Exercise 1 of SSD 6 -- C/C++

难得见到iCarnegie课程里的Exercise有如此生动的描述，居然还有007！汗……
C/C++一直是我的薄弱环节，这次多亏有高人相助~
现在看来这个题出得实在是奥妙无穷，佩服老外！！搞了我2个礼拜茶不思饭不香，终于理解其中的意思了。唉~ 谁让自己是转专业的，C++连课都没听过，内存分配也什么都不懂。不过这次是个开头，以后的题目应该慢慢适应吧，总不会每次都让别人手把手教我的。

<h2>Understanding a Secret Message</h2> <BR /> <p>You have just intercepted an encoded message. The message is a sequence of bits <BR /> which reads as follows in hexadecimal:</p> <BR /> <blockquote> <BR />   <pre>6363636363636363724646636F6D6F72 <BR /> 466D203A65693A7243646E206F54540A <BR /> 5920453A54756F0A6F6F470A21643A6F <BR /> 594E2020206F776F797275744563200A <BR /> 6F786F686E6963736C206765796C656B <BR /> 2C3365737420346E20216F74726F5966 <BR /> 7565636F202061206C61676374206C6F <BR /> 20206F74747865656561727632727463 <BR /> 6E617920680A64746F69766120646E69 <BR /> 21687467630020656C6C786178742078 <BR /> 6578206F727478787863617800783174 <BR /> </pre> <BR /> </blockquote> <BR /> You have no idea how to decode it, but you know that your grade depends on it, so <BR /> you are willing to do <em>anything</em> to extract the message. Fortunately, one <BR /> of your many agents on the field has stolen the source code for the decoder. This <BR /> agent (007) has put the code and the message in the file <BR /> <a href="https://www.icarnegie.com/content/SSD/SSD6/2.0/normal/pg-c-programming-model/assm-exer-c-programming-model/pool-pr-c-programming-model/qn-pr-c-programming-model/handout/secret.cpp"> <BR /> secret.cpp</a>, which you can <BR /> <a href="https://www.icarnegie.com/content/SSD/SSD6/2.0/normal/pg-c-programming-model/assm-exer-c-programming-model/pool-pr-c-programming-model/qn-pr-c-programming-model/handout/secret.cpp"> <BR /> download</a> from the laboratory of your technical staff (Q). <BR /> <p>Q has noticed that the decoder takes four integers as arguments. Executing the <BR /> decoder with various arguments seems to either crash the program or produce unintelligible <BR /> output. It seems that the correct four integers have to be chosen in order for the <BR /> program to produce the decoded message. These four integers are the &quot;secret keys.&quot;</p> <BR /> <p>007 has been unable to find the keys, but from the desk of the encrypting personnel <BR /> he was able to cunningly retrieve the first five characters of the unencoded message. <BR /> These characters are:</p> <BR /> <blockquote> <BR />   <pre>From: <BR /> </pre> <BR /> </blockquote> <BR /> <h3>Assignment</h3> <BR /> Your assignment is to decode the message, and find the keys. <BR /> <h3>Reminders</h3> <BR /> This exercise is not extremely difficult. However, the strategy of trying things <BR /> until something works will be ineffective. Try to understand the material in the <BR /> course, particularly the following: <BR /> <ul> <BR />   <li>Memory contains nothing but bits. Bits are interpreted as integers, characters, <BR />   or instructions by the compiler, but they have no intrinsic type in memory. <BR />   </li> <BR />   <li>The compiler can be strong-armed into interpreting integers as characters, <BR />   or even as instructions, and vice versa. </li> <BR />   <li>Every group of 8 bits (a byte) has an address. </li> <BR />   <li>A pointer in C is merely a stored memory address. </li> <BR />   <li>The activation records for each function call are all together in memory, <BR />   and they are organized in a stack that grows downwards and shrinks upwards on <BR />   function calls and returns respectively. </li> <BR />   <li>The return address of one function as well as the addresses of all of its <BR />   local variables are allocated within one activation record. </li> <BR /> </ul> <BR /> <h3>Strategy</h3> <BR /> The designers of this decoder weren&#39;t very good. They made it possible for us to <BR /> attack the keys in two independent parts. Try to break the first two keys first, <BR /> and do not try to break the third and fourth keys until you have succeeded with <BR /> the first two. <BR /> <p>You can do the first part by specifying only two integer arguments when you execute <BR /> the decoder. If you get the first and second keys right, a message that starts with <BR /> <tt>From:</tt> will appear. This message is not the true message, but a decoy. It <BR /> is useful, however, to let you know that you have indeed broken the first two keys.</p> <BR /> <p>In breaking the first two keys, realize that the function <code>process_keys12</code> <BR /> must be somehow changing the value of the <code>dummy</code> variable. This must <BR /> be so, because the variables <code>start</code> and <code>stride</code> control <BR /> the extraction of the message, and they are calculated from the value of <code>dummy</code>.</p> <BR /> <p>In breaking the third and fourth keys, try to get the code to invoke <code>extract_message2</code> <BR /> <em>instead</em> of <code>extract_message1</code>. This modification must somehow <BR /> be controlled from within the function <code>process_keys34</code>.</p> <BR /> <h3>Files</h3> <BR /> <p>When you are done, write a brief report that includes at least the following:</p> <BR /> <ol> <BR />   <li>The secret message. </li> <BR />   <li>The secret keys. </li> <BR />   <li>One paragraph describing, in your own prose, what <code>process_keys12</code> <BR />   does. For example, you might say that it modifies a specific program variable. <BR />   </li> <BR />   <li>The meaning of the first two keys in terms of variables and addresses in <BR />   the decoder program. For example, you might describe <code>key2</code> by saying <BR />   that its X-Y bits contain the value to which variable <code>start</code> is <BR />   set. Or you might describe <code>key1</code> by saying, for example, that it <BR />   must be set equal to the number of memory addresses separating the address of <BR />   two specific variables. These are only examples. </li> <BR />   <li>One paragraph describing, in your own prose, what <code>process_keys34</code> <BR />   does. </li> <BR />   <li>One paragraph describing the line of source code that is executed when the <BR />   first call to <code>process_keys34</code> returns. </li> <BR />   <li>The meaning of the third and fourth keys in terms of variables and addresses <BR />   in the decoder program. </li> <BR /> </ol> <BR /> Be precise, clear, and brief in each of the points above. Your report should not, <BR /> in any case, be longer than one page. Do not get frustrated if this takes a little <BR /> longer than you expected: brief and clear text often requires more time to write <BR /> than rambling prose. <BR /> <p>Your teacher can tell you what word processors you may use to write your report. <BR /> Chances are that you can write your report in a number of formats, and for simplicity&#39;s <BR /> sake, you might even want to write it using Notepad</p> <BR />
[Ctrl+A 全部选择 提示：你可先修改部分代码，再按运行]

用到的文件：
 secret.cpp

提交的报告：
 

引用:

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The secret message.
From: CTE
To: You
Excellent!You got everything!

The secret keys.
3 777 -1 45

One paragraph describing, in your own prose, what process_keys12 does. For example, you might say that it modifies a specific program variable.
The function process_keys12 modifies the value of dummy. And then let dummy modifies the value of start and stride.

The meaning of the first two keys in terms of variables and addresses in the decoder program. For example, you might describe key2 by saying that its X-Y bits contain the value to which variable start is set. Or you might describe key1 by saying, for example, that it must be set equal to the number of memory addresses separating the address of two specific variables. These are only examples.
The function process_keys12 is used to modify the value of dummy. So from *(key1 + *key1) = *key2; in the decoder program, we can know that *(key1 + *key1) is the address of dummy and *key2 is the value. Then we can get key1 and key2.

One paragraph describing, in your own prose, what process_keys34 does.
The function process_keys34 modifies the address to which this function returns. Because the function must jump over the sentence: msg1 = extract_message1(start, stride); for containing the value of msg1[0] \0.

One paragraph describing the line of source code that is executed when the first call to process_keys34 returns.
The line of source code msg2 = extract_message2(start, stride); is executed when the first call to process_keys34 returns.

The meaning of the third and fourth keys in terms of variables and addresses in the decoder program.
The function process_keys34 is used to modify the address to which this function returns. So from *(((int *)&key3) + *key3) += *key4;, we can know that *(((int *)&key3) + *key3) is the address in which there is the address this function will returns. *key4 is the times of length of integer the function will jump over.

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作业反馈：
 

引用:

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The Description of process_keys12
Score: 10/12
sets the value of start and stride based on the first and second bytes of the value of dummy.

The Meaning of Key1
Score: 5/12
key1 is the offset of dummy's location from key1's location.

The Meaning of Key3
Score: 3/7
key3 is the offset between between key3's location and the return address in process_keys34.


The Meaning of Key4
Score: 2/7
key4 is the distance between the return address of the second invocation of process_keys34 and the first invocation of process_keys34.

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