decrypt.c

解密算法,通过一组数学公式的计算,把加密后的数据还原

/*
*  Name:    $RCSfile$
*
*  Version: $Revision$
*
*  Created by: chensd
*
*  Purpose: This is a C language practice program
*
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*  Copyright (c) $Date$ Unication Co., Ltd.
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*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

/*
*  Name:       main
*
*  Purpose:    program entry
*
*  Params:     refer to the usage
*
*  Return:     >0 - succeed
*              -1 - met error
*
*  Note:       none
*
*/
int main(int argc, char**argv)
{
    char value[10];
    int data[4];
    int dataValue;
    int strLength;
    int temp;
    int i;
    char *p;

	/*input the value*/
    printf("Please input four integer:\n");
    if(scanf("%s", value) != 1){
		printf( "bad input!\n" );
		return -1;
	}

	/*judge the value have four character*/
    strLength = strlen(value);
    if(strLength != 4){
        printf("The input number is wrong!\n");
		return -1;
    }

    p = value;
    while(*p != '\0'){
		if(isdigit(*p) == 0){
			printf("The input number is wrong!\n");
			return -1;
		}
        p++;
    }

	/*pick-up each number*/
    dataValue = atoi(value);
    data[0] = dataValue/1000;
    data[1] = dataValue%1000/100;
    data[2] = dataValue%1000%100/10;
    data[3] = dataValue%1000%100%10;

	/*change the number position*/
    temp = data[0];
    data[0] = data[2];
    data[2] = temp;
	
    temp = data[1];
    data[1] = data[3];
    data[3] = temp;
    
	/*subtract 7 and output the result*/
    printf("The result is:\n");
    for(i=0; i<4; i++){
        data[i] -= 7;
        if(data[i]<0){
			data[i] += 10;
        }
		printf("%d", data[i]);
    }
   printf("\n");
   
   return 0;
}