
**(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 pB = g^b mod pKey1 = B^a mod pKey2 = 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
```pythonimport stringflag_enc = "H98A9W_H6UM8W_6A_9_D6C_5ZCI9C8I_AJ8H7JJ7"alphabet = string.ascii_uppercase + "0123456789"
p = 13g = 5a = 7b = 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.pyC4354R_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.