Question

I am encrypting a string in objective-c and also encrypting the same string in Java using AES and am seeing some strange issues. The first part of the result matches up to a certain point but then it is different, hence when i go to decode the result from Java onto the iPhone it cant decrypt it.

I am using a source string of “Now then and what is this nonsense all about. Do you know?”
Using a key of “1234567890123456”

The objective-c code to encrypt is the following: NOTE: it is a NSData category so assume that the method is called on an NSData object so ‘self’ contains the byte data to encrypt.

   - (NSData *)AESEncryptWithKey:(NSString *)key {
 char keyPtr[kCCKeySizeAES128+1]; // room for terminator (unused)
 bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

 // fetch key data
 [key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];

 NSUInteger dataLength = [self length];

 //See the doc: For block ciphers, the output size will always be less than or 
 //equal to the input size plus the size of one block.
 //That's why we need to add the size of one block here
 size_t bufferSize = dataLength + kCCBlockSizeAES128;
 void *buffer = malloc(bufferSize);

 size_t numBytesEncrypted = 0;
 CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
            keyPtr, kCCKeySizeAES128,
            NULL /* initialization vector (optional) */,
            [self bytes], dataLength, /* input */
            buffer, bufferSize, /* output */
            &numBytesEncrypted);
 if (cryptStatus == kCCSuccess) {
  //the returned NSData takes ownership of the buffer and will free it on deallocation
  return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
 }

 free(buffer); //free the buffer;
 return nil;
}

And the java encryption code is…

public byte[] encryptData(byte[] data, String key) {
    byte[] encrypted = null;

    Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
    byte[] keyBytes = key.getBytes();

    SecretKeySpec keySpec = new SecretKeySpec(keyBytes, "AES");

    try {
        Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding", "BC");
        cipher.init(Cipher.ENCRYPT_MODE, keySpec);

        encrypted = new byte[cipher.getOutputSize(data.length)];
        int ctLength = cipher.update(data, 0, data.length, encrypted, 0);
        ctLength += cipher.doFinal(encrypted, ctLength);
    } catch (Exception e) {
        logger.log(Level.SEVERE, e.getMessage());
    } finally {
        return encrypted;
    }
}

The hex output of the objective-c code is –

7a68ea36 8288c73d f7c45d8d 22432577 9693920a 4fae38b2 2e4bdcef 9aeb8afe 69394f3e 1eb62fa7 74da2b5c 8d7b3c89 a295d306 f1f90349 6899ac34 63a6efa0

and the java output is –

7a68ea36 8288c73d f7c45d8d 22432577 e66b32f9 772b6679 d7c0cb69 037b8740 883f8211 748229f4 723984beb 50b5aea1 f17594c9 fad2d05e e0926805 572156d

As you can see everything is fine up to –

7a68ea36 8288c73d f7c45d8d 22432577

I am guessing I have some of the settings different but can’t work out what, I tried changing between ECB and CBC on the java side and it had no effect.

Can anyone help!? please….

Answer 1

Since the CCCrypt takes an IV, does it not use a chaining block cipher method (such as CBC)? This would be consistant with what you see: the first block is identical, but in the second block the Java version applies the original key to encrypt, but the OSX version seems to use something else.

EDIT:

From here I saw an example. Seems like you need to pass the kCCOptionECBMode to CCCrypt:

ccStatus = CCCrypt(encryptOrDecrypt,
        kCCAlgorithm3DES,
        kCCOptionECBMode, <-- this could help
        vkey, //"123456789012345678901234", //key
        kCCKeySize3DES,
        nil, //"init Vec", //iv,
        vplainText, //"Your Name", //plainText,
        plainTextBufferSize,
        (void *)bufferPtr,
        bufferPtrSize,
        &movedBytes);

EDIT 2:

I played around with some command line to see which one was right. I thought I could contribute it:

$ echo "Now then and what is this nonsense all about. Do you know?" | openssl enc -aes-128-ecb -K $(echo 1234567890123456 | xxd -p) -iv 0 | xxd 
0000000: 7a68 ea36 8288 c73d f7c4 5d8d 2243 2577  zh.6...=..]."C%w
0000010: e66b 32f9 772b 6679 d7c0 cb69 037b 8740  .k2.w+fy...i.{.@
0000020: 883f 8211 7482 29f4 7239 84be b50b 5aea  .?..t.).r9....Z.
0000030: eaa7 519b 65e8 fa26 a1bb de52 083b 478f  ..Q.e..&...R.;G.