How and why to "practice" a password
You will need to know you can type both the low & high security passwords repeatedly from memory, and I have found this strategy works well:
➤ For the low security password, open up a Text Editor on your host computer — one that doesn't save a "buffer" as you are typing unsaved changes (as an unencrypted LibreOffice or MS Word document would) — to come up with a string of 20+ characters that you can type dozens of times in a row: observing that you've typed them exactly the same. Don't save this file by mistake!
➤ For the Frankenwallet / high security password, you cannot do this — since any password typed on the host computer should be considered "blown" — so your options to explore a good password, and commit it to memory, are more restrictive… e.g.:
- Boot your host computer in the Tails environment and practice it in a text editor there.
- Postpone this step until you're setting up your encrypted partition in the Ubuntu installer... and then open up a text editor in that temporary Linux environment to practice it there.
- Write down in advance what you plan to use as a Frankenwallet password, and throw it away when you have typed the password from memory enough times.
- Note this approach can work poorly for long & complicated passwords, since not all passwords that look good on paper can be easily typed or memorised!
About passwords used for AES and assurances AES is "unbreakable"
AES itself, lacking a practical attack surface as understood by cryptographers for nearly 3 decades, is considered unbreakable in itself (best source: Bruce Schneier: tag AES). Any mitigation attempts seem only to reduce the difficulty to that of trying 2^128 keys: i.e. a job for a cosmic scale computer. Therefore your goal is to have a password, which will be "hashed" to this key, with the same unpredictability as a randomly generated 128-bit number.
Assuming that there are 80 possible characters you can type on the computer keyboard (26 uppercase + 26 lowercase + 10 digits + 18 punctuation marks), this is how we derive the 20 minimum number of characters for your password (assuming it's apparently of random & unrelated characters, with any meaning or relationship existing in your mind alone):
$ bc -l 128 * l(2) / l(80) 20.24698764029213039466
With a 20-character password providing enough key variation that the key can't be brute-forced, all remaining attacks have to be on the password itself. This confirms the absolutely strict requirement that the Frankenwallet password can never, never be entered outside the Frankenwallet (where all data is absolutely encrypted and unobservable to the host computer) or in some other system that is completely stateless and also unobservable.
About AES implementations in the programs you will be using
LibreOffice (for text documents & spreadsheets) and 7z (for archives) have sufficient longevity in the open source community that any real threat — revealing the password entered when data is encrypted or decrypted — would have quite a bit of exposure. So the attack surface into your crypto data can be considered zero as long as nobody has "blown the whistle" on either of these programs having insecure password management… something I have been watching for for 12 years and never found.
Beyond that, the parameters for AES seem only to make a difference mathematically: practically, the cracking problem of brute-forcing the keys is either geological or cosmic (depending on whether you consider time or space). So please stay most aware of the security issues as they relate to passwords themselves: as with all cryptographic systems, the most common attack points are where keys are set, verified, or exchanged… rather than breaking the cipher itself.
➤ This is why the high security requirement has been set so the password to encrypt private key archives, and documents with the seed phrases for user funds, is never entered outside the Frankenwallet and therefore cannot be observed… so any opportunity to intercept the password from a program failure or even a "zero-day bug" never takes place.
➤ Since these files and archives are never encrypted or decrypted outside the Frankenwallet or other stateless device, they cannot be exposed to password entry vulnerabilities… since all an attacker would ever see, when that data emerges in the host environment, is the fully AES-encrypted file.