Signing a DLL (Strong Name)

I was working on a project in C# and I had to sign an assembly with a Strong Name.

Here’s how I did that:

• Go to the project’s properties
• Go to the tab Signing
• Check Sign the assembly
• Choose <New...> from the drop-down list
• Give it a name (I used my solution’s name) and uncheck Protect my key file with a password

The assembly was signed now. However, when I tried to build it, an error occurred:

Assembly generation failed -- Referenced assembly 'Assembly' does not have a strong name

So the DLL had to be signed, too. Luckily, I had the source of the DLL. It’s also possible to sign a DLL without the source, but this won’t always work.

Sign the DLL using the source

• Open the project in Visual Studio
• Follow the steps to sign an assembly above for the DLL (but now use the key you already created by clicking <Browse...> instead of <New...>)
• Build the DLL

If you get the following error:

Friend assembly reference 'Assembly.Tests' is invalid. Strong-name signed assemblies must specify a public key in their InternalsVisibleTo declarations.

Replace the following values in the commands:

Placeholder	Value
{SNK}	The path to the combined key (.snk)
{PUB}	The path to public key (will be created)
{KEY}	The public key in SHA1 format

• Double click the error
• Start Visual Studio’s Developer Command Prompt
• Issue the following commands:

  sn -p "{SNK}" "{PUB}"
  sn -tp "{PUB}"
  

• This will display the public key, copy the public key under Public key (hash algorithm: sha1) Don’t copy the public key token!
• Go back to your project
• Locate the following line:

  [assembly: InternalsVisibleTo("Assembly.Tests")]
  

• Put the public key in the string:

  [assembly: InternalsVisibleTo("Assembly.Tests, PublicKey={KEY}")]
  

• Build the DLL

Sign the DLL without the source

Replace the following values in the commands:

Placeholder	Value
{DLL}	The path to the original DLL
{IL}	The path to .il file (will be created)
{RES}	The path to the .res file (located in the same folder as the .il file)
{KEY}	The path to the .snk file (the key file)
{DLL2}	The path to the signed DLL (will be created)

• Start Visual Studio’s Developer Command Prompt
• Run the following commands:

  ildasm "{DLL}" /out:"{IL}"
  ilasm "{IL}" /res:"{RES}" /dll /key:"{KEY}" /out:"{DLL2}"
  

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The DLL has now been signed with your key. Just re-add the reference to the new, signed DLL in your project and rebuild.