Consider referencing System.Memory

[Rob-Hague]

Perhaps you've already considered it, but in case you haven't:

The library has many duplicate code paths, one using a Span implementation for higher targets (.NET) and one using arrays for lower targets (.NET Framework and .NET Standard 2.0)

Microsoft provide (and support) the System.Memory package, which allows use of Span etc. on .NET Framework and .NET Standard 2.0. It is fairly common for multi-targeting libraries to reference the package on lower targets. The source code is at https://github.com/dotnet/maintenance-packages/tree/main/src/System.Memory.

This reference would allow many code paths (and public API) to be unified.

Practically, the initial change would look like:

diff --git a/crypto/src/BouncyCastle.Crypto.csproj b/crypto/src/BouncyCastle.Crypto.csproj
index 8b7c8852..166dc1ae 100644
--- a/crypto/src/BouncyCastle.Crypto.csproj
+++ b/crypto/src/BouncyCastle.Crypto.csproj
@@ -88,6 +88,10 @@
     <None Include="..\..\README.md" Pack="true" PackagePath="\" />
   </ItemGroup>

+  <ItemGroup Condition=" '$(TargetFramework)' == 'net461' or '$(TargetFramework)' == 'netstandard2.0' ">
+    <PackageReference Include="System.Memory" Version="4.5.5" />
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3">
       <PrivateAssets>all</PrivateAssets>

Example unified diff (note Index/Range operators [..32] not supported):

diff --git a/crypto/src/crypto/modes/ChaCha20Poly1305.cs b/crypto/src/crypto/modes/ChaCha20Poly1305.cs
index 56bef5e9..ca982f9d 100644
--- a/crypto/src/crypto/modes/ChaCha20Poly1305.cs
+++ b/crypto/src/crypto/modes/ChaCha20Poly1305.cs
@@ -783,29 +783,16 @@ private ulong IncrementCount(ulong count, uint increment, ulong limit)
 
         private void InitMac()
         {
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
             Span<byte> firstBlock = stackalloc byte[64];
             try
             {
                 mChacha20.ProcessBytes(firstBlock, firstBlock);
-                mPoly1305.Init(new KeyParameter(firstBlock[..32]));
+                mPoly1305.Init(new KeyParameter(firstBlock.Slice(0, 32)));
             }
             finally
             {
                 firstBlock.Fill(0x00);
             }
-#else
-            byte[] firstBlock = new byte[64];
-            try
-            {
-                mChacha20.ProcessBytes(firstBlock, 0, 64, firstBlock, 0);
-                mPoly1305.Init(new KeyParameter(firstBlock, 0, 32));
-            }
-            finally
-            {
-                Array.Clear(firstBlock, 0, 64);
-            }
-#endif
         }
 
         private void PadMac(ulong count)