（71）is the science of hiding information。Whereas the goal of cryptography is to make data unreadable by a third party。the goal of steganography is to hide the data from a third party。In this article,I will discuss what steganography is,what purposes it serves,and will provide an example using available software。 　　There are a large number of steganographic（2）that most of us are familiar with(especially if you watch a lot of spy movies),ranging from invisible ink and microdots to secreting a hidden message in the second letter of each word of a large body of text and spread spectrum radio communication。With computers and networks,there are many other ways of hiding informations,such as: 　　Covert channels(c,g,Loki and some distributed denial-of-service tools use the Internet Control（3）Protocol,or ICMP,as the communication channel between the“bad guy”and a compromicyed system) 　　Hidden text within Web pages 　　Hiding files in“plain sight”(c,g，what better place to“hide”a file than with an important sounding name in the c:\winnt system32 directory) 　　Null ciphers(c,g,using the first letter of each word to form a hidden message in an otherwise innocuous text) 　　steganography today,however,is significantly more（4）than the example about suggest,allowing a user to hide large amounts of information within image and audio。These forms of steganography often are used in conjunction with cryptography so the information is double protected;first it is encrypted and then hidden so that an advertisement first。find the information(an often difficult task in and of itself)and the decrypted it。 　　The simplest approach to hiding data within an image file is called（5）signature insertion。In this method ,we can take the binary representation of the hidden data and the bit of each byte within the covert image。If we are using 24-bit color the amount and will be minimum and indiscriminate to the human eye。 （71）处的单词应是