fiddler - decode custom encrypted body in inspector by c# on the fly - fiddler

Question

The content is Encrypt with an CUSTOM encrypter. fiddler captured content body is base64 encode string in plain text
The application traffic flow:
request:
base64Encode(customEncryptFromStringTobytes(jsonString) ) -> application -> http server
response:
customDecryptFrombytesToString(base64Decode(jsonString) ) <- application <- http server
I have the encrypt/decrypt class in c#:
string EncryptToBase64(string plainText);
string DecryptFromBase64(string plainText);
I build an exe to do the transform, I wonder how To make fiddler decode request/respose body by this exe on the fly
I want Fiddler show decrypt content in inspector , and encrypt again everytime I [Reissue and Edit(E)] the request.
I found something close but Dont know how to call an exe to do decode.
http://docs.telerik.com/fiddler/KnowledgeBase/FiddlerScript/ModifyRequestOrResponse
update:
I have implement the custom inspector for Fiddler. see the answer below.

Answer 1

I create an Extension for custom Inspector
full example https://github.com/chouex/FiddlerExtensionExample
you will build a dll and copy the file to Inspectors and Scripts folder in fiddler. restart fiddler will load the extenion.
note:
I used pre/post-build script to copy dll and restart fiddler in vs project.
custom inspector:
This Example just beautify the json body.
public class ResponseInspector : Inspector2, IResponseInspector2
{
TextBox myControl;
private byte[] m_entityBody;
private bool m_bDirty;
private bool m_bReadOnly;
public bool bReadOnly
{
get { return m_bReadOnly; }
set
{
m_bReadOnly = value;
// TODO: You probably also want to turn your visible control CONFIG.colorDisabledEdit (false) or WHITE (true) here depending on the value being passed in.
}
}
public void Clear()
{
m_entityBody = null;
m_bDirty = false;
myControl.Text = "";
}
public ResponseInspector()
{
// TODO: Add constructor logic here
}
public HTTPResponseHeaders headers
{
get { return null; // Return null if your control doesn't allow header editing.
}
set { }
}
public byte[] body
{
get { return m_entityBody; }
set
{
// Here's where the action is. It's time to update the visible display of the text
m_entityBody = value;
if (null != m_entityBody)
{
var text = System.Text.Encoding.UTF8.GetString(m_entityBody);
if (!String.IsNullOrEmpty(text) && text.StartsWith("{"))
{
text = JsonConvert.SerializeObject(JsonConvert.DeserializeObject(text), Formatting.Indented);
}
myControl.Text = text;
// TODO: Use correct encoding based on content header.
}
else
{
myControl.Text = "";
}
m_bDirty = false;
// Note: Be sure to have an OnTextChanged handler for the textbox which sets m_bDirty to true!
}
}
public bool bDirty
{
get { return m_bDirty; }
}
public override int GetOrder()
{
return 0;
}
public override void AddToTab(System.Windows.Forms.TabPage o)
{
myControl = new TextBox(); // Essentially the TextView class is simply a usercontrol containing a textbox.
myControl.Height = o.Height;
myControl.Multiline = true;
myControl.ScrollBars = ScrollBars.Vertical;
o.Text = "TextViewExample";
o.Controls.Add(myControl);
o.Controls[0].Dock = DockStyle.Fill;
}
}
for Traffic Tamper(not mention in question but I think this is useful):
implement AutoTamperResponseBefore() in IAutoTamper2
This Example just replace any text from "xt" to "c1" in every request body

Answer 2

Add a dummy header 
Fiddler-Encoding: base64
 and encode your body using base64 if it contains any binary data. Fiddler will decode the data before transmitting it to the server.
Taken from http://www.fiddlerbook.com/fiddler/help/composer.asp