Decode the access token from JWT Format

Question

We are using nginx server for reverse proxying with openresty openid lua installed.... This means that every request has an Authorization header containing JWT token - access id and refresh token.

Now, what we need to do is decode the access token because I need the sub from the JWT token and log it on nginx server.

Is there a way to decode and log JWT? I looked into the openidc.lua file and I can see that it decodes the id token but I can't see where I can decode the access token.

Any help would be greatly appreciated.

Answer 1

This done the trick for me

local jwt = require "resty.jwt"
local jwt_obj = jwt:load_jwt(res.access_token)
local cjson = require "cjson"
ngx.log(ngx.DEBUG, "res.access_token.sub=", cjson.encode(jwt_obj))

Answer 2

A JWT token (or better said a JWS, a signed token) just consists of two Base64Url encoded JSON structures and a signature.

To see how it works, you an check your token on https://jwt.io

On that site you'll also find links to JWT frameworks for different languages, also for Lua. But to read the contents of a JWT/JWS you just need a Base64Url decoder and a JSON derserializer.