As Bitcoin private key generation evolved, people have attempted to make different ways to make it easier to preserve and retrieve your private key, though will often hit the threshold of human memorization. Bitcoin private keys are long numbers, which most humans wouldn't even attempt to memorize. Using Base-58 encoding, those addresses can get down in length somewhat, but still are close enough to ascii-garbage that the vast majority of people would write it down rather than memorize it.
There was a time when "brainwallets" were a popular alternative. Using people's existing reflexes of "I secure my stuff with a password", a brainwallet took a password and hashed it to arrive at a private key. The issue with this is that if you pick a common password, you end up sharing a Bitcoin account with someone.
BIP39 brought about a key revolution in this space; using words (which are easier for humans to read/type/say) instead of numbers to represent a private key. The overall number of characters goes way up (each word represents a number that's between 0 and 1,024. "1024" only takes four characters to type, but most words in the list are more than four characters long), but the resistance to error is much better.
But how are we on the memorization front? Most all systems that use BIP39 keys encourage users to write them down somewhere secure. Pretty much none of them just say remember this passphrase. Admittedly, to be most secure, a BIP39-implementation has to present 24 words to the user, and most humans aren't at the point of "just memorize these 24 things".
But recently I recalled the xkcd comic on password complexity that implied memorizing random words was easier than random numbers at least.
That example only leads to memorizing 4 words. But I recalled from my childhood something that was was an even longer string of words. The book Yo, Millard Fillmore! presents a way to memorize all the Presidents of the United States through a goofy story that serves as a mnemonic.
But not only that, the authors of this book wove in hints every five Presidents to give you a means to "jump to" a certain point in the order without having to start at the beginning and count. If I asked you what the 15th letter of the alphabet was, would you have to start at "A" and count them off as you recited? Probably, since the way we memorize alphabetical order typically doesn't include numerically where that letter falls in the overall picture. But if you asked me who the tenth president was, I know that boys "typically start wearing ties around age 10", so the part of the story about the funny necktie with a fishing lure attached to it is the 10th in the series, and tie-lure is the mnemonic for Tyler; John Tyler was the 10th President.
So, can we do the same for mnemonic keys from a BIP39 derivation? From the xkcd comic example, phrases of four words are do-able for humans to memorize together. And since a 24-word BIP39 key is divisible into six four-word chunks, let's go with that. Each person will have their own key to memorize, but let's try one together and see how it goes. Here's a random 24-word BIP39 key I generated:
expose battle bitter manage
record about rose salt
recipe beauty lucky album
height foam artist bench
swamp march trouble minor
market rose dog brand
Our goal is to memorize it, and put check-point numbers in there such that when restoring/checking the key, if a system asks for the 17th word, we can get to it relatively easily. Ready? Let's try:
Once upon a time there was an amateur photographer. But this was a long time ago where cameras were boxy, so picture an old camera sitting on a tripod out on a hillside. The photographer takes the lens cap off and waits, to expose the plate.
While waiting for the exposure to set, the photographer looks out at his subject which is a battle taking place down the hill.
The battle rages and one of the soldiers is momentarily blinded by the sun reflecting off the camera lens, and while distracted gets a whallop by an opposing soldier, which makes him bitter towards the photographer.
The bitter soldier is not just any soldier but a high-ranking one, so he detaches himself from combat for a moment and marches up the hill to confront the photographer and manage the situation.
The photographer, getting yelled at by a bitter soldier decides he might need to recall this conversation later, so pulls out a tape recorder to record this conversation. (But the photographer knows he only has about five minutes of tape left in his recorder to record the situation.)
The soldier, who wants the photographer to leave, tells the photographer to do an about face, and march off in a different direction.
The photographer looks off in the direction the soldier is pointing in and sees that there is a rose bush growing there. That might be interesting to photograph, so the photographer moves his gear over there.
When the photographer arrives at the rose bush, he notices a peculiar ring around the bush on the ground. It's salt; someone's put a ring around this bush to keep slugs away.
The salt was put there by a gardener, who actually is now all out of salt and doesn't have enough for the recipe he's trying to bake!
The gardener is cooking a recipe because he has a date tonight with the girl of his dreams; a real beauty (she's a perfect ten), and he wants to impress her!
The gardener doesn't have a fancy meal ready by the time his date arrives, but it turns out she's a rather understanding gal (lucky for him!) and they head out to dinner.
At the restaurant the gardener tries again to impress her by picking out a song for her on the jukebox; it's her favorite album!
The restaurant the gardener and his date are at are well-known for their wine collection. Their collection is so large the wine racks span the whole height of the building and the servers need long ladders to reach it all.
One of the servers mishandles a bottle of champagne while high up on a ladder and it bursts, raining down foam on everyone below!
This causes quite the commotion which distracts an artist outside the restaurant who was painting a picture (the artist is only fifteen years old so isn't used to working under these distracting conditions!), and causes him to smear a swath of paint across his work.
Disgusted that his painting is now ruined, the artist tosses it on a bench and leaves for home.
As the artist heads out of town towards home, what stands in his way is a large, forbidding swamp.
To keep his spirits up as he heads through the gloomy swamp, the artist whistles a cheerful march and high-steps down the trail.
But his whistling attracts the attention of a local police officer on patrol in the swamp, and the artist gets in trouble because he's out past curfew.
Curfew is enforced because the artist is so young; a minor (which you are considered a minor until you're at least age twenty).
The artist's penalty for being out past curfew is to do community service serving in the local market.
The vendor he's working for is the local florist, who sells many fine flowers in bunches, but one single rose is set out as a centerpiece as the biggest and most expensive piece.
A scruffy dog running through the marketplace thinks that rose looks pretty good and snatches it from the market stand!
The artist catches the dog eventually, and as punishment, the dog is given a brand to mark him as a stray and given over to the pound.
How's that? Can you do it? Try reading through it a few times, and then wait a week and come back to it. Still in your head? I find this sort of memorization has much longer "staying power", and makes it much easier to memorize sequences of things. Can you come up with a better story for this mnemonic phrase? Now, who's willing to give it a try for their own passphrase?