App Transport Security (ATS)
ATS establishes best-practice policies for secure network communications using Apple platforms, employing Transport Layer Security (TLS) version 1.2, forward secrecy, and strong cryptography.
Secure Transport API
Use Apple’s secure transport API to employ current versions of the Secure Sockets Layer (SSL), Transport Layer Security (TLS), and Datagram Transport Layer Security (DTLS) cryptographic protocols for network communications.
Starting with iOS 10 and macOS v10.12, the RC4 cipher suite is disabled by default. In addition, Apple recommends that your servers use certificates signed with the SHA-2 cryptographic function.