What is encryption at rest?
Encryption at rest is the protection of stored data, on a laptop, server, database, backup or cloud storage, by converting it into unreadable ciphertext that can only be turned back into usable information with the correct decryption key. It protects patient data when the device or storage itself is lost, stolen or improperly accessed. This entry focuses on the layers of at-rest encryption and the keys behind them; for the at-rest and in-transit pair together, see encryption at rest and in transit, explained.
- "At rest" means stored, as opposed to "in transit", which means moving across a network.
- Encryption at rest comes in layers: full disk, database or storage volume, file, and field. Each protects against different threats.
- Encryption is only as strong as its key management. A key stored next to the data protects very little.
- Under HIPAA, PHI encrypted consistent with HHS guidance is not "unsecured", which can take a lost device out of breach notification.
The layers of encryption at rest
"We encrypt data at rest" can mean very different things. Ask which layer:
- Full-disk encryption. The whole drive of a laptop, desktop or server is encrypted. If the device is stolen while powered off, the data is unreadable. Once a user logs in, the data is readable to anyone using that session, so it does not protect against a compromised account.
- Storage or database encryption. A cloud storage service or database encrypts everything it writes. This protects against stolen disks and some infrastructure-level access, and is standard for reputable cloud services.
- File-level encryption. Individual files, such as a schedule export, are encrypted with their own key or password. Useful when files move between systems or sit in shared folders.
- Field-level encryption. Specific sensitive values, such as phone numbers or dates of birth, are encrypted inside the database, so even people with database access see only ciphertext unless the application decrypts them for an authorized purpose.
Why key management decides everything
An encryption key is the secret that unlocks the data. If an attacker gets both the encrypted data and the key, the encryption did nothing. Good practice means:
- Keys stored separately from the data, ideally in a dedicated key management service or hardware security module.
- Access to keys limited to the systems and people that need them, with every use logged.
- Keys rotated on a schedule and when staff with access leave.
- Recovery planned. Lose the key and you lose the data. Backups of keys need the same protection as the keys.
- Strong, standard algorithms, such as AES with 256-bit keys, rather than homemade schemes.
What HIPAA says
Under the HIPAA Security Rule, encryption of electronic PHI is an addressable implementation specification (45 CFR 164.312(a)(2)(iv)). Addressable does not mean optional: a practice must implement it if reasonable and appropriate, or document why not and use an equivalent alternative. For most practices today, there is no reasonable argument against encrypting laptops, servers and cloud storage. In January 2025, HHS proposed making encryption required, with limited exceptions; check the current status of that rule with your compliance adviser.
The bigger practical reason is breach notification. HHS guidance on rendering PHI unusable, unreadable or indecipherable says that data encrypted consistent with NIST Special Publication 800-111 for storage, with the key not compromised, is not "unsecured" PHI. If an encrypted laptop is stolen and the key was not on it or with it, the loss is generally not a reportable breach. If the laptop was not encrypted, it likely is.
Example: one stolen laptop, two outcomes
Illustrative case: a front-desk laptop is stolen from a car. On it is a schedule export with 3,000 patients' names, phone numbers and appointment dates.
Scenario A: the laptop has full-disk encryption, it was powered off, and the password was not written down with it. The data is unreadable to the thief. After documenting the assessment, the practice is generally not facing breach notification for those 3,000 patients.
Scenario B: no encryption. The practice now faces a presumed breach of unsecured PHI affecting 3,000 people: written notice to each affected patient; because 500 or more people are affected, notice to HHS within 60 days rather than in an annual log; and, if more than 500 residents of one state are involved, notice to prominent media outlets in that state. The cost in staff time, legal advice and patient trust far exceeds the effort of turning encryption on.
The lesson for a small practice: encryption at rest on every device that holds PHI is one of the cheapest risk reductions available.
Common mistakes
- Assuming the cloud handles everything. Your vendor's storage may be encrypted; the exports sitting in staff downloads folders are not.
- Writing the password on the device. A sticky note on the laptop defeats full-disk encryption.
- Forgetting backups and removable media. USB drives and backup disks need encryption too.
- Treating encryption as access control. Once a user is logged in, encrypted data is readable. Strong passwords, multi-factor authentication and audit logs still matter.
- Not asking vendors which layer. "Encrypted at rest" from a vendor should come with details about the layer and how keys are managed.
Encrypted at rest and in transit, under a signed BAA
DocsMD encrypts practice data at rest and in transit and keeps it in U.S. data centers. A Business Associate Agreement is signed before any patient data moves, and patient data is never used to train AI. Our security page has the details.
Questions
What does "at rest" mean?
Data that is stored, on a disk, in a database, in a backup or in cloud storage, as opposed to data moving across a network.
Is encryption at rest required by HIPAA?
It is an addressable specification under the current Security Rule: implement it if reasonable and appropriate, or document why not and use an equivalent. HHS proposed in January 2025 to make it required.
Does encryption at rest protect against hackers?
Against stolen devices and disks, yes. Against an attacker who has taken over a logged-in account, much less. It works alongside access controls, not instead of them.
Is a password-protected spreadsheet encrypted?
Modern spreadsheet password protection for opening a file generally uses encryption, but the strength depends on the software and the password. A dedicated encrypted storage or transfer method is more reliable.