All: Changed release naming scheme ('R5.1' instead of '2020 R5.1').
Networking: More meaningful exception is throw when attempting to use HTTP CONNECT proxy with NTLM authentication on platforms that don't support it.
Networking: Optimized timeout infrastructure in ProxySocket.Connect.
TLS Core: Added VerifyMessage signature algorithm logging in TLS 1.3.
TLS Core: Close/Dispose method called on TLS 1.3 socket ensures that all outstanding IO operations are canceled before the control is returned to the caller.
TLS Core: Synchronous methods on TlsSocket wrap TaskCanceledException to TlsException.
Cryptography: Enhanced implicit operator for conversion of Certificate->X509Certificate2 to retain private keys on non-Microsoft Windows platforms as well.
Cryptography: Optimized memory usage in symmetric encryption transformations based on Windows CNG API.
Cryptography: Substantial speed-up of Chacha20/Poly1305 (used in SSH and TLS). Utilizing AVX2 or Advanced NEON SIMD on Microsoft .NET 5.0 (if available).
Common: Accelerated common byte array operations in .NET 5.0 on devices with AVX2 support.