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

  • I'll confirm Michuelnik's answer; it saved me from having to make a new key pair, thanks. ssh -v helps a lot too. – Chris K Dec 30 '13 at 5:06
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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" )
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    "How the hell is it doing that".. The private key is generated from randomness so we all have a unique key. If you apply the repeatable one-way RSA algorithm on this private key, youll get the unique public key. There is no algorithm that you can perform on the public key thatll get a unique private key. – Sirch Sep 24 '13 at 14:29
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    @Sirch: I thought the decision which key is private and which one is public is pure random since the two keys are equal. What one key encrypts can only be decrypted with the other. And if one key could be obtained from the other this all would not work out. – Michuelnik Sep 25 '13 at 11:42
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    @Michuelnik You can derive the public key from the private key. You cant derive the private key from the public key. Were not talking about the material that it encrypts. – Sirch Sep 25 '13 at 13:35
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    @Michuelnik imho, the question is off topic on SO and on-topic here (and/or Superuser). imho, it shouldn't be marked as duplicate and instead flagged there for migration. But it's covered on both so I like the more complete sharing of information. – Chris K Dec 30 '13 at 20:39
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    As long as id_rsa.pub exists, ssh-keygen -y -e -f id_rsa will not check id_rsa at all but just return the value from id_rsa.pub. So e.g. if you echo 5 > id_rsa to erase the private key, then do the diff, the diff will pass! Also, running ssh-keygen -yef foo where foo is not a valid key (and has no corresponding foo.pub) will block waiting for user input, so be careful using this in a script. – user68519 Jul 10 '15 at 22:45
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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.

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    This really should replace the accepted answer or at least surpass it in terms of upvotes. – thomanski Oct 5 '16 at 15:01
  • Thank you! This command doesn't work with keys with a passphrase, it doesn't ask that interactively. I extracted the two () command contents to files and diff-ed them, that works. – Yaroslav Nikitenko Oct 18 '19 at 10:10
  • @KenWilliams Ken, this is definitely off-topic but IDK how to otherwise reach you; WAY back in 1989 I joined Relational Technology Inc (later known as Ingres Corp.), a leading database company, and, in sum, I was fairly promptly a direct report to Ken Williams, Sr. VP of Customer Support & Services. We've lost track but he might be or have been a relative of yours? Uncle, perhaps? Or even Gramps? If you know anything about this, I'd love to connect somehow. Thanks. – Richard T Aug 21 at 0:35
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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
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  • What if it's still prompting you for a password? Permissions of authorized_keys are correct, the public key has been copied correctly in the authorized_keys, I also did the diff mentioned in the verified answer and keys belong together. – Vasiliki Sep 30 at 15:39

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