Secure Shell (SSH)

Secure Shell (SSH)

• OpenSSH Manual Pages

Guides

• OpenSSH Guidelines - Mozilla

• Secure Secure Shell - stribika

• Securing SSH - CentOS Wiki

Articles

• The default OpenSSH key encryption is worse than plaintext - lvh (Latacora Blog)

• ChaCha20 and Poly1305 in OpenSSH - Damien Miller

• How Facebook does SSH at scale - Marlon Dutra

Tools

• ssh_scan “configuration and policy scanner” (Mozilla)

• Secretive Generate and store SSH keys in the Mac Secure Enclave (ecdsa-sha2-nistp256 keys)

Mobile

If you use SSH on the go often you'll want to look at using Mosh

iOS

• Termius App Store (Free)
• Prompt 2 App Store (£12.99)
• Blink Shell App Store (£17.99)

Examples

Generate Keys

The ssh-keygen utility is used to create new SSH keys on most *nix systems.

ED25519

ssh-keygen -t ed25519 -a 100

• -t: Type of key to generate

• -a: Number of Key Derivation Function (KDF) rounds

Remove Hashed known_hosts Entry

If your client is set to hash known hosts e.g. has the following line in ~/.ssh/config

HashKnownHosts yes

Then your ~/.ssh/known_hosts file will be obfuscated.

To remove a host, when its hosts key changes, you'll need to execute:

ssh-keygen -R example.com

Which will remove all keys associated with that hostname from ~/.ssh/known_hosts.

Configuration

Key Types

Key types are listed in the order of preference below:

• ED25519

• >= 2048bit RSA

• ECDSA

  • DSA and ECDSA both fail catastrophically on bad randomness.
  • Never use DSA keys

  • Avoid ECDSA keys if you can

Client

Permissions

Only allow your user to access ~/.ssh and your private keys, allow group and world to access your public keys.

chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_*
chmod 644 ~/.ssh/id_*.pub

config

  # ~/.shh/config 
  # ssh_config(5) 

  Host * 
  # For all hosts use the following directives 

  Protocol 2 
  # Use only protocol version two 

  IdentitiesOnly yes 
  # By default ssh will send all public keys (identities) in ~/.ssh to the server if you don't specify which key to use with -i 
  # This prevents that by only using the public keys explicitly configured in config or specified with -i 

  VisualHostKey yes 
  # Print an ASCII art representation of the remote host key fingerprint at login and for unknown host keys 

  HashKnownHosts yes 
  # Hash host names and addresses when they are added to ~/.ssh/known_hosts. 
  # ssh-keygen -R hostname 
  # Removes all keys belonging to hostname from a known_hosts file. 
  UseRoaming no 
  # Mitigates CVE-0216-0777 

  # Cryptography 

  KexAlgorithms curve25519-sha256
  # Allow only curve25519
  
  HostKeyAlgorithms ssh-ed25519,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256
  # Allow ed25519, ECDSA and RSA SHA2 keys for client authentication
  # ed25519 is the preferred key type
  # ECDSA for Secretive/ Secure Enclave keys
  # rsa-sha2-* for compatibility

  Ciphers [e-mail]
  # Only use chacha20-poly1305
  # Chacha20-poly1305 is preferred over AES-GCM because the SSH protocol does 
  #   not encrypt message sizes when GCM (or EtM) is in use. 
  #   This allows some traffic analysis even without decrypting the data.
  #   See: http://blog.djm.net.au/2013/11/chacha20-and-poly1305-in-openssh.html

  MACs [e-mail],[e-mail],[e-mail]
  # Only use encrypt then mac (etm) MACs
  # Allow only HMAC-SHA2-512/256 or UMAC-128
  #   https://crypto.stackexchange.com/a/56432

Server

Permissions

Only allow your user to access ~/.ssh and ~/.ssh/authorized_keys.

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

These permissions are required by the StrictModes directive.

sshd_config

# /etc/ssh/sshd_config 
# sshd_config(5) 

AddressFamily inet 
# Only use IPv4 

ListenAddress x.x.x.x 
# Default is to listen on all local addresses 
# Better to specify an actual IP address to listen on 

Protocol 2 
# Only use protocol version 2 

LogLevel VERBOSE 
# Logs user's key fingerprint on login 

HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_rsa_key
# Key files cannot be group/world-accessible 

PermitRootLogin no 
# root user cannot login via SSH 

AuthenticationMethods publickey 
# Only allow public key authentication for login 

Subsystem sftp internal-sftp 
# Use sshd internal SFTP server code (plays nicer with Chroot) 
# See https://serverfault.com/a/660325 for differences with 
# Subsystem sftp /usr/libexec/openssh/sftp-server 
# If you just scp files you can disable this to reduce attack surface 

# Cryptography 

KexAlgorithms curve25519-sha256
# Allow only curve25519

HostKeyAlgorithms ssh-ed25519,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256
# Allow ed25519, ECDSA and RSA SHA2 keys for client authentication
# ECDSA for Secretive/ Secure Enclave keys
# ed25519 is the preferred key type
# rsa-sha2-* for compatibility

Ciphers [e-mail]
# Only use chacha20-poly1305
# Chacha20-poly1305 is preferred over AES-GCM because the SSH protocol does 
#   not encrypt message sizes when GCM (or EtM) is in use. 
#   This allows some traffic analysis even without decrypting the data.
#   See: http://blog.djm.net.au/2013/11/chacha20-and-poly1305-in-openssh.html

MACs [e-mail],[e-mail],[e-mail]
# Only use encrypt then mac (etm) MACs
# Allow only HMAC-SHA2-512/256 or UMAC-128
#   https://crypto.stackexchange.com/a/56432

Debugging sshd Issues

sudo sshd -t
# Test mode. Only check the validity of the configuration file and sanity of the keys.

sudo systemctl restart sshd
# On systemd based systems restart the sshd service

sudo systemctl status sshd
# On systemd based systems print the status of the sshd service