(Android™) Understanding a few ECDSA Public Key Formats

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Describes a few ECDSA public key formats.

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);

    // Here we have the output of the following openssl command:  openssl ec -in key.pem -pubout -text

    // Private-Key: (256 bit)
    // priv:
    //     0e:63:25:8a:73:3c:71:b6:c0:e7:a3:0f:94:b9:74:
    //     e0:be:bd:46:18:be:40:7e:66:9e:21:99:85:0e:ed:
    //     87:2d
    // pub:
    //     04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
    //     b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
    //     39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
    //     2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
    //     12:b7:b8:6a:43
    // ASN1 OID: prime256v1
    // NIST CURVE: P-256
    // writing EC key
    // -----BEGIN PUBLIC KEY-----
    // MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
    // Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
    // -----END PUBLIC KEY-----

    // The public key is shown in two different formats.
    // The first is this:

    // pub:
    //     04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
    //     b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
    //     39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
    //     2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
    //     12:b7:b8:6a:43

    // It is the ANSI X9.63 format.
    // 65-bytes are the uncompressed public key (04 || X || Y) 

    // This is the same public key, but in PEM format

    // -----BEGIN PUBLIC KEY-----
    // MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
    // Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
    // -----END PUBLIC KEY-----

    // It contains ASN.1 that more explicitly identifies the key type.
    // 
    // SEQUENCE (2 elem)
    //   SEQUENCE (2 elem)
    //     OBJECT IDENTIFIER 1.2.840.10045.2.1 ecPublicKey (ANSI X9.62 public key type)
    //     OBJECT IDENTIFIER 1.2.840.10045.3.1.7 prime256v1 (ANSI X9.62 named elliptic curve)
    //   BIT STRING (520 bit) 000001000101110100011010010011111101100110111101010010011001111011100...

    boolean success;

    // PEM format can be loaded into a Chilkat public key object like this:

    CkStringBuilder sbPem = new CkStringBuilder();
    sbPem.Append("-----BEGIN PUBLIC KEY-----\r\n");
    sbPem.Append("MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET\r\n");
    sbPem.Append("Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==\r\n");
    sbPem.Append("-----END PUBLIC KEY-----\r\n");

    CkPublicKey pubKey1 = new CkPublicKey();
    success = pubKey1.LoadFromString(sbPem.getAsString());
    if (success == false) {
        Log.i(TAG, pubKey1.lastErrorText());
        return;
        }

    // The X9.63 format can be loaded like this:
    CkStringBuilder sbHex = new CkStringBuilder();
    sbHex.Append("04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:");
    sbHex.Append("b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:");
    sbHex.Append("39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:");
    sbHex.Append("2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:");
    sbHex.Append("12:b7:b8:6a:43");

    // Get rid of the ":" chars.
    int numReplaced = sbHex.Replace(":","");

    // We'll need to convert hex to base64..
    CkBinData bdKey = new CkBinData();
    bdKey.AppendEncoded(sbHex.getAsString(),"hex");

    CkPublicKey pubKey2 = new CkPublicKey();
    success = pubKey2.LoadFromString(bdKey.getEncoded("base64"));
    if (success == false) {
        Log.i(TAG, pubKey2.lastErrorText());
        return;
        }

    // Let's get the key in pubKey2 as PEM.
    // It should be idential to the PEM above.
    Log.i(TAG, pubKey2.getPem(true));

    // Here's the output.  You can see it's the same as the PEM above..

    // -----BEGIN PUBLIC KEY-----
    // MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
    // Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
    // -----END PUBLIC KEY-----

  }

  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."
  }
}