Encryption at Rest vs. in Transit: What's the Difference?

Encryption at rest protects data while it sits stored on a hard drive, server, or backup file. Encryption in transit protects that same data while it moves across a network, such as when a file uploads to the cloud or an email travels between mail servers. Both convert readable data into scrambled ciphertext using a mathematical algorithm and a key, but they defend against different moments of exposure, and one does not substitute for the other. If your practice handles client Social Security numbers, health records, or financial account data, you need both protections in place to meet requirements like the FTC Safeguards Rule and the HIPAA Security Rule in 2026.

What Encryption at Rest Actually Does

Encryption at rest applies to data that is not currently moving: files on a laptop hard drive, records in a database, or backup archives sitting in cloud storage. The most common standard is AES-256 (Advanced Encryption Standard with a 256-bit key), a symmetric encryption algorithm that the National Institute of Standards and Technology (NIST) documents in its SP 800-111 guide to storage encryption. In practice, this looks like full-disk encryption on a work laptop (BitLocker on Windows, FileVault on Mac), database-level encryption on a practice management system, or an encrypted backup file. If a laptop is lost or a server is accessed without authorization, encryption at rest means the data on it reads as scrambled noise rather than plain text. This is a different concept from symmetric and asymmetric encryption as algorithm types; encryption at rest is about where the protection is applied, and it usually relies on symmetric algorithms because they perform faster on large stored files.

What Encryption in Transit Actually Does

Encryption in transit protects data while it travels between two points: a browser and a website, an email client and a mail server, or a workstation and a cloud application. The dominant protocol is TLS (Transport Layer Security), the successor to SSL, which encrypts the connection so that anyone intercepting the traffic (on public Wi-Fi, a compromised router, or a network they do not own) sees only ciphertext. A website using HTTPS is using TLS to encrypt the connection between the browser and the server. VPNs, SFTP file transfers, and TLS-secured email connections all serve the same purpose: protecting data during the moment it moves. Encryption in transit says nothing about what happens to the data once it lands. A secure TLS connection can deliver a file to a server that stores it in plain text, which is exactly the gap encryption at rest is meant to close.

Why You Need Both, Not Just One

A gap in either layer leaves data readable at a point where it should not be. A firm that encrypts its server but emails a client's tax return as an unencrypted attachment has protected the data at rest and left it exposed in transit. A firm that uses a TLS-secured client portal to collect documents but stores those documents on an unencrypted shared drive has protected the data in transit and left it exposed at rest. Attackers and researchers both look for the weaker link rather than the stronger one, which is one reason encryption belongs in a broader framework rather than standing alone. It maps most directly to the confidentiality leg of the CIA triad (confidentiality, integrity, and availability), but confidentiality controls only work if they are applied consistently across every place data sits and every path it travels.

This is also why encryption gets evaluated alongside other controls during a vulnerability assessment or a penetration test: an assessor checking your environment will look for unencrypted storage, weak or expired TLS certificates, and any place sensitive data crosses the network in the clear, not just whether encryption exists somewhere in the stack.

What Regulations Actually Require

The FTC Safeguards Rule, which applies to tax preparers, accountants, and other businesses classified as financial institutions under the Gramm-Leach-Bliley Act, requires covered businesses to encrypt customer information both in transit over external networks and at rest, per 16 CFR Part 314.4(c)(3), unless your qualified individual determines encryption is infeasible and approves an equivalent compensating control in writing. The HIPAA Security Rule treats encryption as an addressable implementation specification under 45 CFR 164.312, meaning covered healthcare entities must assess whether encryption is reasonable and appropriate for their environment and, if they decide not to implement it, document the alternative measure and the reasoning. IRS Publication 4557Safeguarding Taxpayer Data, recommends that tax professionals encrypt sensitive files and avoid sending unencrypted taxpayer data by email, a recommendation that also runs through the IRS Security Six checklist. None of these rules guarantee that encryption alone prevents a breach; they describe it as one documented control among several, and specific legal obligations for your practice should be confirmed with counsel.

How This Fits Your Broader Security Program

Encryption is one control in a layered program, not a replacement for the others. It works alongside network segmentation, which limits how far an intruder can move if they get past a single defense, and strong password security, which limits who can reach encrypted systems in the first place. A managed IT or security provider can confirm your configuration matches what your policy claims; the difference between a general IT vendor and a dedicated security partner is explained in cybersecurity company vs. MSP. For tax and accounting firms, encryption decisions belong in a documented WISP; Bellator Cyber Guard's custom WISP starts at $749 for firms with up to five users, with larger practices quoted separately, and typically saves 20 to 40 billable hours compared to building the document from scratch.