[ML-DSA]: Windows MLDsaImplementation throws unclear error for missing private key while signing

[vcsjones]

If you attempt to do something like this:

using MLDsa full = MLDsa.GenerateKey(MLDsaAlgorithm.MLDsa65);
using MLDsa pub = MLDsa.ImportSubjectPublicKeyInfo(full.ExportSubjectPublicKeyInfo());
pub.SignData(new byte[1], new byte[pub.Algorithm.SignatureSizeInBytes]);

It will fail with:

 System.Security.Cryptography.CryptographicException : Unknown error (0xc100000d)
  Stack Trace:
     at Interop.BCrypt.BCryptSignHashPqcPure(SafeBCryptKeyHandle key, ReadOnlySpan`1 data, ReadOnlySpan`1 context, Span`1 destination) in E:\code\runtime\src\libraries\Common\src\Interop\Windows\BCrypt\Interop.BCryptSignHash.cs:line 111
   at System.Security.Cryptography.MLDsaImplementation.SignDataCore(ReadOnlySpan`1 data, ReadOnlySpan`1 context, Span`1 destination) in E:\code\runtime\src\libraries\Common\src\System\Security\Cryptography\MLDsaImplementation.Windows.cs:line 38
   at System.Security.Cryptography.MLDsa.SignData(ReadOnlySpan`1 data, Span`1 destination, ReadOnlySpan`1 context) in E:\code\runtime\src\libraries\Common\src\System\Security\Cryptography\MLDsa.cs:line 128

"Unknown error (0xc100000d)" is not a helpful error and we occasionally get reports indicating it is not helpful.

Since this is the bcrypt implementation, we should check for the presence of the secret key.

We should add:

if (!_hasSecretKey)
{
    throw new CryptographicException(SR.Cryptography_MLDsaNoSecretKey);
}

To SignDataCore:

runtime/src/libraries/Common/src/System/Security/Cryptography/MLDsaImplementation.Windows.cs

Lines 37 to 38 in a8c0a03

	protected override void SignDataCore(ReadOnlySpan<byte> data, ReadOnlySpan<byte> context, Span<byte> destination) =>
	Interop.BCrypt.BCryptSignHashPqcPure(_key, data, context, destination);

---

We cannot add the same check to MLDsaCng because we are not guaranteed to know if the secret key is available or not, until we try signing.

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