Sekure Decrypt
The challenge files includes a core dump (core), an ELF binary (dec) and a source code (src.c) written in C. The dec binary might have crash because of segmentation fault and dumped the core.
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$ file *
core: ELF 64-bit LSB core file, x86-64, version 1 (SYSV), SVR4-style, from './dec', real uid: 0, effective uid: 0, real gid: 0, effective gid: 0, execfn: './dec', platform: 'x86_64'
dec: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=daf03fccbc32333244dc0f36e874e27457110af1, for GNU/Linux 3.2.0, with debug_info, not stripped
src.c: C source, ASCII text
The content of ‘src.c’ file is below, which will read ‘flag.enc’ file content, decrypts with AES algorithm and print the decrypted content. The decrypted content might be the flag.
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mcrypt.h>
#include <math.h>
#include <stdint.h>
#include <stdlib.h>
int encrypt(void* buffer, int buffer_len, char* IV, char* key, int key_len) {
MCRYPT td = mcrypt_module_open("rijndael-128", NULL, "cbc", NULL);
int blocksize = mcrypt_enc_get_block_size(td);
if( buffer_len % blocksize != 0 ) {
return 1;
}
mcrypt_generic_init(td, key, key_len, IV);
mcrypt_generic(td, buffer, buffer_len);
mcrypt_generic_deinit (td);
mcrypt_module_close(td);
return 0;
}
int decrypt(void* buffer, int buffer_len, char* IV, char* key, int key_len) {
MCRYPT td = mcrypt_module_open("rijndael-128", NULL, "cbc", NULL);
int blocksize = mcrypt_enc_get_block_size(td);
if( buffer_len % blocksize != 0 ){
return 1;
}
mcrypt_generic_init(td, key, key_len, IV);
mdecrypt_generic(td, buffer, buffer_len);
mcrypt_generic_deinit (td);
mcrypt_module_close(td);
return 0;
}
void* read_file(char* filename, int len) {
FILE *fp = fopen(filename, "rb");
void* data = malloc(len);
fread(data, 1, len, fp);
fclose(&fp);
return data;
}
int main(int argc, char* argv[]) // gcc src.c -o dec -lmcrypt -ggdb
{
char* IV = "AAAAAAAAAAAAAAAA";
char *key = getenv("KEY");
int keysize = 16;
char* buffer;
int buffer_len = 16;
void *ciphertext = read_file("flag.enc", buffer_len);
decrypt(ciphertext, buffer_len, IV, key, keysize);
printf("Decrypted contents: %s\n", ciphertext);
return 0;
}
Looks like we need to recreate this. But we do not have the key of AES algorithm and the cipher text.
The key can be retrieved from the core dump.
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$ strings core | grep KEY
KEY=
KEY=VXISlqY>Ve6D<{#F
In the ‘src.c’ source code above, after reading the content of the ‘flag.enc’ file, it is allocated in memory using malloc(), which means it resides in the heap. To determine the location of the heap, we can use GDB for verification.
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$ gdb -q ./dec
Reading symbols from ./dec...
(gdb) r
Starting program: /home/remnux/HTB/RE/Sekure Decrypt/dec
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7e0be85 in __GI__IO_fread (buf=0x555555559480, size=1, count=16, fp=0x0) at iofread.c:35
35 iofread.c: No such file or directory.
(gdb) info proc mapping
process 3930
Mapped address spaces:
Start Addr End Addr Size Offset objfile
0x555555554000 0x555555555000 0x1000 0x0 /home/remnux/HTB/RE/Sekure Decrypt/dec
0x555555555000 0x555555556000 0x1000 0x1000 /home/remnux/HTB/RE/Sekure Decrypt/dec
0x555555556000 0x555555557000 0x1000 0x2000 /home/remnux/HTB/RE/Sekure Decrypt/dec
0x555555557000 0x555555558000 0x1000 0x2000 /home/remnux/HTB/RE/Sekure Decrypt/dec
0x555555558000 0x555555559000 0x1000 0x3000 /home/remnux/HTB/RE/Sekure Decrypt/dec
0x555555559000 0x55555557a000 0x21000 0x0 [heap]
0x7ffff7d86000 0x7ffff7d89000 0x3000 0x0
0x7ffff7d89000 0x7ffff7dab000 0x22000 0x0 /usr/lib/x86_64-linux-gnu/libc-2.31.so
0x7ffff7dab000 0x7ffff7f23000 0x178000 0x22000 /usr/lib/x86_64-linux-gnu/libc-2.31.so
0x7ffff7f23000 0x7ffff7f71000 0x4e000 0x19a000 /usr/lib/x86_64-linux-gnu/libc-2.31.so
0x7ffff7f71000 0x7ffff7f75000 0x4000 0x1e7000 /usr/lib/x86_64-linux-gnu/libc-2.31.so
0x7ffff7f75000 0x7ffff7f77000 0x2000 0x1eb000 /usr/lib/x86_64-linux-gnu/libc-2.31.so
0x7ffff7f77000 0x7ffff7f7b000 0x4000 0x0
0x7ffff7f7b000 0x7ffff7f81000 0x6000 0x0 /usr/lib/libmcrypt.so.4.4.8
0x7ffff7f81000 0x7ffff7f9a000 0x19000 0x6000 /usr/lib/libmcrypt.so.4.4.8
0x7ffff7f9a000 0x7ffff7fa7000 0xd000 0x1f000 /usr/lib/libmcrypt.so.4.4.8
0x7ffff7fa7000 0x7ffff7fa9000 0x2000 0x2b000 /usr/lib/libmcrypt.so.4.4.8
0x7ffff7fa9000 0x7ffff7fab000 0x2000 0x2d000 /usr/lib/libmcrypt.so.4.4.8
0x7ffff7fab000 0x7ffff7fb2000 0x7000 0x0
0x7ffff7fcb000 0x7ffff7fce000 0x3000 0x0 [vvar]
0x7ffff7fce000 0x7ffff7fcf000 0x1000 0x0 [vdso]
0x7ffff7fcf000 0x7ffff7fd0000 0x1000 0x0 /usr/lib/x86_64-linux-gnu/ld-2.31.so
0x7ffff7fd0000 0x7ffff7ff3000 0x23000 0x1000 /usr/lib/x86_64-linux-gnu/ld-2.31.so
0x7ffff7ff3000 0x7ffff7ffb000 0x8000 0x24000 /usr/lib/x86_64-linux-gnu/ld-2.31.so
0x7ffff7ffc000 0x7ffff7ffd000 0x1000 0x2c000 /usr/lib/x86_64-linux-gnu/ld-2.31.so
0x7ffff7ffd000 0x7ffff7ffe000 0x1000 0x2d000 /usr/lib/x86_64-linux-gnu/ld-2.31.so
0x7ffff7ffe000 0x7ffff7fff000 0x1000 0x0
0x7ffffffde000 0x7ffffffff000 0x21000 0x0 [stack]
0xffffffffff600000 0xffffffffff601000 0x1000 0x0 [vsyscall]
In the output, it is evident that the heap memory is located immediately below the memory region allocated for the ‘dec’ ELF binary.
Now we know where the ‘flag.enc’ file content will be allocated. We can analyze the core dump using pwn tools to find the heap in the core dump.
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$ python3 -q
>>> from pwn import Corefile
>>> core = Corefile('core')
[x] Parsing corefile...
[*] '/home/remnux/HTB/RE/Sekure Decrypt/core'
Arch: amd64-64-little
RIP: 0x7fca32b0f3eb
RSP: 0x7fff92e111e0
Exe: '/home/user/Documents/RE/easy/Sekure Decrypt/release/debug/dec' (0x562e29888000)
Fault: 0x1f204
[+] Parsing corefile...: Done
>>> print('\n'.join([str(mapping) for mapping in core.mappings]))
562e29888000-562e29889000 r--p 1000 /home/user/Documents/RE/easy/Sekure Decrypt/release/debug/dec
562e29889000-562e2988a000 r-xp 1000 /home/user/Documents/RE/easy/Sekure Decrypt/release/debug/dec
562e2988a000-562e2988b000 r--p 1000 /home/user/Documents/RE/easy/Sekure Decrypt/release/debug/dec
562e2988b000-562e2988c000 r--p 1000 /home/user/Documents/RE/easy/Sekure Decrypt/release/debug/dec
562e2988c000-562e2988d000 rw-p 1000 /home/user/Documents/RE/easy/Sekure Decrypt/release/debug/dec
562e2adce000-562e2adef000 rw-p 21000
7fca32ac6000-7fca32ac9000 rw-p 3000
7fca32ac9000-7fca32aee000 r--p 25000 /usr/lib/x86_64-linux-gnu/libc-2.30.so
7fca32aee000-7fca32c66000 r-xp 178000 /usr/lib/x86_64-linux-gnu/libc-2.30.so
7fca32c66000-7fca32cb0000 r--p 4a000 /usr/lib/x86_64-linux-gnu/libc-2.30.so
7fca32cb0000-7fca32cb3000 r--p 3000 /usr/lib/x86_64-linux-gnu/libc-2.30.so
7fca32cb3000-7fca32cb6000 rw-p 3000 /usr/lib/x86_64-linux-gnu/libc-2.30.so
7fca32cb6000-7fca32cba000 rw-p 4000
7fca32cba000-7fca32cc0000 r--p 6000 /usr/lib/libmcrypt.so.4.4.8
7fca32cc0000-7fca32cd9000 r-xp 19000 /usr/lib/libmcrypt.so.4.4.8
7fca32cd9000-7fca32ce6000 r--p d000 /usr/lib/libmcrypt.so.4.4.8
7fca32ce6000-7fca32ce8000 r--p 2000 /usr/lib/libmcrypt.so.4.4.8
7fca32ce8000-7fca32cea000 rw-p 2000 /usr/lib/libmcrypt.so.4.4.8
7fca32cea000-7fca32cf1000 rw-p 7000
7fca32d12000-7fca32d13000 r--p 1000 /usr/lib/x86_64-linux-gnu/ld-2.30.so
7fca32d13000-7fca32d35000 r-xp 22000 /usr/lib/x86_64-linux-gnu/ld-2.30.so
7fca32d35000-7fca32d3d000 r--p 8000 /usr/lib/x86_64-linux-gnu/ld-2.30.so
7fca32d3d000-7fca32d3e000 rw-p 1000
7fca32d3e000-7fca32d3f000 r--p 1000 /usr/lib/x86_64-linux-gnu/ld-2.30.so
7fca32d3f000-7fca32d40000 rw-p 1000 /usr/lib/x86_64-linux-gnu/ld-2.30.so
7fca32d40000-7fca32d41000 rw-p 1000
7fff92df3000-7fff92e14000 rw-p 21000 [stack]
7fff92f4d000-7fff92f50000 r--p 3000
7fff92f50000-7fff92f51000 r-xp 1000 [vdso]
ffffffffff600000-ffffffffff601000 --xp 1000 [vsyscall]
Now we know from core dump that the heap is located at offset 0x562e2adce000 to offset 0x562e2adef000.
I had written a python script that will search the heap for each 16 byte value and AES decrypt to find potential flag.
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from pwn import Corefile
from Crypto.Cipher import AES
def extractPossibleFlag(corePath, startAddress, size, key, iv):
core = Corefile(corePath)
data = core.read(startAddress, size)
for i in range(0, len(data), 16):
chunk = data[i:i+16]
cipher = AES.new(key, AES.MODE_CBC, iv)
decrypted = cipher.decrypt(chunk)
if decrypted.startswith(b'HTB'):
return decrypted
def main():
corePath = 'core'
startAddress = 0x562e2adce000
size = 2000
key = b'VXISlqY>Ve6D<{#F'
iv = b'A' * 16
flag = extractPossibleFlag(corePath, startAddress, size, key, iv)
print(flag)
if __name__ == "__main__":
main()
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$ python3 ./decode.py
[+] Parsing corefile...: Done
[*] '/home/remnux/HTB/RE/Sekure Decrypt/core'
Arch: amd64-64-little
RIP: 0x7fca32b0f3eb
RSP: 0x7fff92e111e0
Exe: '/home/user/Documents/RE/easy/Sekure Decrypt/release/debug/dec' (0x562e29888000)
Fault: 0x1f204
b'HTB{t1m_l3arn_C}'
This post is licensed under CC BY 4.0 by the author.