# Frida Tutorial 1
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**This is a summary of the post**: [https://medium.com/infosec-adventures/introduction-to-frida-5a3f51595ca1](https://medium.com/infosec-adventures/introduction-to-frida-5a3f51595ca1)\
**APK**: [https://github.com/t0thkr1s/frida-demo/releases](https://github.com/t0thkr1s/frida-demo/releases)\
**Source Code**: [https://github.com/t0thkr1s/frida-demo](https://github.com/t0thkr1s/frida-demo)
## Python
Frida allows you to **insert JavaScript code** inside functions of a running application. But you can use **python** to **call** the hooks and even to **interact** with the **hooks**.
This is a easy python script that you can use with all the proposed examples in this tutorial:
```python
#hooking.py
import frida, sys
with open(sys.argv[1], 'r') as f:
jscode = f.read()
process = frida.get_usb_device().attach('infosecadventures.fridademo')
script = process.create_script(jscode)
print('[ * ] Running Frida Demo application')
script.load()
sys.stdin.read()
```
Call the script:
```bash
python hooking.py
```
It is useful to know how to use python with frida, but for this examples you could also call directly Frida using command line frida tools:
```bash
frida -U --no-pause -l hookN.js -f infosecadventures.fridademo
```
## Hook 1 - Boolean Bypass
Here you can see how to **hook** a **boolean** method (_checkPin_) from the class: _infosecadventures.fridademo.utils.PinUtil_
```javascript
//hook1.js
Java.perform(function() {
console.log("[ * ] Starting implementation override...")
var MainActivity = Java.use("infosecadventures.fridademo.utils.PinUtil");
MainActivity.checkPin.implementation = function(pin){
console.log("[ + ] PIN check successfully bypassed!")
return true;
}
});
```
```
python hooking.py hook1.js
```
Mirar: La funcion recibe como parametro un String, no hace falta overload?
## Hook 2 - Function Bruteforce
### Non-Static Function
If you want to call a non-static function of a class, you **first need a instance** of that class. Then, you can use that instance to call the function.\
To do so, you could **find and existing instance** and use it:
```javascript
Java.perform(function() {
console.log("[ * ] Starting PIN Brute-force, please wait...");
Java.choose("infosecadventures.fridademo.utils.PinUtil", {
onMatch: function(instance) {
console.log("[ * ] Instance found in memory: " + instance);
for(var i = 1000; i < 9999; i++){
if(instance.checkPin(i + "") == true){
console.log("[ + ] Found correct PIN: " + i);
break;
}
}
},
onComplete: function() { }
});
});
```
In this case this is not working as there isn't any instance and the function is Static
### Static Function
If the function is static, you could just call it:
```javascript
//hook2.js
Java.perform(function () {
console.log("[ * ] Starting PIN Brute-force, please wait...")
var PinUtil = Java.use("infosecadventures.fridademo.utils.PinUtil");
for(var i=1000; i < 9999; i++)
{
if(PinUtil.checkPin(i+"") == true){
console.log("[ + ] Found correct PIN: " + i);
}
}
});
```
## Hook 3 - Retrieving arguments and return value
You could hook a function and make it **print** the value of the **passed arguments** and the value of the **return value:**
```javascript
//hook3.js
Java.perform(function() {
console.log("[ * ] Starting implementation override...")
var EncryptionUtil = Java.use("infosecadventures.fridademo.utils.EncryptionUtil");
EncryptionUtil.encrypt.implementation = function(key, value){
console.log("Key: " + key);
console.log("Value: " + value);
var encrypted_ret = this.encrypt(key, value); //Call the original function
console.log("Encrypted value: " + encrypted_ret);
return encrypted_ret;
}
});
```
## Important
In this tutorial you have hooked methods using the name of the mathod and _.implementation_. But if there were **more than one method** with the same name, you will need to **specify the method** that you want to hook **indicating the type of the arguments**.
You can see that in [the next tutorial](frida-tutorial-2.md).
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Support HackTricks
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