feat(idtoken): add ParsePayload returning unvalidated token payload (#…

…2136)

idtoken/validate.go

@@ -122,6 +122,23 @@ func Validate(ctx context.Context, idToken string, audience string) (*Payload, e
 	return defaultValidator.validate(ctx, idToken, audience)
 }
 
+// ParsePayload parses the given token and returns its payload.
+//
+// Warning: This function does not validate the token prior to parsing it.
+//
+// ParsePayload is primarily meant to be used to inspect a token's payload. This is
+// useful when validation fails and the payload needs to be inspected.
+//
+// Note: A successful Validate() invocation with the same token will return an
+// identical payload.
+func ParsePayload(idToken string) (*Payload, error) {
+	jwt, err := parseJWT(idToken)
+	if err != nil {
+		return nil, err
+	}
+	return jwt.parsedPayload()
+}
+
 func (v *Validator) validate(ctx context.Context, idToken string, audience string) (*Payload, error) {
 	jwt, err := parseJWT(idToken)
 	if err != nil {
@@ -145,7 +162,7 @@ func (v *Validator) validate(ctx context.Context, idToken string, audience strin
 	}
 
 	if now().Unix() > payload.Expires {
-		return nil, fmt.Errorf("idtoken: token expired")
+		return nil, fmt.Errorf("idtoken: token expired: now=%v, expires=%v", now().Unix(), payload.Expires)
 	}
 
 	switch header.Algorithm {

idtoken/validate_test.go

@@ -231,6 +231,39 @@ func TestValidateES256(t *testing.T) {
 	}
 }
 
+func TestParsePayload(t *testing.T) {
+	idToken, _ := createRS256JWT(t)
+	tests := []struct {
+		name                string
+		token               string
+		wantPayloadAudience string
+		wantErr             bool
+	}{{
+		name:                "valid token",
+		token:               idToken,
+		wantPayloadAudience: testAudience,
+	}, {
+		name:    "unparseable token",
+		token:   "aaa.bbb.ccc",
+		wantErr: true,
+	}}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			payload, err := ParsePayload(tt.token)
+			gotErr := err != nil
+			if gotErr != tt.wantErr {
+				t.Errorf("ParsePayload(%q) got error %v, wantErr = %v", tt.token, err, tt.wantErr)
+			}
+			if tt.wantPayloadAudience != "" {
+				if payload == nil || payload.Audience != tt.wantPayloadAudience {
+					t.Errorf("ParsePayload(%q) got payload %+v, want payload with audience = %q", tt.token, payload, tt.wantPayloadAudience)
+				}
+			}
+		})
+	}
+}
+
 func createES256JWT(t *testing.T) (string, ecdsa.PublicKey) {
 	t.Helper()
 	token := commonToken(t, "ES256")