These methods all come under the heading of steganography which is concealing a message rather than using cryptography and encrypting it. One of the advantages of this approach is that you can conceal the fact that you are sending a message at all. For the exceptionally cautious you could always conceal an encrypted message, and use both
All the techniques assume that you have an opportunity to meet or communicate securely with the recipient to specify which method you will use in advance !
Many countries forbid the use of encryption software, and messages that carry the tell tale signs may make people suspicious. Being information devices there are many ways to hide or camouflage data on a computer. These 10 are offered as a starting point.
- Convert an ascii text file into its binary equivalent and using the command prompt encode the message in the file attributes of a selection of files in a folder. (eg using h (hidden) = 0 and a (archive) = 1. or encode a message in the datestamps of a series of files etc. This could be done by using a batch file to copy a folder listing into a text file and then splitting the columns using a spreadsheet.
- Encode a message into a numerical array in source code of a harmless seeming program, or encode it into other kinds of numeric variables or variable names or comments. For double security you can compile as well and let the recipient use a decompiler.
- Place a message in every tenth bit of an mp3 file or a spreadsheet column or any file type you can think of. (See note)
- Place a message in meta tags, (eg every third word in the keyword tags or whatever), post the page(s) and send the relevant links to the recipient, or encode the message in every fifth word of a blog post about an harmless unconnected subject.
- Write a program that copies a message into a series of windows registry keys but make the order they are occur in the altering program jumbled so they can't be read sequentially until the recipient views the registry.
- Make a series of image files using photoshop or some other application that allows layers. One of the layers in each image consists of blank background with some of the message text of the same colour on it. To read text just change its colour and reassemble the whole message.
- In a folder containing innocuous files such as an ordinary application place a series of files with a unique extension. Use the words of the message as the filename prefixed by a number that gives the order in which they are to be read. Then in a command prompt use a filter to sort and list all the files with the given extension the message can then be read down the page. Zip and send so it looks like you are sharing an ordinary application.
- Hide messages in the id3 lyrics tag of a series of mp3 songs and send them.
- Hide a message in the tags of a dummy user account for an online site eg: flickr or librarything.com etc
- Using a dummy email address create a series of messages that look like spam, for instance offering cheap Viagra etc, and place small parts of a message somewhere in the body text of each email, for instance surrounded by "tags" to denote where the message starts and ends.
Note: Any of the above can be done using a Hex Editor program such as "Free Hex Editor" and copy and paste. Also learning basic command line functions for your operating system is quite easy and very useful.
None of these techniques are foolproof. To preserve secrecy you may have to migrate from one method to another and keep being inventive. They are recommended not to be used for anything illegal (unless the law concerned is a dumb one !).