(Android™) ECDSA Sign and Verify

Demonstrates how to create an ECDSA signature on the SHA256 hash of some data, and then verify.

Chilkat Android™ Downloads Android™ Java Libraries Android C/C++ Libraries

// Important: Don't forget to include the call to System.loadLibrary
// as shown at the bottom of this code sample.
package com.test;

import android.app.Activity;
import com.chilkatsoft.*;

import android.widget.TextView;
import android.os.Bundle;

public class SimpleActivity extends Activity {

  private static final String TAG = "Chilkat";

  // Called when the activity is first created.
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // First load an ECDSA private key to be used for signing.
    CkPrivateKey privKey = new CkPrivateKey();
    boolean success = privKey.LoadEncryptedPemFile("qa_data/ecc/secp256r1-key-pkcs8-secret.pem","secret");
    if (success == false) {
        Log.i(TAG, privKey.lastErrorText());
        return;
        }

    // Sign the SHA256 hash of some data.
    CkBinData bd = new CkBinData();
    success = bd.LoadFile("qa_data/hamlet.xml");
    if (success == false) {
        Log.i(TAG, "Failed to load file to be hashed.");
        return;
        }

    CkCrypt2 crypt = new CkCrypt2();
    crypt.put_HashAlgorithm("sha256");
    crypt.put_EncodingMode("base64");
    String hashStr = crypt.hashBdENC(bd);

    CkEcc ecdsa = new CkEcc();
    CkPrng prng = new CkPrng();
    // Returns ASN.1 signature as a base64 string.
    String sig = ecdsa.signHashENC(hashStr,"base64",privKey,prng);
    Log.i(TAG, "sig = " + sig);

    // The signature is in ASN.1 format (which may be described as the "encoded DSS signature").
    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
    //   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...

    // If you wish, you can get the r and s components of the signature like this:
    CkAsn asn = new CkAsn();
    asn.LoadEncoded(sig,"base64");
    CkXml xml = new CkXml();
    xml.LoadXml(asn.asnToXml());

    Log.i(TAG, xml.getXml());

    // We now have this:
    // <?xml version="1.0" encoding="utf-8"?>
    // <sequence>
    //     <int>6650D422D86BA4A228B5617604E59052591B9B2C32EF324C44D09EF67E5F0060</int>
    //     <int>0CFD9F6AC85042FC70F672C141BA6B2A4CAFBB906C3D907BCCC1BED62B28326F</int>
    // </sequence>

    // Get the "r" and "s" as hex strings
    String r = xml.getChildContentByIndex(0);
    String s = xml.getChildContentByIndex(1);

    Log.i(TAG, "r = " + r);
    Log.i(TAG, "s = " + s);

    // --------------------------------------------------------------------
    // Now verify against the hash of the original data.

    // Get the corresponding public key.
    CkPublicKey pubKey = new CkPublicKey();
    success = pubKey.LoadFromFile("qa_data/ecc/secp256r1-pub.pem");
    if (success == false) {
        Log.i(TAG, pubKey.lastErrorText());
        return;
        }

    // We already have the SHA256 hash of the original data (hashStr) so no need to re-do it..
    CkEcc ecc2 = new CkEcc();
    int result = ecc2.VerifyHashENC(hashStr,sig,"base64",pubKey);
    if (result != 1) {
        Log.i(TAG, ecc2.lastErrorText());
        return;
        }

    Log.i(TAG, "Verified!");

    // Note: If we have only r,s and wish to reconstruct the ASN.1 signature, we do it like this:
    CkXml xml2 = new CkXml();
    xml2.put_Tag("sequence");
    xml2.NewChild2("int",r);
    xml2.NewChild2("int",s);

    CkAsn asn2 = new CkAsn();
    asn2.LoadAsnXml(xml2.getXml());
    String encodedSig = asn2.getEncodedDer("base64");
    Log.i(TAG, "encoded DSS signature: " + encodedSig);

    // You can go to https://lapo.it/asn1js/  and copy/paste the base64 encodedSig into the online tool, then press the "decode" button.
    // You will see the ASN.1 such as this:

    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
    //   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...

  }

  static {
      System.loadLibrary("chilkat");

      // Note: If the incorrect library name is passed to System.loadLibrary,
      // then you will see the following error message at application startup:
      //"The application <your-application-name> has stopped unexpectedly. Please try again."
  }
}