Mike Ash recently posted about code signing. We’d talked about the subject a few days before and that is a great summary of Mike’s thoughts about it.

Code signing on Mac OS X is still evolving, but with the release of the iPhone SDK, the subject is suddenly being thrust into the limelight. To quote the Wikipedia article:

Code signing is the process of digitally signing executables and scripts to confirm the software author and guarantee that the code has not been altered or corrupted since it was signed by use of a cryptographic hash.

In the past, code signing has been used on several platforms with differing intents and implementations; Microsoft, for instance, has a driver signing program that lets only Microsoft-certified device drivers run without warnings on Vista. In contrast, here’s a statement from an Apple engineer on the apple-cdsa mailing list:

Windows “code signing” is not only completely different in design and implementation, it also has a completely different *intent*. The Windows version tries to determine whether *Microsoft* thinks your program (or driver etc.) should be used. Mac OS X Code Signing is a general medium to let the manufacturer and user negotiate as to whether/how they want to trust each other, with the system providing the infrastructure to make this possible.

From what I’ve learned about the subject so far, the Mac OS X implementation seems indeed to be aimed in this direction – there’s very little documentation available. Even less is, of course, available for the iPhone OS implementation, and details are under NDA. But let me try to explain what I learned so far.

Mac OS X code signing is an implementation of the X.509 standard public key infrastructure. It depends on a cryptographical algorithm which gives reasonable assurance that signatures cannot be forged with currently wide-available technology. Of course that’s a rapidly moving target, and the twiddly mathematical details are beyond the scope of this post (and, in parts, of my poor brain too).

So, you can have signed executables (single binary files) and signed bundles (which can applications, frameworks, plug-ins and so forth). A signed bundle is easily distinguished by the presence of an extra “CodeResources” file inside the bundle’s Contents folder. This file is a XML file containing, among other data, a dictionary that contains a hash code for every file inside a bundle. It also has a list of “optional” files or folders; this allows you to remove things like unwanted localizations from an application without invalidating the signature. Of course, if you later re-add a localization its hash codes must match the one inside the file. Additionally, a signature is added to the bundle’s executable.

If you check Mac OS X’s system folder and applications, you’ll find that, beginning with Leopard, every framework, application and command-line tool is signed. This signature, by various convoluted means, is checkable (currently only by Apple’s codesign command-line tool). It also points back at Apple’s “root certificate“. This certificate is is a component in X.509’s chain of trust tree, which basically tells you which authorities certify that a particular signature is valid; in effect, when you reach the root of such a tree you have to decide whether the root certificate can be trusted. (X.509 also can be used for distributed, or peer-to-peer, trust networks.) Let’s see how you can check this in practice. Open Terminal and try this:

$ codesign -vvvv /Applications/Safari.app
/Applications/Safari.app: valid on disk
/Applications/Safari.app: satisfies its Designated Requirement

(as usual, type the line after the $ prompt and you should see the resulting lines.)

So this means that the Safari application is signed and its signature is intact. Otherwise, you’d get either a “code object is not signed” message, or if the signature has been tampered with, a “code or signature modified” message. Now let’s try a small variation on the command:

$ codesign -dvvvv /Applications/Safari.app
Format=bundle with Mach-O universal (i386 ppc7400)
CodeDirectory v=20001 size=7621 flags=0x0(none) hashes=375+3 location=embedded
Signature size=4064
Authority=Software Signing
Authority=Apple Code Signing Certification Authority
Authority=Apple Root CA
Info.plist entries=23
Sealed Resources rules=9 files=283
Internal requirements count=1 size=68

This gives more information. Specifically, we see that there are 3 “certification authorities” included into the signature, and that there is one “internal requirement”. To see this requirement, try:

$ codesign -dr- /Applications/Safari.app
host => anchor apple and identifier "com.apple.translate"
# designated => identifier "com.apple.Safari" and anchor apple

This is admittedly hard to wrap your neurons around. From what I understood, the “requirements” are actually a little language that establishes what conditions the signed bundle should satisfy. But satisfy to able to… what? Now there’s where everything is (on purpose) still a little vague. “anchor apple” apparently means that the basic requirement is that Apple’s root certificates be present and trusted. Open the Keychain Access utility, click on “certificates”, and you’ll see somewhere in the list a certificate called “Apple Root Certificate Authority”. You can double-click on it to see details.

But what is all this useful for? Your proverbial “Aunt Tillie” will certainly not be interested in typing commands into Terminal, following certificate chains, and reading up on trust audit procedures. Apple’s keychain software tries to make all that as transparent as possible. Simply logging into Mac OS X can open parts of the keychain, and when Aunt Tillie goes to her banking site, the bank’s certificate should be checked against the keychain’s certificates and with the issuing certification authority, and everything should “just work”. For that to be reliable, Safari’s code signature should also be checked – otherwise it could be hacked by some malicious entity to say that the bank’s site checks out OK even when it’s some phishing site masquerading as that bank!

Signatures and certificates are also checked by the parental control preference panel, by Mail when it receives signed e-mail, and so forth. You may be familiar with the dialog asking if a recently-updated application may access the keychain to store or retrieve passwords; for signed applications, this dialog will be issued only once. If a newly-installed update has the same signature as the previous version, which had already been authorized to access the keychain, it’s assumed to also be trusted to do so.

A similar scenario applies to critical code such as the codesign executable itself. Ideally, such code should refuse to run if it’s modified; Apple has some facilities already in place to do so, and even to detect if an executable ihas been modified while it’s already running by some malicious code injection. However, that sort of thing isn’t widely used yet; mostly because it would break certain facilities, like Input Managers, that people currently rely on.

One aspect I haven’t touched upon yet is the so-called “self-signed certificate”. This is a certificate which is its own root certificate, like the top certificates of the various ‘big” authorities. The difference is that these authorities are large companies that undergo serious auditions and verifications. As an example, Quay is currently self-signed; I’m still evaluating how to publish my certificate in a way that easily allows my users to check if new versions have indeed been signed by myself. Let’s run one of the same verifications on Quay:

$ codesign -dvvvv -r- /Applications/Quay.app
Format=bundle with Mach-O universal (i386 ppc)
CodeDirectory v=20001 size=245 flags=0x300(kill,hard) hashes=6+3 location=embedded
Signature size=1671
Authority=Rainer Brockerhoff
Signed Time=Mar 8, 2008 08/03 22:28:09
Info.plist entries=14
Sealed Resources rules=4 files=18
host => anchor apple and identifier "com.apple.translate"
# designated => identifier "net.brockerhoff.Quay" and anchor leaf = H"4cbb97c74336f7ee6aa566122a5e7688e1c725dc"

See the difference? It shows only my name as authority, and a SHA1 hashcode at the end. I could publish this hashcode on my site and it would be not too difficult for a technically knowledgeable user to verify that his copy of Quay matches that.

What does this really mean? Actually, not too much. It means that two versions of Quay that give identical names and hashcodes, which additionally match the ones published on this site, giving reasonable assurance that the author of both versions and of the site are the same; but nothing at all beyond that. However, there are ways around that – SHA1 hashcodes can be duplicated with some effort, though perhaps not yet for certificates, and this site doesn’t have a secure (https) variant. A malicious person could easily strip away my code signatures, resign everything with a new self-signed certificate using my name, make a phishing site that looks like mine, and so forth, and to a normal user everything would appear to be OK. Would someone do that for a €7 utility program? Very probably not worth the trouble, but you can’t be sure.

Then again, I could invest in a secure site, an official certificate signed by a trusted authority – perhaps even by Apple itself – and all that would just make the code signature itself more reliable; it wouldn’t necessarily correspond to what extent the code itself can be trusted to always “do the right thing”.

As Mike points out, there’s a huge gap between “signed” and “trusted”. Ultimately you have to trust somebody, or you would be too scared to buy a box at a computer store and open it, much less connect that computer to the Internet or let your children touch it. If I buy a certificate from Apple, and use that to sign my code, you have to trust someone in the certificate chain even if my code seems to “just work”… it wouldn’t necessarily mean that Apple has audited and quadruple-checked my source code. Indeed, it doesn’t even touch the question of whether Apple as a company can be trusted!

All that is still largely theoretical; Mac OS X still runs unsigned code with no problem, even though with some small restrictions regarding use of the keychain, the firewall, and internet access as I mentioned above. But Apple engineers say that in the future code signing will become mandatory. In the next post, I’ll try to discuss the implications of that – especially regarding the upcoming iPhone OS 2.0, where it will be mandatory from the outset.