fix: use Ed25519 for webhook signature verification (x-wh-signature)

GHL signs x-wh-signature webhooks with an Ed25519 key from the
Marketplace app settings, not RSA. The previous RSA implementation
caused all webhook signature checks to fail, blocking every outbound
SMS send.

Changes:
- Replace parseRSAPublicKey + RSA verification with parseEd25519PublicKey
  + ed25519.Verify for x-wh-signature
- Both x-wh-signature (current) and X-GHL-Signature (July 2026) now
  use the same Ed25519 key from GHL_WEBHOOK_PUBLIC_KEY
- Remove unused crypto/rsa, crypto/sha256, crypto imports
- Update webhook_test.go to generate/sign with Ed25519 instead of RSA

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/ghl/webhook.go

@@ -2,10 +2,7 @@ package ghl
 
 import (
 	"context"
-	"crypto"
 	"crypto/ed25519"
-	"crypto/rsa"
-	"crypto/sha256"
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/json"
@@ -31,13 +28,13 @@ type seenEntry struct {
 // WebhookHandler handles inbound GHL webhook deliveries.
 //
 // Signature verification supports two GHL schemes:
-//   - x-wh-signature: RSA-PKCS1v15 + SHA-256 (current GHL scheme)
-//   - X-GHL-Signature: Ed25519 (GHL next-gen scheme, active from July 2026)
+//   - x-wh-signature: Ed25519 (current GHL scheme, key from Marketplace app settings)
+//   - X-GHL-Signature: Ed25519 (GHL next-gen header name, active from July 2026)
 //
+// Both headers use the same Ed25519 public key configured via GHL_WEBHOOK_PUBLIC_KEY.
 // The handler checks X-GHL-Signature first; if absent it falls back to x-wh-signature.
 type WebhookHandler struct {
-	rsaPubKey    *rsa.PublicKey    // for x-wh-signature (RSA+SHA-256)
-	ed25519Key   ed25519.PublicKey // for X-GHL-Signature (Ed25519) — may be nil
+	webhookKey   ed25519.PublicKey // Ed25519 key for both x-wh-signature and X-GHL-Signature
 	castClient   *castclient.Client
 	ghlAPI       *APIClient
 	oauthHandler *OAuthHandler
@@ -46,15 +43,14 @@ type WebhookHandler struct {
 	seenMessages map[string]seenEntry
 }
 
-// NewWebhookHandler parses pubKeyPEM (RSA or Ed25519) and wires all dependencies.
-// ed25519PEM may be empty — if provided it enables the X-GHL-Signature check.
+// NewWebhookHandler parses pubKeyPEM (Ed25519 PKIX) and wires all dependencies.
 func NewWebhookHandler(pubKeyPEM string, castClient *castclient.Client, ghlAPI *APIClient, oauth *OAuthHandler, store TokenStore) (*WebhookHandler, error) {
-	rsaKey, err := parseRSAPublicKey(pubKeyPEM)
+	key, err := parseEd25519PublicKey(pubKeyPEM)
 	if err != nil {
-		return nil, fmt.Errorf("failed to parse webhook RSA public key: %w", err)
+		return nil, fmt.Errorf("failed to parse webhook public key: %w", err)
 	}
 	return &WebhookHandler{
-		rsaPubKey:    rsaKey,
+		webhookKey:   key,
 		castClient:   castClient,
 		ghlAPI:       ghlAPI,
 		oauthHandler: oauth,
@@ -159,28 +155,16 @@ func (h *WebhookHandler) updateStatus(ctx context.Context, webhook OutboundMessa
 	slog.Info("webhook: message status updated", "message_id", webhook.MessageID, "status", status)
 }
 
-// verifyIncomingSignature checks X-GHL-Signature (Ed25519) first, then x-wh-signature (RSA).
+// verifyIncomingSignature checks X-GHL-Signature first, then x-wh-signature.
+// Both use the same Ed25519 key from GHL Marketplace app settings.
 func (h *WebhookHandler) verifyIncomingSignature(r *http.Request, body []byte) bool {
-	if sig := r.Header.Get("X-GHL-Signature"); sig != "" && h.ed25519Key != nil {
+	if sig := r.Header.Get("X-GHL-Signature"); sig != "" {
 		return h.verifyEd25519(body, sig)
 	}
-	return h.verifyRSA(body, r.Header.Get("x-wh-signature"))
+	return h.verifyEd25519(body, r.Header.Get("x-wh-signature"))
 }
 
-// verifyRSA verifies RSA-PKCS1v15 + SHA-256 (current GHL x-wh-signature scheme).
-func (h *WebhookHandler) verifyRSA(body []byte, signatureB64 string) bool {
-	if signatureB64 == "" {
-		return false
-	}
-	sigBytes, err := base64.StdEncoding.DecodeString(signatureB64)
-	if err != nil {
-		return false
-	}
-	hash := sha256.Sum256(body)
-	return rsa.VerifyPKCS1v15(h.rsaPubKey, crypto.SHA256, hash[:], sigBytes) == nil
-}
-
-// verifyEd25519 verifies an Ed25519 signature (GHL X-GHL-Signature scheme, active July 2026+).
+// verifyEd25519 verifies a raw Ed25519 signature (base64-encoded) over body.
 func (h *WebhookHandler) verifyEd25519(body []byte, signatureB64 string) bool {
 	if signatureB64 == "" {
 		return false
@@ -189,7 +173,7 @@ func (h *WebhookHandler) verifyEd25519(body []byte, signatureB64 string) bool {
 	if err != nil {
 		return false
 	}
-	return ed25519.Verify(h.ed25519Key, body, sigBytes)
+	return ed25519.Verify(h.webhookKey, body, sigBytes)
 }
 
 func (h *WebhookHandler) HandleUninstall(w http.ResponseWriter, r *http.Request) {
@@ -230,10 +214,10 @@ func (h *WebhookHandler) HandleUninstall(w http.ResponseWriter, r *http.Request)
 	w.WriteHeader(http.StatusOK)
 }
 
-// parseRSAPublicKey decodes a PEM-encoded PKIX RSA public key.
-// Tolerates literal \n sequences in the input (common when the key is stored
-// as a single line in a .env file).
-func parseRSAPublicKey(pemStr string) (*rsa.PublicKey, error) {
+// parseEd25519PublicKey decodes a PEM-encoded PKIX Ed25519 public key.
+// Tolerates literal \n sequences in the input (common when stored as a single
+// line in a .env file).
+func parseEd25519PublicKey(pemStr string) (ed25519.PublicKey, error) {
 	pemStr = strings.ReplaceAll(pemStr, `\n`, "\n")
 	block, _ := pem.Decode([]byte(pemStr))
 	if block == nil {
@@ -243,9 +227,9 @@ func parseRSAPublicKey(pemStr string) (*rsa.PublicKey, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse public key: %w", err)
 	}
-	rsaPub, ok := pub.(*rsa.PublicKey)
+	ed25519Pub, ok := pub.(ed25519.PublicKey)
 	if !ok {
-		return nil, fmt.Errorf("key is not RSA (got %T)", pub)
+		return nil, fmt.Errorf("key is not Ed25519 (got %T)", pub)
 	}
-	return rsaPub, nil
+	return ed25519Pub, nil
 }

internal/ghl/webhook_test.go

@@ -1,10 +1,8 @@
 package ghl
 
 import (
-	"crypto"
+	"crypto/ed25519"
 	"crypto/rand"
-	"crypto/rsa"
-	"crypto/sha256"
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/pem"
@@ -17,13 +15,13 @@ import (
 	"git.sds.dev/CAST/cast-ghl-plugin/internal/store"
 )
 
-func generateTestKeyPair(t *testing.T) (*rsa.PrivateKey, string) {
+func generateTestKeyPair(t *testing.T) (ed25519.PrivateKey, string) {
 	t.Helper()
-	privKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	pubKey, privKey, err := ed25519.GenerateKey(rand.Reader)
 	if err != nil {
-		t.Fatalf("failed to generate RSA key: %v", err)
+		t.Fatalf("failed to generate Ed25519 key: %v", err)
 	}
-	pubBytes, err := x509.MarshalPKIXPublicKey(&privKey.PublicKey)
+	pubBytes, err := x509.MarshalPKIXPublicKey(pubKey)
 	if err != nil {
 		t.Fatalf("failed to marshal public key: %v", err)
 	}
@@ -31,13 +29,9 @@ func generateTestKeyPair(t *testing.T) (*rsa.PrivateKey, string) {
 	return privKey, string(pemBlock)
 }
 
-func signPayload(t *testing.T, privKey *rsa.PrivateKey, body []byte) string {
+func signPayload(t *testing.T, privKey ed25519.PrivateKey, body []byte) string {
 	t.Helper()
-	hash := sha256.Sum256(body)
-	sig, err := rsa.SignPKCS1v15(rand.Reader, privKey, crypto.SHA256, hash[:])
-	if err != nil {
-		t.Fatalf("failed to sign payload: %v", err)
-	}
+	sig := ed25519.Sign(privKey, body)
 	return base64.StdEncoding.EncodeToString(sig)
 }