SecureCMerge: Secure PDF Merging over Untrusted Servers

Abstract

Merging two or more PDF files is an operation that is commonly performed by users, most often using freely available online tools, such as pdfmerge, or cloud-based services, such as cloudconvert. These free online servers cannot always be trusted and can often pose great risk to the confidentiality of the PDF files. In this paper, we present a method, called SecureCMerge, to merge the PDF files using free online merge sites in a secure way. Our core idea is to encrypt the content (text and image blocks) in the PDF files using Shamir's Secret Sharing (SSS) scheme before uploading it to a PDF merge server. We also show that the SSS scheme is merge homomorphic and the proposed method, by virtue of using the SSS scheme, provides information theoretic security. Experimental results demonstrate that the proposed method accomplishes secure PDF merging at an acceptable overhead in computation time and size.