What encryption and decryption algorithms are used in GSM

Question

I’m trying to understand how GSM networks secure communication. What encryption and decryption algorithms are commonly used in GSM, and how effective are they against modern threats? Have there been any known vulnerabilities?

Answer 1

GSM (Global System for Mobile Communications) uses encryption to secure voice and data communication between mobile devices and network infrastructure. 

The encryption is focused on ensuring confidentiality and authenticity. However, some algorithms have known vulnerabilities.

1. Encryption Algorithms in GSM

A5 Family of Algorithms

The A5 family is used to encrypt voice and data traffic over the air interface in GSM networks.

• A5/0 (No Encryption)
  Used in some regions or for specific purposes where encryption is disabled.

• A5/1

  • Designed for use in Europe.
  • A stream cipher based on a combination of three LFSRs (Linear Feedback Shift Registers).
  • Known Vulnerabilities:
    • Broken by cryptanalysis using brute force or time-memory trade-offs.
    • Real-time attacks have been demonstrated, making it unsuitable for modern use.

• A5/2

  • A weaker version of A5/1 designed for export to countries with stricter encryption regulations.
  • Known Vulnerabilities:
    • Broken easily with minimal computational effort.
    • Considered highly insecure and deprecated.

• A5/3 (KASUMI)

  • Based on the KASUMI block cipher from the 3GPP standards.
  • Provides stronger security than A5/1 and A5/2.
  • Vulnerabilities:
    • Academic attacks have shown weaknesses in reduced-round versions of KASUMI, but practical exploitation remains challenging.

• A5/4

  • An enhanced version of A5/3 with improvements for better security.
  • Less widely deployed due to legacy infrastructure constraints.

2. Key Management and Authentication

GSM encryption relies on a shared secret key (Ki) stored in the SIM card and the home network’s Authentication Center (AuC).

• COMP128 Algorithm:
  Used for authentication and key generation (Kc) in earlier GSM implementations.
  • Vulnerabilities: Flaws in COMP128-1 allowed attackers to clone SIM cards, leading to its replacement with COMP128-2 and COMP128-3.

3. Implementation Process

1. Authentication:

   • The network sends a random challenge (RAND) to the mobile device.
   • The SIM uses Ki and RAND to compute a response (SRES) using a hash algorithm.
   • If SRES matches the network’s computation, the user is authenticated.

2. Key Generation:

   • The ciphering key (Kc) is derived from Ki and RAND using the COMP128 algorithm.

3. Encryption:

   • Traffic is encrypted on the air interface using the Kc and one of the A5 algorithms.

4. Vulnerabilities in GSM Encryption

Despite its widespread use, GSM encryption has notable vulnerabilities:

• Lack of End-to-End Encryption:
  Encryption only secures the air interface. Once traffic reaches the operator's network, it is decrypted, leaving it vulnerable to interception.

• Weak Algorithms:

  • A5/2 and A5/1 are outdated and vulnerable to real-time attacks.
  • A5/3 offers better security but has academic vulnerabilities.

• Replay and Man-in-the-Middle (MitM) Attacks:
  Attackers can exploit GSM’s lack of mutual authentication (only the user is authenticated, not the network).

• IMSI Catchers:
  Devices like fake base stations (e.g., Stingrays) can trick phones into connecting to them, bypassing encryption.

5. Modern Threats and Alternatives

• Threats:
  With advances in computational power and cryptanalysis, older GSM encryption standards are inadequate against modern attackers.

• Alternatives:

  • Transition to 3G or 4G (LTE), which use stronger encryption (e.g., UMTS encryption based on AES).
  • Mutual authentication mechanisms in newer standards provide additional security.