Tags: vigener
Rating:
# Vigenere (crypto 100)
###ENG
[PL](#pl-version)
In the task we get a ciphertext:
```
LMIG}RPEDOEEWKJIQIWKJWMNDTSR}TFVUFWYOCBAJBQ
```
And information that this is Vigener Cipher with alphabet:
```
ABCDEFGHIJKLMNOPQRSTUVWXYZ{}
```
And the md5 of plaintext is `f528a6ab914c1ecf856a1d93103948fe`
We of course know the flag prefix `SECCON{` so we can instantly recover the prefix of the key:
```python
def get_key_prefix(alphabet, ct, known_pt):
result = ""
for i in range(len(known_pt)):
plain = known_pt[i]
cipher = ct[i]
key = alphabet[alphabet.index(cipher) - alphabet.index(plain)]
result += key
return result
```
which gives us `VIGENER`
Next we can just brute-force the missing 4 bytes of the key:
```python
def decode(alphabet, ct, key):
result = ""
for i in range(len(ct)):
c = ct[i]
k = key[i % len(key)]
if k != "?":
p = alphabet[alphabet.index(c) - alphabet.index(k)]
else:
p = "?"
result += p
return result
def worker(data):
c, alphabet, ct, key = data
key += c
for suffix in itertools.product(alphabet, repeat=4):
new_key = key + "".join(suffix)
pt = decode(alphabet, ct, new_key)
if hashlib.md5(pt).hexdigest() == "f528a6ab914c1ecf856a1d93103948fe":
print(pt)
return pt
def main():
alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ{}"
ct = "LMIG}RPEDOEEWKJIQIWKJWMNDTSR}TFVUFWYOCBAJBQ"
key_prefix = get_key_prefix(alphabet, ct, "SECCON{")
print('key prefix ', key_prefix)
print(brute(worker, [(c, alphabet, ct, key_prefix) for c in alphabet]))
if __name__ == '__main__':
freeze_support()
main()
```
Which gives us almost instantly `SECCON{ABABABCDEDEFGHIJJKLMNOPQRSTTUVWXYYZ}`
###PL version
W zadaniu dostajemy zaszyfrowany tekst:
```
LMIG}RPEDOEEWKJIQIWKJWMNDTSR}TFVUFWYOCBAJBQ
```
I informacje że to szyfr Vigenera z alfabetem:
```
ABCDEFGHIJKLMNOPQRSTUVWXYZ{}
```
Mamy też md5 plaintextu: `f528a6ab914c1ecf856a1d93103948fe`
I oczywiście znamy prefix flagi `SECCON{` więc możemy od razu odzyskać prefix klucza:
```python
def get_key_prefix(alphabet, ct, known_pt):
result = ""
for i in range(len(known_pt)):
plain = known_pt[i]
cipher = ct[i]
key = alphabet[alphabet.index(cipher) - alphabet.index(plain)]
result += key
return result
```
co daje nam `VIGENER`
Następnie możemy brute-forcować brakujące 4 bajty klucza:
```python
def decode(alphabet, ct, key):
result = ""
for i in range(len(ct)):
c = ct[i]
k = key[i % len(key)]
if k != "?":
p = alphabet[alphabet.index(c) - alphabet.index(k)]
else:
p = "?"
result += p
return result
def worker(data):
c, alphabet, ct, key = data
key += c
for suffix in itertools.product(alphabet, repeat=4):
new_key = key + "".join(suffix)
pt = decode(alphabet, ct, new_key)
if hashlib.md5(pt).hexdigest() == "f528a6ab914c1ecf856a1d93103948fe":
print(pt)
return pt
def main():
alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ{}"
ct = "LMIG}RPEDOEEWKJIQIWKJWMNDTSR}TFVUFWYOCBAJBQ"
key_prefix = get_key_prefix(alphabet, ct, "SECCON{")
print('key prefix ', key_prefix)
print(brute(worker, [(c, alphabet, ct, key_prefix) for c in alphabet]))
if __name__ == '__main__':
freeze_support()
main()
```
Co od razu daje nam `SECCON{ABABABCDEDEFGHIJJKLMNOPQRSTTUVWXYYZ}`
