Java - Input byte has incorrect ending byte at 300

I have written a JAVA based utility to store my data in excel encrypted (AES) and upload it to FTP Server and reverse mechanism for the same to download it and decrypt back to plain text.

Now, the encryption side is working perfectly, but when I decrypt the same. It throws the below exception:

Error while decrypting: java.lang.IllegalArgumentException: Input byte array has incorrect ending byte at 300

I have tried to change the encoding scheme but nothing worked.

My Code

public void downloadFileFromFTP () {


		int SFTPPORT = 22;
		String SFTPHOST = "";
	    String SFTPUSER = "";
        String SFTPPASS = "";
        String SFTPWORKINGDIR = "";

		Session session = null;
		Channel channel = null;
		ChannelSftp channelSftp = null;

		try {
			JSch jsch = new JSch();
			
			session = jsch.getSession(SFTPUSER, SFTPHOST, SFTPPORT);
			
			session.setPassword(SFTPPASS);
			
			java.util.Properties config = new java.util.Properties();
			
			config.put("StrictHostKeyChecking", "no");
			
			session.setConfig(config);
			
			session.connect();
			
			channel = session.openChannel("sftp");
			
			channel.connect();
			
			channelSftp = (ChannelSftp) channel;
			
			channelSftp.cd(SFTPWORKINGDIR);
			
			byte[] buffer = new byte[1024];
			
			BufferedInputStream bis = new BufferedInputStream(channelSftp.get("test.csv"));
			
			File newFile = new File("D:\\FMS_Encryption\\dec2.csv");
			
			OutputStream os = new FileOutputStream(newFile);
			
			BufferedOutputStream bos = new BufferedOutputStream(os);
			
			int readCount;
			
			while ((readCount = bis.read(buffer)) > 0) {
			
				String encryptedRow = new String(buffer, StandardCharsets.UTF_8);
				
				String decryptedRow = decrypt( encryptedRow.trim(), "FMS" );
				
				bos.write(decryptedRow.getBytes(), 0, readCount);
			}
			
			bis.close();
			
			bos.close();
		} 
		catch (Exception ex) {
			ex.printStackTrace();
		}

	}

ENCRYPTION/DECRYPTION CODE

public static String encrypt(String strToEncrypt, String secret) 
    {
        try
        {
            setKey(secret);
            Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
            cipher.init(Cipher.ENCRYPT_MODE, secretKey);
            return Base64.getEncoder().encodeToString(cipher.doFinal(strToEncrypt.getBytes("UTF-8")));
        } 
        catch (Exception e) 
        {
            System.out.println("Error while encrypting: " + e.toString());
        }
        return null;
    }
	public static String decrypt(String strToDecrypt, String secret) 
    {
        try
        {
            setKey(secret);
            Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
            cipher.init(Cipher.DECRYPT_MODE, secretKey);
            return new String(cipher.doFinal(Base64.getDecoder().decode(strToDecrypt)));
        } 
        catch (Exception e) 
        {
            System.out.println("Error while decrypting: " + e.toString());
        }
        return null;
    }

Now in download function, when I change the line String decryptedRow = decrypt( encryptedRow, "FMS" ); to String decryptedRow = decrypt( encryptedRow.trim(), "FMS" ); it throws:

java.lang.ArrayIndexOutOfBoundsException
	at java.lang.System.arraycopy(Native Method)
	at java.io.BufferedOutputStream.write(Unknown Source)
	at IbftFileEncryption.downloadFileFromFTP(IbftFileEncryption.java:426)
	at IbftFileEncryption.main(IbftFileEncryption.java:521)

You are trying to process your encrypted data in small chunks, but your encryption/decryption methods are expecting one complete encrypted block.

You are using PKCS5Padding padding with your AES cipher, so your encrypted data is padded to align with the block size of AES when the final call to doFinal(...) occurs during encrypt. This padding is then removed when the final call to doFinal(...) is called during decrypt. If you try to decrypt data where this padding don't exist, the decrypt will fail.

In your code you and up calling doFinal(...) too many times ..

In the same way you are also using base64 decoding on chunks without knowing if you have a complete block.

If you download the full file into a String before decrypting it might work.