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Course 14 - Wi-Fi Pentesting | Episode 5: WEP Cracking: Packet Injection and Replay Attacks (ARP, Chopchop, Fragmentation, and SKA)

CyberCode Academy

About this episode

In this lesson, you’ll learn about:
  • Why WEP cracking depends on Initialization Vectors (IVs)
  • How packet injection accelerates WEP cracking
  • The most reliable WEP injection technique (ARP Replay)
  • Alternative injection methods for idle networks
  • The conceptual difference between Chopchop and Fragmentation attacks
  • Why Shared Key Authentication (SKA) changes the attack strategy
  • How attackers adapt when fake authentication is blocked
Forcing IV Generation on WEP Networks Cracking WEP depends on collecting a large number of Initialization Vectors (IVs). On busy networks, IVs are generated naturally through traffic. However, on idle networks, attackers must force the access point to generate new packets, which in turn generates new IVs. This episode explains three primary packet injection methods, followed by a special technique for Shared Key Authentication (SKA) networks. 1. ARP Request Replay Attack (Most Reliable Method) This is considered the most effective and dependable method for accelerating IV collection. Conceptual Overview
  • The attacker monitors the network.
  • A special ARP request packet is captured.
  • This ARP packet is:
    • Replayed repeatedly back into the network.
  • Each replay forces the access point to:
    • Respond with a new encrypted packet
    • Generate a new IV
This results in:
  • A rapid increase in the IV count
  • Enough data to crack:
    • 64-bit WEP keys
    • 128-bit WEP keys
Key Requirement
  • The attacker must first associate with the target network
  • Without association:
    • The access point will ignore injected packets
2. Chopchop Attack (For Low-Traffic Networks) This method is useful when:
  • The network has no connected clients
  • There is very little traffic
  • No ARP packets are naturally available
How the Chopchop Attack Works (Conceptually)
  • A single encrypted packet is captured.
  • The attacker attempts to:
    • Recover part of the keystream
  • Even a partial keystream (around 80–90%) can be sufficient.
  • Using this partial keystream:
    • A new forged ARP packet is created.
  • This forged packet is then:
    • Injected into the network
    • Forces the access point to generate new encrypted packets
    • Rapidly increases the IV count
This method:
  • Does not rely on existing ARP traffic
  • Works even when the network is almost completely idle
3. Fragmentation Attack This attack is similar in concept to Chopchop, but with an important difference. Key Characteristics
  • Instead of recovering a partial keystream:
    • The attacker recovers the entire 1,500-byte PRGA
  • Once the full PRGA is obtained:
    • A forged packet is created
    • The packet is injected into the network
    • IV generation increases rapidly
Comparison with Chopchop
  • Requires:
    • Better signal quality
    • Being physically closer to the access point
  • Advantages:
    • Much faster than Chopchop
    • More reliable once PRGA is fully obtained
4. Cracking WEP Networks Using Shared Key Authentication (SKA) Most WEP networks use:
  • Open Authentication
However, some rare networks use:
  • Shared Key Authentication (SKA)
Why SKA Is Different
  • In SKA:
    • The router refuses association
    • Unless the correct WEP key is already known
  • This means:
    • The standard fake authentication technique fails
    • Traditional ARP replay cannot be initiated normally
Modified ARP Replay Attack for SKA Networks To bypass SKA restrictions:
  • The attacker must rely on:
    • An already connected legitimate client
How the Bypass Works (Conceptually)
  • The attacker:
    • Observes a connected client
    • Takes note of that client’s MAC address
  • The ARP replay attack is then:
    • Performed using the victim’s MAC address
  • The access point believes:
    • The traffic is coming from the authorized client
  • This allows:
    • Rapid packet generation
    • IV collection without fake authentication
    • Successful WEP key recovery
This method works for:
  • SKA-based WEP networks
  • Standard WEP networks as well
Key Educational Takeaways
  • WEP security fails because:
    • IVs are too small
    • Keystreams get reused
  • Packet injection exists purely to:
    • Speed up IV generation
  • ARP Replay is:
    • The most reliable injection method
  • Chopchop and Fragmentation are:
    • Backup techniques for idle networks
  • Shared Key Authentication:
    • Does not fix WEP’s cryptographic weakness
    • Only changes the attack strategy


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Course 14 - Wi-Fi Pentesting | Episode 5: WEP Cracking: Packet Injection and Replay Attacks (ARP, Chopchop, Fragmentation, and SKA)

CyberCode Academy

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11:50

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