This is a tool for encrypting and decrypting BIP-39 seed phrases. To my knowledge, there is no other tool that takes a seed phrase and a password and simply returns a new seed phrase. There are reasons for that, good cryptographic reasons, but I wanted to do it anyway. I wanted to be able to store my key using one of the existing products (steel capsule, steel plate, etc), but I really hated the idea of making such a permanent impression of such sensitive data. So here we are.
We convert the input seed prase back into pure entropy. The entropy is hashed with the password plus a hard coded salt. The hash is repeated until the time limit is reached. Then end result hash is used as a key to encrypt the entropy using a symmetric cipher.
It's technically pretty bad to hash with a constant salt. The salt is supposed to be generated randomly and stored with the hash. But I didn't want to store the salt (I wanted 256 bits of output), so it is what it is.
This is why it's important to use a strong password and as high of a time limit as you can tolerate. I wouldn't use less than an hour. A day is probably better. A week is probably overkill.
It's also really important that you remember your thread count if you use threads. Different thread counts will produce different results. If you forget your thread count, you will not be able to decrypt your seed phrase. If you want to be really safe, don't set the option and let the default option be used. I won't change the default option except maybe once a decade.