Typing in sample code was all the rage back when I first learned assembly language. The programs were short. Most of them I could understand based on my limited knowledge, but I still enjoyed seeing the result and wondering how they did it. Hopefully, I’d learn something.
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\nOne such program I typed in completely surprised me. The output displayed wasn’t to be found anywhere inside the code. Somehow, the programmer was able to get a text message generated from what looked like random data. It took me a while, but eventually I gleaned what was going on.<\/p>\n
The sample code below works along a similar principle to the assembly language code I looked at years ago. You see an integer array of what appears to be various positive and negative values.<\/p>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n int x, c;\r\n signed int delta[] = {\r\n 72, 29, 7, 0, 3, -79, 70, 12, -3, -2,\r\n -77, 36, 29, 13, -78, 39, 40, 0, -4, -2,\r\n 5, -77, -23, -10\r\n };\r\n\r\n x = c = 0;\r\n while(x<24)\r\n {\r\n c += delta[x];\r\n putchar(c);\r\n x++;\r\n }\r\n return(0);\r\n}<\/pre>\nThe values in the delta[]<\/code> array aren’t ASCII codes. Yet, when you run the program, you see the following text output:<\/p>\nHello from Dan Gookin!<\/code><\/p>\nWhat happens in the while<\/em> loop is that values are read from the delta[]<\/code> array and output via a putchar()<\/em> function at Line 16. The first value is an ASCII code, 72, for the letter H<\/code>. Instead of the next element being an ASCII code, however, its value is added to the current ASCII value. Or, in the case of a negative number, the value is subtracted.<\/p>\nThe effect is that 29 (the second element in the delta[]<\/code> array) is added to 72, yielding 101 or the ASCII code for the letter e<\/code>. The next element, adds 7 to that value, which becomes letter l<\/code>. That’s the way the string is processed, one character after another.<\/p>\nI’m not certain whether this method was the one used as an example in Assembly language years ago, nor is it any form of clever encryption. In fact, the only conundrum I faced was how to tell when the string was terminated. Rather than use some value such as 1000, I just counted the bytes in the string and used that value to terminate output; (x<24)<\/code> at Line 13.<\/p>\nThis program is actually the second one I wrote. After all, you need a way to convert a string of text into positive and negative values. To do that I wrote a program that swallows a string and then outputs the values added and subtracted. Next week’s Lesson<\/a> details how the program works.<\/p>\n","protected":false},"excerpt":{"rendered":"Since I first learned to program, I’ve been fascinated by programs that hide text. Continue reading