gistfile1.js

var http = require('http'),
  httpSignature = require('http-signature'),
  crypto = require('crypto');

var data = JSON.stringify({ foo: 'bar' });
var md5 = crypto.createHash('md5');
md5.update(data);

var options = {
  host: 'localhost',
  port: 3000,
  path: '/samples/123/items',
  method: 'POST',
  headers: {
    'content-type': 'application/json',
    'content-md5': md5.digest('base64')
  }
};

var req = http.request(options, function (res) {
  console.log('STATUS: ' + res.statusCode);
  console.log('HEADERS: ' + JSON.stringify(res.headers));
  
  res.setEncoding('utf8');
  res.on('data', function (chunk) {
    console.log('BODY: ' + chunk);
  });
  res.on('end', function () {
    process.exit(1);
  });
}).on('error', function (err) {
  console.error(err);
  process.exit(1);
});

var signOptions = {
  keyId: 'someKey',
  algorithm: 'hmac-sha256',
  key: 'someSecret',
  headers: ['request-line', 'date', 'content-type', 'content-md5']
};

try {
  httpSignature.sign(req, signOptions);
} catch(err) {
  console.error(err);
  process.exit(1);
}

req.write(data)
req.end();