ANSI X9.62 elliptic curve prime256v1 (aka secp256r1, NIST P-256), SHA512withECDSA Signature verification using Java.

Main.java

import java.security.*;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;

/*

ANSI X9.62 elliptic curve prime256v1 (aka secp256r1, NIST P-256), SHA512withECDSA Signature verification using Java.

## Some useful OpenSSL commands in order to create keys and sign messages:

Generating new EC key using OpenSSL:
openssl ecparam -name prime256v1 -genkey -noout -out key.pem

Signing message 'tolga' using key 'key.pem' with sha512 digest:
echo -n 'tolga' | openssl dgst -sha512 -sign key.pem | openssl enc -base64

Example keys:
-----BEGIN EC PRIVATE KEY-----
MHcCAQEEILgvfksXdWWNCZJznzSbAW6jSJEcoyw8kFnIpmkCPVmBoAoGCCqGSM49
AwEHoUQDQgAEPP4jo2krvRpe0P/cXUdsTElQWjJrbLc0Wu+7/nyg+iT8JHlk38f4
g8sGD6fJogK7AuFgKp8R+w8eCDr9JgC89w==
-----END EC PRIVATE KEY-----

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEPP4jo2krvRpe0P/cXUdsTElQWjJr
bLc0Wu+7/nyg+iT8JHlk38f4g8sGD6fJogK7AuFgKp8R+w8eCDr9JgC89w==
-----END PUBLIC KEY-----
*/

public class Main {

    public static void main(String[] args) throws Exception {
        String b64PublicKey = "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEPP4jo2krvRpe0P/cXUdsTElQWjJrbLc0Wu+7/nyg+iT8JHlk38f4g8sGD6fJogK7AuFgKp8R+w8eCDr9JgC89w==";
        String b64Signature = "MEUCIHZ7HA6tyCmqxmPAsYwBAQaswxm+2wA8mHFKtp8vW/qEAiEArW2fd9gEfnHCEOb4dTnXSxnKdZQels90/dq71Sw7Pl8=";
        String dataStr = "tolga";

        // Convert String data to byte array.
        byte[] data = dataStr.getBytes();
        byte[] publicKeyData = Base64.getDecoder().decode(b64PublicKey);
        byte[] signatureData = Base64.getDecoder().decode(b64Signature);

        PublicKey publicKey = KeyFactory.getInstance("EC").generatePublic(new X509EncodedKeySpec(publicKeyData));
        Signature sig = Signature.getInstance("SHA512withECDSA");

        sig.initVerify(publicKey);
        sig.update(data);

        boolean verify = sig.verify(signatureData);

        System.out.println("Verification: " + verify);
    }
}