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

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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

2 Answers 2

up vote 10 down vote accepted

Question is already answered on stackoverflow: http://stackoverflow.com/questions/274560/how-do-you-test-a-public-private-keypair

(Can I mark a question as duplicate on another stackexchange network?)

I would prefer the ssh-keygen -y -f <private key> way instead of the accepted answer on SO.

ssh-keygen -y -f <private key> takes a private key an 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...)

Needed this myself and used the following bash-oneliner. 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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No answer has been accepted yet. And I like yours better. –  Michael Hampton Sep 11 '12 at 18:03
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@MichaelHampton: now I understand your comment. "the accepted answer" should refer to the question on stackoverflow not this question... –  Michuelnik Sep 12 '12 at 5:38
    
"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

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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