Tags: reverse engineering libgmp
Rating:
## Frog Fractions 2(reversing, 5 points, 65 solves)
Turns out Frog Fractions 2 is not battletoads
We're given a 64bit elf file, let's start by disassembling it and describing important points in the program.
The program uses a library called libgmp to handle big numbers, it makes the debugging less friendly.
After analysing the binary come we come up with a replacement script in python:
```python
data = [] #scraped data array from binary
primes = [] #init primes somewhere
v11 = 1019
line = "A sample line input" #our input
def factor_print(a1):
v4 = a1
for x in range(200):
c = 0
while(v4 % primes[x] == 0):
v4 = v4/primes[x];
c = c+1
if(c != 0):
sys.stdout.write(chr(c))
print
for i in range(len(line)):
v11 = v11* pow(primes[i], ord(line[i]))
j =0
while(j < len(data)/2):
v6 = data[j*2]
v7 = data[j*2+1]
v10 = v11*v6
if(v10 % v7 == 0):
v11 = v10/v7
j = 0
else:
j = j+1
factor_print(v11)
```
While searching through the binary we stumble into two big, interesting integers: 62834...(6955 chars) and 50209...(8423 chars).
What's interesting about them is that if we run them through our `factor_print` function we get the 2 messages available:
`Nope! You need to practice your fractions!`
`Congratulations! Treat yourself to some durians!`
So we need to find a line that reproduces the second output message, we do that by reversing the function:
```python
def crack(v11):
j = 0
while(j < len(data)/2):
v7 = data[j*2]
v6 = data[j*2+1]
v10 = v11*v6
if(v10 % v7 == 0):
v11 = v10/v7
j = 0
else:
j = j+1
factor_print(v11)
```
Which gives us the desired output: `KEY{(By the way, this challenge would be much easier with a cybernetic frog brain)}`
The full source is available [here](https://gist.github.com/nazywam/0634494b4b1c0ffd46a3)
