Verify Apple ID token signature against JWKS

Fixes #25

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/auth/apple.go

@@ -2,11 +2,15 @@ package auth
 
 import (
 	"context"
+	"crypto/rsa"
 	"crypto/x509"
+	"encoding/base64"
 	"encoding/json"
 	"encoding/pem"
 	"fmt"
 	"io"
+	"math/big"
+	"net/http"
 	"os"
 	"time"
 
@@ -81,11 +85,61 @@ func (p *AppleProvider) getUserInfo(ctx context.Context, token *oauth2.Token) (*
 		return nil, fmt.Errorf("missing id_token")
 	}
 
-	// Parse without verification since we already got the token from Apple
+	// Fetch Apple's JWKS
-	parser := jwt.NewParser(jwt.WithoutClaimsValidation())
+	resp, err := http.Get("https://appleid.apple.com/auth/keys")
-	parsed, _, err := parser.ParseUnverified(idToken, jwt.MapClaims{})
 	if err != nil {
-		return nil, fmt.Errorf("parse id_token: %w", err)
+		return nil, fmt.Errorf("fetch apple JWKS: %w", err)
+	}
+	defer resp.Body.Close()
+
+	var jwks struct {
+		Keys []struct {
+			Kty string `json:"kty"`
+			Kid string `json:"kid"`
+			Use string `json:"use"`
+			Alg string `json:"alg"`
+			N   string `json:"n"`
+			E   string `json:"e"`
+		} `json:"keys"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&jwks); err != nil {
+		return nil, fmt.Errorf("decode apple JWKS: %w", err)
+	}
+
+	// Parse and verify the token
+	parsed, err := jwt.Parse(idToken, func(t *jwt.Token) (interface{}, error) {
+		kid, ok := t.Header["kid"].(string)
+		if !ok {
+			return nil, fmt.Errorf("missing kid header")
+		}
+
+		for _, key := range jwks.Keys {
+			if key.Kid == kid {
+				// Decode RSA public key from JWK
+				nBytes, err := base64.RawURLEncoding.DecodeString(key.N)
+				if err != nil {
+					return nil, fmt.Errorf("decode key N: %w", err)
+				}
+				eBytes, err := base64.RawURLEncoding.DecodeString(key.E)
+				if err != nil {
+					return nil, fmt.Errorf("decode key E: %w", err)
+				}
+
+				e := 0
+				for _, b := range eBytes {
+					e = e*256 + int(b)
+				}
+
+				return &rsa.PublicKey{
+					N: new(big.Int).SetBytes(nBytes),
+					E: e,
+				}, nil
+			}
+		}
+		return nil, fmt.Errorf("key %s not found in JWKS", kid)
+	})
+	if err != nil {
+		return nil, fmt.Errorf("verify id_token: %w", err)
 	}
 
 	claims, ok := parsed.Claims.(jwt.MapClaims)
@@ -99,7 +153,7 @@ func (p *AppleProvider) getUserInfo(ctx context.Context, token *oauth2.Token) (*
 	return &OAuthUserInfo{
 		ProviderUserID: sub,
 		Email:          email,
-		Name:           email, // Apple may not provide name in id_token
+		Name:           email,
 	}, nil
 }