Add diff to InvalidChallengeTsError

client.go

@@ -218,6 +218,7 @@ func ChallengeTs(window time.Duration) Criterion {
 		if diff := now().Sub(r.ChallengeTs); diff > window {
 			return &InvalidChallengeTsError{
 				ChallengeTs: r.ChallengeTs,
+				Diff:        diff,
 			}
 		}
 		return nil

client_test.go

@@ -325,6 +325,7 @@ func TestVerify(t *testing.T) {
 			},
 			expected: &InvalidChallengeTsError{
 				ChallengeTs: now().Add(-time.Second),
+				Diff:        time.Second,
 			},
 		},
 		{

errors.go

@@ -58,8 +58,9 @@ func (e *InvalidScoreError) Error() string {
 // window.
 type InvalidChallengeTsError struct {
 	ChallengeTs time.Time
+	Diff        time.Duration
 }
 
 func (e *InvalidChallengeTsError) Error() string {
-	return fmt.Sprintf("invalid reCAPTCHA: invalid challenge timestamp: %s", e.ChallengeTs)
+	return fmt.Sprintf("invalid reCAPTCHA: invalid challenge timestamp: %s (%s old)", e.ChallengeTs, e.Diff)
 }