[API Proposal]: Add CompositeMLDsa property to CngAlgorithm and CngAlgorithmGroup

[PranavSenthilnathan]

Background and motivation

Windows CNG now supports Composite ML-DSA. This API proposal exposes the relevant constants in .NET so they can be used in our CNG APIs (like CngKey.Create).

API Proposal

public sealed partial class CngAlgorithm
{
    [Experimental("SYSLIB5006", UrlFormat="https://aka.ms/dotnet-warnings/{0}")]
    public static CngAlgorithm CompositeMLDsa { get; }
}

public sealed partial class CngAlgorithmGroup
{
    [Experimental("SYSLIB5006", UrlFormat="https://aka.ms/dotnet-warnings/{0}")]
    public static CngAlgorithmGroup CompositeMLDsa { get; }
}

We omitted this from the original Composite ML-DSA proposal (#118320) because we did not know how Windows would expose Composite ML-DSA in CNG.

The documentation for the CngAlgorithm constant is here.

Note that the constant for CngAlgorithmGroup is not documented (it should be here) but it can be inferred from the rest. We have already done the same for ML-DSA.

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[bartonjs]

Video

• It was asked if Experimental is still needed, and we hedged with "yes"

public sealed partial class CngAlgorithm
{
    [Experimental("SYSLIB5006", UrlFormat="https://aka.ms/dotnet-warnings/{0}")]
    public static CngAlgorithm CompositeMLDsa { get; }
}

public sealed partial class CngAlgorithmGroup
{
    [Experimental("SYSLIB5006", UrlFormat="https://aka.ms/dotnet-warnings/{0}")]
    public static CngAlgorithmGroup CompositeMLDsa { get; }
}