Doubt Related With Query String value encryption

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ASP.NET

Hi all

Im encrypting and passing id from one page to another
in test1.aspx
listing all attachemt when click on preview in one item im passing id of that field after encryption to next page say test2.aspx
Response.Redirect("~/test/test2.aspx?id=" + ls_encry);
in page test2 i used code to decrypt this and im getting all these well
problem coming when i passed this url to next system
www.mydom.com/test/test2.asp?id=FFvruvzDN%2bA%3d
login page comes .. after successfull login redirect to test2 page but in this case decryption not works showing bad data

why coming so i tried many encryption and decrption algorithams but result is same in all cases.
wht i noticed is
hcrEC2WtGoE= :-guery value in first working case
hcrec2wtgoe= :- query value in not working case all characters converted to lower case
Any solution for this problem

Thanks in advance
Amritha