ssl：
Syntax: ssl on | off; Default: ssl off; Context: http, server
ssl_certificate：
Syntax: ssl_certificate file; Default: — Context: http, server
当前虚拟主机的pem格式的证书文件；
ssl_certificate_key：
Syntax: ssl_certificate_key file; Default: — Context: http, server
当前虚拟主机上与其证书相匹配的私钥文件；
举例：
server {
listen 443 ssl;
server_name example.com;
ssl_certificate example.com.rsa.crt;
ssl_certificate_key example.com.rsa.key;
ssl_certificate example.com.ecdsa.crt;
ssl_certificate_key example.com.ecdsa.key;
...
}
ssl_protocols：
Syntax: ssl_protocols [SSLv2] [SSLv3] [TLSv1] [TLSv1.1] [TLSv1.2] [TLSv1.3]; Default: ssl_protocols TLSv1 TLSv1.1 TLSv1.2; Context: http, server
ssl_session_cache：
Syntax: ssl_session_cache off | none | [builtin[:size]] [shared:name:size]; Default: ssl_session_cache none; Context: http, server
参数值说明：
off： the use of a session cache is strictly prohibited: nginx explicitly tells a client that sessions may not be reused.
none： the use of a session cache is gently disallowed: nginx tells a client that sessions may be reused,
but does not actually store session parameters in the cache.
builtin： a cache built in OpenSSL; used by one worker process only. The cache size is specified in sessions.
If size is not given, it is equal to 20480 sessions. Use of the built-in cache can cause memory fragmentation.
在OpenSSL中构建的缓存；仅由一个worker进程使用；缓存大小在会话中指定；如果没有给出大小，则等于20480个会话；
使用内置缓存可能导致内存碎片；
shared： a cache shared between all worker processes. The cache size is specified in bytes; one megabyte can store about 4000 sessions.
Each shared cache should have an arbitrary name. A cache with the same name can be used in several virtual servers.
Both cache types can be used simultaneously, for example:
所有worker进程之间共享的session cache；缓存大小以字节为单位指定；一个兆字节可以存储4000个session会话；
每个共享缓存都应该有一个任意的名称；具有相同名称的缓存可以在多个虚拟服务器中使用；
"builtin"、 "shared"这两种缓存类型可以同时使用；
举例： ssl_session_cache builtin:1000 shared:SSL:10m;
ssl_session_timeout：
Syntax: ssl_session_timeout time; Default: ssl_session_timeout 5m; Context: http, server
客户端侧的连接可以复用”ssl_session_cache”中缓存的ssl参数的有效时长；
举例：
server {
listen 443 ssl;
keepalive_timeout 65;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
#ssl_ciphers AES128-SHA:AES256-SHA:RC4-SHA:DES-CBC3-SHA:RC4-MD5;
ssl_certificate /usr/local/nginx/conf/cert.pem;
ssl_certificate_key /usr/local/nginx/conf/cert.key;
ssl_session_cache shared:SSL:10m;
ssl_session_timeout 10m;
...
}