HMAC-SHA256 sample code.

client.js

var http, crypto, sharedSecret, query, signature;

http = require("http");
crypto = require("crypto");

sharedSecret = "super-secret";
body = { "test": "123" };
signature = crypto.createHmac("sha256", sharedSecret).update(body).digest("hex");

http.post({
    path: "localhost:1337/request",
    body: body
    headers: {
        "X-Signature": signature
    }
}, function (res) {
    console.log(res.statusCode);
});

server.js

var http, url, crypto, sharedSecret;

http = require("http");
url = require("url");
crypto = require("crypto");

sharedSecret = "super-secret";

http.createServer(function (req, res) {
    var retrievedSignature, parsedUrl, computedSignature;
 // Deal with CORS.
    res.setHeader("Access-Control-Allow-Origin", "*");
        retrievedSignature = req.headers["x-signature"];
     // Recalculate signature.
        parsedUrl = url.parse(req.url);
        computedSignature = crypto.createHmac("sha256", sharedSecret).update(req.body).digest("hex");
     // Compare signatures.
        if (computedSignature === retrievedSignature) {
            res.writeHead(200, {
                "Content-Type": "text/plain"
            });
            res.end("Hello World\n");
        } else {
            res.writeHead(403, {
                "Content-Type": "text/plain"
            });
            res.end("Get Out\n");
        }
}).listen(1337);

console.log("Server running on port 1337");