Document CompositeMLDsa update to draft-08 breaking change

[Copilot]

CompositeMLDsa was updated from draft-07 to draft-08 of the Composite ML-DSA for X.509 PKI specification in .NET 10.0.0. This update introduces incompatibilities in signature validation and key import/export formats across the draft boundary.

Changes

• Breaking change document: Created docs/core/compatibility/cryptography/10.0/composite-mldsa-draft-08.md documenting the behavioral change, version introduced (10.0.0), and affected APIs
• TOC and index updates: Added entry to docs/core/compatibility/toc.yml and docs/core/compatibility/10.0.md
• Library documentation: Updated docs/core/whats-new/dotnet-10/libraries.md to reference draft 8 instead of draft 7

The document notes that draft-08 and draft-09 remain compatible, and emphasizes that CompositeMLDsa is marked [Experimental] pending specification finalization.

Fixes #120077

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Breaking change]: Update Composite ML-DSA to draft 8</issue_title>
<issue_description>### [Breaking change]: Update Composite ML-DSA to draft-08

Description

CompositeMLDsa has moved from draft-07 to draft-08 of the Composite ML-DSA for use in X.509 Public Key Infrastructure specification.

The draft-08 format is not compatible with the draft-07 signatures, and key export/import formats are also incompatible across the draft-07/draft-08 boundary.

Version

This breaking change was introduced between .NET 10 RC2 and .NET 10.0.0.

Previous behavior

Signatures were generated and validated according to draft-07 of Composite ML-DSA for use in X.509 Public Key Infrastructure.

Public key and private key export and import used the format from draft-07 of Composite ML-DSA for use in X.509 Public Key Infrastructure.

New behavior

Signatures were generated and validated according to draft-08 of Composite ML-DSA for use in X.509 Public Key Infrastructure.

Public key and private key export and import used the format from draft-08 of Composite ML-DSA for use in X.509 Public Key Infrastructure.

draft-08 and draft-09 are compatible

Type of breaking change

• Behavioral change: Signatures generated with earlier versions of .NET 10 will cause VerifyData to always return false. Signatures generated with .NET 10 will likewise fail to verify for .NET 10 Preview and .NET 10 RC releases.
• Behavioral change: Keys that were generated with earlier versions and exported may not import in .NET 10.0.0. Similarly, keys generated and exported in .NET 10.0.0 may not import in previous versions.

Reason for change

Staying current with the underlying specification.

Recommended action

The CompositeMLDsa class is marked as [Experimental], in part, because the specification is not yet complete.
Developers should not yet be depending on this class in production.

Any previously generated keys and signatures should be discarded.

Affected APIs

The following APIs are affected by this change:

• System.Security.Cryptography.CompositeMLDsa

Additional resources

• Pull Request dotnet/docs#120077</issue_description>

Comments on the Issue (you are @copilot in this section)

@PranavSenthilnathan In new behaviors section:

Signatures were generated and validated according to draft-07 of Composite ML-DSA for use in X.509 Public Key Infrastructure.

Should be something like "Signatures are generated and validated according to draft-08 of Composite ML-DSA for use in X.509 Public Key Infrastructure." Same for the keys.

I think keys generated and exported with the draft-08 spec can be imported with draft-07, but it's fine to leave it as is. People shouldn't be mixing the two anyway.</comment_new>
<comment_new>@bartonjs
Copy/paste bug fixed 😄</comment_new>

• Fixes [Breaking change]: Update Composite ML-DSA to draft 8 #48901

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/cryptography/10.0/composite-mldsa-draft-08.md	CompositeMLDsa updated to draft-08
docs/core/compatibility/toc.yml	docs/core/compatibility/toc
docs/core/whats-new/dotnet-10/libraries.md	What's new in .NET libraries for .NET 10

[Copilot]

Pull Request Overview

This PR documents a breaking change where CompositeMLDsa was updated from draft-07 to draft-08 of the Composite ML-DSA specification in .NET 10 GA. The change updates documentation to reflect the new draft version and adds a breaking change article.

• Updated library documentation to reflect draft 8 support
• Added comprehensive breaking change documentation
• Updated TOC and compatibility index to include the new breaking change article

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
docs/core/whats-new/dotnet-10/libraries.md	Updated draft version reference from 7 to 8 with clarification
docs/core/compatibility/toc.yml	Added new breaking change entry to the Cryptography section
docs/core/compatibility/cryptography/10.0/composite-mldsa-draft-08.md	New breaking change documentation for draft-08 update
docs/core/compatibility/10.0.md	Added breaking change entry to the Cryptography table