What is meant by key clustering in cryptography?

Key clustering refers to the phenomenon where two different cryptographic keys generate the same ciphertext when encrypting a given plaintext. This can present significant security vulnerabilities because if an attacker can derive or guess one of the keys, they could potentially decrypt messages encrypted with the other key as well, leading to a compromised security environment.

In the context of cryptography, it's crucial that each key yields a unique ciphertext for a unique plaintext to maintain confidentiality and prevent unauthorized access. Key clustering disrupts this principle, making it a serious concern during the design and implementation of encryption algorithms and systems.

The other options do not accurately describe key clustering. Combining multiple keys for encryption, increasing key length, or sharing a key among multiple users address different aspects of cryptography and security measures, but do not reflect the specific issue presented by key clustering.