CODEWITHSUNDEEP
javac#encryptionaes
Dinusha
Dinusha

Reputation: 716

AES 256 encryption by passing IV

I am tiring to converting encription logic in to java which written in C#. I pass a custom IV as parameter. 

 private static byte[] iv = new byte[] { 116,209,63,154,85,66,37,82,96,104,131,192,103,140,125,176 };

But, In java can not hold more then 128 in a byte array. In C# it is possible. 

        this.secretKeySpec = new SecretKeySpec(key, "AES");
        this.iv = new IvParameterSpec(iv);
        this.cipher = Cipher.getInstance(ALGORITHM_AES256);

In javax.crypto IvParameterSpec constactory only allow byte array as "iv". Can anyone please help for pass above byte[] as iv parameter in java.

Upvotes: 0

Views: 771

Answers (2)

Maarten Bodewes
Maarten Bodewes

Reputation: 94018

Alternative to Eugene's proposal: simply explicitly convert all values - or the values above 127 - by casting to a byte. It's also a good idea to use hexadecimals for binary data.

byte[] test = new byte[] { 0, (byte) 0, 0x00, (byte) 0x00, (byte) 128, (byte) 0x80, (byte) 256, (byte) 0x100 };

Note that - as the value of the last two bytes should make clear - this does allow you to specify values that are out of range without raising a warning.

Alternatively, you could use a hexadecimal or base64 decoder, so you can create more dense byte array representation using strings.

Upvotes: 1

Eugene Podskal
Eugene Podskal

Reputation: 10401

The problem is that Java does not support unsigned datatypes, while .NET does.

Unsigned byte is from 0 to 255. This is .NET Byte struct
Signed byte is from -128 to 127. This is Java byte primitive datatype or .NET SByte struct.

The data in .Net code stores the iv as unsigned values, while Java code does not allow you to do it directly.

Solutions:

1) You could just manually subtract 128 from each byte in .NET code, thus making it to represent the same bit pattern(IV is just sequence of bits - it does not matter whether they stored as signed or unsigned values - it is the bits that must match):

byte[] iv = new byte[] { -8, 81, ...};

2) Or you could store the same data in an int datatype array and convert them to byte values using the following code.

int[] ivInInts = new int[] { 116,209,63,154,85,66,37,82,96,104,131,192,103,140,125,176 };

byte[] iv = new byte[ivInInts.getLength()];

for (int i = 0; i < ivInInts.getLength(); i++)
   iv = (byte)(ivInInts[i] - 128);

EDIT:

If you have some doubts about binary equivalence of numbers you could read about two's complement numbers:

65 = 0100 0001 as unsigned byte = 0100 0001 as signed byte
-65 = cannot be represented as unsigned byte = 1011 1111 as signed byte
193 = 1011 1111 as unsigned byte = cannot be represented as signed byte

Upvotes: 2

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