Windows CNG virtualization-based security

[krwq]

Fixes: #102492

One of the Windows 11 builds has added framework to help secure Windows keys with virtualization-based security (VBS). With this new capability, keys can be protected from admin-level key theft attacks with negligible effect on performance, reliability, or scale.

Blog post:
https://techcommunity.microsoft.com/t5/windows-it-pro-blog/advancing-key-protection-in-windows-using-vbs/ba-p/4050988

Win API:
https://learn.microsoft.com/en-us/windows/win32/api/ncrypt/nf-ncrypt-ncryptcreatepersistedkey

The proposal is to extend existing CngKeyCreationOptions API to include the new flags.

API Proposal

namespace System.Security.Cryptography;

[Flags]
public enum CngKeyCreationOptions : int
{
    // existing:
    // None = 0x00000000,
    // MachineKey = 0x00000020,            // NCRYPT_MACHINE_KEY_FLAG
    // OverwriteExistingKey = 0x00000080,  // NCRYPT_OVERWRITE_KEY_FLAG

    // new APIs:
    PreferVbs = 0x00010000,             // NCRYPT_PREFER_VBS_FLAG
    RequireVbs = 0x00020000,            // NCRYPT_REQUIRE_VBS_FLAG
    UsePerBootKey = 0x00040000,         // NCRYPT_USE_PER_BOOT_KEY_FLAG
}

Example usage

// Note: this API is Windows only

using System.Security.Cryptography;

CngKeyCreationParameters cngCreationParams = new()
{
    Provider = CngProvider.MicrosoftSoftwareKeyStorageProvider,
    KeyCreationOptions = CngKeyCreationOptions.RequireVbs | CngKeyCreationOptions.OverwriteExistingKey,
};

using (CngKey key = CngKey.Create(CngAlgorithm.ECDsaP256, "mySoftwareKey", cngCreationParams))
using (ECDsaCng ecdsa = new ECDsaCng(key))
{
    // do stuff with the key
}

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.