Encryptor/Decryptor for iOS
Provides an easy-to-use, Objective-C interface to the AES functionality of CommonCrypto. Simplifies correct handling of password stretching (PBKDF2), salting, and IV. For more information on these terms, see "Properly encrypting with AES with CommonCrypto," and iOS 6 Programming Pushing the Limits, Chapter 15. Also includes automatic HMAC handling to integrity-check messages.
RNCryptor is an abstract class. Concrete subclasses include:
RNEncryptor,RNDecryptor: Writer and reader for the RNCryptor data format.RNOpenSSLEncryptor,RNOpenSSLDecryptor: Writer and reader for the OpenSSL format. This format is not recommended due to its weak security settings, but is available for compatibility.
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.
RNCryptor suports asynchronous use, specifically designed to work with NSURLConnection.
This is also useful for cases where the encrypted or decrypted data will not comfortably fit in memory.
If the data will comfortably fit in memory, 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.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.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];
}
}];
}
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.
Full API information is available at http://rnapier.github.com/RNCryptor/doc/html/Classes/RNCryptor.html.
Details on the data format are available at https://github.com/rnapier/RNCryptor/wiki/Data-Format.
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 10.7+ and iOS 5+. For more information on backporting, read and comment on Issue #22.
The current file format is v2. 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.
RNCryptor has several design goals, in order of importance:
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.
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.
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 here: http://www.daemonology.net/blog/2009-07-31-thoughts-on-AES.html. 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 http://robnapier.net/blog/mode-rncryptor-767 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. http://security.stackexchange.com/questions/4781/do-any-security-experts-recommend-bcrypt-for-password-storage We use 10k rounds of PBKDF2 which represents about 80ms on an iPhone 4.
RNCryptor endeavors to be implemented as simply as possible, avoiding tricky code. It is designed to be easy to read and code review.
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.
Without sacrificing other goals, it is preferable to read the output format of RNCryptor on other platforms.
- v2.1 Switches to file format v2 to deal with Issue #44.
- v2.0 adds asynchronous modes.
- v2.1 backports
RNCryptorto older versions of Mac OS X (and possibly iOS).
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* Copyright (c) 2008 Damien Bergamini <damien.bergamini@free.fr>
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