How do I validate an RSA SSH public key file (id_rsa.pub)?

Question

Is there a command I can use to verify the public key (id_rsa.pub), just the format only.

Sometimes I have added a new linebreak, sometimes the file missed the ssh-rsa prefix, so is there a command to validate with?

Solution

You can use ssh-keygen for this. Despite its name it can do many more things than generating keys:

dennis@lightning:~$ ssh-keygen -l -f .ssh/id_rsa.pub 
2048 68:df:b2:22:d8:43:5d:36:75:c1:d8:59:c0:8c:22:e8  Dennis Kaarsemaker (RSA)
dennis@lightning:~$ ssh-keygen -l -f foo.txt
foo.txt is not a public key file.

Comments

[Quazi Irfan]

So I have to have the fingerprint beforehand to validate the public key?

[vidstige]

@QuaziIrfan no, you do not need the fingerprint beforehand. The returncode of ssh-keygen will tell you whether it's valid or not.

[vidstige]

Does not work for keys with passwords :-(

[slhck]

Note that this also works on a file containing multiple keys, such as ~/.ssh/authorized_keys.

[Jacopo]

Note that ssh-keygen will also accept options before ssh-xxx (as in authorized_keys), which can potentially be dangerous: command="xxx", ...

[Amory]

The [-E fingerprint_hash] option is also useful if you want something other than the default, e.g. -E md5 or -E sha512