Introduction to Key Encapsulation Mechanism API in Java

Key Encapsulation Mechanism (KEM) is nothing but it is a cryptographic technique used to securely encapsulate the symmetric cryptographic keys within the asymmetric key pairs. In Java, Java Cryptography Architecture (JCA) provides the APIs for implementing the various cryptographic operations which are including the Key Encapsulation Mechanism. Java doesn't have the built-in API specification named the "Key Encapsulation Mechanism API".

Here, we can implement the process of implementing a basic Key Encapsulation Mechanism using Java's Cryptographic APIs.

Prerequisites:

The following are the prerequisites to implement the Key Encapsulation Mechanism API in Java:

• Java Development Kit (JDK)
• Understanding of Java Cryptography Architecture (JCA)
• Knowledge of Asymmetric and Symmetric Cryptography
• Java Cryptography Extension (JCE)
• Security Considerations
• Java IDE (Optional)

Implementation of KEM API in Java

Step 1: Set up Eclipse Project

1. Open Eclipse and create a new Java project.
2. Name the project as "KeyEncapsulationExample".

Step 2: Generating Asymmetric Key Pair

1. Create a Class file and name it as "AsymmetricKeyGenerator" in the src folder.
2. Open the Class file and write the below code to implement the Asymmetric Key Generator Pair.

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;

public class AsymmetricKeyGenerator {
    public static KeyPair generateKeyPair() throws NoSuchAlgorithmException {
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(2048); // Key size
        return keyGen.generateKeyPair();
    }
}

Explanation of the above Program:

Here, Generating Asymmetric Key Pair is generates the RSA asymmetric key pair using the KeyPairGenerator. The size of the key is set to 2048 bits.

Step 3: Generating Symmetric Key

1. Create a Class file and name it as "SymmetricKeyGenerator" in src folder.
2. Open the Class file and write the below code to implement the Symmetric Key Generator.

import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import java.security.NoSuchAlgorithmException;

public class SymmetricKeyGenerator {
    public static SecretKey generateSymmetricKey() throws NoSuchAlgorithmException {
        KeyGenerator keyGen = KeyGenerator.getInstance("AES");
        keyGen.init(256); // Key size
        return keyGen.generateKey();
    }
}

Explanation of the above Program:

Here, Generating Symmetric Key is generates a 256-bit AES symmetric key using keyGen object. AES keys are typically 128, 192, or 256 bits in size.

Step 4: Encrypting Symmetric Key with Public Key

1. Create a Class file and name it as "KeyEncapsulation" in src folder.
2. Open the Class file and write the below code to implement the Encrypting Symmetric Key with Public Key.

import javax.crypto.Cipher;
import java.security.Key;
import java.security.PublicKey;

public class KeyEncapsulation {
    public static byte[] encapsulateKey(Key symmetricKey, PublicKey publicKey) throws Exception {
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, publicKey);
        return cipher.doFinal(symmetricKey.getEncoded());
    }
}

Explanation of the above Program:

1. Here, The KeyEncapsulation class contains a method encapsulateKey. This method is encrypts a symmetric key using RSA encryption with the provided public key.
2. This method is takes the symmetric key and a public key as input parameters and initializes a cipher for RSA encryption and encrypts the byte array representation of the symmetric key with use the public key.
3. Finally, it returns encrypted key as a byte array.
   

Step 5: Decrypting Encrypted Key with Private Key

1. Create a Class file and name it as "KeyDecapsulation" in src folder.
2. Open the Class file and write the below code to implement the Decrypting Encrypted Key with Private Key.

import javax.crypto.Cipher;
import java.security.Key;
import java.security.PrivateKey;

public class KeyDecapsulation {
    public static byte[] decapsulateKey(byte[] encryptedKey, PrivateKey privateKey) throws Exception {
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.DECRYPT_MODE, privateKey);
        return cipher.doFinal(encryptedKey);
    }
}

Explanation of the above Program:

Here, Decrypting Encrypted Symmetric Key with Private Key is initialize the "Cipher" object for RSA decryption with private key obtained from the key pair. After that, it decrypts the encrypted symmetric key using the RSA decryption.

Step 6: Putting it all Together in Main

1. Create a Class file and name it as "Main" in src folder.
2. Open the Class file and write the below code.

import java.security.KeyPair;
import java.security.PrivateKey;
import java.security.PublicKey;
import javax.crypto.SecretKey;

public class Main {
    public static void main(String[] args) throws Exception {
        // Step 1: Generate Asymmetric Key Pair
        KeyPair keyPair = AsymmetricKeyGenerator.generateKeyPair();
        PublicKey publicKey = keyPair.getPublic();
        PrivateKey privateKey = keyPair.getPrivate();
        
        // Step 2: Generate Symmetric Key
        SecretKey symmetricKey = SymmetricKeyGenerator.generateSymmetricKey();
        
        // Step 3: Encrypt Symmetric Key with Public Key
        byte[] encryptedKey = KeyEncapsulation.encapsulateKey(symmetricKey, publicKey);
        
        // Step 4: Decrypt Encrypted Key with Private Key
        byte[] decryptedKey = KeyDecapsulation.decapsulateKey(encryptedKey, privateKey);
        
        // Verify if the keys match
        if (java.util.Arrays.equals(symmetricKey.getEncoded(), decryptedKey)) {
            System.out.println("Keys match! Encapsulation and Decapsulation successful.");
        } else {
            System.out.println("Keys don't match! Encapsulation and Decapsulation failed.");
        }
    }
}

Step 7: Run the Code

1. After completion of implement the code, you need to run the code.
2. For run the code, right click on Java project and then select Run As > Java Application.
3. The output will be shown in console window in your Eclipse as shown below.

Output:

Here, the output is indicates the symmetric key encapsulation and decapsulation process was successfully completed and that the decrypted symmetric key matches the original one, it describes the correctness of Key Encapsulation Mechanism implementation.

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