Thursday, December 10, 2015

CommonCrypto

So Apple has a cryptography library called CommonCrypto (docs). Importantly, it is not a framework.

Here is a quote from the older documentation (last year):

Common Crypto
In OS X v10.5 and later and iOS 5.0 and later, Common Crypto provides low-level C support for encryption and decryption. Common Crypto is not as straightforward as Security Transforms, but provides a wider range of features, including additional hashing schemes, cipher modes, and so on.

Normally, I would have passed on this in favor of, say, trying to build openssl and then access that. (I recall seeing something from tptacek about it along the lines of use something more standard, don't trust that Apple will implement the basics correctly). Judging by history, that would be pretty sage advice.

But ... I came across a wonderful blog post here, which describes not only how to use CommonCrypto from Swift, and not only shows an in-progress library wrapping CommonCrypto, but also describes a hack that allows Swift playgrounds to have access to CommonCrypto. Too cool for words!

The basics: open a new Xcode project in Swift and call it MyApp. We need a "bridging header", we get this by adding a new Objective C class, then Xcode will ask if we want this header, and we say yes. Ungratefully, we promptly delete the dummy Objective C class. In the header, add:

#import <CommonCrypto/CommonCrypto.h>


That's it. For example, I put this code into crypto.swift



and call it from the AppDelegate in applicationDidFinishLaunching. The debugger prints:

e4d909c290d0fb1ca068ffaddf22cbd0

If I put the same text in a file and do

> md5 msg.txt
MD5 (msg.txt) = e4d909c290d0fb1ca068ffaddf22cbd0
>


Of course, what we've done here is the usual C hack of allocating a buffer and passing a pointer to the buffer into the program that will write into it. Except that in Swift these are not your father's pointers. You can't do arithmetic. And they have that label "Unsafe". Reminds me of Maverick.

Now for the cool part. Deep within Xcode there are one (or more for some people) SDKs. Software Development Kits.

> xcrun --show-sdk-path --sdk macosx
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform\
/Developer/SDKs/MacOSX10.11.sdk

and deep within that is:

> ls /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform\
/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks
AGL.framework
AVFoundation.framework
AVKit.framework
Accelerate.framework
...

As I said, CommonCrypto is not a framework. But we can fake things like it is one. We make a directory in that place, call it CommonCrypto.framework and inside that put module.map with some paths:

module CommonCrypto [system] {
header "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform\
/Developer/SDKs/MacOSX10.11.sdk/usr/include/CommonCrypto/CommonCrypto.h"
header "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform\
/Developer/SDKs/MacOSX10.11.sdk/usr/include/CommonCrypto/CommonRandom.h"
export *
}

(Those line breaks should work, but I don't have them in my original).

I did this:

cd /Applications/Xcode.app/Contents/Developer\
/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk\
/System/Library/Frameworks
> sudo mkdir CommonCrypto.framework
Password:
> sudo cp ~/Desktop/module.map CommonCrypto.framework
>
> cat CommonCrypto.framework/module.map
module CommonCrypto [system] {
...
>


Having "corrupted" the SDK in this way, we can now do things like this: paste that same code into an Xcode playground. I had to change the last line, not sure why just at the moment..



And with this setup, we no longer need the bridging header. I tried just deleting it but Xcode knows. So delete the whole project and make a new one with the same name. Add the code in crypto.swift and put import CommonCrypto in that same file. Then from the AppDelegate, call doIt.

It works!



It's worth pointing out that simply substituting SHA256 for MD5 in the code above works. To check the digest on the command line do:

> openssl dgst -sha256 -hex msg.txt
SHA256(msg.txt)= ef537f25c895bfa782526529a9b63d97aa631564d5d789c2b765448c8635fb6c

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