In-Depth Report on Telecommunication Security

1. The Heart of Telecom Infrastructure: Ki and Subscriber Authentication Architecture

What is Ki?

  • Ki (Key) is the absolute secret key used to identify and authenticate mobile subscribers.
  • It is stored securely within the USIM card and the carrier’s core authentication servers (HLR/HSS/5GC), never exposed externally.
  • Authentication is performed by exchanging a random number (RAND) and a response (SRES) based on the Ki.

If Ki is leaked:
→ Attackers could create a “fake USIM” and successfully authenticate to the network.
→ This leads to risks like call interception, location tracking, and data theft.

Subscriber Authentication Flow

  • 2G (GSM): RAND → Generate and send SRES → Carrier verifies
  • 3G (UMTS) / 4G (LTE): Authentication using AKA protocol and RES response comparison
  • 5G (SA structure): Protect SUPI → Only send encrypted SUCI over the network

Reference: 3GPP TS 33.102

2. 5G NSA vs. 5G SA: Structural Differences and Security Comparison

NSA Architecture (Non-Standalone)

  • Adds 5G radio (NR) to the existing LTE Core (EPC).
  • Subscriber authentication and session management still rely on LTE procedures.
  • IMSI plaintext exposure risk remains.

SA Architecture (Standalone)

  • Fully independent 5G Core (5GC) deployment.
  • Enhanced protection through public key encryption → SUPI is encrypted and transmitted as SUCI.

SUCI (SUPI Concealment):

  • Subscriber devices encrypt SUPI using the carrier’s public key, sending SUCI instead.
  • The carrier decrypts SUCI to retrieve SUPI for authentication.

Reference: 3GPP TS 33.501

3. Technical Analysis of the SKT 2025 Breach

Incident Overview

  • On April 19, 2025, SK Telecom detected signs of a breach in its core network servers.
  • Potential leakage of USIM information affecting approximately 23 million subscribers.

Technical Issues

  • Plaintext transmission risks under NSA-based architecture.
  • Ki leakage enables USIM cloning and SIM swapping attacks.

Potential Attack Scenario

  • Core server infiltration → Access to subscriber database → Create cloned USIM → Hijack personal communications.

References:

4. In-Depth Analysis of Historical Global Cases

Gemalto Hacking Incident

  • 2010–2011: NSA and GCHQ targeted SIM card manufacturer Gemalto.
  • Attempted to steal SIM encryption keys (Ki) on a massive scale.

References:

APT10 Operation Soft Cell

  • Chinese APT10 group infiltrated global telecom core networks.
  • Mass theft of VIP subscribers’ call records and location data.

Reference:

Circles SS7 Eavesdropping

  • Circles, affiliated with NSO Group, sold SS7-based interception systems.
  • At least 25 governments purchased this technology for mass surveillance.

Reference:

5. Historical Limitations of Telecom Security Architecture: Critique on PKI and HSM Absence

Why Wasn’t PKI Implemented in Early Mobile Networks?

During the 2G/3G era, devices faced critical limitations in CPU performance, battery capacity, and network speed.

  • Public key operations like RSA and ECC were impractical with available technology.
  • Devices lacked sufficient computational and energy resources for real-time encryption.

Practical Choice:
→ A simple and fast symmetric key-based (Ki) authentication structure was adopted.

However, the Issues Were:

  • IMSI was transmitted in plaintext, exposing users to IMSI catcher (fake base station) attacks.
  • If Ki stored in USIMs were stolen, USIM cloning and identity spoofing attacks became feasible.
  • Supply chain risks (SIM manufacturers, telecom operators) were underestimated.

Moreover,
there was a lack of Hardware Security Modules (HSM) or equivalent secure hardware protection at that time.

  • Core servers (HLR/HSS) also lacked clear key separation and internal cryptographic hardware processing.
  • If a core server was compromised, large-scale Ki leakage could occur.

Thus, early mobile systems prioritized
“rapid commercialization” and “low-cost deployment” over deep security architecture, resulting in serious structural vulnerabilities.

Why PKI and HSM Are Now Essential

Today:

  • Modern devices can handle public key operations (RSA, ECC) in real time.
  • Network latency and performance have improved sufficiently to support encryption.

To strengthen telecom security today, we must implement:

  • SUPI Encryption (SUCI): Prevent plaintext transmission of subscriber identifiers.
  • TLS Secure Channels: Ensure end-to-end security across internal and external network boundaries.
  • Network Slice-Specific Security Policies: Maintain isolation and protection between services.

And on the server side:

  • Software-based key protection alone is insufficient.
  • HSM or equivalent Secure Environment must be used to:
    • Prevent key leakage
    • Protect boot integrity
    • Detect and resist physical tampering

In short:
Early mobile networks sacrificed security for rapid rollout,
whereas today, trust is the absolute prerequisite for telecom infrastructure.

6. Practical Countermeasures for Individual Users

7. Conclusion

There is no such thing as a perfect network.
But daily, proactive efforts to protect ourselves
remain the only true shield against silent, invisible threats.

Additional Comparative Analysis

  • SKT Breach: Affected ~23 million users; Ki leakage suspected; no abuse reported yet.
  • Gemalto Breach: Global SIM supply chain attack; mass key leakage debated.
  • APT10 Operation Soft Cell: Long-term infiltration of telecom cores by a Chinese APT group.
  • Circles Eavesdropping: SS7 vulnerabilities exploited for covert surveillance on a global scale.

References