SharePoint and Encrypted Connections
A security vulnerability was identified in the SSL 3.0 protocol that could allow an attacker to decrypt data. For enhanced security, some SharePoint features now disable SSL 3.0 connection encryption by default, as well as certain encryption algorithms (for example RC4) with known weaknesses.
SharePoint Server 2016 supports TLS 1.2 connection encryption by default. When you set up an SSL binding in Internet Information Services (IIS) Manager to host your web application, SharePoint uses TLS 1.2 connection encryption if your client application supports it. SharePoint also supports TLS 1.2 connection encryption when connecting to other systems (for example when crawling web sites).
Sharegate and Encrypted Connections
The latest version of Sharegate is built using version 4.5 of the .NET framework. This version supports TLS 1.0 / 1.1 / 1.2 but we strongly suggest to at least have TLS 1.1 support since 1.0 is considered vulnerable. There is also a risk of connections issue with sites only supporting 1.0. Note that SSL 3.0 and RC4 cypher are no longer supported by the .NET framework 4.6.