Quote from: f3tus on Today at 05:00:51 PM

Quote from: bob123 on Today at 03:28:40 PM

That is one reason why it is bad.
Another one is that it leaks bits of the plain text.

Any of these 2 reasons is enough to deem that as a bad design.

So BIP-39 is a stupid and bad idea, saving the wallet's key in an easy and human readable format?

BIP39 is neither security by obscurity nor does it leak plaintext bits.

Quote from: f3tus on Today at 05:00:51 PM

Still not getting it and missing the point. Your alternative is to write down 100-300 random characters on a piece of paper (have fun with that) and then require your family to be above-average computer literate to be able to decrypt it.

These are neither 100-300 chars, nor is it too difficult for an average person to decrypt it using a 3-5 step instruction.

It doesn't make any sense to create a shitty and insecure shift-cipher (which has to be taught how to use) instead for example just a BIP39 passphrase. Most proper wallets can handle this.
That would be way more easy to use than either AES or your ~~shift~~ shit cipher.

In the end it comes down to two pieces which have to be stored: the secret and the (encrypted) data.
Whether this is your plaintext-leaking-mnemonic [data] with the instruction and dates [the secret] or simply the securely (non-leaking) BIP39 passphrase protected mnemonic [data] and the passphrase [the secret] doesn't matter. The difference is, one is secure while the other one isn't.