That's just a matter of knowing the algorithm: If I know how you constructed the password, I know how to make candidate guesses. I don't even need to be able to

That's just a matter of knowing the algorithm: If I know how you constructed the password, I know how to make candidate guesses. I don't even need to be able to work backwards; I just need to be able to reproduce one of your passwords (the same way you'd "reverse" a hash digest.) The best solution is randomness -- that's why secure random number generators are so fundamentally important in cryptography.