frombase64

function frombase64( base64str [, ignore] ) --> str, err

Description

Decodes a base 64 encoded string to a data string. It is possible to ignore characters like \n in base64str via the optional ignore string.

Parameters

base64str

The base 64 encoded string that should be decoded.

ignore

String that represents a optional set of characters that should be ignored in base64str.

Return Values

str

The decoded data string or nil if an error occurs.

err

A message that explains the error, otherwise nil.

Code

--ZFUNC-frombase64-v1
local function frombase64( base64str, ignore ) --> str, err
   local bitMap = {
      A="000000", B="000001", C="000010", D="000011", E="000100", F="000101",
      G="000110", H="000111", I="001000", J="001001", K="001010", L="001011",
      M="001100", N="001101", O="001110", P="001111", Q="010000", R="010001",
      S="010010", T="010011", U="010100", V="010101", W="010110", X="010111",
      Y="011000", Z="011001",
      a="011010", b="011011", c="011100", d="011101", e="011110", f="011111",
      g="100000", h="100001", i="100010", j="100011", k="100100", l="100101",
      m="100110", n="100111", o="101000", p="101001", q="101010", r="101011",
      s="101100", t="101101", u="101110", v="101111", w="110000", x="110001",
      y="110010", z="110011",
      ["0"]="110100", ["1"]="110101", ["2"]="110110", ["3"]="110111",
      ["4"]="111000", ["5"]="111001", ["6"]="111010", ["7"]="111011",
      ["8"]="111100", ["9"]="111101", ["+"]="111110", ["/"]="111111"
   }
   local errchar = "unexpected character at position %d: '%s'"

   if ignore then base64str = base64str:gsub( "["..ignore.."]", "" ) end
   local bitTab = {}
   for i = 1, #base64str do
      local c = string.sub( base64str, i, i )
      if c ~= '=' then
         local bitStr = bitMap[ c ]
         if not bitStr then
            return nil, string.format( errchar, i, c )
         end
         table.insert( bitTab, bitStr )
      end
   end

   local fullBitStr = table.concat( bitTab )
   local len = #fullBitStr
   local pad = len % 8
   fullBitStr = string.sub( fullBitStr, 1, len - pad )
   return fullBitStr:gsub( '........', function ( cc )
      return string.char( tonumber( cc, 2 ) )
   end )
end

return frombase64

Examples

local t = require( "taptest" )
local frombase64 = require( "frombase64" )

--should work with wikipedia examples
-- http://en.wikipedia.org/wiki/Base64
t( frombase64( "TWFu" ), "Man" )
t( frombase64( "bGVhc3VyZS4=" ), "leasure." )
t( frombase64( "cGxlYXN1cmUu" ), "pleasure." )
t( frombase64( "ZWFzdXJlLg==" ), "easure." )
t( frombase64( "c3VyZS4=" ), "sure." )

local long64 = "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb2"..
               "4sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBh"..
               "bmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYn"..
               "kgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGludWVk"..
               "IGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLC"..
               "BleGNlZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBw"..
               "bGVhc3VyZS4="

local longtxt = "Man is distinguished, not only by his reason, but by "..
                "this singular passion from other animals, which is a "..
                "lust of the mind, that by a perseverance of delight in "..
                "the continued and indefatigable generation of knowledge, "..
                "exceeds the short vehemence of any carnal pleasure."

t( frombase64( long64 ), longtxt )

-- http://en.wikipedia.org/wiki/Base64#Padding
t( frombase64( "YW55IGNhcm5hbCBwbGVhcw==" ), "any carnal pleas" )
t( frombase64( "YW55IGNhcm5hbCBwbGVhc3U=" ), "any carnal pleasu" )
t( frombase64( "YW55IGNhcm5hbCBwbGVhc3Vy" ), "any carnal pleasur" )

t()

Inspired by

See also