What are some methods that can be used to hold your private keys in memory?
Memorizing 25 words is rather difficult.
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One good way to memorize something is by telling a story. Unfortunately, turning a bunch of completely random words into one is quite hard. Luckily, if you limit the words just slightly, we get
Let's assume that there are 8192 words in each part of speech 8192 words (it will vary wildly, but that's a rough estimate. When you actually do this, do your research). As it turns out, that 13 bits of entropy. Looking at the above MadLib (making sure not to count (same place), since it doesn't contribute any additional entropy), there are 14 blanks, making for 182 bits of entropy. You would have to make sure to randomly generate the words, not just pick them yourselves, since humans are predictable (and perhaps use dice rolls or a deck of cards as an entropy source, since computers aren't either sometimes). You would use the set of words you choose as an entropy source to generate the private key by hashing them (by lower casing them all and adjoining them with commas).
Note that although monero private keys are 256 bits, the security parameter will only be 128 bits. Therefore, it would be more efficient for an attacker to solve the discrete logarithm problem for your key than to brute force the mad lib.
You don't have to use the above mad lib of course. You could even create your own! I would recommend making it something vivid, so you can visualize it. Also, try adding redudancy similar to the (same place) above, to make it easier to remember (but taking note that the redundant words don't increase the entropy). Error correction can also make it recoverable even if forget a couple of the blanks (in the above example, you could make 4 words error correcting words and still have over 128 bits of entropy, allowing you to forget 4 words and still be fine).
Make sure to that you keep track of the mad lib. The mad lib does not need to be kept a secret, since as noted above the entropy of the words is enough. I would recommend publishing in a public place. I could even imagine a social media platform arising where users share their mnemonic mad libs.