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How To: Inspect SSH Key Fingerprints

As you can imagine, SSH keypairs – combinations of private and public keys – are vital elements of your digital identity as a sysadmin or a developer. And since they can be used for accessing source code repositories and for deploying changes to production environments, you usually have more than one SSH key. That’s why it’s important to know how to inspect SSH key fingerprints.

SSH Key Fingerprints

Key fingerprints are special checksums generated based on the public SSH key. Run against the same key, ssh-keygen command will always generate the same fingerprint.

Because of this property, you can use SSH key fingerprints for three things:

SHA-1 Stands for (Secure Hash Algorithm 1) is a cryptographic hash function which takes an input and generate a 160-bit (i.e 20-byte) hash value known as a message digest – This message digest is of rendered as a hexadecimal number, which is if 40 digits long. SHA-1 is the most widely used of the existing SHA hash functions, and is employed in several widely-used security applications and protocols. In 2005, security flaws were identified in SHA-1, namely that a mathematical weakness might exist, indicating that a stronger hash function would be desirable.

Identify SSH key – fingerprint will stay the same even if you rename the file
Confirm integrity of the SSH key – if you get the same fingerprint from your private SSH key, you can be sure it’s still valid and intact
Validate identity of the SSH key – same fingerprint means you’re dealing with the same key (that you or your solution trusted for specific functionality)

How to Check SSH Fingerprint of a Key

ssh-keygen command takes the identity (SSH key) filename and calculates the fingerprint.

You can start by changing directory into .ssh and checking if you have any SSH keys there already. If not, you should generate a new SSH key.

Let’s run ssh-keygen to confirm the fingerprint of the id_rsa keypair:

Check Fingerprint of the Private SSH Key

By default this command looks for the public key portion (id_rsa.pub file), so it’s not a very good test of integrity or identity of the private key. There is a very real possibility that you have one private key and a separate public key, that are not related to each other.

That’s why for checking the private key you must take it a step further and copy private key (id_rsa) into some other directory where you can use ssh-keygen again:

this time, because there’s no public key file found nearby, the ssh-keygen command will have to open private key. And if it’s passphrase protected (as it always should be), you’ll be asked for the SSH key passphrase:

Old-school SSH fingerprints

If you’ve been using Linux/Unix for more than a couple of years, you probably noticed that ssh-keygen now shows you a different looking fingerprints: they used to be these semicolon-delimited sequences like this:

… instead they now are shown as this:

The reason for this is that by default fingerprints are shown as SHA256 sequences, while in the past they were MD5.

In order to show the SSH fingerprint in MD5 format, just specify this in the command line:

Summary

When you apply the hashing algorithm to an arbitrary amount of data, such as a binary file, the result is a hash or a message digest. This hash has a fixed size. MD5 is a hashing algorithm that creates a 128-bit hash value. SHA-1 is a hashing algorithm that creates a 160-bit hash value.

Use FCIV to compute MD5 or SHA-1 cryptographic hash values

Sha1 Fingerprint Generator App

You can use the File Checksum Integrity Verifier (FCIV) utility to compute the MD5 or SHA-1 cryptographic hash values of a file. For additional information about the File Checksum Integrity Verifier (FCIV) utility, click the following article number to view the article in the Microsoft Knowledge Base:

Sha1 Fingerprint Android Studio

To compute the MD5 and the SHA-1 hash values for a file, type the following command at a command line:

Generate Sha1 Hash

For example, to compute the MD5 and SHA-1 hash values for the Shdocvw.dll file in your %Systemroot%System32 folder, type the following command: