Introduction️
DMR (Digital Mobile Radio) is a digital radio standard developed by the ETSI (European Telecommunications Standards Institute), designed to replace analog radio systems and offer more efficient communications. It operates in TDMA (Time Division Multiple Access) with two time slots within a 12.5 kHz channel, allowing for simultaneous transmissions on the same frequency. It allows additional services beyond voice transmission, such as data sending, like text messages or location reports of terminals. Around voice, it allows both individual calls similar to mobile phone calls and group calls, where registered terminals can participate.️
The AMBE+2 (Advanced Multi-Band Excitation) codec is responsible for voice compression in DMR. It is used due to its high efficiency in bandwidth and robustness against noise and interference. The analog voice is sampled at 8 kHz and parameters such as the fundamental frequency and spectral shape are extracted. Voice information is compressed at a rate of 2450 bps, with an additional 1150 bps for error correction, resulting in 3600 bps in digital format.️
Transmission of voice over DMR is organized in TDMA frames, where each time slot lasts 30 ms and contains the digitally coded voice along with signal metadata. Each DMR voice frame contains 108 bits: 6 AMBE+2 voice blocks (each one consisting of 20 ms compressed voice with 49 bits of voice and 36 bits of error correction), control bits (24 bits), synchronization, and finally, a cyclic redundancy check code (4 bits). These data ensure that receivers can reconstruct the voice robustly, even in the presence of errors.️
DMR supports various encryption methods, outside of the official specification, due to user system communication requirements. Some of them are proprietary, defined by manufacturers such as Motorola or Hytera, and others are defined by the DMR Association, which can be implemented by member manufacturers, and will be analyzed. They include RC4 and AES (128-bit and 256-bit).️
Privacy Indicator (PI) is a component of the DMR protocol that inserts into signaling packets and indicates whether transmission is encrypted, what type of encryption is being used, the index of the key used, and the initialization vector if necessary. It allows receivers to correctly decode encrypted transmissions and is sent at the beginning of each call.️
The concept of Late Entry in DMR allows a radio to join a conversation at any time (after the PI frame has been transmitted), receive previous data and decode the ongoing communication correctly. Voice frames include auxiliary synchronization data to enable this re-connection, sacrificing some bits and degrading communications in some cases. A more detailed description can be found in Motorola’s patent EP2347540B1.️
RC4 Cipher️
RC4 (Rivest Cipher 4) is a stream cipher algorithm designed by Ron Rivest in 1987 for RSA Security. It is a symmetric algorithm that operates on individual bytes, generating a pseudo-random key flow that combines with the data through an XOR operation. It is fast with low resource consumption, ideal for limited hardware, easy to implement in software and since it’s symmetric encryption uses the same key for both encrypting and decrypting. Today, it is considered insecure due to various vulnerabilities, although it improves privacy against unauthorized receivers without technical knowledge in cryptography or access to key recovery tools.️
The DMR protocol ignores the first 256 bytes of the pseudo-random key flow and uses an initialization vector (IV) to minimize vulnerability risks. However, the size of the key, 40 bits, allows for 2^40 different keys, which is approximately 1.099.511.627.776 keys. This number of keys enables a brute-force attack by knowing the content of some voice frames, which can be predictable according to the device manufacturer. Such an attack can be performed in weeks on a home computer.️
Other initialization vectors are predictable and not random, generated by a registration of displacement with linear feedback (LFSR) with the polynomial P(x) = x^32 + x^4 + x^2 + 1, defined in the specification TIA-EIA-102.AAAD.️
Attack based on cryptography such as the prediction of output bytes in the S-Box, partial key attacks that allow recovering parts of the key with sufficient encrypted traffic, previously used for key recovery in the WEP Wi-Fi protocol.️
AES Cipher
The Advanced Encryption Standard (AES) is a symmetric block cipher algorithm adopted as a standard by the National Institute of Standards and Technology in 2001. AES encrypts blocks of 128 bits using a series of transformation rounds that depend on the size of the key. Each round consists of mathematical operations over the data matrix representing the clear text. It offers flexible key sizes: AES-128 (128-bit key, 10 rounds), and AES-256 (256-bit key, 14 rounds). It is secure against brute-force attacks due to its mathematical structure and efficient in hardware and software, making it ideal for high-security applications.️
In DMR, the AES OFB (Output Feedback Mode) is used, which converts the block cipher into a stream cipher, similar to RC4. This allows for encrypting data of variable size and with high resistance to transmission errors. A initialization vector (IV) is used to generate a key flow, errors in transmission only affect one bit, and it is used in real-time data transmission applications. In this case, the first 16 bytes of the flow are discarded during decryption of the data. During transmission, a 32-bit IV is sent that is converted into a 128-bit one using the LFSR with the polynomial P(x) = x^32 + x^22 + x^2 + x + 1, defined in the specification TIA-EIA-102.AAAD.️
The algorithm has not been compromised today, although it is recommended to use a randomly generated key, the use of AES-256 for maximum security and periodic key rotation since being a symmetric encryption, the owners of a DMR terminal could extract the key if it is not properly protected.️
Conclusion️
The article has addressed a brief description of the DMR protocol and its various methods of encryption established by the DMR Association, concluding that it is preferred to use the AES encryption algorithm in its version of 256 to avoid intercepting communications by unauthorized receivers.️