"Simple" Decryption Not Working 100% of the times - sql

Question

I have two functions. An encryption function and a decryption function (see below). The decryption function doesn't work 100% of the time with specific words and I cannot figure out the reason why. Can someone help me figure this out? I am testing this two functions using the following select statements after the functions are in place.
select [dbo].[ufn_EncryptString]('Test1') --This string works
select [dbo].[ufn_DecryptString]('Ôæõ÷µ')
select [dbo].[ufn_EncryptString]('diaz-mayo') --This string doesn't work
select [dbo].[ufn_DecryptString]('äêãý±òçĀ÷')
You would think it has something to do with "-" but there are instances when is just a plain name with no spaces or special character and the string cannot be decrypted.
See functions below:
CREATE FUNCTION [dbo].[ufn_EncryptString] ( #pClearString VARCHAR(100) )
RETURNS NVARCHAR(100) AS
BEGIN
DECLARE #vEncryptedString NVARCHAR(100)
DECLARE #vIdx INT
DECLARE
#vBaseIncrement INT
SET #vIdx = 1
SET #vBaseIncrement = 128
SET #vEncryptedString = ''
WHILE #vIdx <= LEN(#pClearString)
BEGIN
SET #vEncryptedString = #vEncryptedString +
NCHAR(ASCII(SUBSTRING(#pClearString, #vIdx, 1)) +
#vBaseIncrement + #vIdx - 1)
SET #vIdx = #vIdx + 1
END
RETURN #vEncryptedString
END
GO
CREATE FUNCTION [dbo].[ufn_DecryptString] ( #pEncryptedString NVARCHAR(100) )
RETURNS VARCHAR(100) AS
BEGIN
DECLARE #vClearString VARCHAR(100)
DECLARE #vIdx INT
DECLARE #vBaseIncrement INT
SET #vIdx = 1
SET #vBaseIncrement = 128
SET #vClearString = ''
WHILE #vIdx <= LEN(#pEncryptedString)
BEGIN
SET #vClearString = #vClearString +
CHAR(UNICODE(SUBSTRING(#pEncryptedString, #vIdx, 1)) -
#vBaseIncrement - #vIdx + 1)
SET #vIdx = #vIdx + 1
END
RETURN #vClearString
END
GO

Answer 1

I think your function works fine, your example is the issue.
select [dbo].[ufn_DecryptString]('äêãý±òçĀ÷')
Doesn't work... however the following works just fine:
select [dbo].[ufn_DecryptString](N'äêãý±òçĀ÷')
Notice the leading N on the string literal? Since your input parameter is actually a UNICODE string, you need to prefix the literal with N in order to prevent it from being cast to a ASCII string...
From MSDN:
Unicode constants are specified with a leading N: N'A Unicode string'.
Otherwise, when it is converted to ASCII, you're actually passing in äêãý±òçA÷... only the Ā character isn't represented in the CHAR literal, which is why your issue was intermittent.