Verifying authenticity of digital contract documents is important for implementing electric contract over internet. However, digital documents are vulnerable to tampering, since it is difficult to detect evidences of tampering. In this the sis, we proposed a tamper-proofing method for contract documents which is based on embedding and detecting watermarks in contract documents.
For tamper-proofing, we assume that contract documents are image files and watermarks of each contractors are embedded in their own regions of the contract document. Watermarks are randomly generated by using random seedswhich are determined by contents of the contract document and each contractors'private key. And they are embedded in least significant 2 bits of pixel values of the contract document image. They could be detected by using each contractors'public key. If each contractors'watermarks exist exactly in contract documents,we could know that they have not been tampered and approve their authenticity.
