The source of the extra size overhead
The file content is actually base64 encoded twice during the encryption flow.
- When reading the file data, the result is a base64 encoded string. (source code)
- When encrypting, The
sjcl.json.encrypt()will treat the plainText as a normal UTF-8 string rather than a bunch of base64 encoded data. After the encryption, the ciphertext is base64 encoded. (source code)
To reduce the unnecessary size overflow, we could convert the base64 string from Step 1 to the bitArray first, then encrypt it with sjcl.json.encrypt(). However, this introduces an extra step and slows down the encryption process.