[ClickOnce] Fix nonce generation in timestamping of signed manifest.

[sujitnayak]

Fixes
#9505

Summary

SecurityUtilities.SignFile function to sign ClickOnce manifest can fail occasionally during timestamping because the random bytes generated for the nonce can be invalid DER encodings for integer values

Changes Made

To ensure the encoding is not invalid, clear the nonce MSB's most significant bit and set the MSB's least significant bit.

This is a port of the fix made for the same issue in the NuGet client:
NuGet/NuGet.Client@3e55190

Customer Impact

Customers calling SignFile API through the Microsoft.Build.Tasks.Core NuGet package encounter occasional failures because the Nonce generated is invalid per DER encoding. The fix will address this issue of the SignFile API failing intermittently.

Regression?

No

Testing

Failing scenario validated + signing scenarios in ClickOnce validated for regression.

Risk

Low

[rainersigwald]

Looks like the current version of the NuGet code is slightly different: https://github.com/NuGet/NuGet.Client/blame/c13cea4dfd50e7eba1b794e1fea979dd1d6af3bd/src/NuGet.Core/NuGet.Packaging/Signing/Timestamp/Rfc3161TimestampProvider.cs#L245-L270

Does this need similar netcore-specific logic?

[sujitnayak]

Looks like the current version of the NuGet code is slightly different: https://github.com/NuGet/NuGet.Client/blame/c13cea4dfd50e7eba1b794e1fea979dd1d6af3bd/src/NuGet.Core/NuGet.Packaging/Signing/Timestamp/Rfc3161TimestampProvider.cs#L245-L270

Does this need similar netcore-specific logic?

I don't think we need that logic since we only support .NET FX. That logic is for Rfc3161TimestampRequest.CreateFromHash (based on the comment) which we don't call. The .NET FX version PInvokes to CryptRetrieveTimeStamp Win32 API.

[JanKrivanek]

LGTM