Keys: which type to choose, and how to keep them safe
Everything in CryptoPad rests on keys. This guide explains what a key is made of, which of the three types to pick, how to be sure a contact's key is really theirs, and what happens when a key is gone. It applies to iPhone and Android alike.
Two parts: one you keep, one you give away
Every key you make in CryptoPad is a pair.
- The private part stays on your phone. Only it can open messages encrypted for you. The app never shows it, never copies it out and never sends it anywhere — there is no export.
- The public part is what Copy Public Key gives you. Send it to anyone you like: it can only lock messages for you, never open them.
A message encrypted for someone can be opened only by the private part of their key. Not even the sender can read it again once it is encrypted.
The name you give a key is only a label for you. It is not part of the public key, and nobody you send the key to sees it.
Which type to choose
CryptoPad makes three types of key. All three encrypt and decrypt, and all three work between iPhone and Android: a message encrypted on one opens on the other, whatever the type.
| Type | What it is | Good to know |
|---|---|---|
| RSA | RSA with a 4096-bit key | The longest public key; takes a few seconds to make. The default in both apps. |
| NIST | Elliptic curve P-256 | Short and made at once. Works on every supported phone. |
| X25519 | Elliptic curve X25519 | Short and made at once. On Android it needs Android 13 or later. |
If someone you write to has an Android phone older than Android 13, give them an RSA or NIST key, not an X25519 one: their phone may not be able to encrypt for it. If you are not sure which to pick, NIST works everywhere.
The person writing to you does not need a key of the same type as yours. Each of you encrypts for the other's key, whatever its type.
Several keys
You can have as many keys as you like — for example, one per group of people, or a new one when you want to stop using an old one. You never choose which key to decrypt with: CryptoPad tries all of yours.
A contact holds one public key. If someone gives you two keys, add them as two contacts with different names.
Make sure a key is really theirs
A public key travels through ordinary channels — a chat, an email. Someone who controls that channel could swap it for their own and read what you encrypt "for your contact". So before you trust a contact's key, compare it with the person through a different channel: side by side when you meet, or read aloud on a call. Compare the whole key, or at least a long stretch from its middle — not the first or last lines, which look alike for every key of the same type (every RSA key ends the same way).
Do the same for your own key: tell people which key is yours.
When a key is gone
A private part can be used only on the phone that made it. Nobody — including us — can move it to another phone or open a copy of it, so a lost key cannot be recovered. What a phone backup may contain is in the privacy policy.
- Deleting a key is final. Messages encrypted for it can no longer be opened, by anyone.
- Deleting the app makes all your keys unusable. On Android they go with the app at once. On iPhone, if you want them gone at once, delete them in the app before you delete the app.
- A new phone does not bring your keys along, on either platform. Make new keys on the new phone and send your contacts their public parts. Messages encrypted for the old keys open only on the old phone, while it still has them.
When you replace a key, send the new public part to the people who write to you, and ask them to replace your old key in their contacts.
More answers are on the Support page.