Consider the Diffie-Hellman implementation exchange::exchange_keys located in exchange.h. No authenticity check is performed in either the network::recv or network::send functions. Does this present a vulnerability in the key exchange? If so, how might an attacker exploit it?
Recall that the HMAC message authentication code uses a cryptographic hashing algorithm alongside a key, where the hashing algorithm breaks the message into fixed sized chunks which is then fed into a running, fixed length hash value. AES-GCM does not require an HMAC, as it ensures message integrity through the aes::gcm::GHASH function in aes.h. How is this function similar to HMAC, and what advantage does AES-GCM with GHASH have over AES-CTR+HMAC?
In the main program, encrypted messages are exchanged via a shared key in the util::receive_message and util::send_message functions in util.h. When using ECB or CTR mode, main will generate an HMAC value for the messages to ensure that they are not modified in transit. Recall that ECB is not a recommended mode of AES. Does the inclusion of an HMAC alleviate the issues in ECB, and make it safe to use for secure communication? Why or why not?