feat: add cmd/cookie-inspect for decoding and verifying a cookie

cmd/cookie-inspect/main.go

@@ -0,0 +1,145 @@
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"flag"
+	"fmt"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+)
+
+type MainConfig struct {
+	name   string
+	cookie string
+	secret string
+}
+
+func main() {
+	cfg := MainConfig{}
+
+	flag.StringVar(&cfg.secret, "secret", "", "The secret used to sign the cookie (same as in Express)")
+	flag.StringVar(&cfg.name, "name", "", "Optional: cookie name (just for display)")
+	flag.Usage = func() {
+		fmt.Fprintf(os.Stderr, `Usage: express-cookie-parse [flags]
+
+Parses a raw browser Cookie: header string and inspects each cookie.
+Also verifies Express.js signed cookie (cookie-parser) format.
+
+Examples:
+  cookie-inspect "session=s:user123.J%%2BsOPk...; lang=en"
+  echo 'session=s:payload.sig; theme=dark' | cookie-inspector --secret "my-secret"
+
+Flags:
+`)
+		flag.PrintDefaults()
+	}
+	flag.Parse()
+
+	args := flag.Args()
+	if len(args) > 0 {
+		cfg.cookie = args[0]
+	}
+	if cfg.cookie == "" {
+		// Try reading from stdin
+		data, err := os.ReadFile("/dev/stdin")
+		if err != nil || len(data) == 0 {
+			fmt.Fprintln(os.Stderr, "Error: Provide cookie value via --cookie or pipe to stdin")
+			flag.Usage()
+			os.Exit(1)
+		}
+		cfg.cookie = strings.TrimSpace(string(data))
+	}
+	fmt.Println("Cookies:", cfg.cookie)
+
+	var cookies []*http.Cookie
+	c, err := http.ParseSetCookie(cfg.cookie)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to parse Cookie header: %v\n", err)
+		os.Exit(1)
+	}
+	cookies = append(cookies, c)
+
+	for _, c := range cookies {
+		inspectCookie(c, cfg.secret)
+	}
+}
+
+func inspectCookie(c *http.Cookie, secret string) {
+	fmt.Printf("Cookie: %s\n", c.Name)
+	fmt.Printf("%+#v\n\n", c)
+	cookieVal, err := url.QueryUnescape(c.Value)
+	if err != nil {
+		cookieVal = c.Value
+	}
+	fmt.Printf("%s\n\n", c.Value)
+	fmt.Printf("%s\n\n", cookieVal)
+
+	if !strings.HasPrefix(cookieVal, "s:") {
+		fmt.Println("   Not an express cookie (no 's:' prefix)")
+		return
+	}
+
+	fmt.Println("   looks like express.js signed format (s:payload.signature)")
+	payload64, sig64, err := decodeExpress(cookieVal)
+	if err != nil {
+		fmt.Printf("  Decoding express payload failed: %v\n", err)
+		return
+	}
+
+	data64 := payload64
+	parts := strings.Split(payload64, ".")
+	if len(parts) == 3 {
+		data64 = parts[1]
+	}
+	if data, err := base64.StdEncoding.DecodeString(data64); err != nil {
+		fmt.Printf("  Base64 decode failed: %v\n", err)
+	} else {
+		fmt.Printf("  Base64 decoded (std): %s\n", string(data))
+	}
+
+	if secret == "" {
+		fmt.Println("  (Verification skipped — provide --secret to check signature)")
+	} else {
+		if err := verifyExpress(payload64, sig64, secret); err != nil {
+			fmt.Printf("  Verification failed: %v\n", err)
+			return
+		}
+		fmt.Printf("  Verified payload.\n")
+	}
+}
+
+func decodeExpress(signed string) (string, string, error) {
+	withoutPrefix := signed[2:]
+
+	dotIdx := strings.LastIndex(withoutPrefix, ".")
+	if dotIdx == -1 {
+		return "", "", fmt.Errorf("missing '.' separator")
+	}
+
+	payload := withoutPrefix[:dotIdx]
+	sigReceived := withoutPrefix[dotIdx+1:]
+
+	return payload, sigReceived, nil
+}
+
+func verifyExpress(expressPayload, expressSig, secret string) error {
+	if secret == "" {
+		return fmt.Errorf("no secret provided")
+	}
+
+	mac := hmac.New(sha256.New, []byte(secret))
+	mac.Write([]byte(expressPayload))
+	expectedSig := mac.Sum(nil)
+
+	expectedB64 := base64.RawURLEncoding.EncodeToString(expectedSig)
+
+	if !hmac.Equal([]byte(expectedB64), []byte(expressSig)) {
+		return fmt.Errorf("signature mismatch")
+	}
+
+	return nil
+}