The core security assumption of bitcoin self custody is a random and secret private key. As I am not a cryptographer, I cannot say anything particularly interesting here other than: A private key is a big number Randomly creating big numbers is hard for computers and humans A bitcoin seed is the ‘seed’ of this private key and can be used to generate the key in a bitcoin wallet A seed can be represented by a 24 word mnemonic, which is easier for humans to store, transcribe, remember without errors Lunaticion has written more extensively on this problem here