Facebook signed request middleware

facebook.js

var crypto = require('crypto')

module.exports = function(secret) {
    return function(req, res, next) {
        var signedRequest = req.body.signed_request

        if (signedRequest) {
            var parts = signedRequest.split('.', 2)
            var signature = parts[0]
            var payload = parts[1]
            var computedSignature = crypto
                .createHmac('sha256', secret)
                .update(payload)
                .digest('base64')
                .replace(/\+/g, '-')
                .replace(/\//g, '_')
                .replace(/\=/, '')

            if (signature != computedSignature) return req.next(400)

            req.facebook = JSON.parse(new Buffer(payload, 'base64').toString('ascii'))
        }

        next()
    }
}