Move Twilio authn code into its own package

5 files changed, 242 insertions, 0 deletions

diff --git a/pkg/twilio/filter.go b/pkg/twilio/filter.go
new file mode 100644
index 0000000..7d5f6b5
--- /dev/null
+++ b/pkg/twilio/filter.go
@@ -0,0 +1,74 @@
+// SPDX-FileCopyrightText: © 2021 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package twilio
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"encoding/base64"
+	"errors"
+	"log"
+	"net/http"
+	"sort"
+	"strings"
+)
+
+var (
+	ErrBase64            = errors.New("failed to decode X-Twilio-Signature header")
+	ErrMissingHeader     = errors.New("missing X-Twilio-Signature header")
+	ErrSignatureMismatch = errors.New("signature mismatch")
+)
+
+type Filter struct {
+	AuthToken []byte
+	Handler   http.Handler
+}
+
+var _ http.Handler = &Filter{}
+
+func (th *Filter) CheckRequestSignature(r *http.Request) error {
+	hdr := r.Header.Get("X-Twilio-Signature")
+	if len(hdr) == 0 {
+		return ErrMissingHeader
+	}
+	reqSig, err := base64.StdEncoding.DecodeString(hdr)
+	if err != nil {
+		return ErrBase64
+	}
+
+	// See https://www.twilio.com/docs/usage/security#validating-requests
+	// for more details.
+
+	parts := []string{}
+	if r.Method == http.MethodPost {
+		if err := r.ParseForm(); err != nil {
+			return err
+		}
+		for k := range r.PostForm {
+			parts = append(parts, k)
+		}
+		sort.Strings(parts)
+		for i, k := range parts {
+			parts[i] += r.PostForm[k][0]
+		}
+	}
+	s := r.URL.String() + strings.Join(parts, "")
+	h := hmac.New(sha1.New, th.AuthToken)
+	h.Write([]byte(s))
+	ourSig := h.Sum(nil)
+
+	if !hmac.Equal(reqSig, ourSig) {
+		return ErrSignatureMismatch
+	}
+	return nil
+}
+
+func (th *Filter) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if err := th.CheckRequestSignature(r); err != nil {
+		log.Println("Failed to check Twilio signature:", err)
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
+	th.Handler.ServeHTTP(w, r)
+}
diff --git a/pkg/twilio/filter_test.go b/pkg/twilio/filter_test.go
new file mode 100644
index 0000000..c0c737c
--- /dev/null
+++ b/pkg/twilio/filter_test.go
@@ -0,0 +1,114 @@
+// SPDX-FileCopyrightText: © 2021 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package twilio
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFilter_CheckRequestSignature(t *testing.T) {
+	th := &Filter{[]byte("token"), EmptyResponseHandler}
+
+	t.Run("Good Signature (POST)", func(t *testing.T) {
+		assert.NoError(t, th.CheckRequestSignature(newRequest(Post)))
+	})
+
+	t.Run("Good Signature (GET)", func(t *testing.T) {
+		assert.NoError(t, th.CheckRequestSignature(newRequest(Get)))
+	})
+
+	t.Run("Missing Header", func(t *testing.T) {
+		r := newRequest(Post)
+		r.Header.Del("X-Twilio-Signature")
+		assert.ErrorIs(t, th.CheckRequestSignature(r), ErrMissingHeader)
+	})
+
+	t.Run("Bad Base64", func(t *testing.T) {
+		r := newRequest(Post)
+		r.Header.Set("X-Twilio-Signature", "Very suspicious Base64 header.")
+		assert.ErrorIs(t, th.CheckRequestSignature(r), ErrBase64)
+	})
+
+	t.Run("Signature Mismatch", func(t *testing.T) {
+		r := newRequest(Post)
+		r.Header.Set("X-Twilio-Signature", "dpE7iSS3LEQo72hCT34eBRt3UEI=")
+		assert.ErrorIs(t, th.CheckRequestSignature(r), ErrSignatureMismatch)
+	})
+}
+
+func TestFilter_ServeHTTP(t *testing.T) {
+	th := &Filter{[]byte("token"), EmptyResponseHandler}
+
+	t.Run("Good Signature (POST)", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		th.ServeHTTP(w, newRequest(Post))
+		assert.Equal(t, http.StatusOK, w.Code)
+		assert.Equal(t, "text/xml", w.HeaderMap.Get("Content-Type"))
+		assert.Equal(t, "<Response/>", w.Body.String())
+	})
+
+	t.Run("Good Signature (GET)", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		th.ServeHTTP(w, newRequest(Get))
+		assert.Equal(t, http.StatusOK, w.Code)
+		assert.Equal(t, "text/xml", w.HeaderMap.Get("Content-Type"))
+		assert.Equal(t, "<Response/>", w.Body.String())
+	})
+
+	t.Run("Missing Header", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		r := newRequest(Post)
+		r.Header.Del("X-Twilio-Signature")
+		th.ServeHTTP(w, r)
+		assert.Equal(t, http.StatusBadRequest, w.Code)
+	})
+
+	t.Run("Bad Base64", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		r := newRequest(Post)
+		r.Header.Set("X-Twilio-Signature", "Very suspicious Base64 header.")
+		th.ServeHTTP(w, r)
+		assert.Equal(t, http.StatusBadRequest, w.Code)
+	})
+
+	t.Run("Signature Mismatch", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		r := newRequest(Post)
+		r.Header.Set("X-Twilio-Signature", "dpE7iSS3LEQo72hCT34eBRt3UEI=")
+		th.ServeHTTP(w, r)
+		assert.Equal(t, http.StatusBadRequest, w.Code)
+	})
+}
+
+const (
+	Get  = true
+	Post = false
+)
+
+// X-Twilio-Signature can be manually generated with:
+// % echo -n "${SOME_STRING}" | openssl dgst -binary -hmac ${AUTH_TOKEN} -sha1 | base64
+
+func newRequest(get bool) *http.Request {
+	vals := url.Values{
+		"To":   {"Bob"},
+		"From": {"Alice"},
+		"Body": {"A random message."},
+	}.Encode()
+	if get {
+		r := httptest.NewRequest(http.MethodGet, "https://example.test/endpoint?"+vals, nil)
+		r.Header.Set("X-Twilio-Signature", "Hh0ReTk/+7Ea38qZ3Xt1/NQx4i4=")
+		return r
+	}
+	rd := strings.NewReader(vals)
+	r := httptest.NewRequest(http.MethodPost, "https://example.test/endpoint", rd)
+	r.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	r.Header.Set("X-Twilio-Signature", "j61PPnnoUAAsfEnLuwUefOfylf4=")
+	return r
+}
diff --git a/pkg/twilio/handler.go b/pkg/twilio/handler.go
new file mode 100644
index 0000000..11cb0b8
--- /dev/null
+++ b/pkg/twilio/handler.go
@@ -0,0 +1,4 @@
+// SPDX-FileCopyrightText: © 2021 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package twilio
diff --git a/pkg/twilio/sms.go b/pkg/twilio/sms.go
new file mode 100644
index 0000000..0a7dba8
--- /dev/null
+++ b/pkg/twilio/sms.go
@@ -0,0 +1,34 @@
+// SPDX-FileCopyrightText: © 2021 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package twilio
+
+import (
+	"net/http"
+	"time"
+)
+
+type SMS struct {
+	DateReceived   time.Time
+	From, To, Body string
+}
+
+type SMSTee struct {
+	Chan    chan<- SMS
+	Handler http.Handler
+}
+
+var _ http.Handler = &SMSTee{}
+
+func (th *SMSTee) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	select {
+	case th.Chan <- SMS{
+		DateReceived: time.Now(),
+		From:         r.FormValue("From"),
+		To:           r.FormValue("To"),
+		Body:         r.FormValue("Body"),
+	}:
+		th.Handler.ServeHTTP(w, r)
+	case <-r.Context().Done():
+	}
+}
diff --git a/pkg/twilio/util.go b/pkg/twilio/util.go
new file mode 100644
index 0000000..1f1d269
--- /dev/null
+++ b/pkg/twilio/util.go
@@ -0,0 +1,16 @@
+// SPDX-FileCopyrightText: © 2021 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package twilio
+
+import (
+	"fmt"
+	"net/http"
+)
+
+// EmptyResponseHandler writes an empty XML response so Twilio knows not
+// to do anything after a webhook has been called.
+var EmptyResponseHandler = http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "text/xml")
+	fmt.Fprint(w, "<Response/>")
+})