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

[Blumenthal, Uri - 0553 - MITLL]

Oh, and I'd much prefer to stay at the EVP level, rather than invoke BIO primitives for this task.

Sent from my BlackBerry 10 smartphone on the Verizon Wireless 4G LTE network.
  Original Message  
From: Blumenthal, Uri - 0553 - MITLL
Sent: Thursday, March 17, 2016 18:09
To: Viktor Dukhovni; 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?

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). 

Thanks!

Sent from my BlackBerry 10 smartphone on the Verizon Wireless 4G LTE network.
  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...

-- 
Viktor.

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