From dc3c2d5533cea2725add8c4687aae92e2a3f2e01 Mon Sep 17 00:00:00 2001
From: Jon Chen <bsd@voltaire.sh>
Date: Tue, 22 Dec 2020 00:54:59 +0000
Subject: [PATCH] implement interaction request signing verification

---
 interactions.go      | 54 +++++++++++++++++++++++++++++++++++
 interactions_test.go | 68 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 122 insertions(+)
 create mode 100644 interactions.go
 create mode 100644 interactions_test.go

diff --git a/interactions.go b/interactions.go
new file mode 100644
index 0000000..6fc2f55
--- /dev/null
+++ b/interactions.go
@@ -0,0 +1,54 @@
+package discordgo
+
+import (
+	"bytes"
+	"crypto/ed25519"
+	"encoding/hex"
+	"io"
+	"io/ioutil"
+	"net/http"
+)
+
+// VerifyInteraction implements message verification of the discord interactions api
+// signing algorithm, as documented here:
+// https://discord.com/developers/docs/interactions/slash-commands#security-and-authorization
+func VerifyInteraction(r *http.Request, key ed25519.PublicKey) bool {
+	var msg bytes.Buffer
+
+	signature := r.Header.Get("X-Signature-Ed25519")
+	if signature == "" {
+		return false
+	}
+
+	sig, err := hex.DecodeString(signature)
+	if err != nil {
+		return false
+	}
+
+	if len(sig) != ed25519.SignatureSize {
+		return false
+	}
+
+	timestamp := r.Header.Get("X-Signature-Timestamp")
+	if timestamp == "" {
+		return false
+	}
+
+	msg.WriteString(timestamp)
+
+	defer r.Body.Close()
+	var body bytes.Buffer
+
+	// at the end of the function, copy the original body back into the request
+	defer func() {
+		r.Body = ioutil.NopCloser(&body)
+	}()
+
+	// copy body into buffers
+	_, err = io.Copy(&msg, io.TeeReader(r.Body, &body))
+	if err != nil {
+		return false
+	}
+
+	return ed25519.Verify(key, msg.Bytes(), sig)
+}
diff --git a/interactions_test.go b/interactions_test.go
new file mode 100644
index 0000000..caaf28a
--- /dev/null
+++ b/interactions_test.go
@@ -0,0 +1,68 @@
+package discordgo
+
+import (
+	"bytes"
+	"crypto/ed25519"
+	"encoding/hex"
+	"net/http/httptest"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestVerifyInteraction(t *testing.T) {
+	pubkey, privkey, err := ed25519.GenerateKey(nil)
+	if err != nil {
+		t.Errorf("error generating signing keypair: %s", err)
+	}
+	timestamp := "1608597133"
+
+	t.Run("success", func(t *testing.T) {
+		body := "body"
+		request := httptest.NewRequest("POST", "http://localhost/interaction", strings.NewReader(body))
+		request.Header.Set("X-Signature-Timestamp", timestamp)
+
+		var msg bytes.Buffer
+		msg.WriteString(timestamp)
+		msg.WriteString(body)
+		signature := ed25519.Sign(privkey, msg.Bytes())
+		request.Header.Set("X-Signature-Ed25519", hex.EncodeToString(signature[:ed25519.SignatureSize]))
+
+		if !VerifyInteraction(request, pubkey) {
+			t.Error("expected true, got false")
+		}
+	})
+
+	t.Run("failure/modified body", func(t *testing.T) {
+		body := "body"
+		request := httptest.NewRequest("POST", "http://localhost/interaction", strings.NewReader("WRONG"))
+		request.Header.Set("X-Signature-Timestamp", timestamp)
+
+		var msg bytes.Buffer
+		msg.WriteString(timestamp)
+		msg.WriteString(body)
+		signature := ed25519.Sign(privkey, msg.Bytes())
+		request.Header.Set("X-Signature-Ed25519", hex.EncodeToString(signature[:ed25519.SignatureSize]))
+
+		if VerifyInteraction(request, pubkey) {
+			t.Error("expected false, got true")
+		}
+	})
+
+	t.Run("failure/modified timestamp", func(t *testing.T) {
+		body := "body"
+		request := httptest.NewRequest("POST", "http://localhost/interaction", strings.NewReader("WRONG"))
+		request.Header.Set("X-Signature-Timestamp", strconv.FormatInt(time.Now().Add(time.Minute).Unix(), 10))
+
+		var msg bytes.Buffer
+		msg.WriteString(timestamp)
+		msg.WriteString(body)
+		signature := ed25519.Sign(privkey, msg.Bytes())
+		request.Header.Set("X-Signature-Ed25519", hex.EncodeToString(signature[:ed25519.SignatureSize]))
+
+		if VerifyInteraction(request, pubkey) {
+			t.Error("expected false, got true")
+		}
+	})
+}