System Design 101: How Does SSH Work

SSH 是如何工作的？

Introduction

介绍

SSH (Secure Shell) is a widely used network protocol that provides a secure way to access remote machines over an unsecured network. It offers encryption, secure authentication, and data transfer mechanisms, making it a cornerstone of secure remote communication in many IT environments.
SSH（安全外壳）是一种广泛使用的网络协议，它提供了一种通过不安全网络安全访问远程机器的方法。它提供了加密、安全身份验证和数据传输机制，使其成为许多 IT 环境中安全远程通信的基石。

This blog will explain in detail how SSH works, from initiating the connection to maintaining secure communication, covering all the essential components involved in this process.
本博客将详细解释 SSH 的工作原理，从启动连接到维护安全通信，涵盖此过程中的所有重要组件。

Steps in an SSH Connection

SSH 连接的步骤

1. Initiating the Connection

1. 启动连接

Client Initiation (客户端启动):

• The SSH connection begins when the client sends a request to the SSH server. This request typically includes information about the SSH protocol version supported by the client and a list of encryption algorithms it supports.
  SSH 连接始于客户端向 SSH 服务器发送请求。此请求通常包括有关客户端支持的 SSH 协议版本的信息以及其支持的加密算法列表。

Server Response (服务器响应):

• The SSH server responds with its protocol version and a list of supported encryption and compression algorithms. The client and server then agree on which algorithms to use for the session.
  SSH 服务器响应其协议版本及其支持的加密和压缩算法列表。然后客户端和服务器就会话期间使用的算法达成一致。

Example (例子):

• Imagine you’re using a terminal on your computer to connect to a remote server. The command ssh [email protected] would initiate this process.
  想象一下，您正在使用计算机上的终端连接到远程服务器。命令 ssh [email protected] 将启动此过程。

2. Key Exchange

2. 密钥交换

Generating Session Keys (生成会话密钥):

• After agreeing on the encryption algorithms, the client and server use a key exchange algorithm like Diffie-Hellman or Elliptic Curve Diffie-Hellman (ECDH) to securely generate a shared session key. This key is used to encrypt the entire session.
  在协商加密算法后，客户端和服务器使用密钥交换算法（如 Diffie-Hellman 或椭圆曲线 Diffie-Hellman (ECDH)）安全生成共享会话密钥。此密钥用于加密整个会话。

Preventing Man-in-the-Middle Attacks (防止中间人攻击):

• The key exchange process ensures that even if an attacker intercepts the communication, they cannot decrypt the session without the private key. This process also involves verifying the server’s authenticity to prevent man-in-the-middle attacks.
  密钥交换过程确保即使攻击者拦截了通信，他们也无法在没有私钥的情况下解密会话。此过程还涉及验证服务器的真实性以防止中间人攻击。

Example (例子):

• If someone tries to intercept the connection, they would be unable to understand the data because they wouldn’t have the shared session key.
  如果有人试图拦截连接，他们将无法理解数据，因为他们没有共享会话密钥。

3. Authentication

3. 身份验证

Password Authentication (密码认证):

• One of the simplest methods, where the client sends a password to the server over the encrypted channel. The server checks the password against its stored credentials. If they match, the client is authenticated.
  这是最简单的方法之一，客户端通过加密通道将密码发送到服务器。服务器将密码与其存储的凭据进行检查。如果匹配，客户端就会通过身份验证。

Public Key Authentication (公钥认证):

• A more secure method where the client generates a pair of keys (public and private). The public key is sent to the server and stored. During the connection, the server sends a challenge to the client, which must be signed with the private key. If the signature is valid, the client is authenticated.
  这是一种更安全的方法，客户端生成一对密钥（公钥和私钥）。公钥发送到服务器并存储。在连接过程中，服务器向客户端发送挑战，必须使用私钥签名。如果签名有效，客户端将通过身份验证。

Multi-Factor Authentication (多因素认证):

• SSH can be configured to require additional factors for authentication, such as a one-time password (OTP) in addition to a public key, adding an extra layer of security.
  SSH 可以配置为需要其他因素进行身份验证，例如除公钥之外的一次性密码 (OTP)，以增加安全层。

Example (例子):

• Developers often use public key authentication to securely access their servers without needing to type a password every time.
  开发人员通常使用公钥认证安全访问他们的服务器，而不需要每次输入密码。

4. Establishing the Secure Channel

4. 建立安全通道

Encrypting the Session (加密会话):

• Once authentication is successful, the SSH session is established, and all communication between the client and server is encrypted using the session key. This encryption ensures that any data exchanged during the session, such as commands, files, or other sensitive information, is secure.
  身份验证成功后，会建立 SSH 会话，并使用会话密钥加密客户端和服务器之间的所有通信。此加密确保会话期间交换的任何数据（如命令、文件或其他敏感信息）都是安全的。

Data Integrity (数据完整性):

• SSH also ensures data integrity by using message authentication codes (MACs) to verify that the data has not been tampered with during transmission.
  SSH 还通过使用消息认证码 (MAC) 确保数据的完整性，以验证传输过程中数据是否未被篡改。

Example (例子):

• When you’re editing a file on a remote server using SSH, everything you type is encrypted before it is sent to the server, ensuring that no one can intercept and read your data.
  当您使用 SSH 编辑远程服务器上的文件时，您输入的所有内容在发送到服务器之前都会被加密，确保没有人可以拦截并读取您的数据。

SSH Use Cases

SSH 的使用场景

1. Remote Server Management (远程服务器管理):

• System administrators use SSH to securely manage remote servers. They can execute commands, monitor system performance, and make configuration changes without needing physical access to the machine.
  系统管理员使用 SSH 安全地管理远程服务器。他们可以执行命令、监控系统性能并进行配置更改，而无需实际访问机器。

2. Secure File Transfer (安全文件传输):

• SSH supports secure file transfer protocols such as SCP (Secure Copy) and SFTP (SSH File Transfer Protocol), enabling users to transfer files securely between machines.
  SSH 支持安全文件传输协议，如 SCP（安全复制）和 SFTP（SSH 文件传输协议），使用户能够在机器之间安全传输文件。

3. Tunneling and Port Forwarding (隧道和端口转发):

• SSH can create encrypted tunnels to securely forward network traffic from one machine to another, providing secure access to services behind firewalls or on remote networks.
  SSH 可以创建加密隧道，以安全地将网络流量从一台机器转发到另一台机器，从而提供对防火墙后面或远程网络上服务的安全访问。

4. Git and Version Control (Git 和版本控制):

• Developers commonly use SSH to securely connect to Git repositories for version control, ensuring that code changes are transmitted securely.
  开发人员通常使用 SSH 安全连接到 Git 存储库进行版本控制，确保代码更改的安全传输。

Conclusion

结论

SSH is a fundamental tool for securely managing and transferring data across networks, particularly in environments where security is critical. By encrypting the connection, authenticating users, and ensuring data integrity, SSH provides a reliable and secure method for remote communication and data transfer.
SSH 是一种通过网络安全管理和传输数据的基本工具，特别是在安全性至关重要的环境中。通过加密连接、验证用户身份并确保数据完整性，SSH 提供了一种可靠且安全的远程通信和数据传输方法。

Understanding how SSH works and utilizing its capabilities effectively can greatly enhance the security of your remote operations.
理解 SSH 的工作原理并有效利用其功能可以极大地增强远程操作的安全性。