Can somebody explain to me why in the calculations 2 ^ 32 combinations is used instead of 2 ^ 64 for the unknown 64 bits ?
In jgarzik's original implementation, an attacker can pre-generate a rainbow table with 2^32 entries, and that lets them take a shortcut so they only have to try 2^32 bits for any particular scratch card (algorithm is, essentially, "foreach value in 2^32: do some complicated math, then see if the result matches a value in the 2^32-size rainbow table; if it does, you've found the unknown 2^64 bits").