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TEA3
TETRA Encryption Algorithm 3

TEA3, short for TETRA Encryption Algorithm 3, 1 is a stream cipher associated with Terrestrial Trunked Radio (TETRA), a European standard for public and emergency services, standardized by the European Telecommunications Standards Institute (ETSI). Part of the TEA suite of encryption algorithms, it is intended for public safety services outside Europe. It is similar, but not identical, to TEA2. It is likely that it is stronger than TEA1 but not as strong as TEA2, although the latter is by no means obvious. It is widely adopted in non-EU countries like India, China and Mexico [4].

It is currently unknown when and where the TEA3 algorithms was developed, but given the fact that its structure is similar to the TEA1 and TEA2 algorithms, it seems likely that it originates at Philips Crypto BV in Eindhoven (Netherlands). It might however have been modified elsewhere. TEA3 uses the full 80-bit key length and was evaluated by ETSI-SAGE — an international group of experts from commercial and governmental parties. As the algorithm is secret, it has never been submitted for peer-review or in-depth security analysis.

In July 2023, Dutch cyber security firm Midnight Blue revealed that it had managed to extract, isolate and analyse the algorithm from a working TETRA radio as part of its TETRA:BURST research project. In the event, no immediately visible weaknesses were found in the TEA3 algorithm, although two peculiarities were observed that call for further cryptanalysis [4]. In addition to this, other vulnerabilities in the TETRA protocol suite were identified [2][4].

  1. Not to be confused with Tiny Encryption Algorithm.  Wikipedia

Structure
The diagram below shows the structure of the TEA3 key stream generator which consists of two parts: a 64-bit state register (R) and an 80-bit key register (K). The state register (R) is initialised with the Initialisation Vector (IV), whilst the key register (K) is initialised with the original key. The key register is basically a Linear Feedback Shift Register (LFSR) with an S-box lookup table (S). It is only fed with data from itself and produces a key-dependent output, independent from the IV.

General structure of the TEA3 stream cipher. Note that all elements are bytes rather than bits.
Structure of the TEA3 stream cipher

The state register (R) is also a Linear Feedback Shift Register (LFSR) that produces the output key stream byte at the top left (R0). It consists of two parts (R0-R2 and R3-R7) with an XOR inbetween. F1 is a non-linear function that takes two input bytes (R5, R6) and produces one output byte that is mixed with the feedback loop. F2 is also a non-linear function that takes two input bytes (R1, R2) and produces one output byte that is mixed in the middle of the state register (R2-R3). (B) is a simple bit permutation of which the output is mixed with the feedback loop.

The following peculiarities were observed in [4]:

  1. S-box defect
    In the TEA3 S-box (S), two input bytes (
    0x14
    and
    0x9E
    ) map to the same output
    value (
    0xC2
    ), while one output value (
    0xD2
    ) does not occur. Although this could be explained as an unintended mistake — the output values differ only by one bit — this is highly unlikely in the context of a professional (export) algorithm. The exact implications of this 'defect' are not yet clear and call for further analysis.

  2. K0 direct feedback
    Key register K0 is directly mixed with the input of the key register, rather than being fed through the S-box. The implication of this bypass is unclear, but it should be noted that it is not present in the other TEAs.
For a more detailed description of the cipher, please refer to the paper 'All cops are broadcasting: TETRA under scrutiny' by Carlo Meijer, Wouter Bokslag and Jos Wetzels, published in August 2023 in relation to the TETRA:BURST vulnerability disclosures [4].

 Read the paper
 More about TETRA:BURST


Source code
As part of the TETRA:BURST project, Midnight Blue researchers managed to extract and reverse-engineer the firmware from an operational TETRA radio, and construct an equivalent of the code in the C programming language. This source code is now available to researchers [II].

 Download the full source code


Publications
  1. Carlo Meijer, Wouter Bokslag and Jos Wetzels,
    All cops are broadcasting: TETRA under scrutiny

    Paper submitted to Crypto Museum. 9 August 2023.

  2. Full source code of TAA1, TEA1, TEA2 and TEA3 algorithms in C
    Reverse-engineered and used for analysis and real life tests.
    Midnight Blue, 9 August 2023.

  3. All Cops Are Broadcasting, Breaking TETRA after decades in the shadows
    Presentation by Jos Wetzels, Carlo Meijer and Wouter Bokslag at Black Hat 2023.
    Midnight Blue, 9 August 2023
References
  1. Wikipedia, Terrestrial Trunked Radio
    Visited 27 July 2023.

  2. TETRA:BURST
    Midnight Blue, 24 July 2023.
     More

  3. Cees Jansen, TEA co-developer at Philips Crypto BV
    Personal correspondence. Crypto Museum, July 2023.

  4. Carlo Meijer, Wouter Bokslag and Jos Wetzels,
    All cops are broadcasting: TETRA under scrutiny

    Paper submitted to Crypto Museum. 9 August 2023.
Further information
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Crypto Museum. Created: Wednesday 09 August 2023. Last changed: Saturday, 12 August 2023 - 14:03 CET.
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