(Android™) ECDSA Sign Data and Get Raw R and S Values

Demonstrates getting the raw R and S value of an ECDSA signature.

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

    // To create an ECDSA signature, the data first needs to be hashed.  Then the hash
    // is signed.

    CkCrypt2 crypt = new CkCrypt2();
    crypt.put_HashAlgorithm("SHA256");
    crypt.put_Charset("utf-8");
    crypt.put_EncodingMode("base64");

    // Hash a string.
    String hash1 = crypt.hashStringENC("The quick brown fox jumps over the lazy dog");
    Log.i(TAG, "hash1 = " + hash1);

    // -----------------------------------------------------------
    // An ECDSA private key is used for signing.  The public key is for signature verification.
    // Load our ECC private key.
    // Our private key file contains this:

    // 	// -----BEGIN PRIVATE KEY-----
    // 	MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg3J8q/24D1sEKGdP9
    // 	72MGYElLGpw/a56Y3t6pfON3uhShRANCAATlSmoizyhAwoYZAOuFBATl07/1RR54
    // 	a1Dzfm16grxJe666AGKR+bSs24hk7TEpaeCTvT8YOOM3l+xKFg7zq6Q9
    // 	-----END PRIVATE KEY-----

    CkPrivateKey privKey = new CkPrivateKey();
    boolean success = privKey.LoadPemFile("qa_data/ecc/secp256r1-key-pkcs8.pem");
    if (success != true) {
        Log.i(TAG, privKey.lastErrorText());
        return;
        }

    // Sign the hash..
    CkPrng prng = new CkPrng();
    CkEcc ecdsa = new CkEcc();
    String ecdsaSigBase64 = ecdsa.signHashENC(hash1,"base64",privKey,prng);
    if (ecdsa.get_LastMethodSuccess() != true) {
        Log.i(TAG, ecdsa.lastErrorText());
        return;
        }

    // The ECDSA signature is ASN.1 that contains a sequence of 2 large integers (r and s)
    // For example:
    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9
    //   INTEGER (255 bit) 481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5

    Log.i(TAG, "Base64 ECDSA signature = " + ecdsaSigBase64);

    // If the raw R and S values are needed, here's how to get them:
    CkAsn asn = new CkAsn();
    success = asn.LoadEncoded(ecdsaSigBase64,"base64");
    if (success == false) {
        Log.i(TAG, asn.lastErrorText());
        return;
        }

    // The R and X will be in hexidecimal in the XML.
    CkXml xml = new CkXml();
    xml.LoadXml(asn.asnToXml());
    Log.i(TAG, xml.getXml());

    // The XML looks like this:
    // <sequence>
    // <int>792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9</int>
    // <int>481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5</int>
    // </sequence>

    // Copy raw R and S hex values into a Chilkat BinData object.
    CkBinData bd = new CkBinData();
    String r = xml.getChildContent("int[0]");
    String s = xml.getChildContent("int[1]");
    bd.AppendEncoded(r,"hex");
    bd.AppendEncoded(s,"hex");

    Log.i(TAG, "Number of bytes in bd: " + String.valueOf(bd.get_NumBytes()));

  }

  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."
  }
}