Key or Cryptographic Key or Cryptovariable: A piece of data used as input for an encryption algorithm to transform plaintext data into ciphertext or vice versa. The key determines the specific output of an encryption algorithm, thus ensuring that only those who possess the correct key can decrypt the data. Keys are used in a variety of contexts, including secure communication, data protection, and digital signatures.

Key Pair: A key pair in public-key cryptography consists of a private key, which is kept secret, and a public key, which can be shared publicly. The keys are mathematically related, allowing for secure data encryption, digital signatures, and key exchanges. Key pairs enable functions such as SSL/TLS for secure web communications and PGP for secure emails.

Key Performance Indicators (KPIs): Quantifiable measures that are used to evaluate the success or effectiveness of a particular activity or process. These are often used to track progress toward strategic objectives over time. For instance, in a security context, KPIs could include the time taken to detect a threat, the rate of false positive alerts, or the percentage of employees who pass a phishing test.

Key Recovery: The process of retrieving and restoring a lost or compromised cryptographic key. It is used to ensure the availability and integrity of encrypted data in the event of a key loss or failure. Examples of key recovery methods include backup and recovery of keys, secure key escrow, and keyless recovery using a trusted third party.

Key Risk Indicators (KRIs): Metrics used to measure and evaluate the risk level of an organization or system based on predetermined criteria and thresholds. It is used to identify, prioritize, and monitor potential risks to the organization's security and integrity. Examples of security-related KRIs include the number of vulnerabilities, the frequency of security incidents, and the level of access controls.

Key Space: In cryptography, key space refers to the set of all possible keys that can be used with a cryptographic algorithm, defining the limits of potential key values. The size of the key space (i.e., the total number of distinct keys) is determined by the key length, impacting the algorithm's resistance to brute force attacks. For example, a 128-bit keyspace has 2^128 possible keys.