[ACTF新生赛2020]usualCrypt

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  1. 用IDA打开
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  1. 现将混淆复原

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    #include<stdio.h>
    #include<string.h>
    #include<stdlib.h>
    int main(){
    	char byte_40E0AA[] = "KLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    	char byte_40E0A0[] = "ABCDEFGHIJ";
    	int result; // eax
        char v1; // cl

      for ( result = 6; result < 15; ++result )
      {
        v1 = byte_40E0AA[result];
        byte_40E0AA[result] = byte_40E0A0[result];
        byte_40E0A0[result] = v1;
      }
    	
    	printf("%s\n",byte_40E0A0);
    	printf("%s", byte_40E0AA);
    }

    发现一个问题，byte_40E0A0[]只有10到不来15，所以要改进一下

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    #include<stdio.h>
    #include<string.h>
    #include<stdlib.h>
    int main(){
    	char byte_40E0AA[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    	int result; // eax
        char v1; // cl

      for ( result = 6; result < 15; ++result )
      {
        v1 = byte_40E0AA[result];
        byte_40E0AA[result] = byte_40E0AA[result+10];
        byte_40E0AA[result+10] = v1;
      }
    	printf("%s", byte_40E0AA);
    } 
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    ABCDEFQRSTUVWXYPGHIJKLMNOZabcdefghijklmnopqrstuvwxyz0123456789+/

  2. 将大小写复原

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    #include<stdio.h>
    #include<string.h>
    #include<stdlib.h>
    int main(){
    	char v2[] = "zMXHz3TIgnxLxJhFAdtZn2fFk3lYCrtPC2l9" ;
    	int i,j;
    	char v1[strlen(v2)];
    	for(i = 0;i < strlen(v2); i++){
    		if ( (v2[i] < 97 || v2[i] > 122) && (v2[i] < 65 || v2[i] > 90) )
          {
            
              v1[i] = v2[i];
          }
          else if(v2[i] < 97 || v2[i] > 122){
          	v1[i] = v2[i] + 32;
    	  }
          else
          {
                v1[i] = v2[i] - 32;
          }
      }
    	
    	printf("%s",v1);
    } 
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    ZmxhZ3tiGNXlXjHfaDTzN2FfK3LycRTpc2L9

    1. 最后用在线网站解码

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      flag{bAse64_h2s_a_Surprise}