Tags: encoding crypto
Rating:
# Baby Encoder
Here is the challenge code and Description
```
I encoded the flag with my own encoder. I'm sure you can decode it because my encoder is just a baby encoder!
```
```python
from Crypto.Util.number import bytes_to_long
def displace(a, base):
res = []
for i in range(base):
if base + i >= len(a):
for j in range(base - 1, i - 1, -1):
res.append(a[j])
return res
res.append(a[base + i])
res.append(a[i])
for j in range(len(a) - 1, 2 * base - 1, -1):
res.append(a[j])
return res
def flag_encoder(flag):
encoded_flag = []
n = len(flag)
f = [ord(ff) for ff in flag]
for i in range(n):
encoded_flag.append(ord(flag[i]) ^ ord(flag[i - 1]))
for i in range(n):
encoded_flag[i] ^= encoded_flag[n - i - 1]
a = []
for i in range(0, n, 3):
a.append(encoded_flag[i] + encoded_flag[i + 1])
a.append(encoded_flag[i + 1] + encoded_flag[i + 2])
a.append(encoded_flag[i + 2] + encoded_flag[i])
encoded_flag = a
for i in range(1, n):
if i % 6 == 0:
encoded_flag = displace(encoded_flag, i)
encoded_flag = ''.join(chr(encoded_flag[i]) for i in range(n))
return encoded_flag
with open('/home/kourosh/CTF/TMU/crypto/BabyEncoder/flag', 'rb') as f:
flag = f.read().decode('UTF-8')
print(str(bytes_to_long(flag_encoder(flag).encode())))
```
# Solution
This challenge really blowed my mind :')\
It has 4 custom encoding parts which are like below
## step 1
It `xor` every byte of the flag with it's previous byte
```python
encoded_flag = []
n = len(flag)
f = [ord(ff) for ff in flag]
for i in range(n):
encoded_flag.append(ord(flag[i]) ^ ord(flag[i - 1]))
```
to reverse this step I wrote this code
```python
def decode3(data):
last = ord('}')
size = len(data)
new1 = []
for i in range(size-1, -1, -1):
last = last ^ data[i]
new1.append(last)
new1 = reverse(new1)
new2 = []
for i in range(size):
new2.append(new1[(i+1)%size])
return new2
```
## step 2
This step `xor` every byte of the previous steps output with its mirror bytes `(0 with n-1)(1 with n-2)` and ...
```python
for i in range(n):
encoded_flag[i] ^= encoded_flag[n - i - 1]
```
To reverse this step I wrote this code
```python
def decode2(data):
size = len(data)
for i in range(size-1, -1, -1):
data[i] ^= data[size - i - 1]
return(data)
```
## step 3
This step seperate previous steps output into 3 blocks and sum each block with it's next block
```python
a = []
for i in range(0, n, 3):
a.append(encoded_flag[i] + encoded_flag[i + 1])
a.append(encoded_flag[i + 1] + encoded_flag[i + 2])
a.append(encoded_flag[i + 2] + encoded_flag[i])
encoded_flag = a
```
And here is the reversed code to recover
```python
def decode1(data):
size = len(data)
new = []
for i in range(0, size, 3):
a0 = (data[i] - data[i+1] + data[i+2]) // 2
a1 = (data[i+1] - data[i+2] + data[i]) // 2
a2 = (data[i+2] - data[i] + data[i+1]) // 2
new.append(a0)
new.append(a1)
new.append(a2)
return(new)
```
## step 4
Here is the code
```python
def displace(a, base):
res = []
for i in range(base):
if base + i >= len(a):
for j in range(base - 1, i - 1, -1):
res.append(a[j])
return res
res.append(a[base + i])
res.append(a[i])
for j in range(len(a) - 1, 2 * base - 1, -1):
res.append(a[j])
return res
for i in range(1, n):
if i % 6 == 0:
encoded_flag = displace(encoded_flag, i)
```
Actually I didn't realize what this function really does (LOL) because I had limited time to put on it to understand\
But the thing I know is that it just changes bytes positions without any change in bytes values.\
As I saw in previous CTFs These functions usually reaches the same state if we repeat them\
So I used the same function with try and error numbers to find exact iteration number
```python
def displace(a, base):
res = []
for i in range(base):
if base + i >= len(a):
for j in range(base - 1, i - 1, -1):
res.append(a[j])
return res
res.append(a[base + i])
res.append(a[i])
for j in range(len(a) - 1, 2 * base - 1, -1):
res.append(a[j])
return res
test = [ord(f) for f in long_to_bytes(28946494946812141829547706026065914605092406854105997612241563383442514740934913838546119691331952671988567947306226900850151388621540356510466883510328793101483278519506803779932615196763052658252298923048223762802716830885754352726914690907223838594044069643833511017514637891042919056).decode()]
for i in range(1, 30):
test = displace(test, 72)
for i in range(1, 310):
test = displace(test, 66)
for i in range(1, 4290):
test = displace(test, 60)
for i in range(1, 2590):
test = displace(test, 54)
for i in range(1, 37128):
test = displace(test, 48)
for i in range(1, 168):
test = displace(test, 42)
for i in range(1, 18):
test = displace(test, 36)
for i in range(1, 60):
test = displace(test, 30)
for i in range(1, 42):
test = displace(test, 24)
for i in range(1, 36):
test = displace(test, 18)
for i in range(1, 20):
test = displace(test, 12)
for i in range(1, 12):
test = displace(test, 6)
```
Because this function has been called several times with different bases(6,12,18,..,72). so we should repeat it with different bases each base has different iteration number
So here is the final overall decoding process
```python
from Crypto.Util.number import long_to_bytes
def displace(a, base):
res = []
for i in range(base):
if base + i >= len(a):
for j in range(base - 1, i - 1, -1):
res.append(a[j])
return res
res.append(a[base + i])
res.append(a[i])
for j in range(len(a) - 1, 2 * base - 1, -1):
res.append(a[j])
return res
def reverse(lst):
return [ele for ele in reversed(lst)]
def decode1(data):
size = len(data)
new = []
for i in range(0, size, 3):
a0 = (data[i] - data[i+1] + data[i+2]) // 2
a1 = (data[i+1] - data[i+2] + data[i]) // 2
a2 = (data[i+2] - data[i] + data[i+1]) // 2
new.append(a0)
new.append(a1)
new.append(a2)
return(new)
def decode2(data):
size = len(data)
for i in range(size-1, -1, -1):
data[i] ^= data[size - i - 1]
return(data)
def decode3(data):
first = ord('}')
size = len(data)
new1 = []
for i in range(size-1, -1, -1):
first = first ^ data[i]
new1.append(first)
new1 = reverse(new1)
new2 = []
for i in range(size):
new2.append(new1[(i+1)%size])
return new2
test = [ord(f) for f in long_to_bytes(28946494946812141829547706026065914605092406854105997612241563383442514740934913838546119691331952671988567947306226900850151388621540356510466883510328793101483278519506803779932615196763052658252298923048223762802716830885754352726914690907223838594044069643833511017514637891042919056).decode()]
flags = True
for i in range(1, 30):
test = displace(test, 72)
for i in range(1, 310):
test = displace(test, 66)
for i in range(1, 4290):
test = displace(test, 60)
for i in range(1, 2590):
test = displace(test, 54)
for i in range(1, 37128):
test = displace(test, 48)
for i in range(1, 168):
test = displace(test, 42)
for i in range(1, 18):
test = displace(test, 36)
for i in range(1, 60):
test = displace(test, 30)
for i in range(1, 42):
test = displace(test, 24)
for i in range(1, 36):
test = displace(test, 18)
for i in range(1, 20):
test = displace(test, 12)
for i in range(1, 12):
test = displace(test, 6)
print(test)
size = len(test)
test1 = decode1(test)
print(test1)
test2 = decode2(test1)
print(test2)
test3 = decode3(test2)
print(test3)
flag = ''.join(chr(test3[i]) for i in range(size))
print(flag)
```
Here is the output and the flag
```text
[128, 45, 131, 170, 99, 103, 183, 126, 115, 88, 91, 3, 63, 137, 92, 148, 94, 68, 162, 82, 84, 148, 142, 184, 130, 141, 111, 161, 227, 166, 105, 76, 107, 169, 122, 171, 68, 145, 91, 6, 111, 105, 214, 213, 213, 153, 144, 143, 115, 195, 134, 111, 101, 198, 155, 110, 53, 215, 193, 198, 180, 169, 179, 96, 106, 176, 234, 199, 181, 163, 136, 149, 41, 45, 40, 49, 66, 65]
[107, 21, 24, 87, 83, 16, 86, 97, 29, 0, 88, 3, 9, 54, 83, 61, 87, 7, 82, 80, 2, 95, 53, 89, 50, 80, 61, 50, 111, 116, 68, 37, 39, 109, 60, 62, 7, 61, 84, 0, 6, 105, 107, 107, 106, 76, 77, 67, 27, 88, 107, 104, 7, 94, 49, 106, 4, 110, 105, 88, 95, 85, 84, 83, 13, 93, 108, 126, 73, 88, 75, 61, 18, 23, 22, 24, 25, 41]
[41, 25, 24, 22, 23, 18, 61, 75, 88, 73, 126, 108, 93, 13, 83, 84, 85, 95, 88, 105, 110, 4, 106, 49, 94, 7, 104, 107, 88, 27, 67, 77, 76, 106, 107, 107, 105, 6, 0, 84, 59, 110, 85, 87, 7, 107, 104, 7, 111, 55, 89, 85, 87, 108, 104, 95, 91, 108, 57, 10, 88, 2, 105, 0, 59, 84, 111, 38, 73, 69, 42, 107, 2, 68, 65, 0, 12, 66]
[84, 77, 85, 67, 84, 70, 123, 48, 104, 33, 95, 51, 110, 99, 48, 100, 49, 110, 54, 95, 49, 53, 95, 110, 48, 55, 95, 52, 108, 119, 52, 121, 53, 95, 52, 95, 54, 48, 48, 100, 95, 49, 100, 51, 52, 95, 55, 48, 95, 104, 49, 100, 51, 95, 55, 104, 51, 95, 102, 108, 52, 54, 95, 95, 100, 48, 95, 121, 48, 117, 95, 52, 54, 114, 51, 51, 63, 125]
TMUCTF{0h!_3nc0d1n6_15_n07_4lw4y5_4_600d_1d34_70_h1d3_7h3_fl46__d0_y0u_46r33?}
```
```
Flag : TMUCTF{0h!_3nc0d1n6_15_n07_4lw4y5_4_600d_1d34_70_h1d3_7h3_fl46__d0_y0u_46r33?}
```
[solution code](https://github.com/KooroshRZ/CTF-Writeups/blob/main/TMU2021/Crypto/BabyEncoder/solve.py)
> KouroshRZ for **AbyssalCruelty**
