Rating:
**(This challenge isn't available in picoGYM).**
### Intended Solution:
To generate the secret keys from both Alice and Bob in diffie-hellman we have to perform the following operations:
```
A = g^a mod p
B = g^b mod p
Key1 = B^a mod p
Key2 = A^b mod p
```
Having the key we use it to perform a shift in the cipher text.
I created a script to solve the challenge
```python
import string
flag_enc = "H98A9W_H6UM8W_6A_9_D6C_5ZCI9C8I_AJ8H7JJ7"
alphabet = string.ascii_uppercase + "0123456789"
p = 13
g = 5
a = 7
b = 3
def diffie_hellman_private_key(p,g,a,b):
A = pow(g,a,p)
B = pow(g,b,p)
S1 = pow(B,a,p)
S2 = pow(A,b,p)
return S1, S2
def caesar_shift(shift, cipher):
plain_text = ""
for c in cipher:
if c == '_' or c == ' ':
plain_text += c
else:
pos = alphabet.index(c)
pos = (pos + shift) % 36
plain_text += alphabet[pos]
return plain_text
S = diffie_hellman_private_key(p,g,a,b)
print(caesar_shift(-S[0],flag_enc))
```
```shell
❯ python solve.py
C4354R_C1PH3R_15_4_817_0U7D473D_5E3C2EE2
```
**The flag is: picoCTF{C4354R_C1PH3R_15_4_817_0U7D473D_5E3C2EE2}**
### Unintended Solution:
Since the cipher text is encoded in a caesar cipher we could just bruteforce it to get the flag, being the shift of only 5.
