Generate a public key token from an X.509 certificate (for use in ClickOnce manifests).

PublicKeyToken.cs

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Security.Cryptography.X509Certificates;

public static class PublicKeyToken
{
    public static string FromCertificate(X509Certificate cert)
    {
        var publicKeyBlob = GetCspPublicKeyBlob(cert);

        var publicKeyBlobPtr = Marshal.AllocHGlobal(publicKeyBlob.Length);
        Marshal.Copy(publicKeyBlob, 0, publicKeyBlobPtr, publicKeyBlob.Length);

        var cryptDataBlob = new Win32.CryptDataBlob { cbData = (uint)publicKeyBlob.Length, pbData = publicKeyBlobPtr };
        var publicKeyTokenPtr = new IntPtr();
        if (0 != Win32._AxlPublicKeyBlobToPublicKeyToken(ref cryptDataBlob, ref publicKeyTokenPtr))
            throw new Win32Exception(Marshal.GetLastWin32Error());

        Marshal.FreeHGlobal(publicKeyBlobPtr);

        var publicKeyToken = Marshal.PtrToStringUni(publicKeyTokenPtr);
        Win32.HeapFree(Win32.GetProcessHeap(), 0, publicKeyTokenPtr);

        return publicKeyToken;
    }

    private static byte[] GetCspPublicKeyBlob(X509Certificate cert)
    {
        var asnEncodedPublicKey = cert.GetPublicKey();

        uint requiredBytes = 0;
        if (!Win32.CryptDecodeObject(Win32.Pkcs7AsnEncoding | Win32.X509AsnEncoding, Win32.RsaCspPublicKeyBlob, asnEncodedPublicKey, (uint)asnEncodedPublicKey.Length, 0, null, ref requiredBytes))
            throw new Win32Exception(Marshal.GetLastWin32Error());

        var publicKeyBlob = new byte[requiredBytes];
        if (!Win32.CryptDecodeObject(Win32.Pkcs7AsnEncoding | Win32.X509AsnEncoding, Win32.RsaCspPublicKeyBlob, asnEncodedPublicKey, (uint)asnEncodedPublicKey.Length, 0, publicKeyBlob, ref requiredBytes))
            throw new Win32Exception(Marshal.GetLastWin32Error());

        return publicKeyBlob;
    }
}

Win32.cs

using System;
using System.Runtime.InteropServices;

public static class Win32
{
  public const uint X509AsnEncoding = 0x00000001;
  public const uint Pkcs7AsnEncoding = 0x00010000;
  public const uint RsaCspPublicKeyBlob = 19;

  [DllImport("clr.dll", CharSet = CharSet.Auto, SetLastError = true)]
  public static extern int _AxlPublicKeyBlobToPublicKeyToken([In] ref CryptDataBlob pCspPublicKeyBlob, [In, Out] ref IntPtr ppwszPublicKeyToken);

  [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
  public extern static IntPtr GetProcessHeap();

  [DllImport("kernel32.dll", SetLastError = true)]
  [return: MarshalAs(UnmanagedType.Bool)]
  public extern static bool HeapFree([In] IntPtr hHeap, [In] uint dwFlags, [In] IntPtr lpMem);

  [DllImport("crypt32.dll", CharSet = CharSet.Auto, SetLastError = true)]
  [return: MarshalAs(UnmanagedType.Bool)]
  public static extern bool CryptDecodeObject(uint certEncodingType, uint lpszStructType, byte[] pbEncoded, uint cbEncoded, uint flags, [In, Out] byte[] pvStructInfo, ref uint cbStructInfo);

  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
  public struct CryptDataBlob
  {
    internal uint cbData;
    internal IntPtr pbData;
  }
}