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Hermetic Stego allows you to encrypt and hide a message (the contents of a computer file) in another file so that the addition of the message to the container file will not noticeably change the appearance of that file. If the file which has been hidden is deleted then there is no indication that it even exists, but it can still be recovered when you need to get at it. This program allows hiding a file of any size in one or more BMP image files, with or without the use of a user-specified stego/encryption key, so that (i) the presence of the hidden file is undetectable (even by forensic software using statistical methods) and (ii) if a user-specified stego key is used then the hidden file can be extracted only by someone, using this software, who knows that stego key. Later versions of most web browsers will display BMP image files, so it is possible to conceal information within a picture on a web page. Hermetic Stego allows two operations: (i) hiding a message file in single BMP image file or in a set of BMP images and (ii) extracting a message file from a single BMP image file or a set of them. This program differs from others of this sort in two respects: 1. There is no limit on the size of the file to be hidden because Hermetic Stego can hide a message file, not just in a single BMP image file, but in a set of them -- as many images as needed to contain the message file. 2. The bits of the message 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 a hidden message. 3. 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 a hidden message 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 message file bits but also is used to encrypt the message file.