b64.lua

#!/usr/local/bin/lua
-- Based on http://lua-users.org/wiki/BaseSixtyFour by Alex Kloss
-- compatible with lua 5.1
-- http://www.it-rfc.de
-- licensed under the terms of the LGPL2

b64 = {}

-- bitshift functions (<<, >> equivalent)
-- shift left
local function lsh(value,shift)
	return (value*(2^shift)) % 256
end

-- shift right
local function rsh(value,shift)
   -- Lua builds with no floating point don't define math.
   if math then return math.floor(value/2^shift) % 256 end
   return (value/2^shift) % 256
end

-- return single bit (for OR)
local function bit(x,b)
	return (x % 2^b - x % 2^(b-1) > 0)
end

-- logic OR for number values
local function lor(x,y)
	result = 0
	for p=1,8 do result = result + (((bit(x,p) or bit(y,p)) == true) and 2^(p-1) or 0) end
	return result
end

-- Character decoding table
local function toBase64Byte(char)
   ascii = string.byte(char, 1)
   if ascii >= string.byte('A', 1) and ascii <= string.byte('Z', 1) then return ascii - string.byte('A', 1)
   elseif ascii >= string.byte('a', 1) and ascii <= string.byte('z', 1) then return ascii - string.byte('a', 1) + 26
   elseif ascii >= string.byte('0', 1) and ascii <= string.byte('9', 1) then return ascii + 4
   elseif ascii == string.byte('-', 1) then return 62
   elseif ascii == string.byte('_', 1) then return 63
   elseif ascii == string.byte('=', 1) then return nil
   else return nil, "ERROR! Char is invalid for Base64 encoding: "..char end
end


-- function decode
-- decode base64 input to string
function b64.decode(data)
	local chars = {}
	local result=""
	for dpos=0,string.len(data)-1,4 do
		for char=1,4 do chars[char] = toBase64Byte((string.sub(data,(dpos+char),(dpos+char)) or "=")) end
			result = string.format(
		   '%s%s%s%s',
		   result,
		   string.char(lor(lsh(chars[1],2), rsh(chars[2],4))),
		   (chars[3] ~= nil) and string.char(lor(lsh(chars[2],4),
		   rsh(chars[3],2))) or "",
		   (chars[4] ~= nil) and string.char(lor(lsh(chars[3],6) % 192,
		   (chars[4]))) or ""
		)
	end
	return result
end

return b64