I discussed this in 2023. Here’s the narrative:

Three Dutch security experts unveiled the weaknesses—five in total—­in a European radio standard known as TETRA (Terrestrial Trunked Radio), utilized in radios produced by Motorola, Damm, Hytera, and others. This standard has been embedded in radios since the ’90s, but the vulnerabilities remained undisclosed because the encryption methods utilized in TETRA were kept confidential until now.

There’s fresh information:

In 2023, Carlo Meijer, Wouter Bokslag, and Jos Wetzels from the security company Midnight Blue, located in the Netherlands, uncovered weaknesses in encryption methods linked to a European radio standard established by ETSI, called TETRA (Terrestrial Trunked Radio). This standard has been integrated into radio systems crafted by Motorola, Damm, Sepura, and others since the 1990s. The flaws remained publicly unknown until their revelation, as ETSI denied access to the proprietary algorithms for decades.

[…]

However, the same researchers have now identified that a certain implementation of the end-to-end encryption system supported by ETSI possesses a comparable flaw, rendering it just as susceptible to eavesdropping. The encryption method employed for the device they investigated begins with a 128-bit key, which is then condensed to 56 bits before encrypting data, making it more vulnerable to attacks. It remains unclear who employs this version of the end-to-end encryption algorithm, nor whether users of devices with this encryption are aware of the existing security flaw.

[…]

The end-to-end encryption scrutinized by the researchers is intended to function atop TETRA encryption methods.

The experts identified the flaw in the end-to-end encryption (E2EE) only after isolating and reverse-engineering the E2EE algorithm implemented in a radio device produced by Sepura.

These appear to be intentionally designed backdoors.