What are the benefits of using SSH host key fingerprint to verify server identity? (2024)

SSH host keys are cryptographic keys that are generated by each SSH server when it is first set up. They are used to encrypt and authenticate the communication between the server and the client. Each host key consists of a public key and a private key. The public key is shared with the client, while the private key is kept secret by the server. The public key and the private key are mathematically related, but it is practically impossible to derive one from the other.

SSH host key fingerprints are shorter and more human-readable versions of the public keys. They are generated by applying a hashing function, such as MD5 or SHA256, to the public key. The hashing function produces a fixed-length output that is unique for each input. The fingerprint can be displayed in different formats, such as hexadecimal, base64, or bubble babble. The fingerprint is used to identify and verify the server's public key.

When you connect to a new SSH server for the first time, your SSH client will ask you to confirm the server's fingerprint. You can do this by comparing the fingerprint shown by the client with the fingerprint provided by the server administrator or obtained from a trusted source. If they match, you can accept the fingerprint and continue the connection. The client will then store the fingerprint in a file called known_hosts, which keeps track of the servers you have connected to before. The next time you connect to the same server, the client will check the fingerprint against the known_hosts file and skip the confirmation step if it finds a match. If the fingerprint does not match, the client will warn you of a possible man-in-the-middle attack, where someone is trying to intercept or modify your SSH connection.

Using SSH host key fingerprints can provide several benefits for securing your SSH connections. It can prevent connecting to a rogue or compromised server, protect from man-in-the-middle attacks, and save time by only needing to confirm the fingerprint once for each server. After that, your SSH client will recognize and trust the server based on the fingerprint, allowing you to detect any changes in the server's public key and reject the connection if it is suspicious.

If you are a server administrator or a user who wants to generate SSH host key fingerprints, you can use various tools and commands depending on your operating system and SSH software. For example, on Linux, you can use the ssh-keygen command with the -l option to list the fingerprints of the host keys in different formats. On Windows, you can use the PuTTYgen tool to load the host keys and display their fingerprints. You can also use online tools or websites that can generate fingerprints from public keys.

If you need to share your SSH host key fingerprints with other users or clients, there are various methods to choose from that meet your preferences and security requirements. For example, you can publish the fingerprints on a secure website or public key server, send them via email or other encrypted communication channels, or display them on a physical medium like a sticker or card.

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

Secure Shell (SSH)

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