Prepare the Signature for SkAdNetwork iOS14

skAdNetworkSignature.java

package main.java.com.abccompany.example;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
import java.security.*;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Base64;
import java.util.regex.Pattern;

public class SignaturePoc {

    public static void main(String[] args) throws Exception {
        final String AdNetworkVersion = "2.0";
        final String AdNetworkIdentifier = "97r2b46746.skadnetwork"; // ABC AdNetwork ID on Apple
        final String AdNetworkCampaignIdentifier = "99";
        final String ITunesItemIdentifier = "711455226"; // Forge of Empires App Store Page
        final String AdNetworkNonce = "e7b315b5-5d3d-4ceb-bb90-b617dee5e173";
        final String SourceAppStoreIdentifier = "331786748"; // CNN App Store page
        final String ParameterAdNetworkTimestamp = "1598441577"; // The Current Unix Timestamp

        final String prepareSignatureForV1 = AdNetworkIdentifier + '\u2063' + AdNetworkCampaignIdentifier + '\u2063' + ITunesItemIdentifier + '\u2063' + AdNetworkNonce + '\u2063' + ParameterAdNetworkTimestamp;
        final String prepareSignatureForV2 = AdNetworkVersion + '\u2063' + AdNetworkIdentifier + '\u2063' + AdNetworkCampaignIdentifier + '\u2063' + ITunesItemIdentifier + '\u2063' + AdNetworkNonce + '\u2063' + SourceAppStoreIdentifier + '\u2063' + ParameterAdNetworkTimestamp;

        System.out.println("--- Version 1.0 ---");
        signStringMessage(prepareSignatureForV1);
        System.out.println("\n\n--- Version 1.0 ---");
        signStringMessage(prepareSignatureForV2);
    }

    static private String signStringMessage(String msg) throws Exception {
        System.out.println("signStringMessage: " + msg);
        final String ALGO = "SHA256withECDSA";

        KeyFactory kf = KeyFactory.getInstance("EC");
        byte[] keyBytesArray = loadPEM("file.pem");
        PKCS8EncodedKeySpec pkcs8EncodedKey = new PKCS8EncodedKeySpec(keyBytesArray);
        PrivateKey key = kf.generatePrivate(pkcs8EncodedKey);

        /*
         * Create a Signature object and initialize it with the private key
         */
        Signature ecdsa = Signature.getInstance(ALGO);
        ecdsa.initSign(key);
        byte[] strByte = msg.getBytes("UTF-8");
        ecdsa.update(strByte);

        /*
         * Now that all the data to be signed has been read in, generate a
         * signature for it
         */

        byte[] realSig = ecdsa.sign();
        // https://developer.apple.com/documentation/storekit/skadnetwork/generating_the_signature_to_validate_an_installation#3627357
        // Encode the binary signature you generated into a Base64 string.
        String sig = Base64.getEncoder().encodeToString(realSig);
        System.out.println("Signature: " + sig);
        return sig;
    }

    static private byte[] loadPEM(String fileName) throws IOException {
        ClassLoader classloader = Thread.currentThread().getContextClassLoader();
        InputStream in = classloader.getResourceAsStream(fileName);
        String pem = new String(readAllBytes(in), StandardCharsets.ISO_8859_1);
        Pattern parse = Pattern.compile("(?m)(?s)^---*BEGIN.*---*$(.*)^---*END.*---*$.*");
        String encoded = parse.matcher(pem).replaceFirst("$1");
        return Base64.getMimeDecoder().decode(encoded);
    }

    static byte[] readAllBytes(InputStream in) throws IOException {
        ByteArrayOutputStream baos= new ByteArrayOutputStream();
        byte[] buf = new byte[1024];
        for (int read=0; read != -1; read = in.read(buf)) { baos.write(buf, 0, read); }
        return baos.toByteArray();
    }
}