<PackageReference Include="BouncyCastle.Cryptography" Version="2.7.0" />

Org.BouncyCastle.Bcpg.PacketFormat

public enum PacketFormat
OpenPGP Packet Header Length Format.
Current = 1

Always use the current (new) packet format.

Legacy = 0

Always use the old (legacy) packet format.

Roundtrip = 2

Let the individual packet decide the format.