JWT Decode
The goal of this guide is to build a pleasant JWT decoder as a Convert Workflow. We will only be using built-in Nodes. JSON Web Tokens (JWT) are composed of three base64 encoded parts separated by dots. The first is the header, the second is the payload and the third is the signature.
INFO
It is, obviously, also possible to build it easily using a single Javascript Code Node.
For this guide we will use the following token as our input:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5cStep 1: Decoding the Token
This is super easy since Caido offers a Node just for that called JWT Decode. Just drag it into your Workflow and connect it! 🚀
If you hit Save and Run just then, you will see that nothing is displayed in the output.
This is normal since we have not selected what we want as an output! 😬
Go ahead and click on the Convert End Node to select the output, we'll choose $jwt_decode.header.
Now you should see the header displayed in the output when you click on Save and Run. 🔥
Step 2: Displaying the Header and Payload
It would be great if we could see both the header and the payload in one go wouldn't it? This is where the Join Node comes in real handy. Try dragging it into your Workflow and connecting it.
This Node requires a bit of configuration, you need to specify the Left and Right elements of the join and the Separator you want to see in between those elements. We will use a reference to $jwt_decode.header as our left element, a reference to $jwt_decode.payload as our right element and a . as our separator.
Lastly, we need to change our Convert End data to reference our new $join.data, now we are talking! 😎
Step 3: Prettifying the Whole Thing
As a bonus step, we will make things pretty! 🤩
Since we know the first two parts of the JWT are JSON, we can use two JSON Prettify Nodes to format them before we join them.
Since we have two Nodes of the same type, I suggest we rename them and change their aliases. For the first, we prettify the $jwt_decode.header and we give it the alias pretty_header.
We also have to change the Join Node, to use the new references from the pretty Nodes. Namely, $pretty_header.data on the left and $pretty_payload.data on the right.
Conclusion
That's it! Now you should have a nice JWT Decoder all ready to use as a Convert Workflow! 🎉
The full Workflow is provided below, ready to be imported.
Full Workflow
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"description": "",
"edition": 2,
"graph": {
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"nodes": [
{
"alias": "convert_start",
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"display": {
"x": 20,
"y": -250
},
"id": 0,
"inputs": [],
"name": "Convert Start",
"version": "0.1.0"
},
{
"alias": "convert_end",
"definition_id": "caido/convert-end",
"display": {
"x": 20,
"y": 310
},
"id": 1,
"inputs": [
{
"alias": "data",
"value": {
"data": "$join.data",
"kind": "ref"
}
}
],
"name": "Convert End",
"version": "0.1.0"
},
{
"alias": "jwt_decode",
"definition_id": "caido/jwt-decode",
"display": {
"x": 20,
"y": -150
},
"id": 2,
"inputs": [
{
"alias": "data",
"value": {
"data": "$convert_start.data",
"kind": "ref"
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],
"name": "JWT Decode",
"version": "0.1.0"
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{
"alias": "pretty_header",
"definition_id": "caido/json-prettify",
"display": {
"x": 20,
"y": -40
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"id": 3,
"inputs": [
{
"alias": "data",
"value": {
"data": "$jwt_decode.header",
"kind": "ref"
}
}
],
"name": "Prettify Header",
"version": "0.1.0"
},
{
"alias": "pretty_payload",
"definition_id": "caido/json-prettify",
"display": {
"x": 20,
"y": 70
},
"id": 4,
"inputs": [
{
"alias": "data",
"value": {
"data": "$jwt_decode.payload",
"kind": "ref"
}
}
],
"name": "Prettify Paylaod",
"version": "0.1.0"
},
{
"alias": "join",
"definition_id": "caido/join-two",
"display": {
"x": 20,
"y": 190
},
"id": 5,
"inputs": [
{
"alias": "left",
"value": {
"data": "$pretty_header.data",
"kind": "ref"
}
},
{
"alias": "right",
"value": {
"data": "$pretty_payload.data",
"kind": "ref"
}
},
{
"alias": "separator",
"value": {
"data": ".",
"kind": "string"
}
}
],
"name": "Join",
"version": "0.1.0"
}
]
},
"id": "7d3a6af4-eefa-4065-8b37-42263c4a71ed",
"kind": "convert",
"name": "JWT Decode"
}