Forward secrecy for SUPI using Hybrid Post-Quantum Approach

This blog was co-authored with Shubham Kumar (chmodshubham@gmail.com)

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In this blog, we delve into the hybrid post-quantum cryptographic approach for SUPI to SUCI conversion. Hybrid post-quantum cryptography combines both classical and post-quantum cryptographic algorithms to enhance security.

Regardless of whether classical or post-quantum cryptography is employed, the seeds used for key generation are derived from Quantum Random Number Generators (QRNG). This hybrid method not only fortifies the network against quantum threats but also provides forward secrecy, ensuring that past communications remain secure even if current keys are compromised. This is the second method of making the procedure of SUPI concealment quantum secure. The ECIES method that was earlier used in this Core for the security of Subscriber’s identity needs to be replaced with either homogeneous mode or the Hybrid mode of QORE.

Hybrid Post-Quantum(H-PQ) Mode

Let’s see how QORE works in Hybrid Post-Quantum(H-PQ) mode to secure subscriber’s identity from Quantum-Powered IMSI Catchers:

1. Long term keygen: Using a QRNG, 2 seeds of length 64 & 32 bytes respectively, are generated at the Home Network (HN). The 64-byte seed will be used to generate the Long Term (LT) Kyber512/768 keys — HN_KYBER_PRIV_KEY, HN_KYBER_PUB_KEY, the other seed will be used to create a Curve25519 Private Key (HN_ECC_PRIV_KEY) and a Public Key (HN_ECC_PUB_KEY).
2. The Long-term public keys — HN_KYBER_PUB_KEY & HN_ECC_PUB_KEY will then be sent to the UE, with their corresponding private keys stored securely at the 5G-CORE’s UDM.
3. During the initial registration procedure initiated by the UE, an ephemeral Curve25519 (ECC) Key pair will be generated at the UE — eph_priv_key, eph_pub_key. The eph_priv_key will be combined with the HN_ECC_PUB_KEY to form a shared secret — shared_secret1, this is done using the X25519 Elliptic Curve Diffie Hellman (ECDH).
4. Further, the HN_KYBER_PUB_KEY will be encapsulated to obtain a Ciphertext- cipher_text1, and a shared secret — shared_secret2
5. The shared secrets will be passed to Ansi-X9.63 KDF in the following order: shared_secret1 + shared_secret2 (convention), with HN_ECC_PUB_KEY + HN_KYBER_PUB_KEY (in the exact same order) being the shared information for the KDF. This will result in the generation of 3 keys: 1 symmetric encryption key — enc_key, an Initialization Vector — iv, and a MAC key — mac_key.
6. The enc_key & the IV then encrypts the critical part of the SUPI — MSIN, using AES256_CTR, this will result in the SUCI.
7. HMAC-SHA256 will be utilized to create a mac tag (UE_MAC) with the concealed MSIN as the data & the key being — mac_key.
8. The eph_pub_key will be concatenated with the cipher_text1, SUCI & the UE_MAC & will be sent to the UDM.
9. Any traces of the cipher_text1, the eph_priv_key & eph_pub_key will be immediately deleted at the UE.
10. At the UDM, the eph_pub_key will be combined with the HN_ECC_PRIV_KEY using X25519 ECDH to create the corresponding shared_secret1. Also, the cipher_text1 will be decapsulated using HN_KYBER_PRIV_KEY to obtain the shared secret — shared_secret2.
11. These 2 shared secrets will be passed to the Ansi X9.63 KDF (in the same order as shown above) along with HN_ECC_PUB_KEY + HN_KYBER_PUB_KEY being the shared information, obtaining a symmetric decryption key — dec_key, an Initialization Vector — IV, and a MAC Key — mac_key.
12. HMAC-SHA256 will then be utilized on the SUCI with key — mac_key to form the mac tag: UDM_MAC, if it matches the UE_MAC, then the procedure will be carried forward, else it’ll be aborted.
13. The dec_key & the IV will then be used to decrypt the SUCI using AES256_CTR to obtain the corresponding SUPI (having the MSIN). All traces of the cipher_text1, eph_pub_key will be deleted immediately, and the protocol will be completed

Note: QORE in hybrid mode is E2E tested with RAN/Q-RAN and simulated UE that supports Post-Quantum Cryptography.

Therefore we can say that for SUPI concealment QORE operates in two modes: Hybrid and Homogeneous mode.

Hybrid Post-Quantum mode: Combines the post-quantum algorithm Crystal-Kyber

with classical algorithms such as Curve25519 and Secp256.

Homogeneous Post-Quantum mode: Utilizes Crystal-Kyber solely, providing a robust

and secure encryption method.

The solution complies with both 3GPP and NIST standards, ensuring adherence to industry

guidelines and best practices. It supports multiple Encryption Profiles, each providing an

increasing level of security, offering versatility and adaptability to various security requirements.

The solution incorporates a QRNG for key generation, enhancing the security and randomness of encryption keys. Additionally, AES-128 will be replaced with AES-256, further strengthening

encryption. The implementation is designed to be resilient to quantum threats, providing robust

protection against potential future vulnerabilities posed by quantum computing.