PM.Crypt by CryptInc

PM.Crypt encrypts private messages in Firefox. The plaintext stays in the sidebar. The site only receives the finished cipher, which you send yourself.

The other person needs PM.Crypt too. You exchange public keys, compare fingerprints, and then read each other's messages. You never need their secret, and it never reaches you.

Getting started

1. Open the sidebar and create a key. The password can be any length. You can also store the key with no password: anyone with access to that Firefox profile can then see it.
2. Copy the SCT-KEY line and send it to the other person. They send you theirs.
3. Add their key to your contacts and compare the fingerprint by voice or another channel, not through the same site. A site administrator can read private messages.
4. Write the message in the panel. "Insert into field" puts the cipher into the field you just selected: a single-line box, a textarea, or a forum editor, including one inside a frame. Password fields are skipped.
5. Paste an incoming cipher into the panel and choose "Decrypt".

Right-click the page for "Insert my key", "Add someone else's key", "Encrypt", "Decrypt", and "Insert into field". A selected passage keeps its paragraph breaks.

What stays with you

• An X25519 encryption key and an Ed25519 signing key are created on this computer. The key does not expire. You can keep more than one identity.
• New messages use SCT2: the text is sealed with a one-time key, and your long-term key only signs the envelope. Older SCT1 messages still open.
• With a password, the secret on disk is wrapped with AES-GCM. The wrapping key is PBKDF2-SHA-256, 600,000 iterations. While the panel is open, the secret lives only in the browser session. "Lock" wipes it at once. With a password, the panel locks itself after 15 minutes of inactivity.
• A copied cipher or key is cleared from the clipboard after about a minute, if the clipboard has not changed.
• There is a backup file for your keys. The file has its own password.
• The secret is not synced with your Firefox account and is not given to the page script.

What this add-on does not do

• There is no account, no PM.Crypt server, and no data collection. Encryption and decryption happen only in the browser.
• The site sees the cipher you chose to send, and your public key if you sent it in a private message. It does not receive the plaintext.
• A message will not open unless the sender's key is in your contacts. A tampered cipher or key is reported as damaged.
• Inserting text requires access to websites. That access is for placing text in a form, not for sending page contents to the developer.
• If the field cannot be reached or is too short, the cipher is copied to the clipboard. On a site where Firefox does not allow the add-on, paste the text yourself.

Requires Firefox 140 or newer.