我想知道如何才能确定给定文件是否是 SSL 证书。文件扩展名是否足以确定文件是否是证书？
答案1
SSL 证书只不过是一个X.509 版本 3 证书但对于常见用途有一些额外的限制。
主要的限制当然是 X500 通用名称 (CN)，需要将其设置为服务器的名称。顺便说一下，主题备用名称字段可能包含更多名称。
此外，对于大多数证书，扩展密钥用法需要设置为服务器身份验证（对应 OID：1.3.6.1.5.5.7.3.1）。服务器也可以设置客户端身份验证（OID：1.3.6.1.5.5.7.3.2）。
请注意，至少对于 TLS 1.2，这些限制并不明确地在规范中提到过（呃），但你最好确保它们存在除非你的浏览器/客户端开始尖叫谋杀。
要验证文件是否为 (TLS) 证书，最简单的方法可能是使用 Windows 打开它并检查一些其他约束。不过就我个人而言，我更喜欢使用 OpenSSL 命令行，这样您就可以简单地输出为文本：
openssl x509 -text -noout -in stackexchangecom.pem
例如将输出以下内容：
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
07:65:c6:4e:74:e5:91:d6:80:39:ca:2a:84:75:63:f0
Signature Algorithm: sha256WithRSAEncryption
Issuer: C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert SHA2 High Assurance Server CA
Validity
Not Before: Oct 5 00:00:00 2018 GMT
Not After : Aug 14 12:00:00 2019 GMT
Subject: C=US, ST=NY, L=New York, O=Stack Exchange, Inc., CN=*.stackexchange.com
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:
00:f4:a9:3a:35:75:0f:d6:87:17:b7:cf:66:4f:57:
08:c5:a7:41:87:30:8b:d1:84:ea:3f:4d:7f:0d:2a:
2d:50:74:73:57:bd:2a:38:24:fb:01:d3:13:d0:ad:
49:8b:aa:c5:c9:aa:73:46:2a:94:22:10:24:84:4b:
1e:5d:1a:74:30:da:f6:d5:f4:94:c3:85:68:09:bf:
88:98:ee:a0:9c:89:73:a2:59:21:ae:92:ba:23:2d:
f8:2b:25:37:cf:2b:7c:5d:80:fe:99:8d:e2:f0:68:
cf:64:ec:ac:44:93:4b:cb:7a:2e:40:19:b3:b8:e9:
94:ff:61:68:9a:79:a2:10:61:74:da:65:60:6f:77:
af:f0:fa:dc:9e:de:dd:0a:21:7b:96:20:48:b1:dd:
f3:90:f7:97:bd:35:58:71:57:1b:fc:c0:6b:14:4c:
dc:e0:5b:88:ba:98:53:88:96:e8:37:3f:30:1e:ff:
7e:3d:70:17:51:41:fc:4c:44:ab:51:f1:4f:08:a2:
47:c1:df:44:02:83:57:f2:33:d4:d5:32:31:88:2a:
1e:e9:73:79:13:59:8f:c8:68:32:bc:49:da:70:7f:
c7:7a:b2:bf:78:b7:38:e8:be:d8:59:51:91:ca:31:
d6:69:a4:ca:d2:b2:61:2a:09:21:e7:da:ac:58:17:
67:e7
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Authority Key Identifier:
keyid:51:68:FF:90:AF:02:07:75:3C:CC:D9:65:64:62:A2:12:B8:59:72:3B
X509v3 Subject Key Identifier:
9A:8A:C1:6E:C1:F2:4D:FA:D9:7B:02:D4:8F:B3:03:AC:6A:3D:C6:58
X509v3 Subject Alternative Name:
DNS:*.stackexchange.com, DNS:stackexchange.com, DNS:stackoverflow.com, DNS:*.stackoverflow.com, DNS:stackauth.com, DNS:sstatic.net, DNS:*.sstatic.net, DNS:serverfault.com, DNS:*.serverfault.com, DNS:superuser.com, DNS:*.superuser.com, DNS:stackapps.com, DNS:openid.stackauth.com, DNS:*.meta.stackexchange.com, DNS:meta.stackexchange.com, DNS:mathoverflow.net, DNS:*.mathoverflow.net, DNS:askubuntu.com, DNS:*.askubuntu.com, DNS:stacksnippets.net, DNS:*.blogoverflow.com, DNS:blogoverflow.com, DNS:*.meta.stackoverflow.com, DNS:*.stackoverflow.email, DNS:stackoverflow.email, DNS:stackoverflow.blog
X509v3 Key Usage: critical
Digital Signature, Key Encipherment
X509v3 Extended Key Usage:
TLS Web Server Authentication, TLS Web Client Authentication
X509v3 CRL Distribution Points:
Full Name:
URI:http://crl3.digicert.com/sha2-ha-server-g6.crl
Full Name:
URI:http://crl4.digicert.com/sha2-ha-server-g6.crl
X509v3 Certificate Policies:
Policy: 2.16.840.1.114412.1.1
CPS: https://www.digicert.com/CPS
Policy: 2.23.140.1.2.2
Authority Information Access:
OCSP - URI:http://ocsp.digicert.com
CA Issuers - URI:http://cacerts.digicert.com/DigiCertSHA2HighAssuranceServerCA.crt
X509v3 Basic Constraints: critical
CA:FALSE
CT Precertificate SCTs:
Signed Certificate Timestamp:
Version : v1(0)
Log ID : A4:B9:09:90:B4:18:58:14:87:BB:13:A2:CC:67:70:0A:
3C:35:98:04:F9:1B:DF:B8:E3:77:CD:0E:C8:0D:DC:10
Timestamp : Oct 5 02:24:01.827 2018 GMT
Extensions: none
Signature : ecdsa-with-SHA256
30:46:02:21:00:F0:9A:77:1B:F8:ED:C0:78:40:E3:AF:
37:DB:3F:47:6D:C4:7A:91:1B:48:8E:3F:32:E0:1D:7F:
9B:CA:79:4E:FD:02:21:00:DA:06:77:3B:C9:F3:B9:45:
5A:9D:15:BD:7E:0E:A6:81:FB:0B:D3:C3:67:FD:91:A6:
EF:73:BF:17:72:06:5F:65
Signed Certificate Timestamp:
Version : v1(0)
Log ID : 87:75:BF:E7:59:7C:F8:8C:43:99:5F:BD:F3:6E:FF:56:
8D:47:56:36:FF:4A:B5:60:C1:B4:EA:FF:5E:A0:83:0F
Timestamp : Oct 5 02:24:02.054 2018 GMT
Extensions: none
Signature : ecdsa-with-SHA256
30:46:02:21:00:D0:8D:F9:95:06:AF:BF:CB:68:01:2B:
F7:84:F7:1E:A3:CF:D8:53:67:9B:48:7E:19:12:B5:2F:
39:7C:C0:31:7A:02:21:00:C0:2E:36:4C:AE:3B:8B:74:
E8:48:84:80:C5:A2:6A:52:59:B8:09:E4:43:0D:BD:19:
C7:88:04:6F:2B:D4:0A:77
Signature Algorithm: sha256WithRSAEncryption
00:93:ce:f7:ff:ed:90:b3:02:9f:25:24:27:fa:26:5e:65:cf:
2e:88:68:3d:f6:99:9d:d3:4f:04:d9:c9:86:12:ba:8d:cc:f7:
25:2b:d2:0d:6c:f8:f0:c6:5f:73:22:04:dc:5e:91:7f:52:d0:
55:55:2d:59:ed:7a:3c:de:a7:ec:18:c3:dd:33:36:2d:dc:5f:
a1:42:94:18:2e:19:46:17:ee:49:7f:6c:7a:65:bd:73:8d:3f:
da:33:71:8c:74:68:be:e8:e3:d5:f9:81:e5:ff:08:14:7b:8e:
4d:ea:44:6e:0d:99:d5:2f:5e:bb:f9:6d:e5:da:70:fe:99:28:
4e:ff:bc:6a:c0:78:99:bb:3d:06:1f:20:47:46:9e:62:e3:76:
e5:1f:4b:e0:eb:bb:09:f2:0b:8d:f3:5a:5a:a6:ea:58:da:fe:
fc:15:cb:d1:f2:3d:04:2d:f8:32:7a:1b:56:a6:31:77:bf:32:
92:ab:fa:d8:da:c3:17:4d:8c:d2:3e:a3:1e:92:cb:1e:1c:d8:
52:31:85:3a:5b:0f:61:f6:9c:8c:69:59:f0:f6:f6:a1:a9:fe:
e7:28:71:dc:0b:65:51:4d:48:24:41:f9:fd:c8:39:a6:04:ea:
34:9d:0f:17:81:fa:5d:eb:9f:cf:6b:15:5f:06:7b:8a:7c:49:
17:05:fa:4c
假设证书为 PEM 格式，对于二进制格式，您可以添加-inform DER，可以使用命令检查使用哪一个file，该命令通常存在于 Linux 和 Cygwin 上（如果安装正确）。
就我个人而言，我不太相信文件扩展名。文件扩展名错误实在是太容易了，文件扩展名是非常松散的打字系统方式。
好的，现在您已验证该文件是 X.509 证书，具体如下：结构。但是，您可能还想进一步验证证书链直至受信任的证书。并且您可能还想执行其他验证，例如检查证书在正确的日期是否仍然有效。这里是关于如何做到这一点的指针。
为了完整起见，您可能还需要检查 OCSP 状态，以查看证书颁发机构 (CA) 是否撤销了证书。
如果证书可以由对手生成，或者它是由公司中的某个开发人员生成的（自签名）测试证书，那么您肯定需要这个。