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I'm building a server app in C++ that needs to accept a certificate containing an ECDSA public key. It must validate the certificate and, upon verification, use the public key contained in the certificate to authenticate a message sent along with the certificate.

I have all this working using ECDSA keypairs generated on the fly - i.e. my code is working nicely - but now I need to do the certificate piece.

And I figured I could use OpenSSL's command-line to create the certificate which is installed on the client (along with the ECDSA private key in a separate file).

Can anyone help?

If you haven't chosen a curve, you can list them with this command:

openssl ecparam -list_curves
I picked secp256r1 for this example.  Use this to generate an EC private key if you don't have one already:
openssl ecparam -out ec_key.pem -name secp256r1 -genkey 
And then generate the certificate.  Your certificate will be in cert.pem.
openssl req -new -key ec_key.pem -x509 -nodes -days 365 -out cert.pem
See also:  req, ecparam
                I think you mean secp256r1. According to this post it is referred to as prime256v1 in OpenSSL.
– oliverdm
                Oct 29, 2014 at 11:27
                One liner: openssl req -new -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -x509 -nodes -days 365 -out cert.pem -keyout cert.pem
– Sam Bull
                May 31, 2020 at 16:04
                Fixed one-liner: openssl req -new -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -x509 -nodes -days 365 -out cert.pem -keyout key.pem
– rustyx
                Dec 23, 2020 at 0:29
                For a bit more context, see RFC 4492 tools.ietf.org/search/rfc4492 and this recommendations document secg.org/sec2-v2.pdf
– Luis E.
                Nov 18, 2021 at 15:53
        
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