Lua: Vigenere Cipher

local fCharacterMap = {}
local fKeyIndex = 1
local fKey = ""
local CHARACTERS = 26

local function isLower( str )
  return type(str) == "string" and str == str:lower() or false
end

local function isAlpha( str )
  return type(str) == "string" and (str:gsub("%a", "")) == "" or false
end

local function vigenere( aKey )
  for row = 1, CHARACTERS do
    local lChar = 65 + row
    if lChar > 90 then
        lChar = 65
    end
    fCharacterMap[row] = {}
    for col = 1, CHARACTERS do
      fCharacterMap[row][col] = string.char( lChar )
      lChar = lChar + 1
      if lChar > 90 then
        lChar = 65
      end
    end
  end

  for i = 1, #aKey do
    local str = aKey:sub( i, i )
    if isAlpha( str ) then
      fKey = fKey..str:upper()
    end
  end

  fKeyIndex = 1
end

local function encode( aCharacter )
  if isAlpha( aCharacter ) then
    local islower = isLower( aCharacter )
    aCharacter = aCharacter:upper()

    local keyChar = fKey:sub( fKeyIndex, fKeyIndex )
    local lRow = keyChar:byte() - 64
    local lColumn = aCharacter:byte() - 64

    aCharacter = fCharacterMap[lRow][lColumn]

    if islower then
      aCharacter = aCharacter:lower()
    end

    fKeyIndex = fKeyIndex + 1
    if fKeyIndex == #fKey + 1 then
      fKeyIndex = 1
    end
  end

  return aCharacter
end

local function decode( aCharacter )
  if isAlpha( aCharacter ) then
    local islower = isLower( aCharacter )
    aCharacter = aCharacter:upper()

    local keyChar = fKey:sub( fKeyIndex, fKeyIndex )
    local lRow = keyChar:byte() - 64

    for lColumn = 1, CHARACTERS do
      if aCharacter == fCharacterMap[lRow][lColumn] then
        aCharacter = string.char( 64 + lColumn )
        break
      end
    end

    if islower then
      aCharacter = aCharacter:lower()
    end

    fKeyIndex = fKeyIndex + 1
    if fKeyIndex == fKey:len() + 1 then
      fKeyIndex = 1
    end
  end

  return aCharacter
end

function encodeString( aString, aKey )
  vigenere( aKey )
  result = ""

  for i = 1, #aString do
    result = result..encode( aString:sub( i, i ) )
  end

  return result
end

function decodeString( aString, aKey )
  vigenere( aKey )
  result = ""

  for i = 1, #aString do
    result = result..decode( aString:sub( i, i ) )
  end

  return result
end