What are the best practices for choosing and changing symmetric keys?

When selecting symmetric keys, it is critical to ensure that they are generated using a source of sufficient entropy to prevent predictability, which could lead to vulnerabilities. Cryptographically secure pseudorandom number generators (CSPRNGs) are typically used for this purpose. The key length is also important; it should be chosen based on the security requirements and the expected lifetime of the encryption system. For modern security standards, a minimum of 128 bits is generally recommended, with 256 bits being preferable for environments requiring higher security. Moreover, keys should be unique and not reused across different systems or applications.

The distribution of symmetric keys must be conducted securely to prevent interception by unauthorized parties. This often involves using a secure channel for transmission or employing an asymmetric encryption system where only the recipient can decrypt the key. For scenarios involving multiple parties, a key distribution center can be used to centrally manage and distribute keys securely. It's also crucial to authenticate the identities of both the sender and recipient of a key to prevent man-in-the-middle attacks. Authentication can be achieved through mechanisms like digital certificates or pre-shared secrets, depending on the environment.

Symmetric keys should only be used for their intended purposes to minimize the risk of key compromise. For example, a key used for encrypting database data should not be used for session encryption in a web application. Implementing strict usage policies helps in mitigating the risk of cross-purpose key exposure. Operational restrictions, such as limiting the number of operations or the amount of data that can be encrypted with a single key, are also important. This not only helps in reducing the risk of cryptographic attacks like birthday attacks but also limits the damage in case a key is compromised.

Symmetric keys should only be used for their intended purposes to minimize the risk of key compromise. For example, a key used for encrypting database data should not be used for session encryption in a web application. Implementing strict usage policies helps in mitigating the risk of cross-purpose key exposure. Operational restrictions, such as limiting the number of operations or the amount of data that can be encrypted with a single key, are also important. This not only helps in reducing the risk of cryptographic attacks like birthday attacks but also limits the damage in case a key is compromised.

Key rotation is the practice of replacing old keys with new ones at regular intervals or under specific conditions, such as after a breach. This limits the amount of data that can be compromised with a single key and reduces the lifetime over which attacks can be developed. Rotation policies should define clear timelines for key changes, depending on the sensitivity of the data and regulatory requirements. Automated key rotation systems can help ensure timely replacement of keys without significant manual intervention. It is crucial, however, to update all systems and backups that depend on the rotated keys and to securely archive old keys if necessary for audit or decryption purposes.

When a key is no longer needed, it must be destroyed in a manner that prevents its recovery. This process should be irreversible. Physical destruction methods can be used for keys stored on physical media, such as shredding or degaussing. For keys stored digitally, overwriting the key material with random data multiple times is recommended. Proper documentation and verification of the destruction process are critical to ensure that keys are not inadvertently left in a recoverable state. This should be part of a formal key management lifecycle policy, regularly reviewed to adapt to new technologies and threats.

In addition to these practices, it is important to implement a comprehensive key management policy that includes training for personnel involved in cryptographic processes. Awareness and training help prevent human errors that could lead to key mismanagement or security breaches. Additionally, legal and compliance considerations should be assessed regularly to ensure that key management practices meet all relevant data protection and privacy laws. Business continuity and disaster recovery plans should include strategies for cryptographic key recovery in the event of a system failure or disaster. These plans must ensure that keys can be securely recovered and restored without compromising their integrity.