crypto: harden API surface

Make it explicit that expiration is not being tested with Decrypt, and
test that the expiration is meaningful with DecryptWithExpiration.

auth/middleware.go

@@ -215,7 +215,7 @@ type AuthCookie struct {
 
 func cookieAuth(cookie *http.Cookie, w http.ResponseWriter,
 	r *http.Request, next http.Handler) {
-	payload := crypto.Decrypt([]byte(cookie.Value))
+	payload := crypto.DecryptWithoutExpiration([]byte(cookie.Value))
 	if payload == nil {
 		authError(w, "Invalid authentication cookie", http.StatusForbidden)
 		return

crypto/crypto.go

@@ -63,12 +63,15 @@ func Encrypt(payload []byte) []byte {
 	return msg
 }
 
-func Decrypt(payload []byte) []byte {
+func DecryptWithoutExpiration(payload []byte) []byte {
 	return fernet.VerifyAndDecrypt(payload,
 		time.Duration(0), []*fernet.Key{fernetKey})
 }
 
 func DecryptWithExpiration(payload []byte, expiry time.Duration) []byte {
+	if expiry == 0 {
+		panic(fmt.Errorf("DecryptWithExpiration given expiration of zero. Use DecryptWithoutExpiration if you really meant it."))
+	}
 	return fernet.VerifyAndDecrypt(payload, expiry, []*fernet.Key{fernetKey})
 }
 

crypto/crypto_test.go

@@ -57,7 +57,7 @@ func TestEncrypt(t *testing.T) {
 	assert.NotNil(t, enc)
 	assert.NotEqual(t, enc, []byte("Hello, world!"))
 
-	dec := Decrypt(enc)
+	dec := DecryptWithoutExpiration(enc)
 	assert.NotNil(t, dec)
 	assert.Equal(t, dec, []byte("Hello, world!"))
 }

model/cursor.go

@@ -20,7 +20,7 @@ func (cur *Cursor) UnmarshalGQL(v interface{}) error {
 	if !ok {
 		return fmt.Errorf("cursor must be strings")
 	}
-	plain := crypto.Decrypt([]byte(enc))
+	plain := crypto.DecryptWithoutExpiration([]byte(enc))
 	if plain == nil {
 		return fmt.Errorf("Invalid cursor")
 	}