Server#

where is sshd config?# 

# location 
/etc/ssh/sshd_config

# list all authorized public keys for a user
cat ~/.ssh/authorized_keys

# allow multiple sessions
MaxSessions 10

# enable public key authentication
PubkeyAuthentication yes

# disable password authentication
PasswordAuthentication no

How to check ssh session or connections?# 

# check tcp connection
sudo netstat -tnpa | grep sshd

# check ssh session
sudo who

Client#

refer to Generating a new SSH key

# generate ssh key pair
ssh-keygen -t ed25519 -C "your_email@example.com"

# copy public key to server
ssh-copy-id -i ~/.ssh/id_ed25519.pub user@server_ip

SSH Handshake#

Basically, ssh handshake includes 4 phases (totally 5 RTT):

  • tcp handshake
  • algorithms negotiation
  • key exchange (for ECDH calculation, similar to TLS)
  • authentication (SSH verifies server identity via TOFU known_hosts, client identity via public key authorized_keys, whereas TLS uses CA to verify server identity)
  Client                                        Server
  -------------------- TCP 3-Way Handshake --------------------
  SYN                          -------->
                               <--------                SYN+ACK
  ACK                          -------->

  -------------------- Algorithm Negotiation ------------------
  Banner + SSH_MSG_KEXINIT     -------->
                               <--------  Banner + SSH_MSG_KEXINIT

  -------------------- Key Exchange ---------------------------
  SSH_MSG_KEX_ECDH_INIT        -------->
                               <--------      SSH_MSG_KEX_ECDH_REPLY
                               <--------             SSH_MSG_NEWKEYS
  SSH_MSG_NEWKEYS              -------->

  -------------------- Authentication -------------------------
  SSH_MSG_SERVICE_REQUEST      -------->
                               <--------      SSH_MSG_SERVICE_ACCEPT
  SSH_MSG_USERAUTH_REQUEST     -------->
                               <--------     SSH_MSG_USERAUTH_SUCCESS

  -------------------- Encrypted Session ----------------------
  Application Data             <------->            Application Data
Term Meaning
Banner Version string exchange (e.g. SSH-2.0-OpenSSH_9.0), identifies protocol version and implementation
SSH_MSG_KEXINIT Both sides advertise supported algorithms (kex, cipher, MAC)
SSH_MSG_KEX_ECDH_INIT Client sends its ephemeral EC public key to start DH exchange
SSH_MSG_KEX_ECDH_REPLY Server sends its EC public key + Host Key + Signature
SSH_MSG_NEWKEYS Signal to switch to the newly derived symmetric key
SSH_MSG_SERVICE_REQUEST Client requests the ssh-userauth sub-protocol
SSH_MSG_SERVICE_ACCEPT Server confirms the auth service is ready
SSH_MSG_USERAUTH_REQUEST Client sends username + public key + signature
SSH_MSG_USERAUTH_SUCCESS Server confirms authentication passed

SSH vs TLS#

for tls details, refer to https

TLS 1.3 TLS 1.2 SSH
RTT after TCP 1 RTT 2 RTT 4 RTT
Server Identity Certificate signed by CA Certificate signed by CA Host Key stored in ~/.ssh/known_hosts (TOFU, Trust On First Use)
Client Identity Optional mTLS certificate Optional mTLS certificate Public key in ~/.ssh/authorized_keys
Trust Model Centralized CA hierarchy Centralized CA hierarchy Trust On First Use (TOFU)
Key Exchange ECDHE inside ClientHello Negotiated after hello Separate KEXINIT then ECDH
Forward Secrecy Mandatory Optional Yes, ephemeral DH keys
User Authentication No No Separate phase (public key / password)
Encryption Start After 1 RTT After 2 RTT After key exchange phase