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[原创]长城杯初赛wp xiran_encrypto
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发表于: 2024-10-7 14:35 5961
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该题为正常ctf题与恶意脚本相结合的题目,cha为常规的re题目,clickme为恶意样本,在cha中拿到信息,然后根据clickme的逻辑解密
cha的核心为比较经过换表base64然后与自己xor之后与内存中的32字节相比较
求解思路:
爆破脚本:
clickme为babyuk家族,主要加密逻辑也类似
核心加密逻辑在main_encrypt_file中
解密脚本:
cha中的私钥如果纯静态的分析 只有爆破才行 能不能在cha运行时把 私钥dump出来?
在尝试的过程中发现cha中,在.init_array中存在一个特殊的初始化函数,在这个函数中做了两个(或者一个)反调试:
在bypass这两个点之后,进入main之后 发现私钥对应的内存实际为0,这也和在ida中找不到它的xref呼应,我觉得这里作者的意思就是故意让程序运行不到main,所以正确做法只能爆破,不然直接进内存dump太简单了。
convert_table
=
b
'APet8BQfu9CRgv+DShw/ETixFUjyGVkzHWl0IXm1JYn2KZo3Lap4Mbq5Ncr6Ods7'
encrypted_flag
=
[
0xD9
,
0x40
,
0x6F
,
0xCA
,
0x3D
,
0x8F
,
0x53
,
0xB1
,
0x8B
,
0x34
,
0x92
,
0x8E
,
0xF7
,
0x19
,
0x94
,
0x61
,
0x68
,
0x71
,
0x55
,
0xB6
,
0xCE
,
0x5B
,
0x71
,
0x1A
,
0x79
,
0x42
,
0x9D
,
0x02
,
0x93
,
0x38
,
0xAD
,
0x1F
,
0xD3
,
0x24
,
0x48
,
0xFF
,
0x21
,
0xA2
,
0x24
,
0xBE
,
0x95
,
0x3A
,
0xC1
,
0xD2
]
def
convert(flag,index):
group
=
(index
/
/
4
)
*
3
if
index
%
4
=
=
0
:
return
convert_table[flag[group] >>
2
]
if
index
%
4
=
=
1
:
return
convert_table[(flag[group] <<
4
&
0x30
)
+
(flag[group
+
1
] >>
4
)]
if
index
%
4
=
=
2
:
return
convert_table[(flag[group
+
1
] <<
2
&
0x3c
)
+
(flag[group
+
2
] >>
6
)]
if
index
%
4
=
=
3
:
return
convert_table[flag[group
+
2
] &
0x3f
]
def
brup(flag):
cur_len
=
len
(flag)
if
cur_len !
=
0
:
res
=
flag[cur_len
-
1
] ^ convert(flag,cur_len
-
1
)
if
res !
=
encrypted_flag[cur_len
-
1
]:
return
if
cur_len
=
=
32
:
sum
=
0
for
i
in
range
(
len
(flag)):
sum
=
19
*
sum
+
flag[i]
sum
=
sum
&
0xffffffff
if
sum
=
=
0xD033A96A
:
for
i
in
flag:
print
(i,end
=
','
)
return
for
i
in
range
(
256
):
flag.append(i)
brup(flag)
flag.pop()
flag
=
bytearray()
brup(flag)
convert_table
=
b
'APet8BQfu9CRgv+DShw/ETixFUjyGVkzHWl0IXm1JYn2KZo3Lap4Mbq5Ncr6Ods7'
encrypted_flag
=
[
0xD9
,
0x40
,
0x6F
,
0xCA
,
0x3D
,
0x8F
,
0x53
,
0xB1
,
0x8B
,
0x34
,
0x92
,
0x8E
,
0xF7
,
0x19
,
0x94
,
0x61
,
0x68
,
0x71
,
0x55
,
0xB6
,
0xCE
,
0x5B
,
0x71
,
0x1A
,
0x79
,
0x42
,
0x9D
,
0x02
,
0x93
,
0x38
,
0xAD
,
0x1F
,
0xD3
,
0x24
,
0x48
,
0xFF
,
0x21
,
0xA2
,
0x24
,
0xBE
,
0x95
,
0x3A
,
0xC1
,
0xD2
]
def
convert(flag,index):
group
=
(index
/
/
4
)
*
3
if
index
%
4
=
=
0
:
return
convert_table[flag[group] >>
2
]
if
index
%
4
=
=
1
:
return
convert_table[(flag[group] <<
4
&
0x30
)
+
(flag[group
+
1
] >>
4
)]
if
index
%
4
=
=
2
:
return
convert_table[(flag[group
+
1
] <<
2
&
0x3c
)
+
(flag[group
+
2
] >>
6
)]
if
index
%
4
=
=
3
:
return
convert_table[flag[group
+
2
] &
0x3f
]
def
brup(flag):
cur_len
=
len
(flag)
if
cur_len !
=
0
:
res
=
flag[cur_len
-
1
] ^ convert(flag,cur_len
-
1
)
if
res !
=
encrypted_flag[cur_len
-
1
]:
return
if
cur_len
=
=
32
:
sum
=
0
for
i
in
range
(
len
(flag)):
sum
=
19
*
sum
+
flag[i]
sum
=
sum
&
0xffffffff
if
sum
=
=
0xD033A96A
:
for
i
in
flag:
print
(i,end
=
','
)
return
for
i
in
range
(
256
):
flag.append(i)
brup(flag)
flag.pop()
flag
=
bytearray()
brup(flag)
package main
import
(
"crypto/sha256"
"os"
"golang.org/x/crypto/chacha20"
"golang.org/x/crypto/curve25519"
)
func main() {
file
, err :
=
os.OpenFile(
"flag.png.xiran"
,os.O_RDWR,
0
)
if
err !
=
nil {
panic(err)
}
fileinfo,err :
=
file
.Stat()
if
err !
=
nil {
panic(err)
}
filesize :
=
fileinfo.Size()
var start int64
=
0
end :
=
filesize
privateKeyBob :
=
[
32
]byte{
144
,
16
,
46
,
165
,
100
,
230
,
124
,
220
,
241
,
66
,
166
,
239
,
164
,
119
,
237
,
82
,
49
,
23
,
47
,
236
,
139
,
107
,
73
,
98
,
43
,
49
,
237
,
80
,
249
,
117
,
245
,
115
}
publicKeyAlice :
=
make([]byte,
32
,
32
)
offset,err :
=
file
.Seek(filesize
-
32
-
6
,
0
)
if
err !
=
nil {
panic(err)
}
file
.ReadAt(publicKeyAlice,offset)
var sharedKeyAlice [
32
]byte
curve25519.ScalarMult(&sharedKeyAlice, &privateKeyBob, (
*
[
32
]byte)(publicKeyAlice))
var key [
32
]byte
=
sha256.Sum256(sharedKeyAlice[:])
var res [
32
]byte
=
sha256.Sum256(key[:])
nonce :
=
res[
10
:
22
]
cipher, err :
=
chacha20.NewUnauthenticatedCipher(key[:],nonce)
if
err !
=
nil {
panic(err)
}
buffer
:
=
make([]byte,
0x100000
,
0x100000
)
for
start < end {
file
.ReadAt(
buffer
,start)
cipher.XORKeyStream(
buffer
,
buffer
)
file
.WriteAt(
buffer
,start)
start
+
=
0xA00000
}
if
err :
=
file
.Close(); err !
=
nil {
panic(err)
}
}
package main
import
(
"crypto/sha256"
"os"
"golang.org/x/crypto/chacha20"
"golang.org/x/crypto/curve25519"
)
func main() {
file
, err :
=
os.OpenFile(
"flag.png.xiran"
,os.O_RDWR,
0
)
if
err !
=
nil {
panic(err)
}
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最后于 2024-10-7 14:38
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wx_Dispa1r
非常支持你的观点!
2024-10-29 18:47
東陽不列山
+1
感谢你的贡献,论坛因你而更加精彩!
2024-10-8 01:36
一笑人间万事
感谢你的贡献,论坛因你而更加精彩!
2024-10-8 01:09
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