API Proposal: export SPKI on ECDiffieHellmanPublicKey

[vcsjones]

Proposal:

namespace System.Security.Cryptography {
    public abstract partial class ECDiffieHellmanPublicKey {
        public virtual bool TryExportSubjectPublicKeyInfo(
            Span<byte> destination,
            out int bytesWritten);

        public virtual byte[] ExportSubjectPublicKeyInfo();
    }
}

In #413 it was noted that the ECDiffieHellmanPublicKey type does not expose {Try}ExportSubjectPublicKeyInfo. This would allow other APIs to return instances of this type instead of ECDiffieHellman with the export faculties.

/cc @bartonjs

[terrajobst]

Video

• Makes sense. The implementation will throw NotSupportedException.

namespace System.Security.Cryptography
{
    public abstract partial class ECDiffieHellmanPublicKey
    {
        public virtual bool TryExportSubjectPublicKeyInfo(Span<byte> destination, out int bytesWritten);
        public virtual byte[] ExportSubjectPublicKeyInfo();
    }
}

[bartonjs]

Moving the milestone to Future because it's not a commitment for .NET 5. But PRs are welcome.