BUUCTF-RE-SimpleRev

拿到题目，是64位ELF

IDA查看

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unsigned __int64 Decry()
{
  char v1; // [rsp+Fh] [rbp-51h]
  int v2; // [rsp+10h] [rbp-50h]
  int v3; // [rsp+14h] [rbp-4Ch]
  int i; // [rsp+18h] [rbp-48h]
  int v5; // [rsp+1Ch] [rbp-44h]
  char src[8]; // [rsp+20h] [rbp-40h]
  __int64 v7; // [rsp+28h] [rbp-38h]
  int v8; // [rsp+30h] [rbp-30h]
  __int64 v9; // [rsp+40h] [rbp-20h]
  __int64 v10; // [rsp+48h] [rbp-18h]
  int v11; // [rsp+50h] [rbp-10h]
  unsigned __int64 v12; // [rsp+58h] [rbp-8h]

  v12 = __readfsqword(0x28u);
  *(_QWORD *)src = 'SLCDN';
  v7 = 0LL;
  v8 = 0;
  v9 = 'wodah';
  v10 = 0LL;
  v11 = 0;
  text = join(key3, (const char *)&v9);
  strcpy(key, key1);
  strcat(key, src);
  v2 = 0;
  v3 = 0;
  getchar();
  v5 = strlen(key);
  for ( i = 0; i < v5; ++i )
  {
    if ( key[v3 % v5] > 64 && key[v3 % v5] <= 90 )
      key[i] = key[v3 % v5] + 32;
    ++v3;
  }
  printf("Please input your flag:", src);
  while ( 1 )
  {
    v1 = getchar();
    if ( v1 == '
' )
      break;
    if ( v1 == ' ' )
    {
      ++v2;
    }
    else
    {
      if ( v1 <= 96 || v1 > 122 )
      {
        if ( v1 > 64 && v1 <= 90 )
          str2[v2] = (v1 - 39 - key[v3++ % v5] + 97) % 26 + 97;
      }
      else
      {
        str2[v2] = (v1 - 39 - key[v3++ % v5] + 97) % 26 + 97;
      }
      if ( !(v3 % v5) )
        putchar(32);
      ++v2;
    }
  }
  if ( !strcmp(text, str2) )
    puts("Congratulation!
");
  else
    puts("Try again!
");
  return __readfsqword(0x28u) ^ v12;
}

可以看到text是最终比较的结果，经过join处理后，其结果为killshadow

join:

 key经过拼接，为ADSFKNDCLS后经处理，变为小写

然后写脚本

text = 'killshadow'
key = 'adsfkndcls'
key_ = ''
v3=0
for i in range(len(key)):
        flag = ord(key[v3%len(key)])
        if flag > 64 and flag <=90:
                key_ += chr(ord(key[v3%len(key)])+32)
        v3=v3+1
ans = ''
for i in range(len(key)):
        for j in range(0,15):
                tmp=chr(ord(text[i]) - 97 + 26*j - 97 + ord(key [v3%len(key)]) + 39)
                if ord(tmp)>64 and ord(tmp) <91:
                        ans+=tmp
                        break
        v3=v3+1
print ans

得出flag为KLDQCUDFZO