ALTER function [dbo].[ConvertBigIntToVarBinary]((c)param BigInt)
returns varbinary(128) AS
begin
 
declare (c)result varchar(128)    
set (c)result = ''

while 1=1 
  begin 	
    select (c)result= convert(char(1),(c)param % 2)+(c)result, (c)param = convert(bigint, ((c)param / 2)) 
    if (c)param = 0 return convert(varbinary(128), replace(replace(reverse((c)result),'0','00'),'1','01'),2)
  end  
 return convert(varbinary(128),'00',2)
end

ALTER function [dbo].[ConvertBigIntToVarBinary1](@param bigint)
returns varbinary(1000) AS
begin

       declare @iPower int;
       declare @result varbinary(1000);
       declare @powerOf2 bigint;

       set @result = 0x;
       set @powerOf2 = 1;
       set @iPower = 0;

       while @powerOf2 <= @param
         begin
              set @result = @result + CAST(@param / @powerOf2 % 2 AS binary(1));

              set @iPower = @iPower + 1;
              
              set @powerOf2 = power(2., @iPower);
         end    
       return @result
end

declare @b bigint = 31;
declare @out varbinary(128) = 0x;

-- simple inline tally table
with cteTen as
	( select N from (values(1),(1),(1),(1),(1),(1),(1),(1),(1),(1)) as t(N) ),
cteNums as
	( select N = 0
	union all
	select	top (63) -- the 64th bit would be bigger than a bigint max value
			N = row_number() over(order by (select null))
	from cteTen as cte1
	cross join cteTen as cte2 ),
-- the meat of the query
cteInit as
	( select	cte.N
		,	ValThis = power(convert(decimal(20,0), 2), N)
		,	ValNext = power(convert(decimal(20,0), 2), N + 1)
	from cteNums as cte )	
	
select	@out = @out + convert(varbinary(1), case when (@b - (floor(@b / cte.ValNext)) * cte.ValNext) >= cte.ValThis then 1 else 0 end)
from cteInit as cte
where cte.ValThis <= @b
order by cte.N desc;

select @out;