Update Composite ML-DSA to draft 8

[PranavSenthilnathan]

Composite ML-DSA spec release draft 8 recently, and this PR updates our implementation with the changes. The draft diff is here.

• Domain separators depend on the algo name instead of OID so they will be stable even when the final OIDs are assigned by IANA. They are now called "labels" instead.
• OIDs have been updated (still not final).
• Signatures don't have a randomizer anymore.
• ECPrivateKey component of the private key must omit publicKey and parameters.
• KeyGen does not need to be timing invariant.
• New test vectors

[Copilot]

Pull Request Overview

This PR updates the Composite ML-DSA implementation to align with draft 8 of the specification. The key changes include removing randomizers from signatures, updating domain separators to use algorithm names (now called "labels"), and enforcing stricter ECPrivateKey formatting requirements.

• Removes 32-byte randomizers from signature format, reducing signature sizes
• Updates domain separators to use algorithm names instead of OIDs for stability
• Enforces that ECPrivateKey components must omit publicKey and parameters fields

Reviewed Changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/EccKeyFormatHelper.cs	Adds inline validation logic for ECPrivateKey format, replacing external helper function
src/libraries/System.Security.Cryptography/src/System.Security.Cryptography.csproj	Removes reference to shared EccKeyFormatHelper file
src/libraries/Microsoft.Bcl.Cryptography/src/Microsoft.Bcl.Cryptography.csproj	Removes reference to shared EccKeyFormatHelper file
src/libraries/Common/tests/System/Security/Cryptography/CompositeMLDsaAlgorithmTests.cs	Updates signature size expectations by removing 32-byte randomizer
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaTestsBase.cs	Updates signature validation tests to reflect new format without randomizer
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaTestHelpers.cs	Updates OIDs and ECPrivateKey size calculations
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaMockImplementation.cs	Updates signature length assertions
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaFactoryTests.cs	Updates private key import tests for new ECPrivateKey restrictions
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaContractTests.cs	Updates signature bounds calculations
src/libraries/Common/src/System/Security/Cryptography/Oids.cs	Updates OID values to match draft 8
src/libraries/Common/src/System/Security/Cryptography/EccKeyFormatHelper.cs	Deletes shared helper file
src/libraries/Common/src/System/Security/Cryptography/CompositeMLDsaManaged.cs	Major refactoring to remove randomizer handling and update domain separators to labels
src/libraries/Common/src/System/Security/Cryptography/CompositeMLDsaManaged.ECDsa.cs	Enforces ECPrivateKey format restrictions by rejecting parameters and publicKey fields
src/libraries/Common/src/System/Security/Cryptography/CompositeMLDsaAlgorithm.cs	Removes randomizer constants and updates signature size calculations

[bartonjs]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/18021553476

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[ericstj]

📋 Breaking Change Documentation Required

Create a breaking change issue with AI-generated content

Generated by Breaking Change Documentation Tool - 2025-10-03 12:19:42

[ericstj]

📋 Breaking Change Documentation Required

Create a breaking change issue with AI-generated content

Generated by Breaking Change Documentation Tool - 2025-10-03 12:50:54

[bartonjs]

Added the breaking change doc. I clicked on the auto-gen link, but threw away most of its content, since it was trying to describe the behavioral changes, and to justify them (FWIW, the justification was entirely bogus hallucination). It'd probably be more useful for a change other than "moved to the current version of a spec" 😄.