Key Space Clumping: A phenomenon in cryptography where potential keys are unevenly distributed within the key space, creating clusters that may be more likely to be chosen or attacked. This can weaken cryptographic systems by reducing the effective key space size, making it more susceptible to brute force attacks. Robust cryptographic systems strive to avoid key space clumping to ensure the equal likelihood of all keys and maintain strong security.

Key Stretching: The process of applying a cryptographic function multiple times to a password or key to increase its effective length and security. It is used to protect against brute-force attacks and dictionary attacks by making it computationally infeasible to guess or crack the key. Examples of key stretching algorithms include PBKDF2 and Argon2.

KillDisk malware: A type of destructive malware that erases or corrupts data on a system, often used to disrupt operations or as part of a larger cyberattack. Examples of KillDisk attacks include attempts to extort money through data destruction threats or to cause system outages, such as the Petya and NotPetya incidents.

Kiosk service point: A physical location where customers can access a company's services or products, often through the use of a self-service terminal. It is commonly found in airports, shopping malls, and other public areas. Examples include a ticket kiosk at an amusement park or a rental car kiosk at an airport.

Known-good Data (Testing): A testing methodology that uses a pre-determined set of input data with expected outcomes to verify system performance and functionality. This data set reflects typical input conditions, enabling accurate assessment of whether a system operates correctly. In security testing, it ensures systems identify and manage suspicious or harmful inputs effectively.

Known-plaintext attack: A known-plaintext attack is a method of cryptanalysis where the attacker has access to both the plaintext (unencrypted message) and its corresponding ciphertext (encrypted message). It is used to exploit weaknesses in encryption algorithms by analyzing how the plaintext is transformed into ciphertext, with the aim of discovering the encryption key. Examples include attacks on historical ciphers like the Vigenère cipher.