基于 OpenSSL 的 CA 建立及证书签发

建立 CA

建立 CA 目录结构

按照 OpenSSL 的默认配置建立 CA ，需要在文件系统中建立相应的目录结构。相关的配置内容一般位于 /usr/ssl/openssl.cnf 内，详情可参见 config (1) 。在终端中使用如下命令建立目录结构：

$ mkdir -p ./demoCA/{private,newcerts}
$ touch ./demoCA/index.txt
$ echo 01 > ./demoCA/serial

产生的目录结构如下：

.
`-- demoCA/
    |-- index.txt
    |-- newcerts/
    |-- private/
    `-- serial

生成 CA 证书的 RSA 密钥对

首先，我们要为 CA 建立 RSA 密钥对。打开终端，使用如下命令生成 RSA 密钥对：

$ openssl genrsa -des3 -out ./demoCA/private/cakey.pem 2048

参数解释

genrsa

用于生成 RSA 密钥对的 OpenSSL 命令。

-des3

使用 3-DES 对称加密算法加密密钥对，该参数需要用户在密钥生成过程中输入一个口令用于加密。今后使用该密钥对时，需要输入相应的口令。如果不加该选项，则不对密钥进行加密。

-out ./demoCA/private/cakey.pem

令生成的密钥对保存到文件 ./demoCA/private/cakey.pem 。

2048

RSA 模数位数，在一定程度上表征了密钥强度。

该命令输出如下，用户应输入自己的密钥口令并确认：

Generating RSA private key, 2048 bit long modulus
................................................+++
.........................+++
e is 65537 (0x10001)
Enter pass phrase for ./demoCA/private/cakey.pem:<enter your pass-phrase>
Verifying - Enter pass phrase for ./demoCA/private/cakey.pem:<re-enter your pass-phrase>

生成 CA 证书请求

为了获取一个 CA 根证书，我们需要先制作一份证书请求。先前生成的 CA 密钥对被用于对证书请求签名。

$ openssl req -new -days 365 -key ./demoCA/private/cakey.pem -out careq.pem

参数解释

req

用于生成证书请求的 OpenSSL 命令。

-new

生成一个新的证书请求。该参数将令 OpenSSL 在证书请求生成过程中要求用户填写一些相应的字段。

-days 365

从生成之时算起，证书时效为 365 天。

-key ./demoCA/private/cakey.pem

指定 ./demoCA/private/cakey.pem 为证书所使用的密钥对文件。

-out careq.pem

令生成的证书请求保存到文件 careq.pem 。

该命令将提示用户输入密钥口令并填写证书相关信息字段，输出如下：

Enter pass phrase for ./demoCA/private/cakey.pem:<enter you pass-phrase>
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:CN
State or Province Name (full name) [Some-State]:ZJ
Locality Name (eg, city) []:HZ
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Some Ltd. Corp.
Organizational Unit Name (eg, section) []:Some Unit
Common Name (eg, YOUR name) []:Someone
Email Address []:[email protected]

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

对 CA 证书请求进行签名

在实际应用中，用户可以通过向知名 CA 递交证书请求来申请证书。但是在这里，我们需要建立的是一个根 CA ，只能由我们自己来对证书请求进行签名。所以我们让 OpenSSL 使用证书请求中附带的密钥对对该请求进行签名，也就是所谓的“ self sign ”：

$ openssl ca -selfsign -in careq.pem -out cacert.pem

参数解释

ca

用于执行 CA 相关操作的 OpenSSL 命令。

-selfsign

使用对证书请求进行签名的密钥对来签发证书。

-in careq.pem

指定 careq.pem 为证书请求文件。

-out ./demoCA/cacert.pem

指定 ./demoCA/cacert.pem 为输出的证书。

该命令要求用户输入密钥口令并输出相关证书信息，请求用户确认：

Using configuration from /usr/lib/ssl/openssl.cnf
Enter pass phrase for ./demoCA/private/cakey.pem:<enter your pass-phrase>
Check that the request matches the signature
Signature ok
Certificate Details:
        Serial Number: 2 (0x2)
        Validity
            Not Before: Jan 16 13:05:09 2008 GMT
            Not After : Jan 15 13:05:09 2009 GMT
        Subject:
            countryName = CN
            stateOrProvinceName = ZJ
            organizationName = Some Ltd. Corp.
            organizationalUnitName = Some Unit
            commonName = Someone
            emailAddress = [email protected]
        X509v3 extensions:
            X509v3 Basic Constraints:
                CA:FALSE
            Netscape Comment:
                OpenSSL Generated Certificate
            X509v3 Subject Key Identifier:
                75:F5:3C:CC:C1:5E:6D:C3:8B:46:A8:08:E6:EA:29:E8:22:7E:70:03
            X509v3 Authority Key Identifier:
                keyid:75:F5:3C:CC:C1:5E:6D:C3:8B:46:A8:08:E6:EA:29:E8:22:7E:70:03

Certificate is to be certified until Jan 15 13:05:09 2009 GMT (365 days)
Sign the certificate? [y/n]:y


1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated

一步完成 CA 证书请求生成及签名

以上两个步骤可以合二为一。利用 ca 命令的 -x509 参数，通过以下命令同时完成证书请求生成和签名从而生成 CA 根证书：

$ openssl req -new -x509 -days 365 -key ./demoCA/private/cakey.pem -out ./demoCA/cacert.pem

参数解释

req

用于生成证书请求的 OpenSSL 命令。

-new

生成一个新的证书请求。该参数将令 OpenSSL 在证书请求生成过程中要求用户填写一些相应的字段。

-x509

生成一份 X.509 证书。

-days 365

从生成之时算起，证书时效为 365 天。

-key ./demoCA/private/cakey.pem

指定 cakey.pem 为证书所使用的密钥对文件。

-out ./demoCA/cacert.pem

令生成的证书保存到文件 ./demoCA/cacert.pem 。

该命令输出如下，用户应输入相应的字段：

Enter pass phrase for ./demoCA/private/cakey.pem:
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:CN
State or Province Name (full name) [Some-State]:ZJ
Locality Name (eg, city) []:HZ
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Some Ltd. Corp.
Organizational Unit Name (eg, section) []:Some Unit
Common Name (eg, YOUR name) []:Someone
Email Address []:[email protected]

至此，我们便已成功建立了一个私有根 CA 。在这个过程中，我们获得了一份 CA 密钥对文件 ./demoCA/private/cakey.pem 以及一份由此密钥对签名的 CA 根证书文件 ./demoCA/cacert.pem ，得到的 CA 目录结构如下：

.
|-- careq.pem
`-- demoCA/
    |-- cacert.pem
    |-- index.txt
    |-- index.txt.attr
    |-- index.txt.old
    |-- newcerts/
    |   `-- 01.pem
    |-- private/
    |   `-- cakey.pem
    |-- serial
    `-- serial.old

注：如果在 CA 建立过程中跳过证书请求生成的步骤，则不会产生 careq.pem 文件。

签发证书

下面我们就可以利用建立起来的 CA 进行证书签发了。

生成用户证书 RSA 密钥对

参照 CA 的 RSA 密钥对生成过程，使用如下命令生成新的密钥对：

$ openssl genrsa -des3 -out userkey.pem
Generating RSA private key, 512 bit long modulus
....++++++++++++
...++++++++++++
e is 65537 (0x10001)
Enter pass phrase for userkey.pem:<enter your pass-phrase>
Verifying - Enter pass phrase for userkey.pem:<re-enter your pass-phrase>

生成用户证书请求

参照 CA 的证书请求生成过程，使用如下命令生成新的证书请求：

$ openssl req -new -days 365 -key userkey.pem -out userreq.pem
Enter pass phrase for userkey.pem:<enter your pass-phrase>
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:CN
State or Province Name (full name) [Some-State]:ZJ
Locality Name (eg, city) []:HZ
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Some Ltd. Corp.
Organizational Unit Name (eg, section) []:Some Other Unit
Common Name (eg, YOUR name) []:Another
Email Address []:[email protected]

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

签发用户证书

现在，我们可以用先前建立的 CA 来对用户的证书请求进行签名来为用户签发证书了。使用如下命令：

$ openssl ca -in userreq.pem -out usercert.pem

参数解释

ca

用于执行 CA 相关操作的 OpenSSL 命令。

-in userreq.pem

指定用户证书请求文件为 userreq.pem 。

-out usercert.pem

指定输出的用户证书文件为 usercert.pem 。

该命令要求用户输入密钥口令并输出相关证书信息，请求用户确认：

Using configuration from /usr/lib/ssl/openssl.cnf
Enter pass phrase for ./demoCA/private/cakey.pem:<enter your pass-phrase>
Check that the request matches the signature
Signature ok
Certificate Details:
        Serial Number: 2 (0x2)
        Validity
            Not Before: Jan 16 14:50:22 2008 GMT
            Not After : Jan 15 14:50:22 2009 GMT
        Subject:
            countryName               = CN
            stateOrProvinceName       = ZJ
            organizationName          = Some Ltd. Corp.
            organizationalUnitName    = Some Other Unit
            commonName                = Another
            emailAddress              = [email protected]
        X509v3 extensions:
            X509v3 Basic Constraints:
                CA:FALSE
            Netscape Comment:
                OpenSSL Generated Certificate
     &nb sp;      X509v3 Subject Key Identifier:
                97:E7:8E:84:B1:45:27:83:94:A0:DC:24:79:7B:83:97:99:0B:36:A9
            X509v3 Authority Key Identifier:
                keyid:D9:87:12:94:B2:20:C7:22:AB:D4:D5:DF:33:DB:84:F3:B0:4A:EC:A2

Certificate is to be certified until Jan 15 14:50:22 2009 GMT (365 days)
Sign the certificate? [y/n]:y


1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated

至此，我们便完成了 CA 的建立及用户证书签发的全部工作。不妨把所有 shell 命令放到一起纵览一下：

# 建立 CA 目录结构
mkdir -p ./demoCA/{private,newcerts}
touch ./demoCA/index.txt
echo 01 > ./demoCA/serial

# 生成 CA 的 RSA 密钥对
openssl genrsa -des3 -out ./demoCA/private/cakey.pem 2048

# 生成 CA 证书请求
openssl req -new -days 365 -key ./demoCA/private/cakey.pem -out careq.pem

# 自签发 CA 证书
openssl ca -selfsign -in careq.pem -out ./demoCA/cacert.pem

# 以上两步可以合二为一
openssl req -new -x509 -days 365 -key ./demoCA/private/cakey.pem -out ./demoCA/cacert.pem

# 生成用户的 RSA 密钥对
openssl genrsa -des3 -out userkey.pem

# 生成用户证书请求
openssl req -new -days 365 -key userkey.pem -out userreq.pem

# 使用 CA 签发用户证书
openssl ca -in userreq.pem -out usercert.pem

了解了这些基础步骤之后，就可以通过脚本甚至 makefile 的方式来将这些工作自动化。 CA.pl 和 CA.sh 便是对 OpenSSL 的 CA 相关功能的简单封装，在 Debian 系统中，安装了 OpenSSL 后，可以在 /usr/lib/ssl/misc/ 目录下找到这两个文件。而 makefile 的解决方案则可以参考这里。

参考文献

• The Open–source PKI Book: A guide to PKIs and Open–source Implementations, Initialisation of the Certification Authority
  http://ospkibook.sourceforge.net/docs/OSPKI-2.4.7/OSPKI-html/initialisation.htm
• OpenSSL: Documents, OpenSSL(1)
  http://www.openssl.org/docs/apps/openssl.html
• OpenSSL Certificate Authority Setup
  http://sial.org/howto/openssl/ca/