Lua script to decode jwt token

decode-jwt.lua

-- Json Parsing based on https://gist.github.com/tylerneylon/59f4bcf316be525b30ab
-- Base64 decoding based on wikipedia description of 8/6bit encoding.
-- base64 char array.. note final 2 chars are for RFC4648-URL encoding
-- as per JWT spec section 2 terminology 'Base64url Encoding'
local alpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_'
-- convert to 6 char long binary string. (max int 64!)
local function toBinaryString(int)
    if int > 64 then error("Bad number " .. int .. " to convert to binary") end
    local remaining = tonumber(int)
    local bits = ''
    for i = 5, 0, -1 do
        local pow = 2 ^ i
        if remaining >= pow then
            bits = bits .. '1'
            remaining = remaining - pow
        else
            bits = bits .. '0'
        end
    end
    return bits
end
local function fromBinaryString(bits) return tonumber(bits, 2) end
local function decodeBase64(encoded)
    local bitstr = ''
    local decoded = ''
    -- decode chars into bitstring
    for i = 1, string.len(encoded) do
        local offset, _ = string.find(alpha, string.sub(encoded, i, i))
        if offset == nil then
            error("Bad base64 character " .. string.sub(encoded, i, i))
        end
        bitstr = bitstr .. toBinaryString(offset - 1)
    end
    -- decode bitstring back to chars
    for i = 1, string.len(bitstr), 8 do
        decoded = decoded ..
                      string.char(fromBinaryString(string.sub(bitstr, i, i + 7)))
    end
    return decoded
end
-- json handling 
local json = {}

-- Returns pos, did_find; there are two cases:
-- 1. Delimiter found: pos = pos after leading space + delim; did_find = true.
-- 2. Delimiter not found: pos = pos after leading space;     did_find = false.
-- This throws an error if err_if_missing is true and the delim is not found.
local function skip_delim(str, pos, delim, err_if_missing)
    pos = pos + #str:match('^%s*', pos)
    if str:sub(pos, pos) ~= delim then
        if err_if_missing then
            error('Expected ' .. delim .. ' near position ' .. pos)
        end
        return pos, false
    end
    return pos + 1, true
end

-- Expects the given pos to be the first character after the opening quote.
-- Returns val, pos; the returned pos is after the closing quote character.
local function parse_str_val(str, pos, val)
    val = val or ''
    local early_end_error = 'End of input found while parsing string.'
    if pos > #str then error(early_end_error) end
    local c = str:sub(pos, pos)
    if c == '"' then return val, pos + 1 end
    if c ~= '\\' then return parse_str_val(str, pos + 1, val .. c) end
    -- We must have a \ character.
    local esc_map = {b = '\b', f = '\f', n = '\n', r = '\r', t = '\t'}
    local nextc = str:sub(pos + 1, pos + 1)
    if not nextc then error(early_end_error) end
    return parse_str_val(str, pos + 2, val .. (esc_map[nextc] or nextc))
end

-- Returns val, pos; the returned pos is after the number's final character.
local function parse_num_val(str, pos)
    local num_str = str:match('^-?%d+%.?%d*[eE]?[+-]?%d*', pos)
    local val = tonumber(num_str)
    if not val then error('Error parsing number at position ' .. pos .. '.') end
    return val, pos + #num_str
end

json.null = {} -- one-off table to represent the null value.
function json.parse(str, pos, end_delim)
    pos = pos or 1
    if pos > #str then error('Reached unexpected end of input.') end
    local pos = pos + #str:match('^%s*', pos) -- Skip whitespace.
    local first = str:sub(pos, pos)
    if first == '{' then -- Parse an object.
        local obj, key, delim_found = {}, true, true
        pos = pos + 1
        while true do
            key, pos = json.parse(str, pos, '}')
            if key == nil then return obj, pos end
            if not delim_found then
                error('Comma missing between object items.')
            end
            pos = skip_delim(str, pos, ':', true) -- true -> error if missing.
            obj[key], pos = json.parse(str, pos)
            pos, delim_found = skip_delim(str, pos, ',')
        end
    elseif first == '[' then -- Parse an array.
        local arr, val, delim_found = {}, true, true
        pos = pos + 1
        while true do
            val, pos = json.parse(str, pos, ']')
            if val == nil then return arr, pos end
            if not delim_found then
                error('Comma missing between array items.')
            end
            arr[#arr + 1] = val
            pos, delim_found = skip_delim(str, pos, ',')
        end
    elseif first == '"' then -- Parse a string.
        return parse_str_val(str, pos + 1)
    elseif first == '-' or first:match('%d') then -- Parse a number.
        return parse_num_val(str, pos)
    elseif first == end_delim then -- End of an object or array.
        return nil, pos + 1
    else -- Parse true, false, or null.
        local literals = {
            ['true'] = true,
            ['false'] = false,
            ['null'] = json.null
        }
        for lit_str, lit_val in pairs(literals) do
            local lit_end = pos + #lit_str - 1
            if str:sub(pos, lit_end) == lit_str then
                return lit_val, lit_end + 1
            end
        end
        local pos_info_str = 'position ' .. pos .. ': ' ..
                                 str:sub(pos, pos + 10)
        error('Invalid json syntax starting at ' .. pos_info_str)
    end
end

local function decode_jwt(jwt)
    local i = 0
    local result = {}
    for match in (jwt .. '.'):gmatch("(.-)%.") do
        result[i] = decodeBase64(match)
        i = i + 1
    end
    -- header
    local head = json.parse(result[0])
    -- claims
    local claims = json.parse(result[1])
    return {head = head, claims = claims}
end

Hey, could you share what's the license this is shared under? Can I use it in private project?