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==

Update: I’ve posted the working code for this here.

Answer 1

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.