File Stream Encryption with Bouncy Castle

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Encryption is a funny thing, it’s essential to do and do well , but it well is complicated this is unfortunate since it’s so important to get right. When you want to include encryption in your code more often then not you discover that its not very approachable.It’s kind of ironic isn’t it?

While I can’t simplify encryption much, I hope to make it more approachable. This example walks through the simple task of encrypting a file using a password. As usual, when it comes to code, nothing teaches better then code.

The tldr

  • Sample code can be grabbed from My GitHub sample repo.
  • EncryptionUtil uses a password to create an AES Key for Symmetric encryption of an InputStream
  • Bouncy Castle was used for Encryption, it works great, has more options and is easier to work with then the java libs imho.
  • Check out the Spock test to see how to use EncryptionUtilSpec
  • Code as is, no warranty, Creative Commons, do what ya want with it!
  • Stay encrypted folks!

Turning a Password Into a Key

I’ll start off with the EncryptionUtil class. Quick note: This is written in Groovy but for the most part you can add semicolons and rename the files to .java if you feel more comfortable with Java. (I’ll spare you my Groovy evangelism!)

Create a key from your password

This example is going to use Symmetric encryption to encrypt the file so first step is to convert that easy to remember password into a key for use in encrypting/decrypting the file.

KeyParameter getAesKey(String passphrase){ 
        byte[] rawKey

            if(passphrase == null || passphrase.isEmpty()) 
                throw new InvalidParameterException("passphrase is null or empty")

            PBEKeySpec keySpec = new PBEKeySpec(passphrase.toCharArray(), pwdSalt, iterations, keyLength)  // [1]
            SecretKeyFactory keyFactory = SecretKeyFactory.getInstance(KEY_ALGORITHM) [2]
            rawKey = keyFactory.generateSecret(keySpec).getEncoded() [3]
        } catch (Exception e){
            log.error "Key factory init failed with the following error: /n${e.toString()}"

        new KeyParameter(rawKey)

The method call getAesKey("yourPassword") does the job of turning your password into an encryption Key using the PBEKeySpec which stands for Password Based Encryption. There are really only 3 things happening here. [1] Set up the KeySpec using the password and salt. [2] Set up the encryption factory using the specified algorithm “PBEWITHSHA256AND256BITAES-CBC-BC” and [3] Generating the key as a byte array.

The salt and other spices

Things to note, the pwdSalt is an array of random generated bytes using the SecureRandom java object. The key to the salt is that its public and you have to keep it to unencrypt! Typically this can be written unencrypted at the beginning of the file, but just for simplicity I have not done so, instead keeping this salt stored as a constant in the sample, in a real implementation you want to write it to the file and generate a new one with every encryption task. (Pleases don’t skip this step when implementing this for multiple users!)

Other things to note are the iterations and key length params being passed in, the iterations is the number of times the password gets hashed and the key length is just specifying how many bits to generate for the password, here I’m using 128bit.

The Algo

I’m using “PBEWITHSHA256AND256BITAES-CBC-BC” to create the key and its worth noting what that very long String is all about. It’s really just a long concatenated set of Algo specifications. PBEWITHSHA256 is saying password based encryption using Sha256. Additionally I’m adding 256 AES just to be extra secure and CBC and BC refer to using Block Cypher which means that the encryption is done in distinct blocks. This becomes somewhat relevant when discussing Initialization Vectors which comes up later when encrypting the actual file.

All done! you have taken your “MyS3cr3tPassword” and turned it into something that looks more like this: [-49, -25, -2, 112, 35, -60, -36, -89, 90, 1, -85, -117, -65, -16, -125, -5, 42, -115, 1, 116, 108, 126, 16, 2, -16, 82, -37, -30, -17, 83, -16, -82]

Encrypting The File

Now that we have the Key generation out of the way its tie fro the fun part, turning your file into a bunch of gibberish! Lets look at the encrypt method.

    def encodeStream(InputStream inputStream, OutputStream streamOut) throws GeneralSecurityException{
        byte[] ivData = new byte[AES_NIVBITS/8] //[1]
        new SecureRandom().nextBytes(ivData)

        //Select encrypt algo and padding: AES with CBC and PCKS7
        //Encrypt input stream using key+iv
        KeyParameter keyParam = getAesKey(this.password) //[2]
        CipherParameters params = new ParametersWithIV(keyParam, ivData)

        BlockCipherPadding padding = new PKCS7Padding()
        BufferedBlockCipher blockCipher = new PaddedBufferedBlockCipher(new CBCBlockCipher(new AESEngine()), padding) // [3]
        blockCipher.init(true, params)

        CipherOutputStream cipherOut = new CipherOutputStream(streamOut, blockCipher) //[4]
        IOUtils.copy(inputStream, cipherOut) 

This is where the fun happens, note that this method takes an InputStream and returns an OutputStream containing the encrypted data. The reason for this is to allow the management of the content to be separated from the encryption. Handy if your reading a URL and not a file.

Let’s walk through the encryption [1] Initializing an Initialization Vector, this is generated from the SecureRandom object and is used to initiate the block cipher. The IV should be unique and never reused. Also this is a public value we will prepend to the output stream and later used to decrypt the file.

[2] Here we are just grabbing the encrypted password mentioned earlier and adding them to the CipherParameters that help Bouncy Castle encryption figure Algorithm to use.

[3] Time to set up the BlockCipher, here we are specifying PKCS7 Padding and AES. Of interest is the BufferedBlockCipher which allows buffering in the encryption so your not trying to encrypt the full file at once, this is pretty important when your attempting to encrypt large files! One reason I picked CBC encryption was that prior Algos using GCS couldn’t manage files over 64mb, ouch.

[4] The next 2 lines make it happen, CipherOutputStream allows you to encrypt a stream and isn’t something you’ll see in most examples that simply encrypt a short String. Finally Apache project helps us again with a handy commons-io Stream copy util. Don’t copy the streams yourself, let Apache do it, trust me.

De-crypting The File

OK got it encrypted! Now to turn that gibberish back into something useful!

def decryptStream(InputStream encStream, OutputStream unEcnOutStream){
        //Extract the IV, which si stored in the next N bytes at the start of fileStream.
        int nIvBytes = AES_NIVBITS/ 8 as int
        byte[] ivBytes = new byte[nIvBytes], 0, nIvBytes) // [1]

        KeyParameter keyParam = getAesKey(this.password)
        CipherParameters params = new ParametersWithIV(keyParam, ivBytes)
        BlockCipherPadding padding = new PKCS7Padding()
        BufferedBlockCipher blockCipher = new PaddedBufferedBlockCipher(new CBCBlockCipher(new AESEngine()), padding)
        blockCipher.init(false, params) [2]

        CipherInputStream cipherIn = new CipherInputStream(encStream, blockCipher) [3]
        IOUtils.copy(cipherIn, unEcnOutStream) 

Not surprisingly, Decrypting the file is pretty much reversing what the encrypting did, as mentioned [1] we grab that unencrypted IV value and read it from the stream so we can reinitialize everything. [2] Set up our Buffered Cipher. NOTE: init(false, params) the false indicates decrypt mode, true is encrypt. It’s not a very obvious detail and can trip you up if you miss it. Finally [3] a CipherInputStream to decrypt the stream and Apache copies it into the OutputStream! Funny how similar and simple reversing the code is!

Summary and Tests

To tie this all together, I wrote a Spock test to provide an example and better demonstrate usage, the full test code is in the file EncryptionUtilSpec but here’s the highlights.

    def "Test encrypting a File" () {
        EncryptionUtil encryptionUtil = new EncryptionUtil(password)
        File inFile = new File('./src/test/resources/starWarsIpsum.txt')
        def unEncLine
        inFile.withReader { unEncLine = it.readLine() }

        InputStream inStr = inFile.newInputStream()
        FileOutputStream fileOutputStream = new FileOutputStream(encOutFie)

        fileOutputStream.with {fileStream ->
            encryptionUtil.encodeStream(inStr, fileStream)

        File encOutFile = new File(encOutFie)

        assert !encOutFile.text .contains(unEncLine)

        String decInput = ""
        FileOutputStream decFileOutStream = new FileOutputStream(decryptFileName)

        InputStream encInStr = encOutFile.newInputStream()
        encryptionUtil.decryptStream(encInStr, decFileOutStream)

        File decFile = new File(decryptFileName)
        assert decFile.exists()
        assert decFile.text .contains(unEncLine)

The test reads the unencrypted Star Wars Ipsum file, encrypts it to a file and then reads the encrypted file and outputs an unencrypted copy of the original. Magic math!

Hope this helps someone along in there own encryption project and remember: Encrypt all the things!

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