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Matt Green, a Johns Hopkins University professor who specializes in cryptography, explained that real-world constraints-including the specific codecs in use, vagaries in the way encoded audio is transcoded, and compression of packet headers-can make it difficult to obtain the full digital plaintext of a call.

AdvertisementДemonstration of the ReVoLTE attack in a commercial LTE network. Once in possession of this so-called вЂњ keystream, the attacker uses it to recover the plaintext of the target call. The attacker then compares the encrypted and plaintext traffic from the second call to deduce the cryptographic bits used to encrypt the call. The second phase requires a Commercial Off-TheShelf (COTS) phone and knowledge of the victimвЂ™s phone number along with his/her current position (i.e., radio cell). However, our attacker model does not require the possession of any valid key material of the victim. Furthermore, the adversary can decode recorded traffic up to the encryption data (PDCP) when she has learned the radio configuration of the targeted eNodeB. For the first phase, the adversary must be capable of sniffing radiolayer transmissions in downlink direction, which is possible with affordable hardware for less than $1,400. The attack consists of two main phases: the recording phase in which the adversary records the target call of the victim, and the call phase with a subsequent call with the victim. When the first call ends, the attacker quickly initiates what the researchers call a keystream call with the target and simultaneously sniffs the encrypted traffic and records the unencrypted sound, commonly known as plaintext. The attack seizes on this error by capturing the encrypted radio traffic of a targetвЂ™s call, which the researchers call the target or first call. The implementation error ReVoLTE exploits is the tendency for base stations to use some of the same cryptographic material to encrypt two or more calls when theyвЂ™re made in close succession. The base station on the other end will then encrypt the call as itвЂ™s transmitted to the other party. The base station then decrypts the traffic to allow it to be passed to any circuit-switched portion of a cellular network. VoLTE encrypts call data as it passes between a phone and a base station. вЂњWe introduced the ReVoLTE attack, which enables an adversary to eavesdrop and recover encrypted VoLTE calls based on an implementation flaw of the LTE protocol.вЂќ

вЂњData confidentiality is one of the central LTE security aims and a fundamental requirement for trust in our communication infrastructure,вЂќ the researchers, from Ruhr University Bochum and New York University, wrote in a paper presented Wednesday at the 29th USENIX Security Symposium. The result is a threat to the privacy of a growing segment of cell phone users. Because of an error in the way many carriers implement VoLTE, the attack converts cryptographically scrambled data into unencrypted sound. Their technique, dubbed ReVoLTE, uses a software-defined radio to pull the signal a carrierвЂ™s base station transmits to a phone of an attackerвЂ™s choosing, as long as the attacker is connected to the same cell tower (typically within a few hundred meters to few kilometers) and knows the phone number. Now, researchers have demonstrated a weakness that allows attackers with modest resources to eavesdrop on calls. VoLTE does all of this while also providing a layer of security not available in predecessor cellular technologies. VoLTE also uses the same IP standard used to send data over the Internet, so it has the ability to work with a wider range of devices. VoLTE, short for Voice over LTE, provides up to three times the capacity of the earlier 3G standard, resulting in high-definition sound quality thatвЂ™s a huge improvement over earlier generations. The emergence of mobile voice calls over the standard known as Long Term Evolution (LTE) has been a boon for millions of cell phone users around the world.