before jumping into this I would recommend doing the math because 64^6 (as far as I understood you corrected your first number above) is quite a huge number.
You would need to have a huge amount of bitcoin in that wallet and a lot of hardware to throw at it (don't forget about the energy cost etc too) to justify this investment/time/cracking.
but it is of course much better compared to hashing algorithms like scrypt etc, therefore it could be feasible with "just" sha256
I think with GPU support it would be much more efficient and a mask like -1 ?d?l?u ?1?1?1?1?1?1 would be done in just a couple of minutes with a modern GPU for -m 1400 (without the elliptic curve operations)... but I agree with undeath that adding elliptic curve code is probably not worth it either and you should do the speed test with already available crackers to get a feeling how long it could take (but should be doable)
You would need to have a huge amount of bitcoin in that wallet and a lot of hardware to throw at it (don't forget about the energy cost etc too) to justify this investment/time/cracking.
but it is of course much better compared to hashing algorithms like scrypt etc, therefore it could be feasible with "just" sha256
I think with GPU support it would be much more efficient and a mask like -1 ?d?l?u ?1?1?1?1?1?1 would be done in just a couple of minutes with a modern GPU for -m 1400 (without the elliptic curve operations)... but I agree with undeath that adding elliptic curve code is probably not worth it either and you should do the speed test with already available crackers to get a feeling how long it could take (but should be doable)