A simple example of decrypting a symmetrically encrypted file (via GPG)

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>co.ntier.security</groupId>
    <artifactId>gpg-example</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <org.bouncycastle.version>1.46</org.bouncycastle.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.bouncycastle</groupId>
            <artifactId>bcmail-jdk15</artifactId>
            <version>${org.bouncycastle.version}</version>
        </dependency>
        <dependency>
            <groupId>org.bouncycastle</groupId>
            <artifactId>bcpg-jdk15</artifactId>
            <version>${org.bouncycastle.version}</version>
        </dependency>
        <dependency>
            <groupId>org.bouncycastle</groupId>
            <artifactId>bcprov-jdk15</artifactId>
            <version>${org.bouncycastle.version}</version>
        </dependency>

        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.4</version>
        </dependency>
    </dependencies>



    
</project>

Symmetric.java

import org.apache.commons.io.FileUtils;
import org.bouncycastle.bcpg.SymmetricKeyAlgorithmTags;
import org.bouncycastle.openpgp.*;
import org.bouncycastle.openpgp.examples.ByteArrayHandler;
import org.bouncycastle.openssl.PEMReader;
import org.bouncycastle.util.io.Streams;

import java.io.*;
import java.security.NoSuchProviderException;
import java.security.Security;

public class Symmetric {


    private static final String FOLDER = "/Users/dave/Desktop/encrypted/";
    private static final String PASS = "abc123";

    public static void main(String[] args) throws IOException, PGPException, NoSuchProviderException {
        Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());

        File encryptedFile = new File(FOLDER + "symmetric.txt.gpg");
        byte[]  encryptedByteArray = FileUtils.readFileToByteArray(encryptedFile);

        byte[] decryptedByteArray = ByteArrayHandler.decrypt(encryptedByteArray, PASS.toCharArray());
        String decryptedString = new String(decryptedByteArray);

        System.out.println(decryptedString);

        System.out.println();

        byte[] encryptedAgain = ByteArrayHandler.encrypt(decryptedByteArray, PASS.toCharArray(), "foobar.txt", SymmetricKeyAlgorithmTags.AES_256, true);
        String encryptedAgainString = new String(encryptedAgain);
        System.out.println(encryptedAgainString);





        byte[] decryptedAgainByteArray = ByteArrayHandler.decrypt(encryptedAgain, PASS.toCharArray());
        String decrypteAgaindString = new String(decryptedAgainByteArray);
        System.out.println(decrypteAgaindString);

    }

}