RNCryptor

Cross-language AES Encryptor/Decryptor data format.

The primary target is Objective-C, but implementations are available in C++, C#, Java, PHP, Python, Javascript, and Ruby.

The data format includes all the metadata required to securely implement AES encryption, as described in "Properly encrypting with AES with CommonCrypto," and iOS 6 Programming Pushing the Limits, Chapter 15. Specifically, it includes:

• AES-256 encryption
• CBC mode
• Password stretching with PBKDF2
• Password salting
• Random IV
• Encrypt-then-hash HMAC

Basic Objective-C Usage

The most common in-memory use case is as follows:

NSData *data = [@"Data" dataUsingEncoding:NSUTF8StringEncoding];
NSError *error;
NSData *encryptedData = [RNEncryptor encryptData:data
                                   	withSettings:kRNCryptorAES256Settings
                                          password:aPassword
                                             error:&error];

This generates an NSData including a header, encryption salt, HMAC salt, IV, ciphertext, and HMAC. To decrypt this bundle:

NSData *decryptedData = [RNDecryptor decryptData:encryptedData
                                    withPassword:aPassword
                                           error:&error];

Note that RNDecryptor does not require settings. These are read from the header.

Asynchronous use

RNCryptor suports asynchronous use, specifically designed to work withNSURLConnection. This is also useful for cases where the encrypted or decrypted data will not comfortably fit in memory. If the data will comfortably fit inmemory, asynchronous operation is best acheived using dispatch_async().

To operate in asynchronous mode, you create an RNEncryptor or RNDecryptor, providing it a handler. This handler will be called as data is encrypted or decrypted. As data becomes available, call addData:. When you reach the end of the data call finish.

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
  [self.encryptedData addData:[self.cryptor addData:data]];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
  [self.cryptor finish];
}

// Other connection delegates

- (void)decryptionDidFinish {
  if (self.cryptor.error) {
    // An error occurred. You cannot trust encryptedData at this point
  }
  else {
    // self.encryptedData is complete. Use it as you like
  }
  self.encryptedData = nil;
  self.cryptor = nil;
  self.connection = nil;
}

- (void)decryptRequest:(NSURLRequest *)request {
  self.encryptedData = [NSMutableData data];
  self.connection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
  self.cryptor = [[RNDecryptor alloc] initWithPassword:self.password
                                               handler:^(RNCryptor *cryptor, NSData *data) {
                                                   [self.decryptedData appendData:data];
                                                   if (cryptor.isFinished) {
                                                     [self decryptionDidFinish];
                                                   }
                                                 }];
}

Async and Streams

When performing async operations on streams, the data can come very quickly (particularly if you're reading from a local file). If you use RNCryptor in a naïve way, you'll queue a work blocks faster than the engine can process them and your memory usage will spike. This is particularly true if there's only one core, such as on an iPad 1. The solution is to only dispatch new work blocks as the previous work blocks complete.

// Make sure that this number is larger than the header + 1 block.
// 33+16 bytes = 49 bytes. So it shouldn't be a problem.
int blockSize = 32 * 1024;

NSInputStream *cryptedStream = [NSInputStream inputStreamWithFileAtPath:@"C++ Spec.pdf"];
NSOutputStream *decryptedStream = [NSOutputStream outputStreamToFileAtPath:@"/tmp/C++.crypt" append:NO];

[cryptedStream open];
[decryptedStream open];

// We don't need to keep making new NSData objects. We can just use one repeatedly.
__block NSMutableData *data = [NSMutableData dataWithLength:blockSize];
__block RNEncryptor *decryptor = nil;

dispatch_block_t readStreamBlock = ^{
  [data setLength:blockSize];
  NSInteger bytesRead = [cryptedStream read:[data mutableBytes] maxLength:blockSize];
  if (bytesRead < 0) {
    // Throw an error
  }
  else if (bytesRead == 0) {
    [decryptor finish];
  }
  else {
    [data setLength:bytesRead];
    [decryptor addData:data];
    NSLog(@"Sent %ld bytes to decryptor", (unsigned long)bytesRead);
  }
};

decryptor = [[RNEncryptor alloc] initWithSettings:kRNCryptorAES256Settings
                                         password:@"blah"
                                          handler:^(RNCryptor *cryptor, NSData *data) {
                                            NSLog(@"Decryptor recevied %ld bytes", (unsigned long)data.length);
                                            [decryptedStream write:data.bytes maxLength:data.length];
                                            if (cryptor.isFinished) {
                                              [decryptedStream close];
                                              // call my delegate that I'm finished with decrypting
                                            }
                                            else {
                                              // Might want to put this in a dispatch_async(), but I don't think you need it.
                                              readStreamBlock();
                                            }
                                          }];

// Read the first block to kick things off    
readStreamBlock();

I'll eventually get this into the API to simplify things. See Cyrille's SO question for more discussion. Pull requests welcome.

Building

Comes packaged as a static library, but the source files can be dropped into any project. The OpenSSL files are not required.

Requires Security.framework.

Supports 10.6+ and iOS 4+.

The current file format is v3. To read v1 files (see Issue #44), you need to set the compile-time macro RNCRYPTOR_ALLOW_V1_BAD_HMAC. It is not possible to write v1 files anymore.

Design considerations

RNCryptor has several design goals, in order of importance:

Easy to use correctly for most common use cases

The most critical concern is that it be easy for non-experts to use RNCryptor correctly. A framework that is more secure, but requires a steep learning curve on the developer will either be not used, or used incorrectly. Whenever possible, a single line of code should "do the right thing" for the most common cases.

This also requires that it fail correctly and provide good errors.

Reliance on CommonCryptor functionality

RNCryptor has very little "security" code. It relies as much as possible on the OS-provided CommonCryptor. If a feature does not exist in CommonCryptor, then it generally will not be provided in RNCryptor.

Best practice security

Wherever possible within the above constraints, the best available algorithms are applied. This means AES-256, HMAC+SHA1, and PBKDF2:

• AES-256. While Bruce Schneier has made some interesting recommendations regarding moving to AES-128 due to certain attacks on AES-256, my current thinking is in line with Colin Percival. PBKDF2 output is effectively random, which should negate related-keys attacks against the kinds of use cases we're interested in.

• AES-CBC mode. This was a somewhat complex decision, but the ubiquity of CBC outweighs other considerations here. There are no major problems with CBC mode, and nonce-based modes like CTR have other trade-offs. See "Mode changes for RNCryptor" for more details on this decision.

• Encrypt-then-MAC. If there were a good authenticated AES mode on iOS (GCM for instance), I would probably use that for its simplicity. Colin Percival makes good arguments for hand-coding an encrypt-than- MAC rather than using an authenticated AES mode, but in RNCryptor mananging the HMAC actually adds quite a bit of complexity. I'd rather the complexity at a more broadly peer-reviewed layer like CommonCryptor than at the RNCryptor layer. But this isn't an option, so I fall back to my own Encrypt-than-MAC.

• HMAC+SHA256. No surprises here.

• PBKDF2. While bcrypt and scrypt may be more secure than PBKDF2, CommonCryptor only supports PBKDF2. NIST also continues to recommend PBKDF2. We use 10k rounds of PBKDF2 which represents about 80ms on an iPhone 4.

Code simplicity

RNCryptor endeavors to be implemented as simply as possible, avoiding trickycode. It is designed to be easy to read and code review.

Performance

Performance is a goal, but not the most important goal. The code must be secure and easy to use. Within that, it is as fast and memory-efficient as possible.

Portability

Without sacrificing other goals, it is preferable to read the output format of RNCryptor on other platforms.

Version History

• v2.2 Switches to file format v3 to deal with Issue #77.
• v2.1 Switches to file format v2 to deal with Issue #44.
• v2.0 adds asynchronous modes.
• v2.1 backports RNCryptor to older versions of Mac OS X (and possibly iOS).

LICENSE

Except where otherwise indicated in the source code, this code is licensed under the MIT License:

Permission is hereby granted, free of charge, to any person obtaining a 
copy of this software and associated documentation files (the "Software"),
to deal in the Software without restriction, including without limitation
the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following conditions:
 
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.

Portions of this code, indicated in the source, are licensed under the following license:

/*-
 * Copyright (c) 2008 Damien Bergamini <[email protected]>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
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 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

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