What are Host Keys?

About

Host keys are a key pairs for a host.

Each host can have one host key for each algorithm.

You should get an SSH host key fingerprint along with your credentials from a server administrator in order to prevent man-in-the-middle attacks.

Management

Create

The host keys are usually automatically generated when an SSH server is installed but you can recreate them on the server with keygen

for Cryptography - RSA (Rivest–Shamir–Adleman)

ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

2048 0d:f4:2e:7b:be:e1:4c:0b:06:52:2b:4d:a4:51:1f:74 /etc/ssh/ssh_host_rsa_key.pub (RSA)

Others

ssh-keygen -l -f /etc/ssh/ssh_host_dsa_key
ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key

Location

Client

On the client to trust the host, you would need to add it in the known_hosts file

Server

On the server: Host keys are stored in the /etc/ssh/ directory.

Key Pair

The host keys are almost always stored in the following files:

/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key

Public

ls /etc/ssh/*.pub

/etc/ssh/ssh_host_dsa_key.pub
/etc/ssh/ssh_host_ecdsa_key.pub
/etc/ssh/ssh_host_ed25519_key.pub
/etc/ssh/ssh_host_rsa_key.pub

cat /etc/ssh/ssh_host_rsa_key.pub

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDwnFlKBk9t/pfd7vQGBrk+JNghDFJRa7G0rLrIoXUTtxconS7qZsZyTfEeh+l2cowWpKxpf03KyxouT7B3IdNjwXWHNKloQcE/8FYLhlcVw+Aicji5oUMdTfTberzCluYIyz4PUwE8rmgNejQbVLhl+lOfoacPMUJ4P6ahQ71WFTmFTEZ+CFnAmHeqtG7scl53DkICnXPkZ+57ymh1WM9h0kYFxVcZBXg7BOZpI/wrXkuWRXU6wpYHS481q/pjJTiLXZGIQhcXg1/inOCMgC1B9W92Rq/aviknWSS98/xQi2JbduG7Jw9Dn523Rs3nutUAQ6CMK7owjVtnvb54Bs1B root@buildbot-bbiwebhosting-builder-wheezy-amd64

Format

Fingerprint

The fingerpint of a public key consists of a hash (SHA256, MD5) of the public key. This is the same data that is base64 encoded to form the body of the public key file.

See The Secure Shell (SSH) Public Key File Format

Documentation / Reference

The Secure Shell (SSH) Public Key File Format