encrypt data on iphone and dencrypt on a .net web application

Question

Hey Guys,

I was to encrypt data on the device and send it by http to our web server then decrypt the data on out .net web app. Is this possible? If yes, which encryption method I should use? and if their are any articles out there?

Thanks

Answer 1

SSL should be the standard solution for HTTP encryption. NSURLConnection supports it out of the box (just load an https:// request), so you would just have to set up your server accordingly.

Answer 2

If SSL is not an option use AES encryption in CBC mode. 128 encryption bit is all you need and you can use anything (0 is acceptable) as the IV.

Answer 3

As you don't want to use SSL (and I agree there are many good reasons not to do so) you can use the built in CommonCrypto framework to encrypt just the data you need to. Here is a simple NSData category to encrypt arbitrary data:

@implementation NSData (AES256)

- (NSData*) encryptedWithKey: (NSString *) key;
{
    // 'key' should be 32 bytes for AES256, will be null-padded otherwise
    char keyBuffer[kCCKeySizeAES128+1]; // room for terminator (unused)
    bzero( keyBuffer, sizeof(keyBuffer) ); // fill with zeroes (for padding)

    [key getCString: keyBuffer maxLength: sizeof(keyBuffer) encoding: NSUTF8StringEncoding];

    // encrypts in-place, since this is a mutable data object
    size_t numBytesEncrypted = 0;

    size_t returnLength = ([self length] + kCCKeySizeAES256) & ~(kCCKeySizeAES256 - 1);

    // NSMutableData* returnBuffer = [NSMutableData dataWithLength:returnLength];
    char* returnBuffer = malloc(returnLength * sizeof(uint8_t) );

    CCCryptorStatus result = CCCrypt(kCCEncrypt, kCCAlgorithmAES128 , kCCOptionPKCS7Padding | kCCOptionECBMode,
             keyBuffer, kCCKeySizeAES128, nil,
             [self bytes], [self length], 
             returnBuffer, returnLength,
             &numBytesEncrypted);

    if(result == kCCSuccess)
     return [NSData dataWithBytes:returnBuffer length:numBytesEncrypted];
    else 
     return nil;

}

@end

Note that this also turns on ECB Mode which you may not want. Also remember that the data that comes back from this call is not suitable for use in URLs you will have to base 64 encode it.