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How Secure Is Your Wireless Network? Safeguarding Your Wi-Fi LAN by CCNA Lee Barken - CISSP MCP, CPA

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IV Collisions

When an IV is reused, we call this a collision. When a collision occurs, the combination of the shared secret and the repeated IV results in a key stream that has been used before. Since the IV is sent in clear text, an attacker who keeps track of all the traffic can identify when collisions occur. A number of attacks become possible upon the discovery of IV collisions.

A key stream attack is a method of deriving the key stream by analyzing two packets derived from the same IV. Simply stated, XORing the two cipher texts together will equal XORing the two plain texts together. Figure 3.4 shows this in detail.

Figure 3.4. A key stream attack.

In the upper left, we have taken 8 bits (plain text 1) and XORed them with our key stream. ...

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