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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
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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 mainimport (
"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 mainimport (
"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
被SleepAlone编辑
，原因：
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