Question

I have an encrypted mp4 using Rijndael and I am decrypting in C# in the following manner.

System.Security.Cryptography.Rijndael crypt = System.Security.Cryptography.Rijndael.Create();

crypt.Key = convertedSecureString;

byte[] initializationVectorLength = new byte[sizeof(int)];
CryptoStream cryptostream = new CryptoStream(inputStream, crypt.CreateDecryptor(), CryptoStreamMode.Read);
byte[] buffer = new byte[1024];
int len;
while ((len = cryptostream.Read(buffer, 0, buffer.Length)) > 0)
{
    outputStream.Write(buffer, 0, len);
    buffer = new byte[1024];
}

outputStream.Flush();
cryptostream.Close();
inputStream.Close();

Now I need to convert this code to Java/Android equivalent. I am not sure where to start frankly. I am confused by so many options – some say use Bouncy Castle, some say Apache Commons, some the native Java lib. How do I do this. And what do I do about CryptoStream etc?

UPDATE

I am using the following the code in C# for assigning the key

byte[] convertedSecureString = new byte[this.key.Length];
IntPtr ptr = System.Runtime.InteropServices.Marshal.SecureStringToBSTR(this.key);

for (int i = 0, j = 0; i < this.key.Length * UnicodeByteLength; i = i + UnicodeByteLength, j++)
{
    convertedSecureString[j] = System.Runtime.InteropServices.Marshal.ReadByte(ptr, i);
}

try
{
    crypt.Key = convertedSecureString;
}

where key is secure. I have the equivalent unsecure key in Java. How do i convert this piece of code to Java

UPDATE

  Rfc2898DeriveBytes newKey = new Rfc2898DeriveBytes(crypt.Key.ToString(), crypt.IV);
                Array.Copy(newKey.GetBytes((int)crypt.KeySize / 8), crypt.Key, (int)crypt.KeySize / 8);
                Array.Copy(newKey.GetBytes((int)crypt.BlockSize / 8), crypt.IV, (int)crypt.BlockSize / 8);

I am using this in C# to mod the key using Rfc 2898 and derive the bytes – I cant find an equivalent in Java – I found here Java equivalent of C#'s Rfc2898DerivedBytes in the second comment – but what values do I give for iterator and dklen?

Answer 1

You need to get Cipher object. Here is one way of getting it, using byte[] aesKey, and byte[] iv (initialization vector, must always be 16 bytes for AES).

// aesKey is 128, 196 or 256-bit key (8, 12 or 16 byte array)
SecretKeySpec key = new SecretKeySpec(aesKey, "AES");

// initialization vector
IvParameterSpec ivSpec = new IvParameterSpec(iv);

// create and initialize cipher object
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING");
cipher.init(Cipher.DECRYPT_MODE, key, ivSpec);

Once you have Cipher object in decrypt mode, you can feed it with encrypted data using update methods, and it will return you plain-text data. When you are done, you must call one of doFinal methods to get final block.

Alternatively, you can create CipherInputStream using your Cipher object, and original input stream that supplies encrypted data. You read data from CipherInputStream, which in turn reads data from original input stream, decrypts it, and returns you the plain-text data.

For encrypting, you need to pass Cipher.ENCRYPT_MODE into Cipher.init method, and use CipherOutputStream instead.

Update: full example, more or less equivalent to original C# code:

// Algorithm, mode and padding must match encryption.
// Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING");

// If you have Bouncycastle library installed, you can use
// Rijndael/CBC/PKCS7PADDING directly.
Cipher cipher = Cipher.getInstance("Rijndael/CBC/PKCS7PADDING");

// convertedSecureString and initVector must be byte[] with correct length
cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(convertedSecureString, "AES"),
    new IvParameterSpec(initVector));

CipherInputStream cryptoStream = new CipherInputStream(inputStream, cipher);
byte[] buffer = new byte[1024];
int len = cryptoStream.read(buffer, 0, buffer.length);
while (len > 0) {
    outputStream.write(buffer, 0, len);
    len = cryptoStream.read(buffer, 0, buffer.length);
}

outputStream.flush();
cryptoStream.close();
// no need to close inputStream here, as cryptoStream will close it

By default, Java doesn’t support Rijndael algorithm (AES may or may not work, see the other answer) and PKCS7 padding. You will need to install Bouncycastle extension for that and then just use Cipher.getInstance("Rijndael/CBC/PKCS7PADDING");. Why CBC and PKCS7 Padding? Those seem to be defaults for System.Security.Cryptography.Rijndael class. If I got that wrong, use correct mode and padding for your situation.