Ex 10 task 2i) - Which algorithm computes the MAC?

Question

Hi,

I’m having trouble understanding, why chacha20-poly1305@openssh.com is used to compute the MAC in the provided pcapng file.

Could you please explain how it works and why this choice was made?

Many thanks in advance!

Answer 1

Hi,

ChaCha20-Poly1305 was chosen as the encryption algorithm.
You can see this in Wireshark by clicking on one of the encrypted messages.
This is a AEAD cipher, meaning that it includes the authentication method (MAC).
This was talked about in TLS 1.3.

Now this is not directly defined in the original RFC, however,
https://datatracker.ietf.org/doc/html/rfc4253#section-6.4 links to
https://datatracker.ietf.org/doc/html/rfc4251#section-6
which defines how organizations can use their own algorithms.
OpenSSH adheres to this standard with chacha20-poly1305@openssh.com.

The draft
https://www.ietf.org/id/draft-josefsson-ssh-chacha20-poly1305-openssh-01.html
also briefly mentions in section 2 how the negotiation works for this encryption algorithm.

The "chacha20-poly1305" offers both encryption and authentication. As such, no separate MAC is required. If the "chacha20-poly1305" cipher is selected in key exchange, the offered MAC algorithms are ignored and no MAC is required to be negotiated.

I hope that clears that up.

Kind regards,
Sebastian