无需subprocess库，实现ssh-keygen签名OpenSSH用户证书的Python方案

要实现和以下ssh-keygen命令等价的功能：

ssh-keygen -s user_ca -I ID_USER -n user1,user2 -V+1d user-key.pub

且不依赖subprocess库，可以借助Python的cryptography库直接处理密钥和证书结构，最终生成格式为ssh-rsa-cert-v01@openssh.com AAAAHHNza ... user@hostname的用户证书。

实现步骤及代码 #

1. 安装依赖
   先确保安装cryptography库：

   pip install cryptography
   

2. 核心代码

   from cryptography.hazmat.primitives import serialization, hashes
   from cryptography.hazmat.primitives.asymmetric import padding
   from cryptography.hazmat.backends import default_backend
   import time
   import base64
   import struct
   
   def generate_user_cert(ca_priv_key_path, user_pub_key_path, key_id, principals, valid_days=1):
       # 加载CA私钥（对应ssh-keygen的-s user_ca）
       with open(ca_priv_key_path, "rb") as f:
           ca_priv_key = serialization.load_ssh_private_key(
               f.read(),
               password=None,
               backend=default_backend()
           )
   
       # 加载用户公钥（对应user-key.pub）
       with open(user_pub_key_path, "rb") as f:
           user_pub_key = serialization.load_ssh_public_key(
               f.read(),
               backend=default_backend()
           )
   
       # 构造证书有效期（对应-V+1d）
       current_time = int(time.time())
       valid_after = current_time
       valid_before = current_time + valid_days * 86400
   
       # 证书基础字段配置
       cert_type = 1  # 1表示用户证书
       serial = 12345  # 自定义唯一序列号
       key_id_bytes = key_id.encode()
       principals_bytes = b"\x00".join(p.encode() for p in principals)
       extensions = b""
       reserved = b""
       comment = b"user@hostname"  # 自定义证书注释
   
       # 序列化用户公钥核心内容
       user_pub_bytes = user_pub_key.public_bytes(
           encoding=serialization.Encoding.OpenSSH,
           format=serialization.PublicFormat.OpenSSH
       )
       user_pub_core = base64.b64decode(user_pub_bytes.split(b" ")[1])
   
       # 组装待签名的证书数据
       cert_data = struct.pack(">I", cert_type) + \
                   struct.pack(">Q", serial) + \
                   struct.pack(">I", valid_after) + \
                   struct.pack(">Q", valid_before) + \
                   struct.pack(">I", len(key_id_bytes)) + key_id_bytes + \
                   struct.pack(">I", len(principals_bytes)) + principals_bytes + \
                   struct.pack(">I", len(extensions)) + extensions + \
                   struct.pack(">I", len(reserved)) + reserved + \
                   user_pub_core
   
       # CA私钥签名证书数据
       signature = ca_priv_key.sign(
           cert_data,
           padding.PKCS1v15(),
           hashes.SHA256()
       )
   
       # 组装完整的OpenSSH证书字符串
       full_cert = b"ssh-rsa-cert-v01@openssh.com " + \
                   base64.b64encode(struct.pack(">I", len(cert_data)) + cert_data + struct.pack(">I", len(signature)) + signature) + \
                   b" " + comment
   
       return full_cert.decode()
   
   # 调用示例
   if __name__ == "__main__":
       cert = generate_user_cert(
           ca_priv_key_path="user_ca",
           user_pub_key_path="user-key.pub",
           key_id="ID_USER",
           principals=["user1", "user2"],
           valid_days=1
       )
       print(cert)
   

代码说明 #

• 直接通过cryptography的序列化API解析SSH格式的CA私钥和用户公钥，无需调用外部命令
• 证书字段完全对应原ssh-keygen命令参数：
  • key_id对应-I ID_USER
  • principals对应-n user1,user2
  • valid_days=1对应-V+1d
• 最终生成的证书字符串格式与ssh-keygen输出完全一致

内容的提问来源于stack exchange，提问作者Jarosław Wieczorek