AES interoperability between .Net and iPhone?

Question

I need to encrypt a string on the iPhone and send it to a .Net web service for decryption. I am able to encrypt/decrypt on the iPhone and with .Net, but the encrypted strings from the iPhone cannot be decrypted by .Net. The error I get is "Padding is invalid and cannot be removed." The .Net code is from: http://blog.realcoderscoding.com/index.php/2008/07/dot-net-encryption-simple-aes-wrapper/ The iPhone code uses the sample code from: http://nootech.wordpress.com/2009/01/17/symmetric-encryption-with-the-iphone-sdk/ AFAIK my key settings are the same: ``` result.BlockSize = 128; // iPhone: kCCBlockSizeAES128 result.KeySize = 128; // kCCBlockSizeAES128 result.Mode = CipherMode.CBC; result.Padding = PaddingMode.PKCS7; // kCCOptionPKCS7Padding ``` I tried different ways of generating ciphertext. hello/hello is: e0PnmbTg/3cT3W+92CDw1Q== in .Net yrKe5Z7p7MNqx9+CbBvNqQ== on iPhone and "openssl enc -aes-128-cbc -nosalt -a -in hello.txt -pass pass:hello" generates: QA+Ul+r6Zmr7yHipMcHSbQ== Upd…

Accepted Answer

At the very least, you are using differing initialization vectors (IV).

The .Net code uses the key for IV.

```
private static AesCryptoServiceProvider GetProvider(byte[] key)
{
    //Set up the encryption objects
    AesCryptoServiceProvider result = new AesCryptoServiceProvider();
    byte[] RealKey = Encryptor.GetKey(key, result);
    result.Key = RealKey;
    result.IV = RealKey;
    return result;
}
```

and

```
private static byte[] GetKey(byte[] suggestedKey, AesCryptoServiceProvider p)
{
    byte[] kRaw = suggestedKey;
    List kList = new List();
    for (int i = 0; i < p.LegalKeySizes[0].MinSize; i += 8 )
    {
        kList.Add(kRaw[i % kRaw.Length]);
    }
    byte[] k = kList.ToArray();
    return k;
}
```

which should probably be: `kList.Add(kRaw[(i / 8) % kRaw.Length]);`. Otherwise a key whose length % 8 == 0 will use the same letter repeatedly, doh!

Thus the IV (and key) used by .Net is: `hleolhleolhleolh`. This is not part of the API, but rather due to the wrapper code that you pointed at (which has a serious bug in it...).

The iPhone code uses 0 for IV.

```
// Initialization vector; dummy in this case 0's.
uint8_t iv[kChosenCipherBlockSize];
memset((void *) iv, 0x0, (size_t) sizeof(iv));
```

openssl by default prepends a randomly generated salt (which is why the output is longer!).

The openssl output is more secure since it is prepending a random initialization vector. It looks like the first few bytes of the base64 decoded string is "Salted__". You can also ask openssl to not use a salt (-nosalt) and / or provide an IV (-iv).

Essentially, openssl, .Net, and the iPhone are using the same encryption, you just need to be careful how you initialize the APIs with the encryption key and the initialization vector.

AES interoperability between .Net and iPhone?

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