This questions pops up a lot in the crypto space and it’s not easy to comprehend how safe seed phrases really are, so here is an attempt.

Seed phrases are just a method to calculate private keys. A private key is what “unlocks” your funds on the blockchain, since they are encrypted (that’s why we call it cryptocurrency). MochiJump made a really nice post about how seed phrases are used to generate private keys: Have you ever wondered how mnemonic seed phrases are generated for a crypto wallet?

How many seed phrases to find a match?

A seed phrase with 12 words can chose out of 2048 different words, which means in worst case you’d need to try 204812 combinations which is roughly 5×10^39 – 1039 is a 1 with 39 zeros, so this number is a 5 with 39 other digits.

The private key itself has 256 Bit (it’s binary, so made of 256 0s and 1s), so there are 2256 combinations which is even more: 10^77.

Some Youtuber with a 3D-Printer made a machine to show what 1 Googol is like (1 Googol is 10100):

Each gear needs to turn 10 times to make the next gear make 1 turn.

You can watch the Youtube video to see how slow even the third wheel is moving (you would need to turn first gear 100 times to achieve one turn). Fourth wheel would need 1000 turns, fith 10,000 and so on.

To find all combinations of a 12-word seed phrase, you would have to make the 39th gear turn 5 times. Even if you turn the first one really fast, let’s say 1000rpm, this would still take 1,902,587,500,000,000,000,000,000,000,000 years!

A really fast supercomputer you say? Well, my 16-core CPU can do 15,000 seeds per second. If you could find a supercomputer which can do 15,000,000,000,000 seeds per second, this would still take 11,509,635,000,000,000,000 years.

Personally, I don’t think a 24-word seed phrase (~79th gear) makes any difference in security. Quiet the opposite, it’s much harder to remember it so you would have to be sure to never lose your metal plate or whatever you’ve put it on.