Home / server / Questions / 426394
Accepted
Ryan
Asked: 2012-09-12 08:24:06 +0800 CST

How to check if an RSA public / private key pair match

  • 772

I have two files, id_rsa and id_rsa.pub. What command can be used to validate if they are a valid pair?

linux

4 Answers

  1. Best Answer

    I would prefer the ssh-keygen -y -e -f <private key> way instead of the accepted answer of How do you test a public/private DSA keypair? on Stack Overflow.

    ssh-keygen -y -e -f <private key> takes a private key and prints the corresponding public key which can be directly compared to your available public keys. (Hint: beware of comments or key-options.)

    (How the hell is it doing that? I can only hope the public key is encoded directly or indirectly in the private key...)

    I needed this myself and used the following Bash one-liner. It should output nothing if the keys belong together. Apply a little -q to the diff in scripts and diff only sets the return code appropriately.

    PRIVKEY=id_rsa
TESTKEY=id_rsa.pub
diff <( ssh-keygen -y -e -f "$PRIVKEY" ) <( ssh-keygen -y -e -f "$TESTKEY" )
    • 179

  2. Depending on where you get the public key file you are testing, the accepted answer may give false positive results. This is because of the behavior described in the comment by @drewbenn. Specifically, when the -e option is used with the private key file as the -f option parameter, it simply parrots (but reformats) what's in the associated public key file.

    In other words, 

    ssh-keygen -y -f id_rsa

    (apparently) generates the public key value, and

    ssh-keygen -y -e -f id_rsa

    simply and outputs (and reformats) the key in the existing id_rsa.pub whatever it is.

    In my case, I have to verify that the pair has not been corrupted. So, I decided to compare the following:

    ssh-keygen -y -f id_rsa | cut -d' ' -f 2

    with

    cut -d' ' -f 2 id_rsa.pub

    Therefore:

    diff <(cut -d' ' -f 2 id_rsa.pub) <(ssh-keygen -y -f id_rsa | cut -d' ' -f 2)

    Perhaps this is not as flexible, but it is better for my needs. Maybe it helps someone else.

    • 69

  3. The easiest is to compare fingerprints as the public and private keys have the same. Visual comparison is pretty easy by putting the two commands on same line:

    ssh-keygen -l -f PRIVATE_KEY; ssh-keygen -l -f PUBLIC_KEY

    Programmatically, you'll want to ignore the comment portion so

    diff -s <(ssh-keygen -l -f PRIVATE_KEY | cut -d' ' -f2) <(ssh-keygen -l -f PUBLIC_KEY | cut -d' ' -f2)
    • 11

  4. If they're on your local system, stick id_rsa.pub in your $HOME/.ssh/authorized_keys and ssh to localhost using the id_rsa key. If it works, then they match.

    cat $HOME/.ssh/id_rsa.pub >> $HOME/.ssh/authorized_keys
ssh -i $HOME/.ssh/id_rsa localhost
    • 6