Thursday, August 19, 2010

Strong Encryption for Cocoa / Cocoa Touch

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AES is a strong encryption standard that has mostly replaced the aging DES standard. AES is widely used and fairly secure encryption mechanism (but I am not an expert at cryptography by any stretch of the imagination; I'm trusting experts for that opinion). AES supports three different key sizes, 128, 192, and 256 (the larger the key, the more secure the encryption and the more processing power it takes to encrypt or decrypt). Apple uses AES-128 and AES-256 in several places in Mac OS X, including for Disk Image encryption.

There are several public-domain implementations of AES. I chose a public domain implementation of AES by Philip J. Erdelsky to use as the basis some Objective-C categories that make encrypting and decrypting files and data using AES-256 easy.

The first category is on NSFileManager, and allows you to encrypt a file in the filesystem. It takes a file at a particular pathname, encrypts it using a passphrase, and then writes the encrypted contents to a new specified file location. This version has relatively low memory overhead, as it streams the data in chunks both for reading and writing, so only the chunk currently being encrypted is in memory. The category adds two methods to NSFileManager, one for encrypting, the other for decrypting. These methods are the best choice when your source data already exists in the file system, especially on the iPhone, because of how little memory it uses to do the work. Here is an example of using the category on NSFileManager to encrypt a file:
    NSError *error = nil;
    if (![[NSFileManager defaultManager] AESEncryptFile:@"/path/to/input file" toFile:@"/path/to/output file" usingPassphrase:@"My secret password" error:&error])
        NSLog(@"Failed to write encrypted file. Error = %@", [[error userInfo] objectForKey:AESEncryptionErrorDescriptionKey]);

There is also a category on NSData that will let you encrypt a chunk of data that's already in memory. This version creates a new NSData object with the encrypted contents of the original NSData instance. If your data is already in memory, and you want an encrypted or decrypted version of it, then the NSData methods are the way to go. Here is an example of using encrypting an NSData object with AES:
    NSData *encryptedData = [data AESEncryptWithPassphrase:@"My secret password"];

Pretty easy, huh? Okay, now, this is a symmetric block cypher, it is not public-key encryption, so if you store your passphrase as a string in your application (as opposed to making the user enter it or storing it in the keychain) then you're giving somebody the ability to decrypt your encrypted application data, so just be forewarned.

Also, I make no warranties about how secure this is. As far as I know, AES-256 has not been broken yet, however I cannot say for certaint that there are no weaknesses in the AES implementation I've used. I don't see any obvious problems but I am not a cryptographer. I haven't heard of any weaknesses in this particular implementation, but any use of this is completely at your own risk.

Here is a zip file containing the two categories and the AES implementation. Just add these to your Xcode project, include the appropriate headers, and encrypt away.

Oh, and, one more important thing: If you use this in an iPhone application that you plan to sell on the App Store, it may lengthen the review process, as you will have to declare that you are using encryption, and will likely have to create and upload a CCATS form and wait for Apple to review it before your app will go up for sale. Read the iTunes Connect Developer Guide for more information on CCATS and the process before deciding to use this in an iPhone application for sale, please.

1 comment:

  1. I'm sorry to see the link doesn't work any more.


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