for the fraud and aggression quick keys respectively. For the fraud and aggression tones, it is also recommend to use 440 Hz slow repeating tone (blank space 500 milli-seconds) and 880 Hz fast repeating tone (blank space 250 milliseconds) respectively. The tone options are as follows: 440 Hz, 620 Hz, 880 Hz, and 1060 Hz.

The parameters page provides an aid or help menu and the last tab within parameters describes briefly the tool kit the version and the history of the transport application tool kit to date.

TETRA Enhanced Data Service (TEDS)

The TETRA Association, working with ETSI, developed the TEDS standard, a wideband data solution, which enhances TETRA with a much higher capacity and throughput for data. In addition to those provided by TETRA, TEDS uses a range of adaptive modulation schemes and a number of different carrier sizes from 25 kHz to 150 kHz. Initial implementations of TEDS will be in the existing TETRA radio spectrum, and will likely employ 50 kHz channel bandwidths as this enables an equivalent coverage footprint for voice and TEDS services. TEDS performance is optimised for wideband data rates, wide area coverage and spectrum efficiency.^[17]

Advances in DSP technology have led to the introduction of multi-carrier transmission standards employing QAM modulation. WiMAX, Wi-Fi and TEDS standards are part of this family.

Refer also to:

• JSR-118;
• Mobile Information Device Profile, JSR-37;
• Wireless Messaging API, JSR120;
• Connected Limited Device Configuration, JSR-139; and
• Technology for the Wireless Industry, JTWI-185.