Encryption algorithms are deterministic. This means that the same input will always result in the same output. This is a good thing, given that, when decrypting, you want to be able to recover the exact same message that was encrypted. Unfortunately, this makes encryption weaker, as it makes it vulnerable to cryptanalysis and known-text attacks.

To face this issue, Initialization Vectors (IVs) were implemented. An IV is an extra piece of information that is different each time the algorithm is executed. It is used to generate the encryption key or to preprocess the cleartext, usually through an XOR operation. This way, if two messages are encrypted with the same algorithm and the same key, but a different IV, the resulting ciphertexts will be different. IVs are attached to the ciphertext, as the recipient has no way of knowing them beforehand.

The golden rule, especially for stream ciphers, is never to repeat IVs. The RC4 implementation of the Wired Equivalent Privacy (WEP) authentication in wireless networks uses a 24-bit (3 bytes) IV that permits duplicated keystreams in a short period of time. Having a known text, such as a DHCP request, sent through the network multiple times with the same IV allows an attacker to recover the keystreams, and multiple keystreams/IV pairs can be used to recover the shared secret.