Function Encrypt(theNumber)
	Dim n, szEnc, t, HiN, LoN, i
	n = CDbl((theNumber + 1570) ^ 2 - 7 * (theNumber + 1570) - 450)
	If n < 0 Then szEnc = "R" Else szEnc = "J"
	n = CStr(abs(n))
	For i = 1 To Len(n) step 2
	  t = Mid(n, i, 2)
	  If Len(t) = 1 Then
	   szEnc = szEnc & t
	   Exit For
	  End If
	  HiN = (CInt(t) And 240) / 16
	  LoN = CInt(t) And 15
	  szEnc = szEnc & Chr(Asc("M") + HiN) & Chr(Asc("C") + LoN)
	Next
	Encrypt = szEnc
End Function

Function Decrypt(theNumber)
	On Error Resume Next
	Dim e, n, sign, t, HiN, LoN, NewN, i
	e = theNumber
	If Left(e, 1) = "R" Then sign = -1 Else sign = 1
	e = Mid(e, 2)
	NewN = ""
	For i = 1 To Len(e) step 2
	  t = Mid(e, i, 2)
	  If Asc(t) >= Asc("0") And Asc(t) <= Asc("9") Then
	   NewN = NewN & t
	   Exit For
	  End If
	  HiN = Mid(t, 1, 1)
	  LoN = Mid(t, 2, 1)
	  HiN = (Asc(HiN) - Asc("M")) * 16
	  LoN = Asc(LoN) - Asc("C")
	  t = CStr(HiN Or LoN)
	  If Len(t) = 1 Then t = "0" & t
	  NewN = NewN & t
	Next
	e = CDbl(NewN) * sign
	Decrypt = CLng((7 + sqr(49 - 4 * (-450 - e))) / 2 - 1570)
End Function

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These are two function used to encrypt and decrypt the ID you don't want to show (for example the id of the user to be saved in the cookies...)

ASP function to encrypt ID (number only)

ASP