PKCS#8 support for ML-DSA

[PranavSenthilnathan]

includes import/export for:

• PKCS#8
• Encrypted PKCS#8
• PEM

The implementation is very similar to SLH-DSA overall and to ML-KEM for the seed related logic.

Contributes to #113502

[Copilot]

Pull Request Overview

This pull request adds support for PKCS#8, Encrypted PKCS#8, and PEM import/export for ML‑DSA, extending functionality already present in related modules (SLH‑DSA and ML‑KEM).

• Introduces new XML and code files for ML‑DSA private key ASN.1 encoding/decoding.
• Modifies key implementation classes (MLDsaOpenSsl, MLDsaImplementation) to propagate seed and secret key information.
• Adds new overloads and tests (including IETF test cases) to cover the new PKCS#8 support.

Reviewed Changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
System.Security.Cryptography.Tests.csproj	Added AsnXml and Compile entries for new ML‑DSA private key files and test helpers.
MLDsaOpenSsl*.cs, MLDsaImplementation*.cs	Extended constructors and key generation/import methods to handle seed and secret key flags.
Common/src/System/Security/Cryptography/MLDsaPkcs8.cs	Implements PKCS#8 export logic for ML‑DSA keys using rented buffers.
Common/src/System/Security/Cryptography/Asn1/*	Added generated ASN.1 encoder/decoder files for both and seed/expanded key representations.
Common/tests/System/Security/Cryptography/AlgorithmImplementations/MLDsa/*	Introduced new test cases and helper methods for verifying the new functionality.
Interop/Unix/System.Security.Cryptography.Native/Interop.EvpPkey.MLDsa.cs	Updated native interop to return additional boolean flags for seed and secret key presence.

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[PranavSenthilnathan]

This was computed by comparing openssl genpkey -algorithm ML-DSA-87 | openssl pkey -pubout -outform DER | wc with the public key size for the algo.

[PranavSenthilnathan]

Computed with openssl genpkey -algorithm ML-DSA-87 | openssl pkey -provparam ml-dsa.output_formats=priv-only -outform DER | wc and compared to secret key size.

[bartonjs]

Why offset at the beginning like this?

[PranavSenthilnathan]

It's how it looks in the IETF doc (it's in the middle of an ASN pretty print). Flushing to the left looked weird too so I just kept it like this.