In smart grid systems, secure in-network data aggregation approaches have been introduced to efficiently collect aggregation data, while preserving data privacy of individual meters. Nevertheless, it is also important to maintain the integrity of aggregate data in the presence of accidental errors and internal/external attacks. To ensure the correctness of the aggregation against unintentional errors, we introduce an end-to-end signature scheme, which generates a homomorphic signature for the aggregation result. The homomorphic signature scheme is compatible with the in-network aggregation schemes that are also based on homomorphic encryption, and supports efficient batch verifications of the aggregation results. Next, to defend against suspicious/compromised meters and external attacks, we present a hop-by-hop signature scheme and an incremental verification protocol. In this approach, signatures are managed distributedly and verification is only triggered in an ex post facto basis - when anomalies in the aggregation results are detected at the collector. The incremental verification process starts from the collector, and traces the anomaly in a breath-first manner. The abnormal node is identified within O(logN) iterations. Therefore, the verification process is computationally inexpensive, while ensuring faithfulness and undeniability properties.