文件加密传输其实就是将文件以二进制格式进行传输。
其中加密文件主要由：源文件二进制文件、源文件数字摘要、数字签名、特征码等等组成。
摘要可确认文件的唯一性，数字签名则是对摘要进行了加密。

本文主要记录使用RSA加密方式

其中生成RSA密钥主要介绍二种方式：

1、安装openssl情况下使用Linux命令生成

2、Java代码实现

1、查看openssl版本
　 openssl version -a

2、生成私钥
　　openssl genrsa -out rsa_private_key.pem 2048 
　　会生成rsa_private_key.pem私钥文件，私钥文件不能使用

3、生成公钥
　　openssl rsa -in rsa_private_key.pem -out rsa_public_key.pem -puboutopenssl pkcs8 -topk8 -inform PEM -outform DER -in private_key.pem -out private_key.der -nocrypt
　　私钥文件不能使用

4、私钥文件PKCS#8编码
　　openssl pkcs8 -topk8 -in rsa_private_key.pem -out pkcs8_rsa_private_key.pem
　　此处生成的私钥文件方可用于Java

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;

import org.apache.commons.codec.binary.Base64;

public class RSAEncrypt {
    /**
     * 字节数据转字符串专用集合
     */
    private static final char[] HEX_CHAR = {‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘,
            ‘7‘, ‘8‘, ‘9‘, ‘a‘, ‘b‘, ‘c‘, ‘d‘, ‘e‘, ‘f‘};
    private static final String PRIVATE_BEGIN = "-----BEGIN PRIVATE KEY-----";
    private static final String PRIVATE_END = "-----END PRIVATE KEY-----";
    private static final String PUBLIC_BEGIN = "-----BEGIN PUBLIC KEY-----";
    private static final String PUBLIC_END = "-----END PUBLIC KEY-----";


    /**
     * 1、随机生成密钥对
     *
     * @param filePath 密钥存放目录
     */
    public void genKeyPair(String filePath) {
        // KeyPairGenerator类用于生成公钥和私钥对，基于RSA算法生成对象
        KeyPairGenerator keyPairGen = null;
        try {
            keyPairGen = KeyPairGenerator.getInstance("RSA");
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }
        // 初始化密钥对生成器，密钥大小为96-1024位
        keyPairGen.initialize(1024, new SecureRandom());
        // 生成一个密钥对，保存在keyPair中
        KeyPair keyPair = keyPairGen.generateKeyPair();
        // 得到私钥
        RSAPrivateKey privateKey = (RSAPrivateKey) keyPair.getPrivate();
        // 得到公钥
        RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();
        try {
            // 得到公钥字符串
            Base64 base64 = new Base64();
            String publicKeyString = new String(base64.encode(publicKey.getEncoded()));
            // 得到私钥字符串
            String privateKeyString = new String(base64.encode(privateKey.getEncoded()));
            // 将密钥对写入到文件
            FileWriter pubfw = new FileWriter(filePath + "\\publicKey.pem");
            FileWriter prifw = new FileWriter(filePath + "\\privateKey.pem");
            BufferedWriter pubbw = new BufferedWriter(pubfw);
            BufferedWriter pribw = new BufferedWriter(prifw);
            pubbw.write(publicKeyString);
            pribw.write(privateKeyString);
            pubbw.flush();
            pubbw.close();
            pubfw.close();
            pribw.flush();
            pribw.close();
            prifw.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 2、从本地文件中读取公钥
     *
     * @param path 公钥路径
     * @return 公钥字符串
     * @throws Exception 异常信息
     */
    public String loadPublicKeyByFile(String path) throws Exception {
        try {
            BufferedReader br = new BufferedReader(new FileReader(path));
            String readLine = null;
            StringBuilder sb = new StringBuilder();
            while ((readLine = br.readLine()) != null) {
                // 去除公钥头部底部
                if (!readLine.equals(PUBLIC_BEGIN) && !readLine.equals(PUBLIC_END)) {
                    sb.append(readLine);
                }
            }
            br.close();
            return sb.toString();
        } catch (IOException e) {
            throw new Exception("公钥数据流读取错误");
        } catch (NullPointerException e) {
            throw new Exception("公钥输入流为空");
        }
    }

    /**
     * 3、字符串公钥转公钥对象
     *
     * @param publicKeyStr 公钥字符串类型
     * @return 公钥对象
     * @throws Exception 异常信息
     */
    public RSAPublicKey loadPublicKeyByStr(String publicKeyStr)
            throws Exception {
        try {
            Base64 base64 = new Base64();
            byte[] buffer = base64.decode(publicKeyStr);
            KeyFactory keyFactory = KeyFactory.getInstance("RSA");
            X509EncodedKeySpec keySpec = new X509EncodedKeySpec(buffer);
            return (RSAPublicKey) keyFactory.generatePublic(keySpec);
        } catch (NoSuchAlgorithmException e) {
            throw new Exception("无此算法");
        } catch (InvalidKeySpecException e) {
            throw new Exception("公钥非法");
        } catch (NullPointerException e) {
            throw new Exception("公钥数据为空");
        }
    }

    /**
     * 4、从本地文件中读取私钥
     *
     * @param path 私钥文件路径
     * @return 私钥字符串
     * @throws Exception 异常信息
     */
    public String loadPrivateKeyByFile(String path) throws Exception {
        try {
            BufferedReader br = new BufferedReader(new FileReader(path));
            String readLine = null;
            StringBuilder sb = new StringBuilder();
            while ((readLine = br.readLine()) != null) {
                //去除私钥头部底部
                if (!readLine.equals(PRIVATE_BEGIN) && !readLine.equals(PRIVATE_END)) {
                    sb.append(readLine);
                } else {
                }
            }
            br.close();
            return sb.toString();
        } catch (IOException e) {
            throw new Exception("私钥数据读取错误");
        } catch (NullPointerException e) {
            throw new Exception("私钥输入流为空");
        }
    }


    /**
     * 5、字符串公钥转公钥对象
     *
     * @param privateKeyStr 私钥字符串类型
     * @return 私钥对象
     * @throws Exception 异常信息
     */
    public RSAPrivateKey loadPrivateKeyByStr(String privateKeyStr)
            throws Exception {
        try {
            Base64 base64 = new Base64();
            byte[] buffer = base64.decode(privateKeyStr);
            PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(buffer);
            KeyFactory keyFactory = KeyFactory.getInstance("RSA");
            return (RSAPrivateKey) keyFactory.generatePrivate(keySpec);
        } catch (NoSuchAlgorithmException e) {
            throw new Exception("无此算法");
        } catch (InvalidKeySpecException e) {
            throw new Exception("私钥非法");
        } catch (NullPointerException e) {
            throw new Exception("私钥数据为空");
        }
    }

    /**
     * 6、公钥加密过程
     *
     * @param publicKey     公钥
     * @param plainTextData 明文数据
     * @return
     * @throws Exception 加密过程中的异常信息
     */
    public byte[] encrypt(RSAPublicKey publicKey, byte[] plainTextData)
            throws Exception {
        if (publicKey == null) {
            throw new Exception("加密公钥为空, 请设置");
        }
        Cipher cipher = null;
        try {
            // 使用默认RSA
            cipher = Cipher.getInstance("RSA");
            // cipher= Cipher.getInstance("RSA", new BouncyCastleProvider());
            cipher.init(Cipher.ENCRYPT_MODE, publicKey);
            byte[] output = cipher.doFinal(plainTextData);
            return output;
        } catch (NoSuchAlgorithmException e) {
            throw new Exception("无此加密算法");
        } catch (NoSuchPaddingException e) {
            e.printStackTrace();
            return null;
        } catch (InvalidKeyException e) {
            throw new Exception("加密公钥非法,请检查");
        } catch (IllegalBlockSizeException e) {
            throw new Exception("明文长度非法");
        } catch (BadPaddingException e) {
            throw new Exception("明文数据已损坏");
        }
    }

    /**
     * 7、私钥加密过程
     *
     * @param privateKey    私钥
     * @param plainTextData 明文数据
     * @return
     * @throws Exception 加密过程中的异常信息
     */
    public byte[] encrypt(RSAPrivateKey privateKey, byte[] plainTextData)
            throws Exception {
        if (privateKey == null) {
            throw new Exception("加密私钥为空, 请设置");
        }
        Cipher cipher = null;
        try {
            // 使用默认RSA
            cipher = Cipher.getInstance("RSA");
            cipher.init(Cipher.ENCRYPT_MODE, privateKey);
            byte[] output = cipher.doFinal(plainTextData);
            return output;
        } catch (NoSuchAlgorithmException e) {
            throw new Exception("无此加密算法");
        } catch (NoSuchPaddingException e) {
            e.printStackTrace();
            return null;
        } catch (InvalidKeyException e) {
            throw new Exception("加密私钥非法,请检查");
        } catch (IllegalBlockSizeException e) {
            throw new Exception("明文长度非法");
        } catch (BadPaddingException e) {
            throw new Exception("明文数据已损坏");
        }
    }

    /**
     * 8、私钥解密过程
     *
     * @param privateKey 私钥
     * @param cipherData 密文数据
     * @return 明文
     * @throws Exception 解密过程中的异常信息
     */
    public byte[] decrypt(RSAPrivateKey privateKey, byte[] cipherData)
            throws Exception {
        if (privateKey == null) {
            throw new Exception("解密私钥为空, 请设置");
        }
        Cipher cipher = null;
        try {
            // 使用默认RSA
            cipher = Cipher.getInstance("RSA");
            // cipher= Cipher.getInstance("RSA", new BouncyCastleProvider());
            cipher.init(Cipher.DECRYPT_MODE, privateKey);
            byte[] output = cipher.doFinal(cipherData);
            return output;
        } catch (NoSuchAlgorithmException e) {
            throw new Exception("无此解密算法");
        } catch (NoSuchPaddingException e) {
            e.printStackTrace();
            return null;
        } catch (InvalidKeyException e) {
            throw new Exception("解密私钥非法,请检查");
        } catch (IllegalBlockSizeException e) {
            throw new Exception("密文长度非法");
        } catch (BadPaddingException e) {
            throw new Exception("密文数据已损坏");
        }
    }

    /**
     * 9、公钥解密过程
     *
     * @param publicKey  公钥
     * @param cipherData 密文数据
     * @return 明文
     * @throws Exception 解密过程中的异常信息
     */
    public byte[] decrypt(RSAPublicKey publicKey, byte[] cipherData)
            throws Exception {
        if (publicKey == null) {
            throw new Exception("解密公钥为空, 请设置");
        }
        Cipher cipher = null;
        try {
            // 使用默认RSA
            cipher = Cipher.getInstance("RSA");
            // cipher= Cipher.getInstance("RSA", new BouncyCastleProvider());
            cipher.init(Cipher.DECRYPT_MODE, publicKey);
            byte[] output = cipher.doFinal(cipherData);
            return output;
        } catch (NoSuchAlgorithmException e) {
            throw new Exception("无此解密算法");
        } catch (NoSuchPaddingException e) {
            e.printStackTrace();
            return null;
        } catch (InvalidKeyException e) {
            throw new Exception("解密公钥非法,请检查");
        } catch (IllegalBlockSizeException e) {
            throw new Exception("密文长度非法");
        } catch (BadPaddingException e) {
            throw new Exception("密文数据已损坏");
        }
    }

    /**
     * 10、字节数据转十六进制字符串
     *
     * @param data 输入数据
     * @return 十六进制内容
     */
    public String byteArrayToString(byte[] data) {
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < data.length; i++) {
            // 取出字节的高四位 作为索引得到相应的十六进制标识符 注意无符号右移
            stringBuilder.append(HEX_CHAR[(data[i] & 0xf0) >>> 4]);
            // 取出字节的低四位 作为索引得到相应的十六进制标识符
            stringBuilder.append(HEX_CHAR[(data[i] & 0x0f)]);
            if (i < data.length - 1) {
                stringBuilder.append(‘ ‘);
            }
        }
        return stringBuilder.toString();
    }
}

　　 /**
     * 生成加密后文件
     *
     * @param oldFilePath 需要加密文件路径+名称
     * @param newFilePath 加密后文件路径+名称
     * @param privatePath 私钥文件路径+名称
     */
    public void fileEncrypt(String oldFilePath, String newFilePath, String privatePath) {
        ByteUtil byteUtil = new ByteUtil();

        //文件格式：特征码+原始升级包长度+数字签名长度+原始包内容+数字签名
        byte[] code = byteUtil.intToByteArray(0x9F2308DC);
        RSAEncrypt rsaEncrypt = new RSAEncrypt();
        try {
            //1、特征码写入
            OutputStream out = new FileOutputStream(new File(newFilePath));
            out.write(code, 0, 4);

            //2、原始升级包长度写入
            byte[] fileByte = byteUtil.File2byte(oldFilePath);
            int L1 = fileByte.length;
            byte[] a = byteUtil.intToByteArray(L1);
            out.write(a, 0, 4);

            //文件摘要生成
            MsgDigestDemo msgDigestDemo = new MsgDigestDemo();
            MessageDigest md5Digest = MessageDigest.getInstance("MD5");
            md5Digest.update(msgDigestDemo.fileBytes(oldFilePath));
            byte[] md5Encoded = md5Digest.digest();
            log.info("==========MD5摘要:{}==========", Base64.encodeBase64URLSafeString(md5Encoded));

            String privateKey = rsaEncrypt.loadPrivateKeyByFile(privatePath);
            RSAPrivateKey privateKeyfile = rsaEncrypt.loadPrivateKeyByStr(privateKey);

            //生成签名(摘要加密过程)
            byte[] signature = rsaEncrypt.encrypt(privateKeyfile, md5Encoded);

            //3、签名长度
            int L2 = signature.length;
            byte[] c = byteUtil.intToByteArray(L2);
            out.write(c, 0, 4);

            //4、原始升级包内容写入
            out.write(fileByte, 0, L1);
            //5、数字签名写入
            out.write(signature, 0, L2);
            out.flush();
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

public class MsgDigestDemo {
public byte[] fileBytes(String filePath) {
        try {
            File file = new File(filePath);
            ByteArrayOutputStream out = new ByteArrayOutputStream(1024);
            FileInputStream in = new FileInputStream(file);
            byte[] fileByte = new byte[1024];
            int n;
            while ((n = in.read(fileByte)) != -1) {
                out.write(fileByte, 0, n);
            }
            in.close();
            byte[] data = out.toByteArray();
            out.close();
            return data;
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

}