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I have some question to ask, what actually is checksum in mnemonic seed phrases, what's the difference between seed phrase with valid checksum and the one with invalid checksum in respect to wallet vulnerability to be easily attack/brute force.
If you manually create a mnemonic like this, then it is no longer a "seed" but a brainwallet and it is not safe at all because there is no randomness in the entropy that was used at all.
The time it takes to crack it is also in seconds not years because all it takes is someone trying all the combinations of 2048 words where all words are the same (check 2048 seed phrases in total).
I'm getting to understand something here, manually selecting randomness of seed and seed extension( I mean adding additional word making the 13th word) is the same thing? Will it be possible to crack/brute force extended mnemonic seed phrase, if possible, then how long can it takes to be done.
My questions might seem to be out of context, correct me if I'm wrong, I'm learning.