🔑 X-Hub-Signature Vapor middleware 🔑

---

📚 Table of Contents

• Installation
• Getting Started
• Now It Works
• How It Works
• License

📦 Installation

You just need to add this package as a dependency in your Package.swift:

let package = Package(
    name: "MyProject",
    dependencies: [
        .Package(url: "https://github.com/ythecombinator/vapor-x-hub-middleware.git", majorVersion: 1),
        // ...
    ]
    // ...
)

🚀 Getting Started

First of all, add the x-hub-signature middleware into your droplet middlewares array (inside Config/droplet.json):

{
    ...
    "middleware": [
        "error",
        "x-hub-signature",
        ...
    ],
    ...
}

Note: Make sure you insert this middleware after the error one. Otherwise the proper headers and body might not be added to the response.

Next time you boot your application, you will be prompted to add a x-hub-signature.json file inside Config/secrets which is like:

{
    "secret": "my_key",
}

Note: Environment Variables might not be the safest solution for secret data–there's much stuff out there on this. However, following the twelve-factor app manifesto can be a wiser instead of storing this data on a simple .json, so you'd better do something like:

export MY_KEY=my_key

and then:

{
    "secret": "$MY_KEY"
}

Afterwards you can add the middleware using a throwing overload of the initialiser that accepts Vapor's Config:

import XHubMiddleware

let config = try Config()
try config.setup()

// ↓ ↓ ↓ important setup stuff ↓ ↓ ↓

do {
    try config.addConfigurable(middleware: XHubMiddleware(config: config), name: "x-hub-signature")
} catch {
    fatalError("Error creating XHubMiddleware, please check that you've setup x-hub-signature.json correctly.")
}

// ↑ ↑ ↑ important setup stuff ↑ ↑ ↑

let drop = try Droplet(config)
try drop.setup()
try drop.run()
// ...

🎇 Now It Works

A working scenario would be:

Your secret: secret

The payload received from the server:

{
    "example": "payload"
}

When everything goes okay, this middleware doesn't take care of forming the response body –it just returns a 200 HTTP status–since the feedback given by the webhook to pushed events may vary depending on your domain.

Now let's say that the event emitter sent a different X-Hub-Signature than the expected:

Last but not least, we are also aware of malformed–without a valid X-Hub-Signature header–requests:

Yay, for sugary error messages.

🤓 How It Works

The client must generate an HMAC signature of the payload and include that signature in the request headers. The X-Hub-Signature header's value must be sha1=signature, where signature is a hexadecimal representation of a SHA1 signature. The signature must be computed using the HMAC algorithm with the request body as the data and the secret as the key.

This middleware recomputes the SHA1 signature with the shared secret using the same method as the client. If the signature does not match, the request cannot be verified and should probably be dropped.

Originally this technique has been described in the PubSubHubbub spec.

⚖️ License

X-Hub-Signature Vapor middleware is distributed under the MIT License, available in this repository.

All contributions are assumed to be also licensed under the same.