Network coding, whose intermediate nodes reencode packets to forward incoming packets, increases the capacity the multicast network. However there are security issues of confidentiality and integrity which degrade and threat network coding. In this paper, we propose Secure Key Checksum (SKC), a secure mechanism using an encryption key, to provide both confidentiality and integrity. To provide the confidentiality, SKC utilizes a block encryption. For the integrity check, SKC uses a checksum based scheme verifying encoded blocks on-the-fly with low computational cost. Also, SKC uses a combined security key for encryption and checksum. The integrity check of SKC is computationally fast as SKC utilizes linear operations. In addition, SKC provides interoperability with other network coding by using various encryptions of the block cipher.