java加密实现

要实现加解密,最简单实现如下：
核心类Ciper,三步走：（以"RSA"为例）
1.获取Keypair（公钥私钥对）。生成或者获取公钥和私钥。
生成Keypair过程，可以写死到文件里，去读取。或者通过api自动生成。
2.实例化Cipher，约定算法
2.初始化Cipher，定义是加密还是解密。是公约加密啊，还是私钥加密，是公钥解密，还是私钥解密。
3.获取加密或解密的结果。

// KeyPairGenerator类用于生成公钥和私钥对，基于RSA算法生成对象  
KeyPairGenerator keyPairGen = KeyPairGenerator.getInstance("RSA");  
// 初始化**对生成器，**大小为96-1024位  
keyPairGen.initialize(1024,new SecureRandom());  
// 生成一个**对，保存在keyPair中  
KeyPair keyPair = keyPairGen.generateKeyPair();  
RSAPrivateKey privateKey = (RSAPrivateKey) keyPair.getPrivate();   // 得到私钥  
RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();  // 得到公钥  

//**公钥加密**
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher. ENCRYPT_MODE, publicKey);
cipher.doFinal(minwen.getBytes("UTF-8")
//**私钥解密**
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.DECRYPT_MODE, privateKey);
cipher.doFinal(miwen.getBytes("UTF-8")

补充：

a) 由于多平台加解密会存在不兼容的问题。（如：Android系统和windows，linux平台默认支持的加密算法不一致）Ciper实例化，需要使用第三方类 BouncyCastleProvider 代码如下：

Cipher cipher = Cipher.getInstance("RSA/NONE/PKCS1Padding",
                new BouncyCastleProvider().getName());
//算法类型/工作方式/填充方式

b) 为了让字节数组转字符串格式固定，一般需要用到base64转码
apache.commons-codex下的base64类

    //base64 解码
    public static String decode(byte[] bytes) {  
        return new String(Base64.decodeBase64(bytes));  
    }  
  
    //base64 编码
    public static String encode(byte[] bytes) {  
        return new String(Base64.encodeBase64(bytes));  
    }  

c) 如果要打印出公钥，私钥，可通过 java.security.Key获取字节，再转字符串。

/**
     * Returns the key in its primary encoding format, or null
     * if this key does not support encoding.
     *
     * @return the encoded key, or null if the key does not support
     * encoding.
     */
    public byte[] getEncoded();

d) 若有需要在加解密中加入自己的算法逻辑，如字节数组重排等，可见Ciper类其他api。

 /**
     * Finishes a multi-part transformation (encryption or decryption).
     * <p>
     * Processes any bytes that may have been buffered in previous {@code
     * update} calls.
     * <p>
     * The final transformed bytes are stored in the {@code output} buffer.
     *
     * @param output
     *            the output buffer.
     * @param outputOffset
     *            the offset in the output buffer.
     * @return the number of bytes placed in the output buffer.
     * @throws IllegalBlockSizeException
     *             if the size of the resulting bytes is not a multiple of the
     *             cipher block size.
     * @throws ShortBufferException
     *             if the size of the {@code output} buffer is too small.
     * @throws BadPaddingException
     *             if the padding of the data does not match the padding scheme.
     * @throws IllegalStateException
     *             if this cipher instance is not initialized for encryption or
     *             decryption.
     */
    public final int doFinal(byte[] output, int outputOffset)
            throws IllegalBlockSizeException, ShortBufferException,
            BadPaddingException {
        if (mode != ENCRYPT_MODE && mode != DECRYPT_MODE) {
            throw new IllegalStateException();
        }
        if (outputOffset < 0) {
            throw new IllegalArgumentException("outputOffset < 0. outputOffset=" + outputOffset);
        }
        return getSpi().engineDoFinal(null, 0, 0, output, outputOffset);
    }
 /**
     * Finishes a multi-part transformation (encryption or decryption).
     * <p>
     * Processes the bytes in {@code input} buffer, and any bytes that have been
     * buffered in previous {@code update} calls.
     *
     * @param input
     *            the input buffer.
     * @return the final bytes from the transformation.
     * @throws IllegalBlockSizeException
     *             if the size of the resulting bytes is not a multiple of the
     *             cipher block size.
     * @throws BadPaddingException
     *             if the padding of the data does not match the padding scheme.
     * @throws IllegalStateException
     *             if this cipher instance is not initialized for encryption or
     *             decryption.
     */
    public final byte[] doFinal(byte[] input) throws IllegalBlockSizeException,
            BadPaddingException {
        if (mode != ENCRYPT_MODE && mode != DECRYPT_MODE) {
            throw new IllegalStateException();
        }
        return getSpi().engineDoFinal(input, 0, input.length);
    }
 /**
     * Finishes a multi-part transformation (encryption or decryption).
     * <p>
     * Processes the {@code inputLen} bytes in {@code input} buffer at {@code
     * inputOffset}, and any bytes that have been buffered in previous {@code
     * update} calls.
     *
     * @param input
     *            the input buffer.
     * @param inputOffset
     *            the offset in the input buffer.
     * @param inputLen
     *            the length of the input
     * @return the final bytes from the transformation.
     * @throws IllegalBlockSizeException
     *             if the size of the resulting bytes is not a multiple of the
     *             cipher block size.
     * @throws BadPaddingException
     *             if the padding of the data does not match the padding scheme.
     * @throws IllegalStateException
     *             if this cipher instance is not initialized for encryption or
     *             decryption.
     * @throws IllegalArgumentException
     *             if {@code inputOffset} and {@code inputLen} do not specify an
     *             valid chunk in the input buffer.
     */
    public final byte[] doFinal(byte[] input, int inputOffset, int inputLen)
            throws IllegalBlockSizeException, BadPaddingException {
        if (mode != ENCRYPT_MODE && mode != DECRYPT_MODE) {
            throw new IllegalStateException();
        }
        checkInputOffsetAndCount(input.length, inputOffset, inputLen);
        return getSpi().engineDoFinal(input, inputOffset, inputLen);
    }

 /**
     * Finishes a multi-part transformation (encryption or decryption).
     * <p>
     * Processes the {@code inputLen} bytes in {@code input} buffer at {@code
     * inputOffset}, and any bytes that have been buffered in previous {@code
     * update} calls.
     *
     * @param input
     *            the input buffer.
     * @param inputOffset
     *            the offset in the input buffer.
     * @param inputLen
     *            the length of the input.
     * @param output
     *            the output buffer for the transformed bytes.
     * @param outputOffset
     *            the offset in the output buffer.
     * @return the number of bytes placed in the output buffer.
     * @throws ShortBufferException
     *             if the size of the {@code output} buffer is too small.
     * @throws IllegalBlockSizeException
     *             if the size of the resulting bytes is not a multiple of the
     *             cipher block size.
     * @throws BadPaddingException
     *             if the padding of the data does not match the padding scheme.
     * @throws IllegalStateException
     *             if this cipher instance is not initialized for encryption or
     *             decryption.
     * @throws IllegalArgumentException
     *             if {@code inputOffset} and {@code inputLen} do not specify an
     *             valid chunk in the input buffer.
     */
  public final int doFinal(byte[] input, int inputOffset, int inputLen,
            byte[] output, int outputOffset) throws ShortBufferException,
            IllegalBlockSizeException, BadPaddingException {
        if (mode != ENCRYPT_MODE && mode != DECRYPT_MODE) {
            throw new IllegalStateException();
        }
        checkInputOffsetAndCount(input.length, inputOffset, inputLen);
        return getSpi().engineDoFinal(input, inputOffset, inputLen, output,
                outputOffset);
    }

  /**
     * Continues a multi-part transformation (encryption or decryption). The
     * transformed bytes are returned.
     *
     * @param input
     *            the input bytes to transform.
     * @return the transformed bytes in a new buffer, or {@code null} if the
     *         input has zero length.
     * @throws IllegalStateException
     *             if this cipher instance is not initialized for encryption or
     *             decryption.
     * @throws IllegalArgumentException
     *             if the input is {@code null}.
     */
    public final byte[] update(byte[] input) {
        if (mode != ENCRYPT_MODE && mode != DECRYPT_MODE) {
            throw new IllegalStateException();
        }
        if (input == null) {
            throw new IllegalArgumentException("input == null");
        }
        if (input.length == 0) {
            return null;
        }
        return getSpi().engineUpdate(input, 0, input.length);
    }

更多实现可参考：
https://www.cnblogs.com/lz2017/p/6917049.html