MetaData

NET metadata, in the Microsoft .NET framework, refers to code that describes .NET CIL (Common Intermediate Language) code. A .NET language compiler will generate the metadata and store this in the assembly containing the CIL. Metadata describes all classes and class members that are defined in the assembly, and the classes and class members that the current assembly will call from another assembly. The metadata for a method contains the complete description of the method, including the class (and the assembly that contains the class), the return type and all of the method parameters. When the CLR executes CIL it will check to make sure that the metadata of the called method is the same as the metadata that is stored in the calling method. This ensures that a method can only be called with exactly the right number of parameters and exactly the right parameter types.

Attributes

Developers can add metadata to their code through attributes. There are two types of attributes, custom and pseudo custom attributes, and to the developer these have the same syntax. Attributes in code are messages to the compiler to generate metadata. A pseudo custom attribute is metadata that the CLR knows about, for example [Serializable] (which means that an instance of the class can be serialized). The 'pseudo' in pseudo custom attribute refers to the fact that the compiler will not use it to generate custom metadata; instead, it will generate metadata that the CLR knows about.

Example (C#):[Serializable]

public class MyClass
{
...
}

When the compiler sees a custom attribute it will generate custom metadata that is not recognised by the CLR. The developer has to provide code to read the metadata and act on it. For example, the Visual Studio property grid groups together properties of an object that is being viewed using categories; the class developer indicates the category for the object's class by applying the [Category] custom attribute. In this case it is application code — the property grid — that interprets the attribute, not the CLR.

How metadata is stored

Assemblies contain tables of metadata. These tables are described by the CIL specification. The metadata tables will have zero or more entries and the position of an entry determines its index. When CIL code uses metadata it does so through a metadata token. This is a 32-bit value where the top 8 bits identify the appropriate metadata table, and the remaining 24 bits give the index of the metadata in the table. The Framework SDK contains a sample called metainfo that will list the metadata tables in an assembly, however, this information is rarely of use to a developer.

Metadata in an assembly may be viewed using the ILDASM tool provided by the .NET Framework SDK.

Reflection

Reflection is the API used to read .NET metadata. The reflection API provides a logical view of metadata rather than the literal view provided by tools like metainfo. Reflection in version 1.1 of the .NET framework will allow you to inspect the descriptions of classes and their members, and it will allow you to execute code. However, version 1.1 will not allow you to get access to the CIL for a method. Version 2.0 of the framework will allow you to obtain the CIL for a method.

Besides the System.Reflection namespace, the following tools are available for reading .NET metadata and parsing IL:

PostSharp
Mono Cecil

Giving a .NET Assembly a Strong Name

You assign an assembly a strong name by associating the assembly with a pair of 1,024-bit cryptographic public and private keys. The actual process varies slightly, depending on whether the developer has access to the private key. In larger, security-oriented corporations, most developers do not have access to the private key. Instead, only a few members of a final QA or security team can access the private key.

In order to assign one or more assemblies a strong name, you must first create the 1,024-bit public and private key pair. You do this by running the Strong Name Utility (SN.EXE), like so:

sn.exe -k PublicPrivateKeyFile.snk

This randomly creates a pair of 1,024-bit cryptographic keys. You can use these keys for encryption and decryption by using the RSA public/private key algorithm. The resulting key file contains both the public and the private key. You can extract the public key from the file and place it in a separate file like this:

sn.exe -p PublicPrivateKeyFile.snk PublicKeyFile.snk

Typically, you will only perform the above steps once per corporation or division because all assemblies you produce can use the same public and private keys as long as the assemblies have unique friendly text names.

Next, you need to associate the 1,024-bit public key with an assembly. You do this by telling the compiler to read the contents of a key file, extract the public key from the key file, and place the public key into the definition of the assembly's identity. In effect, this makes the public key an extension of the friendly text name of the assembly. This also makes the assembly name globally unique because no other developer will be using the same 1,024-bit public key as part of their assemblies' name.

When you have access to the key file that contains both the public and the private key, you can associate the assembly with a public key and digitally sign the assembly (discussed later) in a single operation by including the following compiler metadata instructions in one of your assembly's source files.

// C#
using System.Reflection;

[assembly: AssemblyDelaySign(false)]
[assembly: AssemblyKeyFile("PublicPrivateKeyFile.snk")]

Alternatively, when you only have access to the key file that contains just the public key, you must enable delay signing of the assembly. Therefore, instead of the above attributes, you need to specify the following attributes.

// C#
using System.Reflection;

[assembly: AssemblyDelaySign(true)]
[assembly: AssemblyKeyFile("PublicKeyFile.snk")]

At this point, your assembly has a strong name. Further, if you specified that the compiler should not delay-sign the assembly (therefore, the compiler did sign the assembly), the assembly is also a valid assembly and you can load it, debug it, and generally use it as you wish.

However, if you specified that the compiler should delay-sign the assembly (therefore, the compiler did not sign the assembly), you will discover that the runtime considers the assembly to be invalid and will not load it or allow you to debug and run it.

When the compiler digitally signs an assembly, it calculates a cryptographic digest of the contents of the assembly. A cryptographic digest is a fancy hash of your assembly's file contents. Let's call this cryptographic digest the compile-time digest of the assembly. The compiler encrypts the compile-time digest using the 1,024-bit private key from your public-private key pair file. The compiler then stores this encrypted compile-time digest into the assembly. Note that this all happens during development.

Sometime later, whenever the .NET loader loads an assembly with a strong name, the loader itself calculates a cryptographic digest of the contents of the assembly. Let's call this digest the runtime digest of the assembly. The loader then extracts the encrypted compile-time digest from the assembly, extracts the public key for the assembly from the assembly itself, and uses the public key to decrypt the previously encrypted compile time digest. The loader then compares the calculated runtime digest to the decrypted compile-time digest. When they are not equal, something or someone has modified the assembly since you compiled it; therefore, the runtime fails the assembly load operation.

Note: Based on the above description, when you do not have access to the private key, you cannot encrypt the compile-time digest, thus cannot sign the assembly. In this case, you must delay-sign the assembly. Setting the delay signing option to true tells the compiler that you'll calculate, encrypt, and store the compile-time digest into the assembly at a later time. Because a delay-signed assembly does not contain a valid digital signature, the runtime will not load it.

However, developers without access to the private key need to be able to debug and test a delay-signed assembly. Therefore, such developers must inform the runtime that it should load an assembly even though it does not contain a valid digital signature.

You can instruct the runtime to skip the digital signature verification process for a particular assembly a specific system by using the following command:

sn.exe -Vr YourAssembly.dll

Technically, this instruction registers the specified assembly for digital signature verification skipping. It is a sticky setting in that the runtime never again verifies the digital signature for that assembly until you unregister it for verification skipping using the inverse command:

sn.exe -Vu YourAssembly.dll

A developer without access to the private key cannot digitally sign an assembly, so must register the assembly for verification skipping in order to debug and test the assembly.

Finally, once the developer determines that the assembly operates correctly, she hands off the assembly to the team that does have access to the private key. That team performs the delay signing operation on the assembly by using the following command:

sn.exe -R YourAssembly.dll PublicPrivateKeyFile.snk

The strong name utility computes the compile-time cryptographic digest for the assembly, encrypts the digest with the private key from the key file, and stores the encrypted digest into the assembly. The assembly will now load successfully on all systems.

Now, let's consider the effect of obfuscation on this process.

One .NET obfuscator, Demeanor for .NET, uses the .NET runtime to load the assembly it is obfuscating to insure that it obfuscates exactly those assemblies the .NET runtime will load when an application executes. Therefore, the assembly to be obfuscated must be loadable. This means that, in order for Demeanor for .NET to load an assembly, one of the following must be true.

1) It must contain a valid digital signature or,
2) You must have enabled digital signature verification skipping for the assembly on the obfuscating system.

Obfuscation modifies the assembly. This means that even when the assembly contains a valid digital signature before the obfuscation process (step one above), the assembly will not contain a valid digital signature after the obfuscation process.

Therefore, after obfuscating an assembly, you must either re-sign the assembly (which computes the proper digest, encrypts it, and stores it into the assembly), or you must only use the assembly on systems with digital signature verification skipping enabled, which isn't really a practical option except for developers debugging the obfuscated code.

Practically, this means that you end up using the delay-signing process when obfuscating an assembly with a strong name. The typical usage pattern for a developer using obfuscation is:

Build the assembly with delay signing enabled.
Enable strong name verification skipping for the assembly (sn.exe -Vr).
Debug and test the assembly.
Obfuscate the assembly.
Debug and test the obfuscated version.
Delay sign the assembly (sn.exe -R).

Alternatively, smaller shops that allow all developers access to the private key do the following:

Build the assembly with delay signing disabled.
Debug and test the assembly.
Obfuscate the assembly.
Delay sign the assembly (sn.exe -R).
Debug and test the obfuscated version.