Decryptbykey Multiple Session Issue

Hi
I'm having some issues using the decryptbykey method via multiple connections. When I run the below test script simultaneously on two machines the sum function is always less then the known amount (ie 14945490 and 36382777). Does anyone know of any locking method or alternative way to sum an encrypted column?

Thanks in advance
Waz







open symmetric key HR01 decryption by password='yes'
DECLARE @Bonus decimal
DECLARE @Salary decimal
DECLARE @Errors int
DECLARE @Success int
DECLARE @LoopCount int
SET @Errors = 0
SET @Success = 0
SET @LoopCount = 0


WHILE (@LoopCount < 40)
BEGIN

SELECT
@Bonus = SUM(convert(float,convert(varchar(80),decryptbykey(Bonus)))),
@Salary = SUM(convert(float,convert(varchar(80),decryptbykey(Salary))))
FROM ChallengeEmployee
WHERE ChallengeID = 5

IF(@Bonus <> 14945490 OR @Salary <> 36382777)
BEGIN
PRINT 'Bonus ' + CAST(@Bonus AS varchar(80))
PRINT 'Salary ' + CAST(@Salary AS varchar(80))
SET @Errors = @Errors + 1
END
ELSE
SET @Success = @Success + 1

SET @LoopCount = @LoopCount + 1

END

PRINT 'Finish'
PRINT 'Errors ' + CAST(@Errors AS varchar(80))
PRINT 'Success ' + CAST(@Success AS varchar(80))
close symmetric key HR01