How to Decrypt Files Encrypted with OpenSSL's AES in Java?

Decrypting Files Encrypted with OpenSSL Using AES in Java

This Java code snippet outlines a solution for decrypting files encrypted using the OpenSSL command with AES encryption:

import java.io.File;
import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.security.GeneralSecurityException;
import java.security.MessageDigest;
import java.util.Arrays;
import java.util.List;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

import org.bouncycastle.util.encoders.Base64;

class OpenSSLDecryptor {
    private static final Charset ASCII = Charset.forName("ASCII");
    private static final int INDEX_KEY = 0;
    private static final int INDEX_IV = 1;
    private static final int ITERATIONS = 1;

    private static final int ARG_INDEX_FILENAME = 0;
    private static final int ARG_INDEX_PASSWORD = 1;

    private static final int SALT_OFFSET = 8;
    private static final int SALT_SIZE = 8;
    private static final int CIPHERTEXT_OFFSET = SALT_OFFSET + SALT_SIZE;

    private static final int KEY_SIZE_BITS = 256;

    public static void main(String[] args) {
        try {
            // Read base 64 encoded file
            File f = new File(args[ARG_INDEX_FILENAME]);
            List<string> lines = Files.readAllLines(f.toPath(), ASCII);
            StringBuilder sb = new StringBuilder();
            for (String line : lines) {
                sb.append(line.trim());
            }
            String dataBase64 = sb.toString();
            byte[] headerSaltAndCipherText = Base64.decode(dataBase64);

            // Extract salt & encrypted text
            byte[] salt = Arrays.copyOfRange(headerSaltAndCipherText, SALT_OFFSET, SALT_OFFSET + SALT_SIZE);
            byte[] encrypted = Arrays.copyOfRange(headerSaltAndCipherText, CIPHERTEXT_OFFSET, headerSaltAndCipherText.length);

            // Specify cipher and digest for EVP_BytesToKey method
            Cipher aesCBC = Cipher.getInstance("AES/CBC/PKCS5Padding");
            MessageDigest md5 = MessageDigest.getInstance("MD5");

            // Create key and IV
            final byte[][] keyAndIV = EVP_BytesToKey(KEY_SIZE_BITS / Byte.SIZE, aesCBC.getBlockSize(), md5, salt, args[ARG_INDEX_PASSWORD].getBytes(ASCII), ITERATIONS);
            SecretKeySpec key = new SecretKeySpec(keyAndIV[INDEX_KEY], "AES");
            IvParameterSpec iv = new IvParameterSpec(keyAndIV[INDEX_IV]);

            // Initialize cipher instance and decrypt
            aesCBC.init(Cipher.DECRYPT_MODE, key, iv);
            byte[] decrypted = aesCBC.doFinal(encrypted);

            String answer = new String(decrypted, ASCII);
            System.out.println(answer);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static byte[][] EVP_BytesToKey(int key_len, int iv_len, MessageDigest md, byte[] salt, byte[] data, int count) {
        // ... [implementation of EVP_BytesToKey method as provided by Ola Bini]
    }
}</string>

Additional Notes:

• This code uses the EVP_BytesToKey method as implemented by Ola Bini.
• The character set used is ASCII.
• It's recommended to explicitly specify the digest algorithm in the OpenSSL command line interface and Java code to avoid algorithm changes that may break the decryption process.

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