SHELL CTF 2022
A beginner-friendly CTF, Hosted By S.H.E.L.L
We got the 11 position on the scoreboard under team name 0xCha0s
One
is this challenge we have a binary file and after decompilation, we got the encryption algorithm
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int __cdecl main(int argc, const char **argv, const char **envp)
{
int v4; // [rsp+Ch] [rbp-604h]
int i; // [rsp+10h] [rbp-600h]
int j; // [rsp+14h] [rbp-5FCh]
int v7; // [rsp+18h] [rbp-5F8h]
int v8; // [rsp+1Ch] [rbp-5F4h]
int v9; // [rsp+20h] [rbp-5F0h]
int v10; // [rsp+24h] [rbp-5ECh]
int k; // [rsp+28h] [rbp-5E8h]
int v12; // [rsp+2Ch] [rbp-5E4h]
int m; // [rsp+30h] [rbp-5E0h]
int v14; // [rsp+34h] [rbp-5DCh]
int v15; // [rsp+34h] [rbp-5DCh]
int v16; // [rsp+38h] [rbp-5D8h]
int n; // [rsp+3Ch] [rbp-5D4h]
int v18[197]; // [rsp+40h] [rbp-5D0h] BYREF
char s1[8]; // [rsp+356h] [rbp-2BAh] BYREF
__int16 v20; // [rsp+35Eh] [rbp-2B2h]
char s[352]; // [rsp+360h] [rbp-2B0h] BYREF
__int64 v22[37]; // [rsp+4C0h] [rbp-150h] BYREF
int v23; // [rsp+5E8h] [rbp-28h]
unsigned __int64 v24; // [rsp+5F8h] [rbp-18h]
v24 = __readfsqword(0x28u);
__isoc99_scanf(&unk_2020, s, envp);
v4 = 0;
for ( i = 0; i < strlen(s); ++i )
{
for ( j = s[i]; j > 0; j /= 2 )
s[v4++ + 48] = j % 2 + 48;
while ( (v4 & 7) != 0 )
s[v4++ + 48] = 48;
}
s[v4 + 48] = 0;
v7 = 2;
memset(v22, 0, sizeof(v22));
v23 = 0;
v8 = 0;
v9 = 0;
while ( v9 < v4 )
{
*(_QWORD *)s1 = 0LL;
v20 = 0;
v10 = 0;
for ( k = 0; k < v7 && v4 > v9 + k; ++k )
{
s1[k] = s[v9 + 48 + k];
++v10;
}
switch ( v10 )
{
case 1:
if ( !strcmp(s1, "0") )
{
*((_BYTE *)v22 + v8) = 97;
}
else if ( !strcmp(s1, "1") )
{
*((_BYTE *)v22 + v8) = 98;
}
break;
case 2:
if ( !strcmp(s1, "00") )
{
*((_BYTE *)v22 + v8) = 99;
}
else if ( !strcmp(s1, "01") )
{
*((_BYTE *)v22 + v8) = 100;
}
else if ( !strcmp(s1, "10") )
{
*((_BYTE *)v22 + v8) = 101;
}
else if ( !strcmp(s1, "11") )
{
*((_BYTE *)v22 + v8) = 102;
}
break;
case 3:
if ( !strcmp(s1, "000") )
{
*((_BYTE *)v22 + v8) = 49;
}
else if ( !strcmp(s1, "001") )
{
*((_BYTE *)v22 + v8) = 50;
}
else if ( !strcmp(s1, "010") )
{
*((_BYTE *)v22 + v8) = 51;
}
else if ( !strcmp(s1, "011") )
{
*((_BYTE *)v22 + v8) = 52;
}
else if ( !strcmp(s1, "100") )
{
*((_BYTE *)v22 + v8) = 53;
}
else if ( !strcmp(s1, "101") )
{
*((_BYTE *)v22 + v8) = 54;
}
else if ( !strcmp(s1, "110") )
{
*((_BYTE *)v22 + v8) = 55;
}
else if ( !strcmp(s1, "111") )
{
*((_BYTE *)v22 + v8) = 56;
}
break;
default:
*((_BYTE *)v22 + v8) = 57;
break;
}
v9 += v10;
++v8;
v7 = (v7 + 1) % 4;
}
v12 = 0;
for ( m = 0; m < v8; m += 2 )
{
if ( *((char *)v22 + m + 1) <= 47 || *((char *)v22 + m + 1) > 57 )
v14 = *((char *)v22 + m + 1) - 97;
else
v14 = *((char *)v22 + m + 1) - 48;
if ( *((char *)v22 + m) <= 47 || *((char *)v22 + m) > 57 )
v15 = 16 * (*((char *)v22 + m) - 97) + v14;
else
v15 = 16 * (*((char *)v22 + m) - 48) + v14;
v18[v12++ + 96] = v15;
}
qmemcpy(v18, "R", 0x174uLL);
v16 = 0;
for ( n = 0; n <= 78; ++n )
{
if ( v18[n] == v18[n + 96] )
++v16;
}
if ( v16 == 79 )
puts("you're good at this!");
else
puts("nope, that's not it.");
return 0;
}
that by create a reveres script for it you got the flag
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arr = [0x52,0x91,0x41,0x91,0x36,0x90,0x44,0x90,0x27,0x91,0x42,0x91,0x36,0x91,0x24,0x90,0x26,0x91,0x44,0x90,0x36,0x91,0x38,0x90,0x52,0x91,0x41,0x90,0x52,0x90,0x52,0x90,0x45,0x91,0x48,0x91,0x45,0x91,0x24,0x90,0x26,0x91,0x27,0x90,0x46,0x91,0x27,0x90,0x58,0x90,0x47,0x90,0x35,0x90,0x27,0x90,0x37,0x91,0x44,0x90,0x46,0x90,0x44,0x90,0x32,0x91,0x46,0x90,0x52,0x90,0x27,0x90,0x57,0x91,0x44,0x91,0x36,0x90,0x47,0x90,0x58,0x90,0x42,0x90,0x52,0x91,0x56,0x90,0x46,0x90,0x46,0x91,0x54]
lookup_table = {
97:"0",
98:"1",
99:"00",
100:"01",
101:"10",
102:"11",
49:"000",
50:"001",
51:"010",
52:"011",
53:"100",
54:"101",
55:"110",
56:"111",
}
c = 2
bits = ''
for i in arr:
m = [i // 16, i % 16]
for j in range(2):
if c == 2:
m[j] = m[j] + 97
elif c == 3:
m[j] = m[j] + 48
elif c == 1:
m[j] = m[j] + 97
elif c == 0:
m[j] = 57
c = (c + 1) % 4
continue
bits += lookup_table[m[j]]
c = (c + 1) % 4
flag_bytes = [bits[i:i+8] for i in range(0, len(bits), 8)]
flag = ''.join(chr(int(i[::-1],2)) for i in flag_bytes)
print(flag)
flag shellctf{s0Me_b4S3_c0nVer51on5_4_U}