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SSH Keys and Agent Cheat Sheet

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SSH key generation, authorized_keys management, ssh-keyscan, host keys, ssh-agent, ssh-add, and certificate basics.

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## Key Generation
Generate an Ed25519 key
↗#
ssh-keygen -t ed25519 -C 'you@example.com'

# Create a modern SSH key pair.

Generate an RSA key
↗#
ssh-keygen -t rsa -b 4096 -C 'you@example.com'

# Create an RSA key pair for compatibility when needed.

Generate a key at a custom path
↗#
ssh-keygen -t ed25519 -f ~/.ssh/id_work_ed25519 -C 'work key'

# Write a key pair to a chosen file.

Generate a key without passphrase
↗#
ssh-keygen -t ed25519 -N '' -f ~/.ssh/id_ci_ed25519

# Skip passphrase prompts, usually only for automation accounts.

Change key passphrase
↗#
ssh-keygen -p -f ~/.ssh/id_ed25519

# Update the passphrase on an existing private key.

Show a key fingerprint
↗#
ssh-keygen -lf ~/.ssh/id_ed25519.pub

# Print the fingerprint of a public key file.

Show randomart for a key
↗#
ssh-keygen -lvf ~/.ssh/id_ed25519.pub

# Display a visual fingerprint representation.

Extract public key from private key
↗#
ssh-keygen -y -f ~/.ssh/id_ed25519 > ~/.ssh/id_ed25519.pub

# Regenerate the public half from a private key.

## Authorized Keys and Host Keys
Install your public key on a server
↗#
ssh-copy-id -i ~/.ssh/id_ed25519.pub user@example.com

# Append your public key to a remote account's authorized_keys.

Install key on custom port
↗#
ssh-copy-id -i ~/.ssh/id_ed25519.pub -p 2222 user@example.com

# Use ssh-copy-id with a non-default SSH port.

Scan a host key
↗#
ssh-keyscan example.com

# Fetch a server host key without logging in.

Add a host key to known_hosts
↗#
ssh-keyscan -H example.com >> ~/.ssh/known_hosts

# Append a scanned host key to your known_hosts file.

Find a host in known_hosts
↗#
ssh-keygen -F example.com

# Search known_hosts for a specific host entry.

Remove a host from known_hosts
↗#
ssh-keygen -R example.com

# Delete stale host key entries for a host.

Hash known_hosts file
↗#
ssh-keygen -H -f ~/.ssh/known_hosts

# Hash hostnames in known_hosts for privacy.

Sign a host key with a CA
↗#
ssh-keygen -s ~/.ssh/ca_host -I host-web01 -h -n web01.example.com /etc/ssh/ssh_host_ed25519_key.pub

# Create a host certificate from a host CA key.

## ssh-agent and ssh-add
Start ssh-agent in current shell
↗#
eval "$(ssh-agent -s)"

# Start an agent and export its environment variables.

Add default key to agent
↗#
ssh-add ~/.ssh/id_ed25519

# Load the default identity into ssh-agent.

List loaded keys
↗#
ssh-add -l

# Show fingerprints of identities currently loaded in the agent.

List public keys in agent
↗#
ssh-add -L

# Print public keys currently loaded in the agent.

Remove one key from agent
↗#
ssh-add -d ~/.ssh/id_ed25519

# Delete a specific key from the agent.

Remove all keys from agent
↗#
ssh-add -D

# Clear all identities from the agent.

Require confirmation before using a key
↗#
ssh-add -c ~/.ssh/id_ed25519

# Load a key that prompts for confirmation on each use.

Set agent key lifetime
↗#
ssh-add -t 1h ~/.ssh/id_ed25519

# Load a key that expires automatically from the agent.

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