Mnemonic words are hard to remember and it is unsafe to put it in online storage like iCloud/Dropbox. To solve this, we can encrypt the mnemonic words with memorable passphrase, then it's more safe to store the result ciphertext in the online storage.

The encryption/decryption is done by WebCrypto API with AES-GCM-256 and you can refer to Mozilla Developer Site for more detail about that API.