How can i check the authenticity of a private key

Mike Rosing eresrch at
Fri May 31 06:18:52 PDT 2002

On Fri, 31 May 2002, surinder pal singh makkar wrote:

> I am a newbie in cryptography. What I have learnt till
> now is that in assymeric cryptography scenario we have
> a private key and we generate the public key
> corresponding to it and then we send it to the central
> agency.

You don't have to send the public key to a repository,
it's just convienient.

> Suppose after sometime I have a private key and the
> public key. Is there some software tool which can tell
> me whether the public key is the same corresponding to
> the private key I am having. Also is there some tool
> which can tell me whether the keys have been curropted
> or not

With ECC you just recompute the public key from the private
key and make sure it matches what's out in public.  With
RSA you just pick some random value (not zero or 1) and
see if r^(e*d) = 1 mod N, or if you know p and q (where
N = p*q) check that e*d = 1 mod (p-1)*(q-1).  It's the
same thing as encrypting/decrypting something to see if
you get the same thing back.  If not, something is wrong.

I'm not sure how you can tell which key might be corrupted.
For the public side, having the key reside in many places
would do it - you can just check that they are all the same.
so it may well be that saving the public key in a private
place for that purpose is also useful.

Patience, persistence, truth,
Dr. mike

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