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Mailvelope is a tool to send and receive encrypted messages and files to other users. It uses PGP public key cryptography with a default private key size of 4096 bytes.
Mailvelope is a browser add-on that you can use in:
Versions: https://mailvelope.com/en/products
Tutorial: https://mailvelope.com/en/help
Generate a public and private key pair first. Export both keys and move the file to your back up.