Add aes-gcm mode to contain the message authentication in network module

[somniumism]

The network module is using the block cipher mode aes-256-cbc.
However, it does not contain message authentication.
Then, I added aes-gcm mode, which is an authenticated message encryption method.

I used the aes-gcm-siv crate to implement the aes-gcm mode. AES-GCM-SIV has been designed
to preserve both privacy and integrity, even if nonces are repeated.

And I added directory /target to .gitignore.

Fixed #5

[sgkim126]

It doesn't matter if you add a new mode but replacing the current mode to another does.

[somniumism]

@sgkim126 AES-GCM-SIV has been designed to preserve both privacy and integrity, even if nonces are repeated. You can confirm this content here: aes-gcm-siv@0.3.0 and AES-GCM-SIV:Informational. And I 'added' the aes-gcm mode, not replace. : ) With @HoOngEe, Could you review this PR?

[sgkim126]

Would you explain why do you take only 12 bytes from the nonce?

[somniumism]

@sgkim126 When using AES-GCM, a 96-bit IV is generally recommended. I referred to the following a answer. I think there is no problem with using only 12 bytes out of 16 bytes because we are using a fixed nonce.

[sgkim126]

If you will use only 12 bytes, IMHO, you should change the type of a nonce parameter as [u8: 12]. Discarding information inside a function may make the user misuse.

[HoOngEe]

Also, nonce reuse is not recommended in AES-GCM-SIV so we should not use a fixed nonce anymore.

[somniumism]

CodeChain is using AES-CBC mode in the Network module. AES-CBC doesn’t contain message authentication, so we need to apply AEAD to avoid the adaptive chosen ciphertext attack. However, in our p2p protocol, we use the nonce shared in the handshake step. So if we use AES-GCM, it leaks the key when the same iv is used twice.

I proposed to apply AES-GCM-SIV because using it can prevent the worst-case in AES-GCM. However, AES-GCM-SIV also recommends that we don’t use iv more than once. Therefore, it is necessary to change our p2p protocol to use AES-GCM-SIV for Foundry.

I investigated examples related to the network or p2p protocol using AES-GCM-SIV to refer to examples in creating the new protocol. However, I don’t find such protocols or examples in real-world applications. It’s a relatively new cipher, so it seems difficult to find a protocol that is using it yet.

Therefore, since there is no immediate way to add new modes or change protocols, I close this PR. Getting rid of the attack possibility and using AEAD is meaningful, so if a new way to apply AEAD is devised, then it will continue from there.