Java Program to Generate EC Keys

EC Cryptography Tutorials - Herong's Tutorial Examples

∟Java Program to Generate EC Keys

This section provides a tutorial example on how to write a Java program to generate EC private-public key pairs.

If you are a Java developer, you can also write a Java program to generate EC private-public key pairs.

Here is my sample Java program that uses java.security.* packages to private-public key pairs with EC or other cryptographic algorithms.

/* JavaKeyPair.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.io.*;
import java.math.*;
import java.security.*;
import java.security.interfaces.*;
class JavaKeyPair {
   public static void main(String[] a) {
      if (a.length<3) {
         System.out.println("Usage:");
         System.out.println("java JavaKeyPair keySize output algorithm");
         return;
      }
      int keySize = Integer.parseInt(a[0]);
      String output = a[1];
      String algorithm = a[2]; // EC, RSA, DSA, ...
      try {
         getKeys(keySize,output,algorithm);
      } catch (Exception e) {
         System.out.println("Exception: "+e);
         return;
      }
   }
   private static void getKeys(int keySize, String output,
         String algorithm) throws Exception {
      KeyPairGenerator kg = KeyPairGenerator.getInstance(algorithm);
      kg.initialize(keySize);
      System.out.println();
      System.out.println("KeyPairGenerator Object Info: ");
      System.out.println("Algorithm = "+kg.getAlgorithm());
      System.out.println("Provider = "+kg.getProvider());
      System.out.println("Key Size = "+keySize);
      System.out.println("toString = "+kg.toString());
      KeyPair pair = kg.generateKeyPair();
      PrivateKey priKey = pair.getPrivate();
      PublicKey pubKey = pair.getPublic();
      String fl = output+".pri";
      FileOutputStream out = new FileOutputStream(fl);
      byte[] ky = priKey.getEncoded();
      out.write(ky);
      out.close();
      System.out.println();
      System.out.println("Private Key Info: ");
      System.out.println("Algorithm = "+priKey.getAlgorithm());
      System.out.println("Saved File = "+fl);
      System.out.println("Size = "+ky.length);
      System.out.println("Format = "+priKey.getFormat());
      System.out.println("toString = "+priKey.toString());
      fl = output+".pub";
      out = new FileOutputStream(fl);
      ky = pubKey.getEncoded();
      out.write(ky);
      out.close();
      System.out.println();
      System.out.println("Public Key Info: ");
      System.out.println("Algorithm = "+pubKey.getAlgorithm());
      System.out.println("Saved File = "+fl);
      System.out.println("Size = "+ky.length);
      System.out.println("Format = "+pubKey.getFormat());
      System.out.println("toString = "+pubKey.toString());
   }
}

Now let's try my sample program with different options.

1. Generate a 256-bit private-public key pairs. The output shows that Java picks the "secp256r1" curve.

herong> java JavaKeyPair 256 1st_ec EC

KeyPairGenerator Object Info:
Algorithm = EC
Provider = SunEC version 17
Key Size = 256
toString = java.security.KeyPairGenerator$Delegate@6d5380c2

Private Key Info:
Algorithm = EC
Saved File = 1st_ec.pri
Size = 67
Format = PKCS#8
toString = sun.security.ec.ECPrivateKeyImpl@3a0141b0

Public Key Info:
Algorithm = EC
Saved File = 1st_ec.pub
Size = 91
Format = X.509
toString = Sun EC public key, 256 bits
  public x coord: 853346207140393763650036399724805522838088855041430...
  public y coord: 745481693560289876495239256879081829182273437238861...
  parameters: secp256r1 [NIST P-256,X9.62 prime256v1] (1.2.840.10045.3.1.7)

herong> dir
  67 1st_ec.pri
  91 1st_ec.pub
  ...

2. Generate a longer private-public key pairs. The output shows that Java picks the "secp521r1" curve.

herong> java JavaKeyPair 521 2nd_ec EC

KeyPairGenerator Object Info:
Algorithm = ec
Provider = SunEC version 17
Key Size = 521
toString = java.security.KeyPairGenerator$Delegate@5ef04b5

Private Key Info:
Algorithm = EC
Saved File = 2nd_ec.pri
Size = 98
Format = PKCS#8
toString = sun.security.ec.ECPrivateKeyImpl@5b7f3667

Public Key Info:
Algorithm = EC
Saved File = 2nd_ec.pub
Size = 158
Format = X.509
toString = Sun EC public key, 521 bits
  public x coord: 518836930523842718148489356314770664342986719875920...
  public y coord: 564229509192138801573886481648145012198584749459934...
  parameters: secp521r1 [NIST P-521] (1.3.132.0.35)

3. Generate a shorter private-public key pairs. Too bad, curve "secp192r1" is not supported.

herong> java JavaKeyPair 192 3rd_ec EC

KeyPairGenerator Object Info:
Algorithm = ec
Provider = SunEC version 17
Key Size = 192
toString = java.security.KeyPairGenerator$Delegate@4459eb14
Exception: java.security.ProviderException: Curve not supported:
  secp192r1 [NIST P-192,X9.62 prime192v1] (1.2.840.10045.3.1.1)