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The art or science of hiding information is named steganography. In one form or another it has been practiced since ancient times. With the development of computers it has become possible to hide the contents of a computer file (the data) in another file (the container file) so that the insertion of the data file into the container file will not noticeably change the properties of that file. If the file which has been hidden is deleted (preferably using a secure deletion program such as Data Destroyer) then there is no indication that it even exists, but it can still be recovered when you need to get at it.
Good candidates for container files are graphics images, since it is possible to make small changes to the bytes specifying pixel color values in such a way that the "stego" image (containing the hidden data) does not look any different from the original image (the one which does not contain the hidden data).
Hermetic Stego allows you to hide a file of any type and of any size in one or more BMP image files, with or without the use of a user-specified stego key (an encryption key), so that (i) the presence of the hidden file is undetectable (even by forensic software using statistical methods) and (ii) if a stego key has been used when hiding the data then that data can be extracted only by someone who knows that stego key.
BMP images containing hidden data can be archived as a ZIP file and sent to another person as an email attachment. If one simply wishes to hide sensitive data (such as a file containing passwords, bank account details, etc.) on one's computer, there are normally plenty of BMP files to be found there.
Another use for Hermetic Stego is to transport sensitive data (e.g., within an Excel spreadsheet) without detection. One can hide the data in a set of BMP images and put those images on a memory stick (a.k.a. thumb drive). After reaching one's destination the data can then be extracted from the images.
Hermetic Stego differs from other steganographic software in three respects:
- There is no limit on the type or the size of the file to be hidden because Hermetic Stego can hide a data file, not just in a single BMP image file, but in a set of them — as many images as needed to contain the data file (but see the note below).
- The bits of the data are inserted into the bytes of the image files randomly and in such a way as to defeat the use of statistical tests to reveal the presence of hidden data.
- The program can be used with a user-specified stego key or with a default stego key; use of a default stego key allows sending hidden data to another person who does not know what that stego key is.
The stego key (user-specified or default) is used not only to facilitate random selection of bytes for hiding data file bits but also is used to encrypt the data file.
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