Technical Note TN2206

OS X Code Signing In Depth

The purpose of this technote is to provide a more in depth view of code signing. It is intended to expand upon the information given in the Code Signing Guide by supplying a more detailed analysis of the technology. The target audience for this document is OS X developers who have read and presumably understand the information given in the Code Signing Guide but want to learn a bit more.

This document is not meant to be applied to code signing on iOS, however. Xcode manages code signing on iOS; viewing the iOS documentation will give a clue as to the similarities.

Code Signing Recap
Self-signed Identities and Self-created Certificate Authorities
Creating a Self-signed Code Signing Certificate using OpenSSL
Code Signing Changes in OS X Mavericks v10.9
Gatekeeper Changes in OS X v10.9.5 and Later
Gatekeeper Changes in OS X v10.10.4 and Later
OS X Code Signing Tips and Tricks
Version 2 Code Signature FAQ
Troubleshooting
Document Revision History

Code Signing Recap

Code signing is a facility by which developers can assign a digital identity to their programs. Apple provides the tools necessary to sign your programs (see the codesign manual page).

Code signing on OS X is an integral part of the development process. Most code signing certificates are provided by Apple or internally provisioned by enterprise IT departments. While tools like Xcode handle much of the certificate management, you can also maintain your signing certificates yourself if your situation calls for it.

In short, code signing is a technology that allows you to dictate how validating mechanisms will interpret your code. Code signing does implement some policy checks. However, policy is mostly set by the specific subsystem carrying out validation; any policy decisions outside of those implemented by OS X subsystems are left up to you and your end users in how you interoperate between a specific set of subsystems.

Trust and Code Signing

Trust is determined by policy. A security trust policy determines whether a particular code identity, which is essentially the designated requirement (DR) for the code, should be accepted for allowing something to happen on the system, e.g., access to a resource or service, after testing for validity. Each OS X subsystem has its own policy, and makes this determination separately. Thus, it makes no sense to ask whether code signing trusts a particular signature. You have to ask based on the subsystem, and it is more meaningful to ask whether a specific subsystem trusts your signature.

In general, most subsystems do not care that your identity certificate chain leads to a trusted anchor, however, some do. Additionally, some subsystems track identities and some don't. Subsystem tracking alludes to how the subsystem verifies an identity after the initial policy decision has been acted upon. For a concrete example, below is a list of commonly-used subsystems that verify code signatures:

Table 1  Examples of OS X subsystems that verify the validity of code.

Subsystem

Function

Initial Policy

Tracking Policy

App Sandbox

Gate access to system resources based on entitlements.

Allow if entitlement is present in the app's code signature.

Initial policy decision is verified against the application's DR.

Gatekeeper

Restrict launching of applications from unidentified developers

A configurable trusted anchor check (Developer ID or Mac App Store).

None (each request is evaluated by policy).

Application Firewall

Restrict inbound network access by applications.

Allow if a trusted anchor check succeeds; otherwise prompt the user.

Initial policy decision is verified against the application's DR.

Parental Controls (MCX)

Restrict what applications a managed user can run.

Explicit administrator decision (no code signing involved in the initial decision).

Initial policy decision is verified against the application's DR.

Keychain Access Controls

Controls what applications can do with specific keychain items.

The creating application is automatically trusted with its item, and determines the access policy using code signing requirements.

Free access to the keychain item by the creating application and tracked with its DR (No automatic tracking for custom ACLs).

Developer Tools Access (DTA)

Restrict what programs are allowed to call DTA APIs (task_for_pid, etc.)

A hard-coded trusted anchor check.

None (each request is evaluated by policy).

The above examples also further emphasize the fact that all policy decisions are determined by a specific subsystem and not by code signing itself. In addition, it highlights the diversity in how code signing can be used by a specific subsystem to carry out policy. For instance:

  • DTA doesn't even have a tracking policy. It simply applies a set requirement to every requester without needing to retain any information.

  • Parental controls show that you don't have to even use code signing at all in order to craft a usable policy.

  • Application Firewall uses code signing for both its initial and tracking policy decisions.

  • The keychain acts on the tracking policy by default but it can also allow arbitrary requirement-bearing ACLs to be added to express arbitrary policies determined by the owner of a specific keychain item.

Many parts of OS X do not care about the identity of the signer. (Gatekeeper is a notable exception.) They care only whether the program is validly signed and stable. Stability is determined through the designated requirement (DR) mechanism, and does not depend on the nature of the certificate authority used. The keychain system and parental controls are examples of such usage. Self-signed identities and homemade certificate authorities (CA) work by default for this case.

Other parts of OS X constrain acceptable signatures to only those drawn from certificate authorities that are trusted on the system performing the validation. For those checks, the nature of the identity certificate used does matter. The Application Firewall is one example of this usage. Self-signed identities and self-created CA will not be verified as being valid for this check unless the verifying system has been told to trust them for Application Firewall purposes.

Please keep in mind that using a signing identity that is system-wide trusted doesn't automatically mean that it's:

  • a requirement for a signature to be valid.

  • going to be ignored by the majority of subsystems.

  • going to matter only to particular checks within particular subsystems.

Code Requirements

A code requirement is a statement that expresses constraints on a validly signed application. Code signature validation can accept a requirement as input which will then be used to check whether the code is validly signed and satisfies the constraints of the requirement. When signing code, it is not normally necessary to concern yourself with code requirements. They will be managed implicitly by codesign and Xcode. However, in rare cases you may need to explicitly override the default settings to achieve particular effects.

To explicitly test whether a program satisfies a particular requirement, use the -R option to the codesign command; for example:

$ # Make a copy of the md5 tool.
$ cp /sbin/md5 .
$ # The copy still satisfies its DR.
$ codesign -vvvv ./md5
./md5: valid on disk
./md5: satisfies its Designated Requirement
$ # And we can check that it's signed by Apple.
$ codesign -vvvv -R="anchor apple" ./md5
./md5: valid on disk
./md5: satisfies its Designated Requirement
./md5: explicit requirement satisfied
$ # Modify the binary.
$ chmod u+w ./md5
$ dd if=/dev/zero bs=1 count=1 seek=8192 conv=notrunc of=./md5
1+0 records in
1+0 records out
1 bytes transferred in 0.000036 secs (27777 bytes/sec)
$ # The modified program no longer satisfies its DR.
$ codesign -vvvv ./md5
./md5: code or signature modified
$ # But we can resign the modified program with our signature.
$ codesign -s my-signing-identity -f ./md5
./md5: replacing existing signature
$ # And the modified program now satisfies its DR.
$ codesign -vvvv ./md5
./md5: valid on disk
./md5: satisfies its Designated Requirement
$ # But not our supplement requirement.
$ codesign -vvvv -R="anchor apple" ./md5
./md5: valid on disk
./md5: satisfies its Designated Requirement
test-requirement: failed to satisfy code requirement(s)

The requirement language is a set of rules that can be chained together using logical operators ("and", "or", "not", and parentheses to denote precedence) to form a requirement expression. Below is the current list of supported rules and their usage:

Table 2  The requirement language syntax.

Rule Syntax Usage

Description

identifier <string>

The signing identifier is exactly the string given

certificate <slot> = <hash>

The certificate in the certificate chain has a SHA-1 hash as given

certificate <slot> trusted

The certificate is trusted as per Trust Settings API for code signing

certificate <slot> [<key>] = <value>

Some part of the certificate has the given value

info [<key>] = <value>

The Info.plist has a key with the given value

cdhash <hash>

The CodeDirectory's SHA-1 hash is the given value

anchor <string>

The root certificate given by its path

anchor trusted

The certificate chain must lead to a trusted root

anchor apple

The certificate chain must lead to an Apple root

Some important things to keep in mind:

  • When you pass a path of a certificate to set as an anchor in the designated requirement (DR) the DR will automatically be transformed to store only the SHA-1 hash of that certificate. When the policy engine then evaluates the validity of the signed program it uses the stored hash value found in the DR to compare to the actual anchor.

  • The signing identifier is also embedded in the DR and will default to the CFBundleIdentifier found in the Info.plist for convenience if one is not supplied explicitly. The identifier has no meaning as far as code signing is concerned, other than as a means to make DRs unique.

  • You may pass in negative integers as an index into the certificate chain array if you want the searching origin to start with the root certificate rather than the leaf as the origin; the value for the leaf certificate and root certificate are 0 and -1, respectively. The distance for the leaf origin (positive integer) is measured by the absolute value of the integer value passed in. Whereas, the distance for the root origin (negative integer) is measured by the absolute value of 1 + the integer value passed in.

  • To manipulate or experiment with requirements, use the csreq command.

Code Designated Requirement

All signed code has a designated requirement (DR). This requirement states, from the perspective of the developer of the program, what constraints a program needs to satisfy in order to be considered an instance of this program. Obviously, every program should satisfy its own DR, e.g., codesign -vv checks for this. More interestingly, a program's DR should also be satisfied by updates, i.e., new versions of that code, and by nothing else. This is how the OS X code signing policy engine recognizes updates and upgrades.

By default, the system synthesizes a suitable DR for your code when you sign your program. This will work fine in most cases. However, you may specify an explicit DR when signing your program, and there are situations where you should do so.

To see what DR a program has:

$ codesign -d -r- /sbin/md5
Executable=/sbin/md5
 
# designated => identifier "com.apple.md5" and anchor apple

Look for the line starting with "designated =>". If it is commented out (starts with a "#" mark), it is implicitly generated. If not, it is explicit.

Use the -r option to the codesign command to explicitly specify a DR when signing; for example:

$ codesign -vvvv -s my-signing-identity -r="designated => anchor trusted" ~/Desktop/CodesignTest
/Users/admin/Desktop/CodesignTest: signed Mach-O thin (i386) [CodesignTest]

If you do create a DR, you are responsible for crafting a suitable requirement to use. For example below is a DR for specifying to the policy engine that it should check that the signer of the program leads to a trusted anchor on the calling system and that the program's identifier matches the supplied parameter string.

$ codesign -vvvv -s my-signing-identity -r="designated => anchor trusted and \
identifier com.foo.bar" ~/Desktop/CodesignTest
/Users/admin/Desktop/CodesignTest: signed Mach-O thin (i386) [CodesignTest]

Certificate Validity

Except as stated in the next paragraph, the code signing and validation engines accept signatures made with expired certificates. This means that your signed code will not become invalid when your certificate expires. In simple applications, you can continue signing with an expired certificate and OS X will continue to accept this. Code signing will reject signatures made with identities that have been revoked according to standard X.509 processing rules (See RFC 3280 for an example). Revocation check instructions have to be embedded in the certificates you want checked. Revocation checking is a per-user preference found in Keychain Access. When revocation checking is configured and enabled the system may still be unable to ascertain revocation status if it is disconnected from the network in which case the validation might succeed instead of fail, i.e., if certificate revocation Keychain Access preferences are set as "Best Attempt" instead of "Require if Cert Indicates".

Developer ID signatures carry cryptographic timestamps by default. Signatures with cryptographic timestamps are validated against the signing time, and signatures made with expired (at signing time) certificates are invalid. The previous discussion still applies to Developer ID signatures without secure timestamps.

Self-signed Identities and Self-created Certificate Authorities

Depending on the policy used by the subsystem in question, a self-signed identity can usually be used (your program will reap all the benefits of being signed by it) as long as that identity is set following the respective policy. Obviously, one big downside in using a self-signed identity is that you will never be able to revoke it, however, it may be sufficient for your organization's certificate policy requirements or if you just want to test out the code signing machinery.

If you decide to create your own CA then you specify an explicit DR naming your own anchor certificate:

Listing 1  Creating a DR

$ codesign -vvvv -s my-signing-identity -r="designated => anchor rootCert and identifier com.foo.bar" \
~/Desktop/CodesignTest
/Users/admin/Desktop/CodesignTest: signed Mach-O thin (i386) [CodesignTest]

By using your own CA you gain the ability to issue new identities at will, since any signing identities issued from your CA will now satisfy the check. You can do this by selecting "Create a Certificate for Someone Else as a Certificate Authority" as option for the Certificate Assistant in Keychain Access. You can also do this with openssl:

$ openssl ca -out cert.rsa -config ./openssl.cnf -infiles request.csr

Just as in a custom created CA, if you buy a signing certificate from a commercial CA you'll want to craft a DR that expresses the CA vendor's issuance policies. For instance, every time your CA reissues you a new certificate your identities will change which is something that your DR should take care to handle.

Creating a Self-signed Code Signing Certificate using OpenSSL

If you already have an SSL identity, i.e., a public and private key pair generated through OpenSSL, and you want to use it for code signing then you will need to use something other than the Certificate Assistant. This is because converting an existing OpenSSL digital identity for use with code signing is currently unsupported by the Certificate Assistant. However, you can solve this problem using openssl. The steps that you need to take are:

Code Signing Changes in OS X Mavericks v10.9

OS X Mavericks v10.9 introduced a number of significant changes to the code signing machinery. Most of these changes apply to the resource envelope of a code signature, where the signature keeps a list of files in a bundle. The pre-Mavericks version, version 1, recorded only files in the Resources directory and ignored the rest. The Mavericks version, version 2, makes the following changes:

A signature may contain multiple versions of resource envelopes. Mavericks generates, by default, both version 2 and version 1 resource envelopes. Mavericks verifies version 2 resource envelopes. Pre-Mavericks systems ignore the version 2 resource envelope and use the version 1 resource envelope. If Mavericks sees only a version 1 signature, it performs version 1 validation.

To determine which version of resource envelope a code signature has, use codesign -dv and note the version of the sealed resources, like this:

$ codesign -dv Chess.app/
[...]
Sealed Resources version=2 rules=15 files=53
[...]

Resource Rules

Systems before OS X Mavericks v10.9 documented a signing feature (--resource-rules) to control which files in a bundle should be sealed by a code signature. This feature has been obsoleted for Mavericks. Code signatures made in Mavericks and later always seal all files in a bundle; there is no need to specify this explicitly any more. This also means that the Code Signing Resource Rules Path build setting in Xcode should no longer be used and should be left blank.

It is thus no longer possible to exclude parts of a bundle from the signature. Bundles should be treated as read-only once they have been signed.

Nested Code

From Mavericks onwards, signatures record nested code by its code signature and embed that information in the (outer) signature's resource envelope, recursively. This means that when a code signature is created, all nested code must already be signed correctly or the signing attempt will fail.

This is not a problem if you follow the standard Xcode build flow, because it will build and sign targets inside out: build the innermost code, copy it into the next-outer bundle, then sign that, etc. Always check that

Nested code is expected in a number of standard locations within a bundle.

Table 3  Standard locations for code inside a bundle

Location

Description

Contents

Top content directory of the bundle

Contents/MacOS

Helper apps and tools

Contents/Frameworks

Frameworks, dylibs

Contents/PlugIns

Plug-ins, both loadable and Extensions

Contents/XPCServices

XPC services

Contents/Helpers

Helper apps and tools

Contents/Library/Automator

Automator actions

Contents/Library/Spotlight

Spotlight importers

Contents/Library/LoginItems

Installable login items

Contents/Library/LaunchServices

Privileged helper tools installed by the ServiceManagement framework

These places are expected to contain only code. Putting arbitrary data files there will cause them to be rejected (since they're unsigned).

Conversely, putting code into other places will cause it to be sealed as data (resource) files. This causes this code to be sealed twice; once in the nested code, and once in the outer signature. This wastes both signing and verification time and storage space. Also, this can break the outer signature of apps that use their own update mechanisms to replace nested code. If this nested code is being treated as resources, the outer signature doesn't know that this nested content is actually code.

Always put code and data into their proper places. This applies to all signed code and is enforced by the code signing machinery regardless of how the code is distributed.

Note that a location where code is expected to reside cannot generally contain directories full of nested code, because those directories tend to be interpreted as bundles. So this occasional practice is not recommended and not officially supported. If you do do this, do not use periods in the directory names. The code signing machinery interprets directories with periods in their names as code bundles and will reject them if they don't conform to the expected code bundle layout.

Store Python, Perl, shell, and other script files and other non-Mach-O executables in your app's Contents/Resources directory. While it's possible to sign such executables and store them in Contents/MacOS, this is not recommended.

In addition, there may not be any content in the top level of a bundle. In other words, if a bundle has a Contents or Versions directory at its top level, there may be no other files or directories alongside them. The one exception is that alongside Versions, there may be symlinks to files in Versions/Current.

The code signing machinery performs some framework checks specifically on frameworks that are nested within other code. It's possible that signing a framework will succeed, but the result fails to validate when placed into another bundle's Frameworks directory. Make sure the framework is structured correctly per the requirements above.

Bundles must have their Info.plist in the proper location. For app bundles, this is in Contents. For frameworks, this is in Versions/Current/Resources.

Gatekeeper Changes in OS X v10.9.5 and Later

Beginning with OS X v10.9.5, there are changes in how Gatekeeper recognizes signed apps. Version 1 signatures created with OS X versions prior to Mavericks will no longer be recognized by Gatekeeper and are considered obsolete.

If your team is using an older version of OS X to build your code, re-sign your app using OS X v10.9 or later using the codesign tool to create version 2 signatures. Apps signed with version 2 signatures will work on older versions of OS X.

Structure your bundle according to the expectations for OS X v10.9 or later:

Gatekeeper Changes in OS X v10.10.4 and Later

Beginning with OS X v10.10.4, Gatekeeper verifies libraries loaded from outside an application bundle.

If an app uses @rpath or an absolute path to link to a dynamic library outside of the app, the app will be rejected by Gatekeeper. This restriction applies to the app’s main executable and any other executable in the bundle, including libraries. This restriction applies even if the path does not exist (and normally causes the dynamic linker to fall back to a library inside the bundle).

If Gatekeeper rejects launching an app for this reason, the system log will note Fails dylib check, like this:

7/8/15 4:00:19.942 PM CoreServicesUIAgent[5940]: File /Applications/MyApp.app/Contents/MacOS/MyApp failed on loadCmd /foo/libLibrary.dylib
7/8/15 4:00:19.942 PM CoreServicesUIAgent[5940]: Fails dylib check

This means that MyApp.app links against /foo/libLibrary.dylib. /foo is not a standard location for libraries on OS X, so the linkage isn’t allowed.

Neither the codesign nor the spctl tool will show the error. The error will only appear in the system log.

This check is performed the first time the app is run. It does not apply to libraries the app loads itself using the dlopen function. It also does not apply to libraries loaded from paths where libraries are expected to reside, such as /System, /Library, and /usr/.

To see which libraries an app references, use the command otool -L MyApp.app/Contents/MacOS/MyApp.

The changes in this section also apply to OS X v10.8.5 if Security Update 2015-005 Mountain Lion has been installed.

OS X Code Signing Tips and Tricks

Checking Gatekeeper Conformance

To test Gatekeeper conformance, you must use OS X v10.9.5 or later. Follow these steps:

  • Package your program the way you ship it, such as in a disk image.

  • Download it from its website, or mail it to yourself, or send it to yourself using AirDrop or Message. This will quarantine the downloaded copy. This is necessary to trigger the Gatekeeper check as Gatekeeper only checks quarantined files the first time they're opened.

    Hint: keep the downloaded .dmg around; it will stay quarantined and you can use it again and again to test.

  • Drag-install your app and launch it.

  • Observe the results.

    Hint: Don't launch from inside the .dmg.

What results did you get?

  • If there is no dialog at all, a step was missed. Check the instructions and repeat the test.

  • If you get the dialog ... is from the Internet with an Open button, the test succeeded.

  • If you're told that only apps from the Mac App Store or registered developers can be installed, your app isn't Developer ID-signed. Use the Xcode Organizer to export a Developer ID-signed copy of your app and repeat the test.

  • If you get any other complaint, your signature is weak or broken. Please review the Version 2 Code Signature FAQ.

This command:

$ codesign --verify --deep --verbose=2 Foo.app

mimics what Gatekeeper does to check your app.

You can also use the check-signature tool to check both apps and installer packages.

Mount the disk image, then run the tool like this:

$ cd "/Volumes/Signature Check"
$ ./check-signature /Path/to/Foo.app /Path/to/Bar.pkg

For each target, the tool will present a simple YES answer if the signature meets Gatekeeper requirements, or NO if it does not.

Read the error messages carefully, with particular attention to the in subcomponent: part which, if present, tells you which nested code is giving you problems.

Understand that this validation will stop on many errors, and thus you must repeat it until you run out of problems.

You can also use the spctl tool to check if Gatekeeper will accept your app's signature. spctl is a command-line interface to the same security assessment policy subsystem that Gatekeeper uses.

Like Gatekeeper, spctl will only accept Developer ID-signed apps and apps downloaded from the Mac App Store by default. It will reject apps signed with Mac App Store development or distribution certificates.

Run spctl on your app like this:

$ spctl -a -t exec -vv Foo.app

This is the output if your app's signature will be accepted:

Foo.app: accepted
source=Developer ID

source will be Mac App Store for apps downloaded from the Mac App Store.

If spctl shows any result other than accepted, you must re-sign your app on Mavericks to ensure Gatekeeper compatibility.

Users can override Gatekeeper's assessment of your app's signature using the steps described in this support article.

Moving Away From Custom Resource Rules

If you used custom resource rules because your program modifies itself as it is running, you should stop doing this as soon as possible. If you transport such a modified app to another computer by web download, email, AirDrop, etc., the modified copy at the receiving end will be quarantined. This will trigger the Gatekeeper check the first time the app is run on the receiving machine, and the app will not be allowed to run because of the broken signature.

If you used custom resource rules because your installation process relies on changing the bundle, your app will be rejected by Gatekeeper on first launch. These modifications are not permitted. Using an installation package instead of a drag-install will get you through Gatekeeper.

To support the creation of droplet-like apps or similar forms of personalization, it is acceptable to place the string representation of a UUID (e.g. 16F2ACA8-AA29-48ED-9ED9-F96F3A230505) into an extended attribute named com.apple.application-instance of the top-level directory of your bundle (e.g. MyApp.app). This will not invalidate the code signature or affect the bundle’s code identity. You may then link this UUID to storage outside the bundle that carries the necessary configuration data, such as NSUserDefaults, a file in the Application Support directory, or a web service.

Placing configuration data directly into extended attributes is not supported.

Changes That Don't Invalidate a Code Signature

There are a few changes you can make to a signed bundle that won't invalidate its signature.

If you have optional or replaceable content you wish to change without invalidating the code signature, nested code can be replaced with equivalent (conforms to the designated requirement) nested code without disturbing the outer signature. This is the design mechanism for indirection in code bundles. It is acceptable to code-sign a bundle containing no main executable, and then treat it as nested code (typically in Contents).

Removing files from .lproj directories inside Contents/Resources will not invalidate the code signature, but adding or changing files will.

Removing the Mach-O slice for a particular architecture from a universal binary will also not invalidate the code signature.

Signing with Xcode

Xcode will run all build phases for a target before it signs that target. This is usually what you want since you shouldn't tamper with the bundle after it's been signed. If for some reason you must do something to the target after signing, create a separate aggregate target, make it depend on the original target, and put your custom build phases there.

Make sure your target dependencies are set correctly so the targets are built and signed in the correct order. Confirm that the Identity section of every target's General pane has the same value for Team so every target is signed with the same identity.

Using the codesign Tool's --deep Option Correctly

When verifying signatures, add --deep to perform recursive validation of nested code. Without --deep, validation will be shallow: it will check the immediate nested content but not check that fully. Note that Gatekeeper always performs --deep style validation.

Note that signing with the combination --deep --force will forcibly re-sign all code in a bundle.

Installer Packages

Bundle-style installer packages are a legacy transition aid that is no longer supported. PackageMaker is also no longer supported. It is now necessary to convert to flat-file installer packages using tools like productbuild.

If you must make short-term modifications to existing .pkg or .mpkg bundles to get them through Gatekeeper, then specify explicit resource rules that declare all files in the package to be resources with no exceptions.

The default codesign behavior for bundle-style installer packages creates a legacy signature format that does not pass Gatekeeper any more.

Use the check-signature tool to check your installer packages as described earlier.

Version 2 Code Signature FAQ

codesign says my signature is weak.

  • Your bundle seal is incomplete: it excludes some files. All version 1 signatures are weak by definition. But if you re-sign as directed, the result should not be weak.

  • You used resource rules to make your signature weak. This is no longer allowed.

I exclude files so I can fix my bundle after I build it.

  • This is no longer allowed. If you must modify your bundle, do it before signing. If you modify a signed bundle, you must re-sign it afterwards.

I write a receipt/license file/magic marker into my bundle after installation.

  • This is no longer allowed.

  • Write data into files outside the bundle as described earlier.

  • Use the extended attribute mechanism described earlier to point your bundle at this data if necessary.

  • Use an online service to store it and use the extended attribute mechanism to point at data there.

I store data in my bundle after I sign it.

  • This is no longer allowed. You will need to locate this data elsewhere.

  • If you write the data before your app runs for the first time, Gatekeeper will reject your app because the signature will be invalid.

  • If you write that data after your app first runs, you're still breaking your signature. Gatekeeper will check your app again if your app is copied to another computer such that the new copy is quarantined.

  • OS X APIs that rely on a valid identity will fail.

  • In general, you should plan for future stricter runtime checks of code validity.

codesign says my code is unsigned when I try to sign it.

  • Make sure all nested code is already signed and its signature is valid. Xcode will take care of this for you if you let it handle your code signing tasks.

  • Make sure your target dependencies are set correctly so the targets are built and signed in the correct order. Confirm that the Identity section of every target's General pane has the same value for Team so every target is signed with the same identity.

The 'code' codesign complains about isn't code. It’s just some data files.

  • If a file is in a code location, it must be code, and it must be signed.

  • Do not put data files into code locations. Move these elsewhere, such as Contents/Resources.

I can't change where my data files are stored without significantly changing my code.

  • You can work around this by moving the files to the correct locations and leave behind symlinks so your code can still find the files.

  • Then fix your code, and remove the symlinks as soon as possible.

codesign says my code is broken when I try to sign it.

  • The signature of your nested code is broken and should be fixed as described earlier.

codesign says my main executable failed strict validation.

  • Your Mach-O executable does not conform to modern Mach-O layout rules.

  • You may be using a third party development product that hasn't been brought up to date, or post-processed your file in unsupported ways.

I wrote some data to the Mach-O file before signing. Is that allowed?

  • No. Do not tamper with Mach-O files, outside of using OS X build tools and Xcode workflows.

I embed my data in the Mach-O file.

  • Use the -sectcreate linker option for this.

codesign says I have 'unsealed content' somewhere.

  • Do not put files or directories into the top directory of an app or framework bundle

  • All content must be inside Contents or Versions respectively as described earlier.

codesign says my bundle format is ambiguous.

  • codesign thinks your bundle looks a bit like an app and a bit like a framework.

  • Perhaps you have both Contents and Versions directories, or files in the top directory that match reserved names for directories in a bundle.

  • Perhaps there's an Info.plist in an odd place.

  • In short, put everything in the correct places.

codesign says an embedded framework is wrong.

  • Check the framework for top-level contents, and for proper placement and contents of its Info.plist.

  • If your framework contains multiple versions, make sure they are all properly signed by the same signing authority.

  • Make sure all symlinks in the top .framework directory are in fact symlinks and point to the matching directory inside Versions/Current.

Troubleshooting

Signing Modifies the Executable

Signing a program will modify its main executable file. There are some situations where this will cause you trouble:

  • If your program has a self-verification mode that detects a change, your code may refuse to run.

  • Appending data to a Mach-O executable is expressly prohibited. Signature verifications on such files will fail.

The obvious solution to these problems is to not meddle with your signed program after you've signed it with codesign. If you're modifying the executable or bundle in any way then the code signing validation engine will obviously pick up on that change and act appropriately with the set policy. If you've set your program to do self-integrity checking then it is possible that your expectation of what constitutes "your program" is likely to have changed due to code signing. More specifically, whether you're checking the entire contents of the Mach-O file or just the aggregation of certain pieces of the file it's highly likely that code signing will break what you believe integrity checking is. For example:

$ # Let's see what the Mach-O file looks like pre-signing:
$ otool -l ~/Desktop/CodesignTest
CodesignTest:
Load command 0
      cmd LC_SEGMENT
  cmdsize 56
  segname __PAGEZERO
   vmaddr 0x00000000
   vmsize 0x00001000
  fileoff 0
 filesize 0
  maxprot 0x00000000
 initprot 0x00000000
   nsects 0
    flags 0x0
[...]
Load command 11
          cmd LC_LOAD_DYLIB
      cmdsize 52
         name /usr/lib/libSystem.B.dylib (offset 24)
   time stamp 2 Wed Dec 31 16:00:02 1969
      current version 111.0.0
compatibility version 1.0.0
$ # Now let's see what it looks like after signing:
$ codesign -vvvv -s my-signing-identity ~/Desktop/CodesignTest
CodesignTest: signed Mach-O thin (i386) [CodesignTest]
$ otool -l CodesignTest
CodesignTest:
Load command 0
      cmd LC_SEGMENT
  cmdsize 56
  segname __PAGEZERO
   vmaddr 0x00000000
   vmsize 0x00001000
  fileoff 0
 filesize 0
  maxprot 0x00000000
 initprot 0x00000000
   nsects 0
    flags 0x0
[...]
Load command 11
          cmd LC_LOAD_DYLIB
      cmdsize 52
         name /usr/lib/libSystem.B.dylib (offset 24)
   time stamp 2 Wed Dec 31 16:00:02 1969
      current version 111.0.0
compatibility version 1.0.0
Load command 12
      cmd LC_CODE_SIGNATURE
  cmdsize 16
 dataoff  12816
 datasize 5232
$ # Notice the extra load command LC_CODE_SIGNATURE at number 12!
$ # Let's check it out even further with pagestuff:
$ pagestuff CodesignTest -a
File Page 0 contains Mach-O headers
[...]
File Page 3 contains local of code signature
File Page 4 contains local of code signature

Signing Frameworks

Seeing as frameworks are bundles it would seem logical to conclude that you can sign a framework directly. Most frameworks contain a single version and can in fact be signed directly. Signing the framework as a whole signs its "Current" version by default.

Multi-versioned frameworks are discouraged in general. However, if you happen to have one, make sure that you sign each specific version as opposed to the whole framework:

$ # This is the wrong way to sign a multi-versioned framework:
$ codesign -s my-signing-identity ../FooBarBaz.framework
$ # This is the right way:
$ codesign -s my-signing-identity ../FooBarBaz.framework/Versions/A
$ codesign -s my-signing-identity ../FooBarBaz.framework/Versions/B

Multi-versioned frameworks are "versioned bundles", and each contained version should be signed and validated separately.

Extended Key Usage

Standard X.509 certificates (RFC 2459) contain object identifiers (OID) which form key usage extensions to define what the public and private key can and cannot be used for. Extended key usages just further refine the key usage extensions. An extension is either critical or non-critical. If the extension is critical than the identity must only be used for indicated purpose(s).

The X.509 certificates and their code signing extended key usage is obviously required for an identity to be used for code signing on OS X. However, the code signing extended key usage should also be the only extended key usage for a certificate (this code signing extended usage is critical) in order to be valid to the OS X code signing subsystem. It is not possible to create one certificate that can be used for both code signing and other purposes.

You can check your certificate to find out whether the code signing extended key usage attribute is present by viewing the certificate in Keychain Access or by using any other X.509 compliant certificate parser. Dumping all the available information about a certificate can also show if the certificate has other usages which will cause it to be an invalid identity for use with code signing on OS X. For example:

$ # Use security and certtool
$ security find-certificate -a -e clarus@apple.com -p > cert.pem
$ certtool d cert.pem
Serial Number : 01
Issuer Name :
   Common Name : Testing Code Signing
   Org : Apple Inc.
   OrgUnit : DTS
   State : CA
   Country : US
   Locality : Cupertino
   Email addrs : clarus@apple.com
Subject Name :
   Common Name : Testing Code Signing
   Org : Apple Inc.
   OrgUnit : DTS
   State : CA
   Country : US
   Locality : Cupertino
   Email addrs : clarus@apple.com
Cert Sig Algorithm : OID : < 06 09 2A 86 48 86 F7 0D 01 01 05 >
   alg params : 05 00
Not Before : 19:27:16 Nov 14, 2007
Not After : 19:27:16 Nov 13, 2008
Pub Key Algorithm : OID : < 06 09 2A 86 48 86 F7 0D 01 01 01 >
   alg params : 05 00
Pub key Bytes : Length 270 bytes : 00 00 00 00 00 00 00 00 ...
CSSM Key :
   Algorithm : RSA
   Key Size : 2048 bits
   Key Use : CSSM_KEYUSE_VERIFY
Signature : 256 bytes : FF FF FF FF FF FF FF FF ...
Other field: : OID : < 06 0C 60 86 48 01 86 F8 4D 02 01 01 01 17 >
Other field: : OID : < 06 0C 60 86 48 01 86 F8 4D 02 01 01 01 16 >
Extension struct : OID : < 06 03 55 1D 0F >
   Critical : TRUE
   usage : DigitalSignature
Extension struct : OID : < 06 03 55 1D 25 >
   Critical : TRUE
   purpose  0 : OID : < 06 08 2B 06 01 05 05 07 03 03 >

Shipping your Signed Code

Code signing uses extended attributes to store signatures in non-Mach-O executables such as script files. If the extended attributes are lost then the program's identity will be broken. Thus, when you ship your script, you must use a mechanism that preserves extended attributes.

One way to guarantee preservation of extended attributes is by packing up your signed code in a read-write disk image (DMG) file before signing and then, after signing, converting to read-only. You probably don't need to use a disk image until the final package stage so another less heavy-handed method would be to use ZIP or XIP files.

Interpreting code signing failures

If signing or validation fails in the codesign command due to problems with nested code, the command will output an additional line

 In subcomponent: path

indicating which nested code caused the problem. Always look for this line to correctly interpret a code signing failure.

If the Xcode Organizer produces a code signing error during distribution signing, the problem may be with improperly placed code or resources and not a problem with your signing identities.



Document Revision History


DateNotes
2015-07-28

Cover changes to linking to dynamic libraries outside of an app bundle.

2014-11-13

Add reference to the new check-signature tool.

2014-10-23

Added v2 signature FAQ and additional guidance for developers transitioning to v2 signatures.

2014-08-26

Added clarifications of Mavericks and Yosemite code signing changes.

2014-07-31

Updated discussion of significant code signing changes in OS X Mavericks. Other editorial changes.

2014-07-28

Added discussion of significant code signing changes in OS X Mavericks. Other editorial changes.

2014-06-24

Omitting files from the signature's seal is deprecated on OS X Mavericks.

2008-08-06

New document that intermediate to expert level overview of OS X code signing that details specific options and gotchas