diffie-hellman

Challenge

Alice and Bob wanted to exchange information secretly. The two of them agreed to use the Diffie-Hellman key exchange algorithm, using p = 13 and g = 5. They both chose numbers secretly where Alice chose 7 and Bob chose 3. Then, Alice sent Bob some encoded text (with both letters and digits) using the generated key as the shift amount for a Caesar cipher over the alphabet and the decimal digits. Can you figure out the contents of the message? Download the message here. Wrap your decrypted message in the picoCTF flag format like: picoCTF{decrypted_message}

Solution

We don't need to know anything about the Diffie-Hellman key exchange algorithm for this challenge. Just use cryptii.com (or any Caesar cipher application). Paste in the message H98A9W_H6UM8W_6A_9_D6C_5ZCI9C8I_D9FF6IFD, then set the alphabet to abcdefghijklmnopqrstuvwxyz0123456789, then increment the shift until the message is clear. The offset I found was 5 (a --> f)

Flag

picoCTF{C4354R_C1PH3R_15_4_817_0U7D473D_84AA1DA8}