How to calculate PublisherID from Publisher?

Question

I'd like to know something about Windows Store and APPX package internals. The package.appxmanifest has an <Identity> element that has a package name, publisher and version attributes, for example

<Identity
    Name="MyCompany.MyGreatApp"
    Publisher="CN=B408A06D-44F7-4860-A12E-644DD44FA743"
    Version="1.0.0.3" />

Apparently, when I open this manifest in VS2013 and go to Packaging tab, it shows me a read-only "Package Family Name" field, which is a concatenation of package name, underscore and something that looks like a strange hash of the publisher string.

MyCompany.MyGreatApp_f08ys7xx9zb3y

How do I calculate this hash (also known as PublisherId)? See also PackageId class or PACKAGE_ID structure.

Here are some sample values for you eager cryptanalysts. It is 13 lowercase letters and digits, so the approximate "quality" is 67 bits. Thank you!

8wekyb3d8bbwe   CN=Microsoft Corporation, O=Microsoft Corporation, L=Redmond, S=Washington, C=US
amge560j0aq9g   CN=C357A519-CEE3-4675-9EF4-44DE1D99A5D6
a2xxwqz7shah6   CN=07AACB4D-E1D7-4606-AF0F-77713A7C52F6
cw5n1h2txyewy   CN=Microsoft Windows, O=Microsoft Corporation, L=Redmond, S=Washington, C=US
54ggd3ev8bvz6   CN=2180B9A4-DDFD-4BFD-8D7E-EADC9C394EF5
azstdzfk4mfqj   CN=246910D1-A42D-4A04-8CF1-0C2A5CD42D4D
rxzpp8adhbvh8   CN=7882B094-0135-443F-8362-164AA239F2A0
pwh22gvzcj20c   CN=9C2E3884-8027-4E71-97C7-BB7731A649A4
q4d96b2w5wcc2   CN=DCD4AC3C-C7E0-46FF-8387-51FDC8CBC467
r6rtpscs7gwyg   CN=54157592-46DE-47CD-AF04-3B89DE46E29B
8xx8rvfyw5nnt   CN=6E08453F-9BA7-4311-999C-D22FBA2FB1B8
kzf8qxf38zg5c   CN=Skype Software Sarl, O=Microsoft Corporation, L=Luxembourg, S=Luxembourg, C=LU
a76a11dkgb644   CN=40886CD1-D5C5-48D6-B914-AB6E72010FFC
6bhtb546zcxnj   CN=BBC567E9-A52C-43A3-A890-F8B17D68310E
46hhcags7zat8   CN=ABF01D82-FF53-447D-B7E8-61B6F2105F68

pd2za7f9waemw   CN=B408A06D-44F7-4860-A12E-644DD44FA740
h0ed56e8a88dc   CN=B408A06D-44F7-4860-A12E-644DD44FA741
wcvtzcf7freyj   CN=B408A06D-44F7-4860-A12E-644DD44FA742
f08ys7xx9zb3y   CN=B408A06D-44F7-4860-A12E-644DD44FA743
85zvc56jp30ec   CN=C408A06D-44F7-4860-A12E-644DD44FA743
x4nmjqajw9mv6   CN=D408A06D-44F7-4860-A12E-644DD44FA743
qrhphajnj16d4   CN=E408A06D-44F7-4860-A12E-644DD44FA743

Answer 1

I was very close in my guess of how the PublisherId value is constructed. I heard some whispering...

The value is a Crockford’s Base32 encoding of the first eight bytes of SHA-256 hash of the publisher string in UTF-16 (little endian). I have a good implementation that I validated using the sample values from my question.

Answer 2

Or you simply use the Kernel32.dll Method PackageFamilyNameFromId:

private static string GetPackageFamilyName(string name, string publisherId)
{
  string packageFamilyName = null;
  PACKAGE_ID packageId = new PACKAGE_ID
  {
    name = name,
    publisher = publisherId
  };
  uint packageFamilyNameLength = 0;
  //First get the length of the Package Name -> Pass NULL as Output Buffer
  if (PackageFamilyNameFromId(packageId, ref packageFamilyNameLength, null) == 122) //ERROR_INSUFFICIENT_BUFFER
  {
    StringBuilder packageFamilyNameBuilder = new StringBuilder((int)packageFamilyNameLength);
    if (PackageFamilyNameFromId(packageId, ref packageFamilyNameLength, packageFamilyNameBuilder) == 0)
    {
      packageFamilyName = packageFamilyNameBuilder.ToString();
    }
  }
  return packageFamilyName;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode, Pack = 4)]
private class PACKAGE_ID
{
  public uint reserved;
  public uint processorArchitecture;
  public ulong version;
  public string name;
  public string publisher;
  public string resourceId;
  public string publisherId;
}


[DllImport("kernel32.dll", CharSet = CharSet.Unicode, ExactSpelling = true)]
private static extern uint PackageFamilyNameFromId(PACKAGE_ID packageId, ref uint packageFamilyNameLength, StringBuilder packageFamilyName);

public static void TestMethod()
{
  GetPackageFamilyName("MyCompany.TestApp", "CN=YourPublisherId");
}