# CSAW checker Write Up
## Details:
Jeopardy style CTF
Category: Reverse Engineering
Comments:
What's up with all the zeros and ones? Where are my letters and numbers? (This is a reversing challenge.)
## Write up:
The first thing I did was look at the encode function:
```py
def encode(plain):
d = 24
x = up(plain)
x = right(x,d)
x = down(x)
x = left(x,d)
return x
```
I saw that I would have to do these function calls in reverse to decode the string. So I then started looking into the individual functions to see which ones I could just call and which ones I would have to rewrite.
```py
def right(x,d):
x = x[d:] + x[0:d]
return x
def left(x,d):
x = right(x,len(x)-d)
return x[::-1]
```
The functions left and right just reorganized the flag based on the length and the d value so these would not need to be rewritten.
```py
def down(x):
x = ''.join(['1' if x[i] == '0' else '0' for i in range(len(x))])
return x
```
The down function just reversed the bits so that also would not need to be rewritten.
```py
def up(x):
x = [f"{ord(x[i]) << 1:08b}" for i in range(len(x))]
return ''.join(x)
```
The up function however converted the flag to bits so I would need to rewrite that. I then added the decode function to the script:
```py
def up(x):
x = [f"{ord(x[i]) << 1:08b}" for i in range(len(x))]
return ''.join(x)
def down(x):
x = ''.join(['1' if x[i] == '0' else '0' for i in range(len(x))])
return x
def right(x,d):
x = x[d:] + x[0:d]
return x
def left(x,d):
x = right(x,len(x)-d)
return x[::-1]
def encode(plain):
d = 24
x = up(plain)
x = right(x,d)
x = down(x)
x = left(x,d)
return x
def decode(enc):
d = 24
enc = left(enc,d)
enc = down(enc)
enc = right(enc,d)
# go 8 bits at a time and turn them into characters
print(''.join([chr(int(''.join(enc[i:i+7]), 2)) for i in range(0, len(enc), 8)]))
def main():
flag = "redacted"
encoded = encode(flag)
print("What does this mean?")
encoded = "1010000011111000101010101000001010100100110110001111111010001000100000101000111011000100101111011001100011011000101011001100100010011001110110001001000010001100101111001110010011001100"
decode(encoded)
if __name__ == "__main__":
main()
```
When run this printed out:
```
What does this mean?
3vers!nG_w@rm_Up}flag{r
```
The flag was:
```
flag{r3vers!nG_w@rm_Up}
```