Unlock Java Encryption: A Hands‑On Guide to Bouncy Castle

1. Introduction

Using cryptographic and digest algorithms in Java is common, but implementing them requires deep knowledge of algorithm rules, bitwise operations, and number‑base conversions. Java provides JCA, JCE, and JSSE, which can be overwhelming, so using a library is advisable.

2. Bouncy Castle

Bouncy Castle is an open‑source JCE provider for Java that offers the most comprehensive set of cryptographic algorithms. Two main Maven artifacts are available:

JDK1.5 version

Lightweight (trimmed) version:

<dependency>
  <groupId>org.bouncycastle</groupId>
  <artifactId>bcprov-jdk15on</artifactId>
  <version>1.66</version>
</dependency>

Full version:

artifactId=bcprov-ext-jdk15on

JDK1.5‑JDK8 version

Lightweight version:

<dependency>
  <groupId>org.bouncycastle</groupId>
  <artifactId>bcprov-jdk15to18</artifactId>
  <version>1.66</version>
</dependency>

Full version:

artifactId=bcprov-ext-jdk15to18

3. Simple Demonstration

The following examples show a commonly used digest algorithm (MD5) and an encryption algorithm (AES).

MD5 digest

/**
 * bouncyCastle md5 digest.
 *
 * @param src the src
 * @return the string
 */
public String bouncyCastleMD5(String src) {
    // initialize MD5 digest
    Digest digest = new MD5Digest();
    // bytes to digest
    byte[] bytes = src.getBytes(StandardCharsets.UTF_8);
    // update
    digest.update(bytes, 0, bytes.length);
    byte[] md5Bytes = new byte[digest.getDigestSize()];
    // finalize
    digest.doFinal(md5Bytes, 0);
    // hex conversion
    return Hex.toHexString(md5Bytes).toUpperCase();
}

You can similarly use SHA256, SHA512, etc.

AES encryption

String src = "felord.cn";
// AES key must be at least 128 bits (16 characters)
String password = "ffffffffffffffff";
// Load Bouncy Castle provider
Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
// Get AES cipher instance
Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding");
// Initialize encryption
cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(password.getBytes(StandardCharsets.UTF_8), "AES"));
byte[] bytes = cipher.doFinal(src.getBytes(StandardCharsets.UTF_8));
String encrypt = Hex.toHexString(bytes).toUpperCase();
System.out.println("encrypt = " + encrypt);

// Decryption
cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(password.getBytes(StandardCharsets.UTF_8), "AES"));
byte[] decode = Hex.decode(encrypt.getBytes(StandardCharsets.UTF_8));
byte[] decrypt = cipher.doFinal(decode);
System.out.println("decrypt = " + new String(decrypt));

This encrypts the string felord.cn and then decrypts it back.

4. Usage Risks

Cryptographic algorithms are often subject to export controls and patents in many countries, including China. Software using them may require compliance checks when exported or used in certain jurisdictions.

Other algorithm demos are available; interested readers can follow the public account and reply with “bouncy” to receive them.