You are totally right...

typin' mistake...

the 'CAST' is to a NVARCHAR(MAX)

I tried a lot whitout that CAST and it returned another 'md5' string... i figure that the problem is in the asigment '@md5 = ...'

the sql corrected would be...

CREATE FUNCTION [dbo].[fn_MD5]

(

@string AS VARCHAR(MAX),

@chars AS INT

)

RETURNS NVARCHAR(32)

AS

BEGIN

-- Declare the return variable here

DECLARE @md5 NVARCHAR(32)

SELECT @md5 = SUBSTRING(CAST(master.dbo.fn_varbintohexstr(HashBytes('MD5',@string )) AS NVARCHAR(MAX)),3,@chars )

-- Return the result of the function

RETURN @md5

END