New encrypted wallet file header:

[ecdsa]

• the file is encrypted with a master key
• the master key is encrypted with one or several passwords
• passwords can be added to or removed from the list
• a hmac of the file is written at the beginning of the file
• passwords can be checked/updated without having to read/write the whole file
• the GUI assumes a single password
• partial writes: encrypted blobs are appended at the end of the file each blob is prefixed by its length can recover from incomplete write: hmac is updated after appending

[SomberNight]

related https://github.com/spesmilo/electrum/issues/5999