Yeah, title says it all, I lost my passphrase from my Bitcoin Core wallet I made in 2009, losing around 3.200.000€ ( 150 BTC ).
I would love to know if there's any possible way I could recover the passphrase, there is no way I remember it or that I'm able to recover it myself.
There is no way to recover a lost passphrase if ...
The passphrase (AKA password) in Bitcoin core is used to encrypt the private keys in the wallet data file. If it were possible to recover the passphrase without any knowledge of it, the wallet developers would have failed, Bitcoin core would have its reputation destroyed and no-one would use it.
It is of fundamental importance to Bitcoin core that it not be possible to do what you ask.
There are programs† which can assist in trying millions of variations of a partially remembered passphrase - but for these to have any reasonable chance of success, the amount of the passphrase that was forgotten or lost must be absolutely minimal. You must know almost all of the passphrase.
You will of course be contacted by people claiming to be able to help but they will all be scammers. Once they have gained your confidence they will promise you your 3.200.000€ in return for a small fee with whatever imaginative fee-name they think you will find plausible.
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The answer that RedGrittyBrick gave is largely right, though wrong at the edges.
For example, there were no password / passphrase complexity requirements for early Bitcoin core wallets. So, it would be perfectly valid to assign the password "42" to your wallet. (I think this is still true in current versions of Bitcoin core).
It's practical to brute force a short password to a wallet.dat file pretty quickly. (By brute force, I mean try every possible combination of characters).
Testing every combination up to 4 characters on a mid-sized server with a couple of 3080 GPUs takes less than 10 minutes; 5 characters takes less than a day; 6 characters takes about 2 months, and the amount of time it takes skyrockets from there.
There are also other techniques that involve testing popular leaked passwords against wallets.
But, most legitimate crypto asset recovery companies will require that you have some idea of what your password could be. Otherwise, the risk increases that this isn't actually your wallet -- that it was stolen, forged, or otherwise purchased online.
You can find legitimate recovery companies. I won't mention them by name, but just make sure that they've been in business for a few years, that their founders are public about who they are, and that they've been profiled by reputable publications.
There are services that will find your password for you as long as its not ridiculously long. I would take great care in ensuring that you chose a good service that wont just try to steal your coins once they've opened your wallet. They use specialised software to try thousands of passwords a second, and any hints you can give them will be very helpful. Also check your old emails for hints of a password.
