How important is entropy?

Based on Wifi WEP Encryption algorith.

Today let me explain why entropy is so important in cryptography based on WEP protocol.

WEP (Wired Equivalent Privacy) was introduced as a security protocol for Wi-Fi networks in the late 1990s. It primarily uses the RC4 stream cipher for encryption. The goal of WEP was to provide comparable confidentiality to that of a traditional wired network, hence the name “Wired Equivalent Privacy”.

To achieve this, WEP uses a set of keys to encrypt data that is sent over the network. The primary method for encryption in WEP is the RC4 stream cipher, which uses a secret key and a public initialization vector (IV) to create a keystream. This keystream is then bitwise XOR’d with the plaintext to create the ciphertext that is sent over the network.

However, WEP is now deprecated due to its numerous vulnerabilities, with the main one being Key Reuse. This issue occurs due to the combination of the secret key with a 24-bit IV to form the seed for RC4. Given that the IV is transmitted in plain text and has a short length, the same IV tends to be reused after roughly 5000 packets on a busy network, leading to key repetition and a major security vulnerability.

In addition to Key Reuse, other notable weaknesses include Weak IVs that reveal key-output correlations, absence of built-in Key Management, and a weak Integrity Check mechanism. All these points contribute to WEP’s overall security vulnerabilities but Key Reuse remains central.

The concept of entropy, or randomness, is crucial in key generation. In WEP’s case, the repeated use of IVs and the static nature of secret keys reduce the overall entropy of the key+IV combination, thus making it easier for attackers to exploit these vulnerabilities. Consequently, modern protocols have moved to improve entropy and security by using larger keys and IVs, along with frequent key changes.