Overview
This lecture discusses the RC4 NOMORE attack, which exposes practical vulnerabilities in the RC4 encryption algorithm used in HTTPS and WPA-TKIP, enabling attackers to decrypt sensitive information such as web cookies and network packets.
RC4 in HTTPS and Its Weaknesses
- RC4 was widely used to encrypt HTTPS traffic but contains exploitable statistical weaknesses.
- Attackers can use these weaknesses to decrypt web cookies, enabling account takeover and unauthorized data access.
- The RC4 NOMORE attack demonstrates cookie decryption within 52–75 hours, making practical exploitation feasible.
- The attack process involves injecting malicious JavaScript, capturing many encrypted requests, and testing likely cookie values.
Demonstration and Impact
- The attack was proven in a lab setting using real devices, marking the first practical exploitation of RC4 in TLS/HTTPS.
- Successfully decrypting a 16-character cookie can be done with a 94% chance if enough encrypted traffic is collected.
- The method applies to any repeatedly encrypted data, not just cookies.
- Following the disclosure, major browsers disabled RC4, reducing its use in HTTPS/TLS to below 1%.
RC4 NOMORE Attack vs Previous Attacks
- Earlier attacks needed over 2000 hours and more requests; RC4 NOMORE reduces this to 75 hours with higher request rates.
- Previous methods targeted password decryption or depended on weaker, less frequently occurring keystream biases.
- RC4 NOMORE uses both Fluhrer-McGrew and Mantin's ABSAB biases, providing greater efficiency and practicality.
Vulnerabilities in Other Protocols
- RC4 weaknesses also affect WPA-TKIP; the attack allows decryption/injection of Wi-Fi traffic in about an hour.
- Any protocol using RC4 is considered vulnerable due to these statistical biases.
Countermeasures
- The only effective defense is to stop using RC4 in all protocols.
- Slowing down traffic generation limits attack speed but does not prevent exploitation.
Key Terms & Definitions
- RC4 — A stream cipher previously used to encrypt internet and Wi-Fi communications.
- HTTPS — Protocol securing web traffic, often with RC4 in the past.
- TLS — Protocol for secure internet communication, sometimes using RC4.
- WPA-TKIP — A Wi-Fi security protocol using RC4, now vulnerable.
- Statistical Bias — Predictable patterns in RC4's output exploitable for decryption.
- Fluhrer-McGrew Bias — Statistical bias affecting two consecutive keystream bytes in RC4.
- Mantin's ABSAB Bias — Bias causing pairs of consecutive RC4 bytes to repeat values.
Action Items / Next Steps