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 and decrypt 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.

Residual risk is the risk that remains after applying risk responses, such as avoidance, mitigation, transfer, or acceptance. It represents the level of exposure that the organisation is willing to tolerate or assume. Residual risk should be aligned with the organisation's risk appetite and risk tolerance, which are determined by senior management. Therefore, the best way to obtain senior management support for investment in a control implementation would be to articulate the reduction in residual risk that the control would achieve. This would demonstrate how the control would help the organisation meet its risk objectives and reduce the likelihood or impact of adverse events. References = ISACA CRISC Review Manual, 7th Edition, Chapter 1, Section 1.3.2, page 25.