Encryption and Decryption in Java

Java provides several algorithms for encryption and decryption of data, including symmetric-key algorithms and asymmetric-key algorithms.

1. **Symmetric-key algorithms:** 
These algorithms use the same key for both encryption and decryption. Some commonly used symmetric-key algorithms in Java include:
	1. Advanced Encryption Standard (AES)
	2. Data Encryption Standard (DES)
	3. Blowfish
	4. Twofish
2. **Asymmetric-key algorithms:**
These algorithms use different keys for encryption and decryption. They are typically used for secure communication between two parties, such as in public key cryptography. Some commonly used asymmetric-key algorithms in Java include:
	1. RSA (Rivest-Shamir-Adleman)
	2. Elliptic Curve Cryptography (ECC)
	3. Diffie-Hellman key exchange

Java provides built-in classes and interfaces for implementing these encryption and decryption algorithms, such as the `javax.crypto` package for symmetric-key algorithms and the `java.security` package for asymmetric-key algorithms. Additionally, third-party libraries and frameworks are also available for implementing encryption and decryption in Java.

#### Advanced Encryption Standard (AES)
The Advanced Encryption Standard (AES) is a symmetric-key encryption algorithm. 
- AES is a block cipher, which means that it encrypts data in fixed-size blocks, with each block being processed independently.
- AES uses a variable-length key that can be either 128, 192, or 256 bits long.
- The algorithm consists of a series of encryption and decryption rounds, with the number of rounds depending on the length of the key. 

##### AES Encryption process
During the encryption process, AES divides the plaintext message into fixed-size blocks and applies a series of substitutions and transformations to each block, based on the key. 
These operations include: 
- substitution with a fixed substitution table,
- a row-shifting operation, 
- a column-mixing operation, and 
- an XOR operation with a round key. 
The resulting output from each round is used as the input to the next round.

##### AES Decryption process
During decryption process, the inverse operations are applied to the ciphertext blocks using the **same key**, in reverse order. 
The decryption process is as follow
- starts with an inverse XOR operation with the last round key,
- an inverse column-mixing operation,
- an inverse row-shifting operation, and 
- an inverse substitution operation.

AES is widely used in a variety of applications, including secure communication protocols, disk encryption, and secure data storage. It has been extensively studied and tested, and is considered to be a secure and reliable encryption algorithm.

##### Sample example shows encrypting and decrypting plain text using AES

In this example, we define two methods - `encrypt()` and `decrypt()` - that use the AES algorithm to encrypt and decrypt a given text using a secret key. 

```
import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;
import java.util.Base64;

public class AESExample {
    
    public static void main(String[] args) throws Exception {
        String plainText = "This is a secret message!";
        String key = "mySecretKey43234";
        
        //Encrypt the text
        byte[] encryptedText = encrypt(plainText, key);
        System.out.println("Encrypted Text: " + new String(encryptedText));
        
        //Decrypt the encrypted text
        String decryptedText = decrypt(encryptedText, key);
        System.out.println("Decrypted Text: " + decryptedText);
    }
    
    public static byte[] encrypt(String plainText, String key) throws Exception {
        SecretKeySpec secretKeySpec = new SecretKeySpec(key.getBytes(), "AES");
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.ENCRYPT_MODE, secretKeySpec);
        byte[] encryptedText = cipher.doFinal(plainText.getBytes());
        return Base64.getEncoder().encode(encryptedText);
    }
    
    public static String decrypt(byte[] encryptedText, String key) throws Exception {
        SecretKeySpec secretKeySpec = new SecretKeySpec(key.getBytes(), "AES");
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.DECRYPT_MODE, secretKeySpec);
        byte[] decryptedText = cipher.doFinal(Base64.getDecoder().decode(encryptedText));
        return new String(decryptedText);
    }
}

```