[openssl-users] Naive: how to generate EC public key from EC private key?

Blumenthal, Uri - 0553 - MITLL uri at ll.mit.edu
Thu Mar 17 22:09:13 UTC 2016

Great! Say, I want to extract the public key and make it available to another entity or module? Possibly DER-encoded, though I'd like to learn how to do both: extract  ASN.1-encoded and raw (assuming it is possible). 


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  Original Message  
From: Viktor Dukhovni
Sent: Thursday, March 17, 2016 17:57
To: openssl-users at openssl.org
Reply To: openssl-users at openssl.org
Subject: Re: [openssl-users] Naive: how to generate EC public key from EC	private key?

> On Mar 17, 2016, at 5:17 PM, Blumenthal, Uri - 0553 - MITLL <uri at ll.mit.edu> wrote:
> I’ve an extremely naïve question. I am generating ephemeral EC keys for ECDH, following the example in https://wiki.openssl.org/index.php/EVP_Key_and_Parameter_Generation
> But it looks like the example ends on generation of the private key:
> /* Generate the key */
> if (!EVP_PKEY_keygen(kctx, &key)) goto err;
> The next step must be obvious, but somehow I can’t figure it out. So my question is: from having EVP_PKEY *privateECKey how do I get EVP_PKEY *publicECKey?

The public key is always there. You either have both the public and private keys
or just the public key. So the same EVP_PKEY object holds both. What you do
next depends on what you want to do with the public key...


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