How to encrypt text messages on iPhone

Updated 2026-10-31

The short answer

It depends on how the message is sent. In the Messages app, iMessage is end-to-end encrypted, SMS and MMS are not, and RCS is encrypted only in some conversations. Apple describes the three on one page: What is the difference between iMessage, RCS, and SMS/MMS?

That protection is about the trip between devices. What happens to the text on each phone afterwards is a separate question, and it is the one Sealbox is built around: it seals the text itself, before any channel touches it, so the channel only ever carries ciphertext.

What the Messages app encrypts

The Messages app sends each message as iMessage, RCS or SMS/MMS. According to Apple:

In practice, a conversation is protected end to end when it goes as iMessage (turned on or off in the Settings app) or when an RCS conversation shows "Encrypted". A green bubble without that label is not protected from a third party reading it while it is sent.

What an encrypted chat leaves open

Apple's wording is about transit: iMessage conversations "can't be read while they're sent between devices." After the message arrives, the text lives on the phone, in the app's database, in backups and in every copy along the way. Sealbox starts from another premise: every message ends up somewhere and stays there, on the device, in app databases, in backups, in every copy along the way. Its answer is to seal the text itself instead of relying on the channel, so the channel never holds it in the clear. The two layers compose: Sealbox adds to any channel, and it does not replace the one you already use.

How to encrypt the text itself with Sealbox

Sealbox is an iPhone app (iOS 26+ at launch). You write in its own composer, not in the Messages app, and what comes out is a sealed block of ciphertext that you send through any app that can carry text.

1. Pair once with each person. Pairing is mutual: each side hands its keys to the other. In person, you exchange them with an animated QR code; remotely, you share the invite through any channel. A remote contact works right away but is marked "unverified" until the two of you compare your security codes, and the app warns you before you send to an unverified contact.
2. Open Sealbox and write in its composer. There is no third-party keyboard, and dictation is blocked by default (dictated text is removed once transcribed, and with server-based dictation the first instant of audio may already have left). The text stays inside Sealbox until you seal it.
3. Pick the contact and seal. Your keys are released only by your biometrics (Face ID or Touch ID). By default the message is view-once: it opens one time, and its key is destroyed on reading.
4. Send the block through any app. Share it to WhatsApp, e-mail or a post; the channel only carries ciphertext. If you prefer, turn on "Generate QR" in the composer and the block comes out as an animated code that the other phone reads with its camera.
5. The other person opens it in Sealbox. They select the received block and choose the Sealbox extension in the share sheet, or point the camera at it if it came as a QR code. To reply, the extension leaves the reply ready and they open Sealbox themselves.

Sending a sealed block by SMS or RCS

The default mode, Quantum, produces blocks that fit messengers, e-mail and pastebins, but not SMS. For SMS or RCS, use Classic or Compact mode (the mode is the one active for the sender, who is you when you write) and turn view-once off for that message. Even then, the smallest block is 220 characters in Classic and 176 in Compact, so expect a long message.

A block sent this way is outside view-once: instead of opening one time, it can be read again for as long as the keys exist.

Classic and Compact blocks do not have the post-quantum protection of Quantum mode, and a block sent this way (outside view-once, in the epoch regime) is signed in the open: whoever holds both public identities can tell whose it is.

What this does not protect

• A skipped verification. Pair remotely and never compare the security codes, and a man-in-the-middle at first contact stays possible; the "unverified" badge stays visible until you verify.
• Metadata. Who you talk to, when, how often, that a Sealbox block went by, and how large it is. The channel generates this, and no content layer can hide it.
• The other person needs Sealbox too. The block is ciphertext bound to its reader, and only the person you paired with can open it.
• iPhone only. iOS 26+ at launch.
• A compromised, unlocked device in active use. Malware that reads your screen as you read is beyond any encryption app.
• Compelled secrets. If someone can force your biometrics with the device in hand, or compel your 24-word recovery phrase, they hold the keys and can read what hasn't been read yet and whatever arrives.
• Text you paste in from another app. Whatever keyboard you used there saw it first. Write inside Sealbox instead.
• A copy the reader keeps. If you allow copying (blocked is the default), the reader can save the message in an encrypted draft on their own device. It is a draft like any other: it goes into the backup, when there is one. View-once does not erase that draft, and you cannot revoke it afterwards.

Questions people ask

Does Sealbox replace the Messages app?

No. It adds to any channel: you keep using the app you already use, and the channel only carries the sealed block.

What can the app that carries the block see?

Only the sealed box, plus the metadata any channel generates. The channel never holds the text in the clear.

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