Imagining for a moment that you even could get it to work...

Let's assume that the "passkey" is exclusively made up of alphanumerics? (no punctuation, and no non-printable characters).

There are 26 letters in the english alphabet in two different cases (upper and lower) for a total of 52 alphabetics.  Ad in another 10 numerics and you're looking at 62 possibilities for each of the 24 positions.

62²⁴ = 1.04 X 10⁴³

Let's say you can get 30 GHash/s out of your cube.

1.04 X 10⁴³ possibilities  /  3.0 X 10¹⁰ hash/sec = 7.43 X 10³² sec to try all possibilities.

That's about 23571733000000000000000000 years to get through all the possibilities. (about 2.4 X 10²⁵)

The age of the solar system is estimated to be only 4.6 X 10⁹ years, so that's more than 5000000000000000 times more than how long the solar system has existed.

To have even a 0.01% chance of success you'll need to run the cube for more than 2357173300000000000000 years.