参考 OpenSSL Cookbook - Creating a Private Certification Authority，使用 OpenSSL 命令行可以创建一个私有CA。

本文则基于python的cryptography库，编写一个简单的私有CA，并用其来签发一套测试使用的证书。

创建自签发的root CA

普通证书的签发需要使用 CA的私钥对其进行私钥加密，root CA由于位于该条信任链的起始，只能使用自身的私钥对其进行签发，即自签发证书。

自签发证书遵循以下步骤

1. 生成私钥
2. 使用私钥对自身进行签发

genRootCA.py：

from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography import x509
from cryptography.x509.oid import NameOID
from cryptography.hazmat.primitives import hashes
import datetime

# 生成 2048 位 rsa 私钥
key = rsa.generate_private_key(
    public_exponent=65537,
    key_size=2048,
)

# 保存私钥到磁盘，并使用 key pass 进行加密
with open("rootca.key.secure", "wb") as f:
    f.write(key.private_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PrivateFormat.TraditionalOpenSSL,
        encryption_algorithm=serialization.BestAvailableEncryption(b"12345678")
    ))

# Various details about who we are. For a self-signed certificate the
# subject and issuer are always the same.
subject = issuer = x509.Name([
    x509.NameAttribute(NameOID.COUNTRY_NAME, u"CN"),
    x509.NameAttribute(NameOID.STATE_OR_PROVINCE_NAME, u"Sichuan"),
    x509.NameAttribute(NameOID.LOCALITY_NAME, u"Chengdu"),
    x509.NameAttribute(NameOID.ORGANIZATION_NAME, u"ctyun elb"),
    x509.NameAttribute(NameOID.COMMON_NAME, u"test root CA"),
])
cert = x509.CertificateBuilder().subject_name(
    subject
).issuer_name(
    issuer
).public_key(
    key.public_key()
).serial_number(
    x509.random_serial_number()
).not_valid_before(
    datetime.datetime.utcnow()
).not_valid_after(
    # Our certificate will be valid for 1 years default
    datetime.datetime.utcnow() + datetime.timedelta(365)
).add_extension(
    x509.SubjectAlternativeName([x509.DNSName(u"localhost")]),
    critical=False,
# 使用root CA的私钥签发root CA证书
).sign(key, hashes.SHA256())
# 保存证书到磁盘
with open("rootca.pem", "wb") as f:
    f.write(cert.public_bytes(serialization.Encoding.PEM))

执行程序

python3 genRootCA.py

得到受密码保护的RSA私钥（rootca.key.secure）和证书（rootca.pem）

使用root CA签发普通证书

root CA我们使用上一步得到的 rootca.key.secure 和 rootca.pem。

普通证书签发遵循以下步骤

1. 加载CA私钥
2. 生成私钥
3. 使用CA私钥对证书进行签发

genCert.py：

from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography import x509
from cryptography.x509.oid import NameOID
from cryptography.hazmat.primitives import hashes
import datetime

# 加载 root CA 的证书与私钥
with open("rootca.pem", 'rb') as f:
    datas = f.read()
    cacert = x509.load_pem_x509_certificate(datas)
with open("rootca.key.secure", 'rb') as f:
    datas = f.read()
    cakey = serialization.load_pem_private_key(datas,b'12345678')

# 生成 2048 位 rsa 私钥
key = rsa.generate_private_key(
    public_exponent=65537,
    key_size=2048,
)

# 保存私钥到磁盘，并使用 key pass 进行加密
with open("www.example.com.key.secure", "wb") as f:
    f.write(key.private_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PrivateFormat.TraditionalOpenSSL,
        encryption_algorithm=serialization.BestAvailableEncryption(b"12345678")
    ))

# 保存私钥到磁盘，不加密
with open("www.example.com.key.unsecure", "wb") as f:
    f.write(key.private_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PrivateFormat.TraditionalOpenSSL,
        encryption_algorithm=serialization.NoEncryption()
    ))

# Various details about who we are. For a self-signed certificate the
# subject and issuer are always the same.
subject = issuer = x509.Name([
    x509.NameAttribute(NameOID.COUNTRY_NAME, u"CN"),
    x509.NameAttribute(NameOID.STATE_OR_PROVINCE_NAME, u"Sichuan"),
    x509.NameAttribute(NameOID.LOCALITY_NAME, u"Chengdu"),
    x509.NameAttribute(NameOID.ORGANIZATION_NAME, u"ctyun elb"),
    x509.NameAttribute(NameOID.COMMON_NAME, u"www.example.com"),
])
cert = x509.CertificateBuilder().subject_name(
    subject
).issuer_name(
    cacert.subject
).public_key(
    key.public_key()
).serial_number(
    x509.random_serial_number()
).not_valid_before(
    datetime.datetime.utcnow()
).not_valid_after(
    # Our certificate will be valid for 1 years default
    datetime.datetime.utcnow() + datetime.timedelta(365)
).add_extension(
    x509.SubjectAlternativeName([x509.DNSName(u"example.com")]),
    critical=False,
# 使用root CA的私钥签发root CA证书
).sign(cakey, hashes.SHA256())
# 保存证书到磁盘
with open("www.example.com.pem", "wb") as f:
    f.write(cert.public_bytes(serialization.Encoding.PEM))

执行程序

python3 genCert.py

得到受密码保护的RSA私钥（www.example.com.key.secure）、不受密码保护的RSA私钥（www.example.com.key.unsecure）和证书（www.example.com.pem）

这里提供一个更加完善的程序：orzPrivateCA