bn_mul_low_recursive
void bn_mul_low_recursive(BN_ULONG *r, BN_ULONG *a, BN_ULONG *b, int n2,
bn_mul_low_recursive(&(t[0]), &(a[0]), &(b[n]), n, &(t[n2]));
bn_mul_low_recursive(&(t[0]), &(a[n]), &(b[0]), n, &(t[n2]));