Java RSA 私钥解密

java的公钥跟私钥使用不一样的文档格式。公钥使用pem格式，去掉文档的头尾说明。编码用X509而不是pkcs8。

JAVA RSA使用的私钥不是pem格式，如果是openssl生成的通用文件，请执行下面命令转换成der文件。

openssl pkcs8 -topk8 -inform PEM -outform DER -in private.pem -out private.der-nocrypt

下面是完整的加密类。

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

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

import javax.crypto.Cipher;

public class RSAUtil {
    private PrivateKey privateKey;
    
    public RSAUtil() throws Exception{
        getPrivateKey();
    }
    
    private void getPrivateKey() throws Exception{
        File file = new File(getClass().getClassLoader().getResource("pkcs8_private.der").getFile());
        FileInputStream fileInputStream = new FileInputStream(file);
        DataInputStream dataInputStream = new DataInputStream(fileInputStream);
        
        
        byte[] keyBytes = new byte[(int)file.length()];
        dataInputStream.readFully(keyBytes);
        dataInputStream.close();
        
        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        this.privateKey =  keyFactory.generatePrivate(keySpec);
    }
    
    public String decryptString(String cipherTextString) throws Exception{
        Base64 base64 = new Base64();
        byte[] cipherText = base64.decode(cipherTextString);
        
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.DECRYPT_MODE, this.privateKey);
        byte[] plainText = cipher.doFinal(cipherText);
        
        String plainTextString = new String(plainText);
        
        return plainTextString;
    }
}