2016下半年信息安全工程師上午真題試題解析第71-75題
71-75、（1）is the science of hiding information. Whereas the goal of cryptography is to make data unreadable by a third party. the goal of steganography（[?steg?'n?gr?f?]隱寫術） 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.
（1）A、Cryptography 
B、Geography
C、Stenography
D、Steganography
（2）A、methods
B、software
C、tools
D、services
（3）A、Member
B、Management
C、Message
D、Mail
（4）A、powerful
B、sophistication（[s??f?st??ke??n]可理解為先進） 
C、advanced
D、easy
（5）A、least（LSB：least significant bit）
B、most
C、much
D、less
信管網解析：
LSB隱寫
LSB全稱least significant bit，最低有效位
PNG文件中的圖像像數(shù)一般是由RGB三原色（紅綠藍）組成，每一種顏色占用8位，取值范圍為0x00~0xFF，即有256種顏色，一共包含了256的3次方的顏色，即16777216 種顏色
人類的眼睛可以區(qū)分約1000萬種不同的顏色
這意味著人類的眼睛無法區(qū)分余下的顏色大約有6777216種
LSB隱寫就是修改RGB顏色分量的最低二進制位（LSB），而人類的眼睛不會注意到這前后的變化
每個像數(shù)可以攜帶3比特的信息
信管網參考答案：A、A、C、B、A
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