Tags: rot base64 vigenere rsa
Rating:
# Left Foot Two Stomps
## Description
> n=960242069 e=347
> c=346046109,295161774,616062960,790750242,259677897,945606673,321883599,625021022,731220302,556994500,118512782,843462311,321883599,202294479,725148418,725148418,636253020,70699533,475241234,530533280,860892522,530533280,657690757,110489031,271790171,221180981,221180981,278854535,202294479,231979042,725148418,787183046,346046109,657690757,530533280,770057231,271790171,584652061,405302860,137112544,137112544,851931432,118512782,683778547,616062960,508395428,271790171,185391473,923405109,227720616,563542899,770121847,185391473,546341739,851931432,657690757,851931432,284629213,289862692,788320338,770057231,770121847
## Solution
We recognize a RSA public key, with a very small `n` which will be easy to factorize.
```python
n = 960242069
e = 347
cipher = [346046109,295161774,616062960,790750242,259677897,945606673,321883599,625021022,731220302,556994500,118512782,843462311,321883599,202294479,725148418,725148418,636253020,70699533,475241234,530533280,860892522,530533280,657690757,110489031,271790171,221180981,221180981,278854535,202294479,231979042,725148418,787183046,346046109,657690757,530533280,770057231,271790171,584652061,405302860,137112544,137112544,851931432,118512782,683778547,616062960,508395428,271790171,185391473,923405109,227720616,563542899,770121847,185391473,546341739,851931432,657690757,851931432,284629213,289862692,788320338,770057231,770121847]
for p in range(3,n,2):
if n % p == 0:
break
q = n // p
print(p, q)
def egcd(a, b):
if a == 0:
return (b, 0, 1)
else:
g, y, x = egcd(b % a, a)
return (g, x - (b // a) * y, y)
def modinv(a, m):
g, x, y = egcd(a, m)
if g != 1:
raise Exception('modular inverse does not exist')
else:
return x % m
d = modinv(e, (p-1)*(q-1))
print(d)
def pow(a, x, n):
if x == 0:
return 1
if x == 1:
return a%n
s = pow(a,x//2,n)
s *= s
if x%2 == 1:
s *= a
return s%n
plaintext = [pow(a, d, n) for a in cipher]
print("".join([chr(c) for c in plaintext]))
```
This gives us the following string:
```
xhBQCUIcbPf7IN88AT9FDFsqEOOjNM8uxsFrEJZRRifKB1E=|key=visionary
```
This is a Vigenere cipher and the key is given, so we can give it to [dCode](https://www.dcode.fr/chiffre-vigenere).
Output: `czJIOHIldUx7QF88MG9FMHxiMGAwNV8wckNjQWZATnxST1Q=`
The `=` in the end looks like `base64` padding, we decode it with [asciitohex](https://www.asciitohex.com/). Output: ``s2H8r%uL{@_<0oE0|b0`05_0rCcAf@N|ROT``.
This is a ROT cipher, we once again give it to [dCode](https://www.dcode.fr/chiffre-rot).
Flag: `DawgCTF{Lo0k_@t_M3_1_d0_Cr4p7o}`
