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Cyber Risk Council
Category: Cryptography & Key Management

Encryption at Rest

Also known as: Data-at-rest encryption, At-rest encryption
Simply put

Encryption at rest is the practice of scrambling stored data so that it cannot be read without the correct key. It protects information saved on disks, drives, and other storage from being exposed if someone gains access to the underlying hardware or files. Without the key, the encrypted data is useless to an attacker.

Formal definition

Encryption at rest refers to applying cryptographic protection to data while it is persisted to storage media such as disks, databases, or file systems, ensuring the stored data remains unreadable without the corresponding decryption key. It is intended to mitigate threats including data breaches, unauthorized access, and physical theft of storage media, and it limits the access that systems and engineers have to the underlying data. It typically operates at the storage or disk layer, so that data cannot be recovered by physically moving media to another machine, and its effectiveness depends heavily on proper key management. Encryption at rest addresses stored data specifically and does not, on its own, protect data in transit or data in use.

Why it matters

Encryption at rest addresses a specific and often underestimated threat: the exposure of stored data when an attacker gains access to the underlying hardware or files rather than to a running application. If someone physically moves a drive to another machine, or gains access to raw storage media or backup files, encryption at rest ensures that data remains unreadable without the correct key. Without that protection, stored files, databases, and documents can be read directly, making the data useful to anyone who obtains the media.

For security leaders, encryption at rest is a foundational control that also limits internal exposure. It restricts the access that systems and engineers have to underlying data, which supports least-privilege objectives and reduces the blast radius of compromised infrastructure accounts. It is a common expectation in data protection programs and is frequently referenced in the context of preventing data breaches, unauthorized access, and physical theft of storage.

A critical caveat for executives and buyers is that encryption at rest is not a complete data protection strategy. It secures stored data specifically and does not, on its own, protect data in transit or data in use. Its effectiveness depends heavily on proper key management; if keys are poorly protected or accessible alongside the encrypted data, the control provides far less value than assumed. A virtual CISO can help frame encryption at rest as one layer within a broader governance and risk approach rather than a standalone guarantee.

Who it's relevant to

Security and risk leaders (including virtual and fractional CISOs)
Encryption at rest is a common component of a data protection program, and security leaders are typically responsible for advising on where it should be applied and how key management should be governed. A virtual CISO can direct and prioritize this control as part of overall risk strategy, while accountability for the underlying decisions and their implementation generally remains with the client organization and its officers.
Organizations handling sensitive stored data
Any organization that stores files, documents, or database records containing sensitive information benefits from encryption at rest as a defense against data breaches, unauthorized access, and physical theft of storage media. Its value depends on organizational maturity and on complementary controls, since it does not address data in transit or data in use.
Engineering and infrastructure teams
Encryption at rest limits the access that systems and engineers have to underlying data, supporting least-privilege objectives. These teams typically implement and operate the encryption and key management mechanisms at the storage layer, and the specific approach may vary by platform or provider.
Buyers evaluating security posture
Executives and procurement teams assessing vendors or their own environments often treat encryption at rest as a baseline expectation. It is important to understand that it protects stored data specifically and does not on its own guarantee comprehensive protection, so it should be evaluated alongside key management practices and other controls.

Inside Encryption at Rest

Data-at-Rest Scope
Encryption at rest applies to data that is stored on persistent media such as disks, databases, backups, file systems, and archival storage, as distinguished from data in transit or data in use. Defining precisely which repositories fall in scope is a foundational step, since coverage often varies across an environment.
Encryption Keys and Key Management
The cryptographic strength of encryption at rest depends heavily on how keys are generated, stored, rotated, and access-controlled. Key management (including options such as customer-managed keys or hardware security modules) is frequently the point where implementations succeed or fail, and it is often treated as a governance concern as much as a technical one.
Full-Disk vs. Field or Column-Level Encryption
Encryption may be applied at different layers, from full-disk or volume encryption to more granular database, field, or column-level encryption. Each layer protects against different threat scenarios and carries different operational trade-offs, so the chosen approach should map to the specific risks being addressed.
Relationship to Compliance Requirements
Standards and regulations such as HIPAA, PCI DSS, GDPR, ISO 27001, and SOC 2 may reference or expect protection of stored sensitive data, and encryption at rest is commonly used to support those expectations. It typically contributes to readiness rather than by itself constituting compliance or certification.
Threat Model Coverage
Encryption at rest is primarily effective against threats involving physical media theft, loss, or improper disposal, and unauthorized access to underlying storage. Its protective value is more limited against threats where an attacker obtains access to a running system with decryption privileges.
Governance and Oversight Role
Decisions about where encryption at rest is required, which key management model to use, and how it aligns with organizational risk appetite are governance activities. A security leader such as a virtual CISO may advise on and direct these decisions, while accountability for them typically remains with the client organization.

Common questions

Answers to the questions practitioners most commonly ask about Encryption at Rest.

Does encryption at rest protect data while it is being accessed or transmitted?
No. Encryption at rest protects data stored on disk or other media when it is not actively being used. It does not protect data in transit between systems, which requires encryption in transit such as TLS, nor does it protect data in use while it is loaded into memory or being processed by an application. A common expert correction is that treating encryption at rest as comprehensive data protection is a mistake; it addresses only one state of data and should be paired with controls for the other states.
If our data is encrypted at rest, does that mean we are protected against breaches and compliant with regulations?
Not necessarily. Encryption at rest reduces the risk that stored data is readable if physical media or storage volumes are stolen or improperly accessed, but it does not prevent breaches that occur through valid credentials, application-layer access, or compromised accounts, since the data is typically decrypted transparently for authorized processes. Regarding compliance, frameworks and regulations such as HIPAA, PCI DSS, or GDPR may reference or expect encryption as a safeguard, but encryption at rest alone does not establish compliance or certification. It is one control among many, and its role varies by framework and how the overall program is assessed.
Where in our environment should encryption at rest typically be applied?
Encryption at rest is often applied at multiple layers, and the appropriate placement depends on the environment and risk tolerance. Common points include full-disk or volume encryption, database-level or column-level encryption, file and object storage encryption, and backups. In many engagements a security leader will advise mapping where sensitive data resides before deciding which layers to encrypt, since encrypting only one layer may leave other copies, such as backups or logs, unprotected. Effectiveness depends on organizational data inventory maturity.
How are encryption keys typically managed for data at rest?
Key management is usually the more consequential decision than the encryption algorithm itself. Approaches vary by provider and environment, and may include cloud-provider managed keys, customer-managed keys, or dedicated key management services and hardware security modules. Considerations often include how keys are generated, stored separately from the encrypted data, rotated, and revoked, as well as who has access to them. A frequent expert point is that if keys are stored alongside or accessible from the same compromised system as the data, the protective value of encryption at rest can be significantly reduced.
Who is accountable for decisions about encryption at rest when a virtual or fractional CISO is engaged?
A virtual or fractional CISO typically advises on encryption strategy, standards, and key management governance and helps direct implementation, but legal and organizational accountability for security decisions generally remains with the client organization and its officers unless a contract specifies otherwise. Hands-on tasks such as configuring encryption on storage systems or administering key management tooling are often out of scope for an advisory engagement and may fall to internal teams or contracted operational providers.
What factors should be considered before implementing encryption at rest across an organization?
Considerations often include a data inventory to identify what should be encrypted, performance and cost implications, key management capabilities, recovery and backup procedures so encrypted data does not become unrecoverable, and how encryption fits with access controls and monitoring. The value and feasibility of implementation may vary by organizational maturity, existing infrastructure, and stakeholder cooperation. Because encryption at rest does not defend against authorized-but-compromised access, it is typically planned as part of a broader control set rather than as a standalone safeguard.

Common misconceptions

Encryption at rest protects data in all states and eliminates the risk of a breach.
Encryption at rest primarily protects stored data against scenarios such as stolen or improperly disposed media and unauthorized access to underlying storage. It generally does not protect data in transit or data in use, and it does not prevent breaches where an attacker gains access to a running system with legitimate decryption privileges. It reduces certain risks rather than guaranteeing breach prevention.
Enabling encryption at rest makes an organization compliant with regulations like HIPAA, PCI DSS, or GDPR.
Encryption at rest may support readiness against expectations in these frameworks, but compliance depends on many additional controls, processes, and documentation. Enabling encryption is not equivalent to achieving or certifying compliance, and no single control should be described as making an organization compliant.
Once encryption at rest is turned on, the data is secure and no further attention is needed.
The security value depends heavily on key management, including how keys are stored, rotated, and access-controlled, as well as which storage layers are actually in scope. Weak or poorly governed key management can undermine the protection, so encryption at rest requires ongoing oversight rather than being a one-time setting.

Best practices

Define and document precisely which data stores, backups, and archives are in scope for encryption at rest, and identify any repositories that remain unencrypted so gaps are visible and risk-based.
Treat key management as a first-class concern by establishing controls for key generation, storage, rotation, and access, and consider options such as customer-managed keys or hardware security modules where appropriate to the risk.
Match the encryption layer, such as full-disk versus field or column-level encryption, to the specific threats being addressed rather than assuming one approach covers all scenarios.
Clarify that encryption at rest addresses stored-data threats and pair it with complementary controls for data in transit and data in use so it is not relied upon as sole protection.
Position encryption at rest as supporting readiness for relevant standards such as HIPAA, PCI DSS, GDPR, ISO 27001, or SOC 2, without asserting that it alone achieves compliance or certification.
Keep governance decisions about scope, key management model, and risk alignment with accountable client stakeholders, using security leadership to advise and direct rather than to assume organizational accountability.