Abstract: In Wireless Sensor Networks (WSNs), code dissemination faces threats from both external attackers and potentially compromised nodes. Security thus becomes a critical requirement for code dissemination protocols. In this study, we propose a secure code dissemination algorithm based on the hash digest and the layered strategy. Compared to the existing algorithms, our main contributions are two-folds: (1) All data packets can be roughly and immediately verified by their hash digest. All image blocks can be exactly verified by their hash value. By the double independent verification, the code image can be securely updated. Compared to directly transmitting all hash value, the message traffic and delay are reduced (2) The hash chain is transmitted layer by layer. The below hash chain is request only if the corresponding image block is failed to verify in the above layer. Therefore, without loss of security, the message traffic is greatly reduced.