ReVoLTE: Researchers eavesdrop on encrypted LTE voice calls

Base stations use encryption keys several times. This allows previously recorded phone calls to be decrypted afterwards. At least in this country, mobile network providers have apparently closed the vulnerability in the meantime.

Researchers at Ruhr University Bochum and New York University Abu Dhabi have discovered a vulnerability in the Voice-over-LTE protocol (VoLTE) that allows the encryption of 4G voice calls to be cracked. The attacks called ReVoLTE are possible because mobile network operators often use the same encryption key for calls that are processed via a base station.

LTE (Image: Shutterstock)In their experiments under real conditions, the researchers found that several cell phone operators are affected. With the GSM Association, which administers the cellular standards, they also developed a fix that has since solved the problem.

VoLTE is the protocol used for voice calls within the 4G standard. It stipulates that cell phone providers secure phone calls with an encryption key of their choice, also known as a stream cipher. There should actually be a separate stream cipher for each call.

However, the researchers found that not every provider follows this requirement. In many cases, a base station’s encryption keys were reused for multiple calls. The researchers also criticized predictable algorithms for generating the keys.

However, the vulnerability can only be exploited with considerable effort and only under certain conditions. First of all, attackers have to intercept and record the data stream of a call conducted via VoLTE. He then has to telephone one of the two call partners himself via the base station in question. In this way he receives the stream cipher reused by the base station.

Another problem: the decryption of the previously recorded conversation depends on the length of the phone call that the attacker then has with one of the victims. For example, if it takes five minutes, five minutes of the recorded conversation can also be deciphered. “The longer the attacker talks to the victim, the more he can decipher the previous conversation,” said David Rupprecht from the Ruhr University in Bochum. The researchers also demonstrate the decryption in a video.

An analysis of randomly selected base stations in Germany showed that around 80 percent of cell phone masts in this country were affected. The GSMA already informed them in December 2019. Renewed tests recently did not reveal any vulnerable base stations.

However, the researchers do not rule out that base stations in other countries are still vulnerable. Because of this, they developed one AndroidApp with which mobile phone providers can check their 4G networks and base stations. It is under an open source license and is available on GitHub.

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