## 3 Attacks on 5G NSA
### Passive Attacks  
In passive attacks on mobile networks, the adversary is only passively involved in the attack. Generally, the adversary silently sniffs the signals/messages exchanged between the UE and the BS using a passive sniffer. Depending upon the attack, the adversary may need to sniff the control and/or the data plane traffic.  
The _Passive Fingerprinting Attack_ allows the adversary to classify the baseband modem of phones by passively analyzing the uplink control traffic [23; 31]. Specifically, the attacker needs to sniff core network capabilities sent in plain text in the Attach Request message. The adversary first needs to build a reference model that classifies the baseband modem based on their capabilities. Later, whenever required, the adversary needs to sniff the capabilities of the UE and compare them to the reference model to classify the baseband modem.  
In a _Passive Localization Attack_, the adversary uses information from unencrypted Timing Advance command responses and the time of arrival of messages at the sniffer to localize a victim UE [23]. The adversary should have access to a combined custom uplink/downlink LTE sniffer to extract the required information [31]. The attack narrows down the location of a victim to two possible areas at the intersection of an ellipse and a wide ring.  
In the _ReVoLTE_ attack [32], the adversary exploits an implementation flaw of reusing key streams in successive calls to recover the contents of an encrypted VoLTE call. Generally, when a UE connects to a base station, a new user-plane key is negotiated for the radio connection. However, if two consecutive calls occur within a single radio connection, key stream reuse may occur, depending on the network operator's implementation. The attack capitalizes on this weakness, taking advantage of the XOR operation used in generating an encrypted ciphertext from the keystream and plain text. In order to perform the attack, the adversary first needs to sniff the first call (target call) directed towards the victim. Subsequently, upon detecting the end of the target call, it needs to immediately place a keystream call (second call) toward the victim. For the second call, the attacker needs to record the unencrypted sound (known plaintext) and sniff encrypted radio traffic. To decrypt the target call, the adversary needs to compute the keystream by XORing the contents of the second call (unencrypted sound and encrypted voice). Once it has access to the keystream, it decrypts the first call by XORing the first call's ciphertext with this keystream. Note that the attacker needs to engage the victim in conversations that last for the same time as the previous call to retrieve all of the previous call's content.  
_Passive attacks_ on 5G NSA that require sniffing control plane data can be carried out using 4G sniffing tools because the non-stand-alone version of 5G uses the control plane from 4G. However, attacks that require sniffing the data plane are currently difficult to perform using open-source tools.  
Sniffing 5G traffic using open-source tools presents greater complexity compared to earlier technologies, particularly concerning hardware requirements. A 5G sniffer should be capable of processing RF signals exceeding 24 GHz, as 5G operates across two frequency ranges: sub-6 GHz and millimeter wave [33]. Notably, even high-end SDRs like the USRP X410 can currently only support frequencies up to 8 GHz.  
Additionally, if the operator plans to send data via both encodeB and gnodeB, the attacker would need two sniffers to record the data, which further complicates the process of passively sniffing 5G NSA traffic.