Frida Tutorial

Installation
Install frida tools:
pip install frida-tools
pip install frida
Download and install in the android the frida server (Download the latest release). 
One-liner to restart adb in root mode, connect to it, upload frida-server, give exec permissions and run it in backgroud:
adb root; adb connect localhost:6000; sleep 1; adb push frida-server /data/local/tmp/; adb shell "chmod 755 /data/local/tmp/frida-server"; adb shell "/data/local/tmp/frida-server &" 
Check if it is working:
frida-ps -U #List packages and processes
frida-ps -U | grep -i <part_of_the_package_name> #Get all the package name
Tutorials
​Tutorial 1​
From: https://medium.com/infosec-adventures/introduction-to-frida-5a3f51595ca1
APK: https://github.com/t0thkr1s/frida-demo/releases
Source Code: https://github.com/t0thkr1s/frida-demo​
Follow the link to read it.
​Tutorial 2​
From: https://11x256.github.io/Frida-hooking-android-part-2/ (Parts 2, 3 & 4)
APKs and Source code: https://github.com/11x256/frida-android-examples​
Follow the link to read it.​
​Tutorial 3​
From: https://joshspicer.com/android-frida-1
APK: https://github.com/OWASP/owasp-mstg/blob/master/Crackmes/Android/Level_01/UnCrackable-Level1.apk​
Follow the link to read it.
You can find some Awesome Frida scripts here: https://codeshare.frida.re/​
Fast Examples
Here you can find the more basic and interesting functionalities of Frida to make a quick script:
Calling Frida from command line
frida-ps -U
​
#Basic frida hooking
frida -l disableRoot.js -f owasp.mstg.uncrackable1
​
#Hooking before starting the app
frida -U --no-pause -l disableRoot.js -f owasp.mstg.uncrackable1
#The --no-pause and -f options allow the app to be spawned automatically,
#frozen so that the instrumentation can occur, and the automatically
#continue execution with our modified code.
Basic Python Script
import frida, sys
​
jscode = open(sys.argv[0]).read()
process = frida.get_usb_device().attach('infosecadventures.fridademo')
script = process.create_script(jscode)
print('[ * ] Running Frida Demo application')
script.load()
sys.stdin.read()
Hooking functions without parameters
Hook the function a() of the class sg.vantagepoint.a.c
Java.perform(function () {
;  rootcheck1.a.overload().implementation = function() {
  rootcheck1.a.overload().implementation = function() {
    send("sg.vantagepoint.a.c.a()Z   Root check 1 HIT!  su.exists()");
    return false;
  };
});
Hook java exit()
var sysexit = Java.use("java.lang.System");
  sysexit.exit.overload("int").implementation = function(var_0) {
    send("java.lang.System.exit(I)V  // We avoid exiting the application  :)");
  };
Hook MainActivity .onStart() & .onCreate()
var mainactivity = Java.use("sg.vantagepoint.uncrackable1.MainActivity");
  mainactivity.onStart.overload().implementation = function() {
    send("MainActivity.onStart() HIT!!!");
    var ret = this.onStart.overload().call(this);
  };
  mainactivity.onCreate.overload("android.os.Bundle").implementation = function(var_0) {
    send("MainActivity.onCreate() HIT!!!");
    var ret = this.onCreate.overload("android.os.Bundle").call(this,var_0);
  };
Hook android .onCreate()
  var activity = Java.use("android.app.Activity");
  activity.onCreate.overload("android.os.Bundle").implementation = function(var_0) {
    send("Activity HIT!!!");
    var ret = this.onCreate.overload("android.os.Bundle").call(this,var_0);
  };
Hooking functions with parameters and retrieving the value
 Hooking a decryption function. Print the input, call the original function decrypt the input and finally, print the plain data:
  function getString(data){
      var ret = "";
      for (var i=0; i < data.length; i++){
          ret += data[i].toString();
        }
      return ret
    } 
  var aes_decrypt = Java.use("sg.vantagepoint.a.a");
  aes_decrypt.a.overload("[B","[B").implementation = function(var_0,var_1) {
    send("sg.vantagepoint.a.a.a([B[B)[B   doFinal(enc)  // AES/ECB/PKCS7Padding");
    send("Key       : " + getString(var_0));
    send("Encrypted : " + getString(var_1));
    var ret = this.a.overload("[B","[B").call(this,var_0,var_1);
    send("Decrypted : " + ret);
​
    var flag = "";
    for (var i=0; i < ret.length; i++){
      flag += String.fromCharCode(ret[i]);
    }
    send("Decrypted flag: " + flag);
    return ret; //[B
  };
Hooking functions and calling them with our input
Hook a function that receives a string and call it with other string (from here)
var string_class = Java.use("java.lang.String"); // get a JS wrapper for java's String class
​
my_class.fun.overload("java.lang.String").implementation = function(x){ //hooking the new function
  var my_string = string_class.$new("My TeSt String#####"); //creating a new String by using `new` operator 
  console.log("Original arg: " +x );
  var ret =  this.fun(my_string); // calling the original function with the new String, and putting its return value in ret variable
  console.log("Return value: "+ret);
  return ret;
};
Getting an already created object of a class
If you want to extract some attribute of a created object you can use this.
In this example you are going to see how to get the object of the class my_activity and how to call the function .secret() that will print a private attribute of the object:
Java.choose("com.example.a11x256.frida_test.my_activity" , {
  onMatch : function(instance){ //This function will be called for every instance found by frida
    console.log("Found instance: "+instance);
    console.log("Result of secret func: " + instance.secret());
  },
  onComplete:function(){}
});