# Post-Quantum Key Exchange and Subscriber Identity Encryption in 5G Using ML-KEM (Kyber) 1
## 2 Motivation and Background
### Background on 5G AKA and Cryptographic Primitives
#### 2.2.1 5G-AKA and SUCI  
Fifth-generation Authentication and Key Agreement (5G-AKA) is a mutual authentication protocol in the 5G cellular network between the user equipment (UE) and the core network (CN). The UE is the beneficiary of the cellular service provision receiving the connectivity access, while the CN is the cellular service provider network providing the connectivity access to the UE. The 5G-AKA protocol consists of two phases: the identification phase and the challenge-response phase. The identification phase starts with a registration request by the UE. Upon powering on, the UE initiates the registration request to the CN by sending its subscription identifier. This identifier is stored on an integrated chip called the Universal Subscriber Identifier Module (USIM). The USIM in the UE also stores a public key of the CN. In 4G, the International Mobile Subscriber Identity (IMSI) is sent in plaintext during authentication initialization. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Security Level** & **Symmetric** & **RSA** & **EC** &
\begin{tabular}{c} **Post-Quantum** \\ **(NIST Round 4)** \\ \end{tabular} \\ \hline
128 & AES-128 & 3072 bits & 256 bits & Kyber-512 \\ \hline
192 & AES-192 & 7680 bits & 384 bits & Kyber-768 \\ \hline
256 & AES-256 & 15,360 bits & 512 bits & Kyber-1024 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Security level for pre-quantum vs. post-quantum ciphers. [PERSON]’s algorithm breaks the pre-quantum RSA and EC ciphers by finding the key in polynomial time. \begin{table}
\begin{tabular}{c c} \hline \hline AKA & Authentication and Key Agreement \\ \hline CN & Core Network \\ \hline ECIES & Elliptic Curve Integrated Encryption Scheme \\ \hline IMSI & International Mobile Subscriber Identity \\ \hline KEM & Key Encapsulation Mechanism \\ \hline KDF & Key Derivation Function \\ \hline MSIN & Mobile Subscriber Identity Number \\ \hline PQC & Post-Quantum Cryptography \\ \hline SUCI & Subscriber Concealed Identifier \\ \hline SUPI & Subscriber Permanent Identifier \\ \hline UE & User Equipment \\ \hline USIM & Universal Subscriber Identity Module \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Acronyms used in this research. In 5G, the identifier is known as a Subscriber Permanent Identifier (SUPI), which is sent in an encrypted format known as an SUCI. The CN public key stored in the USIM encrypts the SUPI into the SUCI. Figure 2 shows the SUCI format. The SUCI contains an SUPI-type indicator, a home network identifier, a protection scheme ID, a home network public-key ID, and a protection scheme's output. The protection scheme's output contains a public key, message authentication code (MAC), and encrypted MSIN (Mobile Subscriber Identity Number), either an SUPI or IMSI. When the UE sends the SUCI to the core network, it decrypts the SUCI using its private key and identifies the subscriber successfully. The SUCI format protection scheme identifies the algorithm used for the encrypt and decrypt function. Table 3 describes the protection schemes, where the protection scheme ID is a numerical value ranging from 0 to 15. It signifies whether a protection scheme is null (0) or non-null (1-15) for the given SUCI value. When an SUPI value is provided, the protection scheme ID determines the corresponding scheme applied, resulting in an output scheme. In the null scheme, no encryption is applied to the provided SUPI. Instead, the output scheme consists of the MSIN component of the IMSI. Our work focuses on the 5G protection schemes beyond the null scheme, which uses encryption to protect the confidentiality of the UE ID. More specifically, we focus on the two schemes called Profile A and Profile B using distinct ciphers. In both Profiles A and B, the ECIES generates the SUCI from a given SUPI value at the UE side. Similarly, at the CN side, the ECIES is utilized to decipher the SUPI value from the received SUCI value.