The most important factor to consider when assessing the residual risk of implementing encryption for data at rest is the key management. Key management is the process of generating, storing, distributing, using, and destroying the cryptographic keys that are used to encrypt anddecrypt the data. Key management is essential for ensuring the security, availability, and integrity of the encrypted data, as well as for complying with the legal and regulatory requirements. Poor key management could result in the loss, theft, compromise, or corruption of the keys, which could lead to unauthorized access, data breach, data loss, or data recovery failure. Therefore, key management must be considered to assess the residual risk, which is the risk that remains after the risk treatment, such as encryption, is applied. Data retention requirements, data destruction requirements, and cloud storage architecture are not as important as key management, as they do not directly affect the encryption and decryption of the data, and they may not introduce significant residual risk. References = CRISC Review Manual, 6th Edition, ISACA, 2015, page 217.