BCrypt Composite ML-DSA by PranavSenthilnathan · Pull Request #129612 · dotnet/runtime

PranavSenthilnathan · 2026-06-19T09:19:20Z

Windows Insider builds support Composite ML-DSA in BCrypt. Documentation of public API can be found here.

Note this changes the default implementation on Windows from managed to BCrypt. On versions that support Composite ML-DSA, this reduces the number of supported algorithms to the 4 listed in the docs above. And for Windows versions that don't support Composite ML-DSA, our default implementation also doesn't support it now.

Fixes #116999

dotnet-policy-service · 2026-06-19T09:20:37Z

Tagging subscribers to this area: @bartonjs, @vcsjones, @dotnet/area-system-security
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Pull request overview

This PR wires up Windows support for Composite ML-DSA via the BCrypt “single key”/PQDSA blob mechanism (including new parameter-set identifiers and blob magic values), and updates platform-support expectations in tests.

Changes:

Add Composite ML-DSA PQDSA blob encode/decode support and parameter-set mapping for Windows composite algorithms.
Implement Windows CompositeMLDsa operations (keygen/import/export/sign/verify) using BCrypt Composite-ML-DSA provider and PQDSA blobs.
Update Windows algorithm identifiers / magic numbers and adjust support-detection tests accordingly.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/src/System.Security.Cryptography.csproj	Removes managed Composite ML-DSA sources from the Windows build item group.
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaFactoryTests.cs	Updates platform support expectations for `IsAlgorithmSupported` on Windows.
src/libraries/Common/src/System/Security/Cryptography/PqcBlobHelpers.cs	Adds composite parameter-set strings plus composite PQDSA blob encode/decode helpers; adjusts stackalloc threshold.
src/libraries/Common/src/System/Security/Cryptography/CompositeMLDsaImplementation.Windows.cs	Replaces Windows PNSE stub with a BCrypt-backed implementation for composite keygen/import/export/sign/verify.
src/libraries/Common/src/System/Security/Cryptography/CompositeMLDsa.cs	Adds an algorithm support check during PKCS#8 private-key import.
src/libraries/Common/src/Interop/Windows/BCrypt/Interop.Blobs.cs	Adds composite ML-DSA public/private magic numbers for PQDSA blobs.
src/libraries/Common/src/Interop/Windows/BCrypt/Interop.BCryptSignHash.cs	Changes PQC “pure” signing helper to return `bytesWritten` and relaxes debug assertion.
src/libraries/Common/src/Interop/Windows/BCrypt/Cng.cs	Adds BCrypt algorithm name constant for Composite-ML-DSA.

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Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

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Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

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Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Copilot

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

+            int written = Interop.BCrypt.BCryptSignHashPqcPure(_key, data, context, destination);
+            Debug.Assert(written == destination.Length);


vcsjones · 2026-06-24T22:41:37Z

+            if (!CompositeMLDsa.IsSupported ||
+                CompositeMLDsa.IsAlgorithmSupported(CompositeMLDsaAlgorithm.MLDsa87WithEd448))
+                throw new SkipTestException("Algorithm is supported on this platform.");


This looks odd. Consider using braces. I generally don't recommend we use braceless ifs if the if condition itself spans multiple lines. (Repeat feedback to other places)

vcsjones · 2026-06-24T22:48:35Z

        internal const string BCRYPT_MLKEM_PARAMETER_SET_768 = "768";
        internal const string BCRYPT_MLKEM_PARAMETER_SET_1024 = "1024";

+        internal const string BCRYPT_COMPOSITE_MLDSA_PARAMETER_SET_44_ECDSA_P256_SHA256 = "44-ECDSA-P256-SHA256";


This basically hardcodes our list of supported composite algorithms. I think that's fine, but just noting that if Windows gets around to adding other things like RSA then we will have to do work to enable them.

vcsjones · 2026-06-24T22:50:58Z

+            return ExportKey(
+                Interop.BCrypt.KeyBlobType.BCRYPT_PQDSA_PRIVATE_BLOB,
+                destination);


This doesn't have to be wrapped and can be a single line.

vcsjones · 2026-06-24T22:54:43Z

            throw new PlatformNotSupportedException();
        }

+        internal static bool TryGetCompositeMLDsaParameterSet(CompositeMLDsaAlgorithm algorithm, [NotNullWhen(true)] out string? parameterSet)


This method signature should wrap.

vcsjones · 2026-06-24T22:57:59Z

            throw new PlatformNotSupportedException();
        }

+        internal static bool TryGetCompositeMLDsaParameterSet(CompositeMLDsaAlgorithm algorithm, [NotNullWhen(true)] out string? parameterSet)


Should the parameterSet just be a nullable property on CompositeMLDsaAlgorithm like MaxPublicKeySizeInBytes? That way this doesn't really even need to exist.

CompositeMLDsaAlgorithm alg = GetTheAlg(); if (alg.CngParameterSetName is null) { throw new NotSupportedException(); } string parameterSet = alg.CngParameterSetName;

PranavSenthilnathan self-assigned this Jun 19, 2026

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build-analysis Bot mentioned this pull request Jun 19, 2026

Unable to pull image from mcr.microsoft.com #117164

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PranavSenthilnathan mentioned this pull request Jun 24, 2026

Update Windows version boundary for Composite ML-DSA #129787

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Comment thread src/libraries/Common/src/System/Security/Cryptography/CompositeMLDsaImplementation.Windows.cs

Comment thread .../Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaFactoryTests.cs Outdated

PranavSenthilnathan force-pushed the cmldsa-bcrypt branch from fadf2c4 to b94a7dc Compare June 24, 2026 03:46

BCrypt Composite ML-DSA

f7063c5

Copilot AI review requested due to automatic review settings June 24, 2026 03:50

PranavSenthilnathan force-pushed the cmldsa-bcrypt branch from b94a7dc to f7063c5 Compare June 24, 2026 03:50

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Comment thread src/libraries/Common/src/System/Security/Cryptography/CompositeMLDsaImplementation.Windows.cs

add assert

30f5a41

PranavSenthilnathan changed the title ~~[WIP] BCrypt Composite ML-DSA~~ BCrypt Composite ML-DSA Jun 24, 2026

PranavSenthilnathan marked this pull request as ready for review June 24, 2026 04:14

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Comment thread src/libraries/Common/src/System/Security/Cryptography/MLDsaImplementation.Windows.cs

Comment on lines +47 to +48

int written = Interop.BCrypt.BCryptSignHashPqcPure(_key, data, context, destination);

Debug.Assert(written == destination.Length);

This was referenced Jun 24, 2026

TestNativeDigits fails for ur-IN on Apple platforms (xunit v3 exposed previously non-running test) #125933

Open

DiagnosticsTest Http2 SendAsync_Success_ConnectionSetupActivityGraphRecorded: Unexpected EOF trying to read request header #129771

Open

M.BCL.Crypto doesn't need managed Composite ML-DSA

b1c9b6a

Copilot AI mentioned this pull request Jun 24, 2026

Fix flaky SendAsync_Success_ConnectionSetupActivityGraphRecorded test (Http1.1 + Http2) #129822

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vcsjones reviewed Jun 24, 2026

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		int written = Interop.BCrypt.BCryptSignHashPqcPure(_key, data, context, destination);
		Debug.Assert(written == destination.Length);

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PranavSenthilnathan commented Jun 19, 2026 •

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