How each one works
Encryption in transit
When data travels across a network, it passes through routers, Wi-Fi hotspots and internet providers. Encryption in transit wraps it so none of them can read or change it on the way. On the web and in apps, that is almost always TLS, which sets up a fresh key for each connection.1
Email adds a twist. Mail servers pass messages to each other with STARTTLS, which upgrades a connection to TLS only if both sides agree.2 An attacker in the middle can strip that offer and force plain text. MTA-STS fixes this by letting a domain publish a policy that says mail must only be delivered to it over valid TLS.3
Encryption at rest
Data at rest is anything stored: files on a laptop, a database, a backup, a mailbox on a server. Encryption at rest scrambles it on the storage itself, so a stolen disk or a copied backup is unreadable without the key.4
| State | Protects against | Typical tools |
|---|---|---|
| In transit | Eavesdroppers and tampering on the network | TLS, HTTPS, STARTTLS with MTA-STS, VPN tunnels |
| At rest | Stolen disks, leaked backups, server breaches | Disk and database encryption, zero-access encryption |
| In use | Exposure while data is being processed | Processing on your own device, secure hardware |
Why the difference matters
Each one leaves a gap the other covers. Data encrypted in transit is decrypted when it arrives, so if the server stores it in plain text, a breach exposes it. Data encrypted at rest still has to travel, and without TLS it can be read on the wire.
The bigger question is who holds the keys. Most services that say “encrypted at rest” encrypt their disks with keys they manage, which stops a thief with a hard drive but not the service itself, a rogue employee or a legal demand. When the keys are created on your device and the service never holds them, that is zero-access encryption. When data stays encrypted with your keys the whole way from sender to recipient, that is end-to-end encryption.
Where you’ll see these terms
- Security pages and privacy policies, which often list both. Check whose keys are used.
- Compliance requirements for health, finance and government data, which usually require both.
- Email, where TLS between servers protects mail in transit and the provider decides how it is stored.
At rest and in transit in Secria
Secria Mail stores your mail under zero-access encryption: your keys are created on your device and only reach our servers encrypted, and mail from other providers is encrypted with your public keys as it arrives, before it is stored. In transit, mail travels over TLS, and our domains publish an MTA-STS policy in enforce mode with TLS reporting. Every message in your mailbox is protected with ML-KEM-1024 and X25519.
Related terms
Sources
- IETF, RFC 8446: The Transport Layer Security (TLS) Protocol Version 1.3 (August 2018).
- IETF, RFC 3207: SMTP Service Extension for Secure SMTP over Transport Layer Security (February 2002).
- IETF, RFC 8461: SMTP MTA Strict Transport Security (MTA-STS) (September 2018).
- NIST, SP 800-111: Guide to Storage Encryption Technologies for End User Devices (November 2007).
Checked October 2026. Secria facts are from our Mail and VPN pages and the whitepaper.
Questions about encryption at rest and in transit
What is the difference between encryption at rest and in transit?
In transit protects data while it moves across a network. At rest protects it while it sits in storage. A service needs both, because each one leaves a gap the other covers.
Is TLS encryption at rest or in transit?
In transit. TLS protects the connection between two machines. Once the data arrives, TLS no longer protects it.
Is AES-256 encryption at rest or in transit?
It can be either. AES-256 is a cipher, not a state. TLS often uses it to protect data in transit, and disk and database encryption use it to protect data at rest.
Is encryption at rest enough?
Not on its own. If the service holds the keys, it can still read your data. Look for zero-access or end-to-end encryption, where only you hold the keys.