MOD calculation / reversing

[raduga_fb]

Hi all.

I am going to prepare a keygen. I need your help.

c = MOD (a * MOD (a*a, b) , b)

(for example, c=48765, b=67890 -> a=12345, can be more valid "a")

"c" and "b" are known. I need to calculate the "a".

"a" and "b" are in 256 digits as Hex, "c" is almost same. They are not Prime.

Thanks,