Encrypt / bDecrypt issues

My solution, 2 tables... And 2 seperate functions (Encrypt / Decrypt)... :(

Graphics 800,600,0,2
SeedRnd (MilliSecs()) 
Global table1 = CreateBank(256)
Global table2 = CreateBank(256)

For i% = 0 To 255
   value% = Rand(0,255)
   For j% = 0 To i
      table_value% = PeekByte(table1, j)
      If value = table_value Then
         j = 0
         value% = Rand(0,255)
      EndIf
   Next
   PokeByte table1, i, value
Next

For i = 0 To 239 Step 16
   a$ = ""
   For j = 0 To 15
      b$ = Str$(PeekByte (table1, i + j))
      a$ = a$ + b$ + String$(" ", 4 - Len(b$))
   Next
   Print a$
Next

For i = 0 To 255
  For j = 0 To 255
     value = PeekByte (table1, j)
     If value = i Then found = found + 1
  Next
Next
Print "Unique :" + found

For i = 0 To 255
  value = PeekByte (table1, i)
  PokeByte table2, value, i
Next

phrase$ = Input$("Type the phrase to encrypt :")
length% = Len(phrase$)

temp_bank1 = CreateBank(length) ; original string
For i = 1 To length
   ascii% = Asc(Mid$(phrase$,i,1))
   PokeByte temp_bank1, i - 1, ascii
Next

temp_bank2 = CreateBank(length) ; result bank

Print "Encrypted :" + encrypt$(temp_bank1, temp_bank2, length)

CopyBank temp_bank2, 0, temp_bank1, 0, length
Print "Decrypted :" + decrypt$(temp_bank1, temp_bank2, length)
WaitKey()
End

Function encrypt$(bank1, bank2, length)
result$ = ""
For i = 1 To length
   ascii% = PeekByte(bank1, i - 1)
   value% = PeekByte(table1, ascii)
   PokeByte bank2, i - 1, value
   result$ = result$ + Chr$(value) ; for display purpose
Next
Return result$
End Function

Function decrypt$(bank1, bank2, length)
result$ = ""
For i = 1 To length
   ascii% = PeekByte(bank1, i - 1)
   value% = PeekByte(table2, ascii)
   PokeByte bank2, i - 1, value
   result$ = result$ + Chr$(value) ; for display purpose
Next
Return result$
End Function

I tought I could make it simpler. In fact, to make this simpler, I would have to swap 2 values, making it a 128 byte encryption key, which would give the answer in the next 128 bytes. That would be really bad! Actually, making a reverse reference from the table seems the logical way of doing this. But for the solution proposed here, I'll try to see if it is possible to not have a cross reference table and just try to compute that out... I'm just a little hazed out on this sort of thing!