We have a secure environment where we sometimes transfer large quantities of unclassified data between machines certified for classified operations. We zip these files to ease the process of data transfer. A concern has been raised about these zip files: specifically, do zip files use a form of block compression that could potentially grab portions of blocks on the hard drive (outside of the files of interest) that might contain classified information?
To illustrate further, let's say that Zip file A contains files B, C and D. Let's assume for purposes of this discussion that the logical block size of the NTFS file system we're working with is 16K bytes, and that the zip algorithm uses a similar block size of 16K.
Given those conditions, files B, C and D will almost certainly have some slack space at the end of them (their size not being exact multiples of 16K). Is there any possibility of that slack space containing information other than what is contained in the files of interest, and if so, is there any possibility of that information making its way into a zip file (not catalogued by the Zip file system itself)?