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secure.go
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secure.go
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// This file is part of the Smart Home
// Program complex distribution https://github.com/e154/smart-home
// Copyright (C) 2016-2021, Filippov Alex
//
// This library is free software: you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 3 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Library General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library. If not, see
// <https://www.gnu.org/licenses/>.
package alexa
import (
"crypto/tls"
"crypto/x509"
"encoding/pem"
"errors"
"io"
"net/http"
"net/url"
"strings"
"time"
)
// HTTPError is a convenience method for logging a message and writing the provided error message
// and error code to the HTTP response.
func HTTPError(w http.ResponseWriter, logMsg string, err string, errCode int) {
if logMsg != "" {
log.Error(logMsg)
}
http.Error(w, err, errCode)
}
// IsValidAlexaRequest handles all the necessary steps to validate that an incoming http.Request has actually come from
// the Server service. If an error occurs during the validation process, an http.Error will be written to the provided http.ResponseWriter.
// The required steps for request validation can be found on this page:
// --insecure-skip-verify flag will disable all validations
// https://developer.amazon.com/public/solutions/alexa/alexa-skills-kit/docs/developing-an-alexa-skill-as-a-web-service#hosting-a-custom-skill-as-a-web-service
func IsValidAlexaRequest(w http.ResponseWriter, r *http.Request) bool {
certURL := r.Header.Get("SignatureCertChainUrl")
// Verify certificate URL
if !verifyCertURL(certURL) {
HTTPError(w, "Invalid cert URL: "+certURL, "Not Authorized", 401)
return false
}
// Fetch certificate data
certContents, err := readCert(certURL)
if err != nil {
HTTPError(w, err.Error(), "Not Authorized", 401)
return false
}
// Decode certificate data
block, _ := pem.Decode(certContents)
if block == nil {
HTTPError(w, "Failed to parse certificate PEM.", "Not Authorized", 401)
return false
}
cert, err := x509.ParseCertificate(block.Bytes)
if err != nil {
HTTPError(w, err.Error(), "Not Authorized", 401)
return false
}
// Check the certificate date
if time.Now().Unix() < cert.NotBefore.Unix() || time.Now().Unix() > cert.NotAfter.Unix() {
HTTPError(w, "Amazon certificate expired.", "Not Authorized", 401)
return false
}
// Check the certificate alternate names
foundName := false
for _, altName := range cert.Subject.Names {
if altName.Value == "echo-api.amazon.com" {
foundName = true
}
}
if !foundName {
HTTPError(w, "Amazon certificate invalid.", "Not Authorized", 401)
return false
}
return true
}
func readCert(certURL string) ([]byte, error) {
certPool, err := x509.SystemCertPool()
if err != nil || certPool == nil {
log.Error("Can't open system cert pools")
}
tr := &http.Transport{
TLSClientConfig: &tls.Config{RootCAs: certPool, InsecureSkipVerify: insecureSkipVerify},
}
hc := &http.Client{Timeout: 2 * time.Second, Transport: tr}
cert, err := hc.Get(certURL)
if err != nil {
return nil, errors.New("could not download Amazon cert file: " + err.Error())
}
defer cert.Body.Close()
certContents, err := io.ReadAll(cert.Body)
if err != nil {
return nil, errors.New("could not read Amazon cert file: " + err.Error())
}
return certContents, nil
}
func verifyCertURL(path string) bool {
link, _ := url.Parse(path)
if link.Scheme != "https" {
return false
}
if link.Host != "s3.amazonaws.com" && link.Host != "s3.amazonaws.com:443" {
return false
}
if !strings.HasPrefix(link.Path, "/echo.api/") {
return false
}
return true
}