Detect if RSA-384 is supported on the platform

[vcsjones]

Contributes to #120353

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.

[bartonjs]

It's draft, but currently LGTM (FWIW).

[Copilot]

Pull Request Overview

This PR centralizes RSA-384 support detection across different RSA provider implementations by introducing a new PlatformSupport.IsRSA384Supported property. The change eliminates duplicated platform detection logic and provides a unified approach for determining RSA-384 support.

Key changes:

• Added centralized RSA-384 support detection in PlatformSupport class
• Replaced provider-specific RSA-384 support checks with calls to the centralized property
• Removed duplicated platform detection code from individual RSA provider classes

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs	Added IsRSA384Supported property with platform-specific detection logic and RSA-384 test parameters
src/libraries/System.Security.Cryptography/tests/DefaultRSAProvider.cs	Replaced custom RSA-384 support detection with centralized PlatformSupport.IsRSA384Supported
src/libraries/System.Security.Cryptography.Csp/tests/RSACryptoServiceProviderProvider.cs	Changed from hardcoded true to use centralized RSA-384 support detection
src/libraries/System.Security.Cryptography.Cng/tests/RSACngProvider.cs	Replaced custom RSA-384 support detection with centralized PlatformSupport.IsRSA384Supported

[vcsjones]

The Windows pipeline did not have these failures so enabled automatic merge.

[vcsjones]

/backport to release/10.0

[vcsjones]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/18232060822

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/18232062344

[github-actions]

@vcsjones backporting to "release/10.0" failed, the patch most likely resulted in conflicts:

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Detected is RSA-384 is supported on the platform
Using index info to reconstruct a base tree...
M	src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs
Falling back to patching base and 3-way merge...
Auto-merging src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs
CONFLICT (content): Merge conflict in src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 Detected is RSA-384 is supported on the platform
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!

[github-actions]

@vcsjones backporting to "release/9.0-staging" failed, the patch most likely resulted in conflicts:

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Detected is RSA-384 is supported on the platform
Using index info to reconstruct a base tree...
M	src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs
Falling back to patching base and 3-way merge...
Auto-merging src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs
Applying: Fix M.B.C compilation
Using index info to reconstruct a base tree...
M	src/libraries/Microsoft.Bcl.Cryptography/tests/Microsoft.Bcl.Cryptography.Tests.csproj
Falling back to patching base and 3-way merge...
Auto-merging src/libraries/Microsoft.Bcl.Cryptography/tests/Microsoft.Bcl.Cryptography.Tests.csproj
CONFLICT (content): Merge conflict in src/libraries/Microsoft.Bcl.Cryptography/tests/Microsoft.Bcl.Cryptography.Tests.csproj
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0002 Fix M.B.C compilation
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!

[vcsjones]

Boo. This would backport cleanly if the patch was squashed.