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2G/3G Attacks

A5/1 Encryption Breaking

CRITICAL SEVERITY
Overview

A5/1 is the encryption algorithm used in 2G GSM networks. Due to its weak 64-bit key and known vulnerabilities, it can be broken in real-time to decrypt voice calls and SMS messages.

Technical Overview

The A5/1 stream cipher was designed in 1987 and has been cryptographically broken. Using rainbow tables and modern computing power, attackers can decrypt A5/1 encrypted communications in seconds. The attack requires capturing the encrypted traffic and the initial keystream.

Impact
Potential consequences of this attack
  • •Complete decryption of voice calls
  • •SMS message interception and decryption
  • •Loss of communication confidentiality
  • •Exposure of sensitive personal and business information
  • •Compliance violations for regulated industries
Attack Vectors
Methods used to execute this attack
  • •Capture encrypted GSM traffic using SDR
  • •Use rainbow tables to crack A5/1 encryption
  • •Real-time decryption with sufficient computing power
  • •Passive monitoring without detection
  • •Targeted surveillance of specific subscribers
Attack Methodology
Step-by-step attack execution process
  1. 1Set up SDR to capture GSM traffic on target frequencies
  2. 2Identify target device by IMSI or TMSI
  3. 3Capture encrypted voice or SMS traffic
  4. 4Extract keystream from captured data
  5. 5Use Kraken or similar tools with rainbow tables
  6. 6Decrypt communications in near real-time
Mitigations & Defense
Protective measures and countermeasures
  • Upgrade to 3G/4G networks with stronger encryption
  • Disable 2G on devices when not needed
  • Use end-to-end encrypted communication apps
  • Implement A5/3 encryption where 2G is necessary
  • Monitor for forced 2G downgrade attacks
  • Deploy network-level encryption for sensitive communications
Real-World Examples
Known incidents and use cases
  • →Karsten Nohl's demonstration of A5/1 cracking at CCC
  • →Intelligence agency surveillance programs
  • →Criminal interception of business communications
  • →Privacy violations in countries with weak telecom regulations

Related Attacks

2G/3G Attacks
IMSI Catching

IMSI catchers are rogue base stations that trick mobile devices into connecting to them, allowing attackers to capture International Mobile Subscriber Identity (IMSI) numbers and intercept communications.

2G/3G Attacks
Fake BTS Attacks

Fake Base Transceiver Station (BTS) attacks involve deploying rogue cell towers that impersonate legitimate network infrastructure to intercept communications and perform man-in-the-middle attacks.

2G/3G Attacks
Downgrade Attacks

Downgrade attacks force mobile devices to connect to older, less secure network technologies (2G) where encryption is weaker and easier to break, enabling various attack vectors.

References & Further Reading