证书概要

证书是一个包含公钥、持有者（申请人、公司）相关信息及证书颁发者（传说中的CA）数字签名的数字文件。这可以形象的类比我们日常生活中工商管理的证书，你去饭馆吃饭的时候会在墙上看到XXX证，然后会有商铺的信息、颁发者的信息（我朝XXX局）、还有日期、盖章等，那么HTTPS中使用的证书可以对应类比

公钥≈商铺的具体地址信息+商铺名称（说这个信息是公开的，欢迎大家来访问吧，用这个公钥加密信息）
持有者≈商铺持有者信息
数字签名≈工商局的盖章
其他≈其他

那么有了证书之后，持有者的就可以大大方方的说我这个网站是安全的，你看经过CA（证书颁发中心）认证的。
Android系统内置了很多HTTPS证书，在Android源代码目录下，如图

证书位置.png

证书字段

X.509证书有多种编码的格式：XXX.pem等， 大多数证书是PEM的，PEM内容采用Base64编码，相对容易复制一些，也方便了我们这些喜欢看大文件就双击打开的观众， 证书字段从古至今已经发展了几个版本，让我们来看一看

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            34:4e:d5:57:20:d5:ed:ec:49:f4:2f:ce:37:db:2b:6d
    Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=US, O=thawte, Inc., OU=Certification Services Division, OU=(c) 2006 thawte, Inc. - For authorized use only, CN=thawte Primary Root CA
        Validity
            Not Before: Nov 17 00:00:00 2006 GMT
            Not After : Jul 16 23:59:59 2036 GMT
        Subject: C=US, O=thawte, Inc., OU=Certification Services Division, OU=(c) 2006 thawte, Inc. - For authorized use only, CN=thawte Primary Root CA
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    00:ac:a0:f0:fb:80:59:d4:9c:c7:a4:cf:9d:a1:59:
                    73:09:10:45:0c:0d:2c:6e:68:f1:6c:5b:48:68:49:
                    59:37:fc:0b:33:19:c2:77:7f:cc:10:2d:95:34:1c:
                    e6:eb:4d:09:a7:1c:d2:b8:c9:97:36:02:b7:89:d4:
                    24:5f:06:c0:cc:44:94:94:8d:02:62:6f:eb:5a:dd:
                    11:8d:28:9a:5c:84:90:10:7a:0d:bd:74:66:2f:6a:
                    38:a0:e2:d5:54:44:eb:1d:07:9f:07:ba:6f:ee:e9:
                    fd:4e:0b:29:f5:3e:84:a0:01:f1:9c:ab:f8:1c:7e:
                    89:a4:e8:a1:d8:71:65:0d:a3:51:7b:ee:bc:d2:22:
                    60:0d:b9:5b:9d:df:ba:fc:51:5b:0b:af:98:b2:e9:
                    2e:e9:04:e8:62:87:de:2b:c8:d7:4e:c1:4c:64:1e:
                    dd:cf:87:58:ba:4a:4f:ca:68:07:1d:1c:9d:4a:c6:
                    d5:2f:91:cc:7c:71:72:1c:c5:c0:67:eb:32:fd:c9:
                    92:5c:94:da:85:c0:9b:bf:53:7d:2b:09:f4:8c:9d:
                    91:1f:97:6a:52:cb:de:09:36:a4:77:d8:7b:87:50:
                    44:d5:3e:6e:29:69:fb:39:49:26:1e:09:a5:80:7b:
                    40:2d:eb:e8:27:85:c9:fe:61:fd:7e:e6:7c:97:1d:
                    d5:9d
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 Key Usage: critical
                Certificate Sign, CRL Sign
            X509v3 Subject Key Identifier: 
                7B:5B:45:CF:AF:CE:CB:7A:FD:31:92:1A:6A:B6:F3:46:EB:57:48:50
    Signature Algorithm: sha1WithRSAEncryption
         79:11:c0:4b:b3:91:b6:fc:f0:e9:67:d4:0d:6e:45:be:55:e8:
         93:d2:ce:03:3f:ed:da:25:b0:1d:57:cb:1e:3a:76:a0:4c:ec:
         50:76:e8:64:72:0c:a4:a9:f1:b8:8b:d6:d6:87:84:bb:32:e5:
         41:11:c0:77:d9:b3:60:9d:eb:1b:d5:d1:6e:44:44:a9:a6:01:
         ec:55:62:1d:77:b8:5c:8e:48:49:7c:9c:3b:57:11:ac:ad:73:
         37:8e:2f:78:5c:90:68:47:d9:60:60:e6:fc:07:3d:22:20:17:
         c4:f7:16:e9:c4:d8:72:f9:c8:73:7c:df:16:2f:15:a9:3e:fd:
         6a:27:b6:a1:eb:5a:ba:98:1f:d5:e3:4d:64:0a:9d:13:c8:61:
         ba:f5:39:1c:87:ba:b8:bd:7b:22:7f:f6:fe:ac:40:79:e5:ac:
         10:6f:3d:8f:1b:79:76:8b:c4:37:b3:21:18:84:e5:36:00:eb:
         63:20:99:b9:e9:fe:33:04:bb:41:c8:c1:02:f9:44:63:20:9e:
         81:ce:42:d3:d6:3f:2c:76:d3:63:9c:59:dd:8f:a6:e1:0e:a0:
         2e:41:f7:2e:95:47:cf:bc:fd:33:f3:f6:0b:61:7e:7e:91:2b:
         81:47:c2:27:30:ee:a7:10:5d:37:8f:5c:39:2b:e4:04:f0:7b:
         8d:56:8c:68
SHA1 Fingerprint=91:C6:D6:EE:3E:8A:C8:63:84:E5:48:C2:99:29:5C:75:6C:81:7B:81

注意：这是我从Android内置证书随便选的一个打开，其中-----BEGIN CERTIFICATE-----和-----END CERTIFICATE-----是证书的Base64编码的格式，这其实就是证书的内容了，而下面那部分是数字证书的明文内容

在Chrome浏览器上访问HTTPS网站也可以看到

证书.png

参考链接：

1、What is SSL and what are Certificates?
http://www.tldp.org/HOWTO/SSL-Certificates-HOWTO/x64.html
2、HTTPS权威指南