SSH secure shell 原理与运用

转： http://www.ruanyifeng.com/blog/2011/12/ssh_remote_login.html

作者： 阮一峰

日期： 2011年12月21日

SSH是每一台Linux电脑的标准配置。

随着Linux设备从电脑逐渐扩展到手机、外设和家用电器，SSH的使用范围也越来越广。不仅程序员离不开它，很多普通用户也每天使用。

SSH具备多种功能，可以用于很多场合。有些事情，没有它就是办不成。本文是我的学习笔记，总结和解释了SSH的常见用法，希望对大家有用。

虽然本文内容只涉及初级应用，较为简单，但是需要读者具备最基本的"Shell知识"和了解"公钥加密"的概念。如果你对它们不熟悉，我推荐先阅读《UNIX / Linux 初学者教程》和《数字签名是什么？》。

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SSH原理与运用

作者：阮一峰

一、什么是SSH？

简单说，SSH是一种网络协议，用于计算机之间的加密登录。

如果一个用户从本地计算机，使用SSH协议登录另一台远程计算机，我们就可以认为，这种登录是安全的，即使被中途截获，密码也不会泄露。

最早的时候，互联网通信都是明文通信，一旦被截获，内容就暴露无疑。1995年，芬兰学者Tatu Ylonen设计了SSH协议，将登录信息全部加密，成为互联网安全的一个基本解决方案，迅速在全世界获得推广，目前已经成为Linux系统的标准配置。

需要指出的是，SSH只是一种协议，存在多种实现，既有商业实现，也有开源实现。本文针对的实现是OpenSSH，它是自由软件，应用非常广泛。

此外，本文只讨论SSH在Linux Shell中的用法。如果要在Windows系统中使用SSH，会用到另一种软件PuTTY，这需要另文介绍。

二、最基本的用法

SSH主要用于远程登录。假定你要以用户名user，登录远程主机host，只要一条简单命令就可以了。

　　$ ssh user@host

如果本地用户名与远程用户名一致，登录时可以省略用户名。

　　$ ssh host

SSH的默认端口是22，也就是说，你的登录请求会送进远程主机的22端口。使用p参数，可以修改这个端口。

　　$ ssh -p 2222 user@host

上面这条命令表示，ssh直接连接远程主机的2222端口。

三、中间人攻击

SSH之所以能够保证安全，原因在于它采用了公钥加密。

整个过程是这样的：（1）远程主机收到用户的登录请求，把自己的公钥发给用户。（2）用户使用这个公钥，将登录密码加密后，发送回来。（3）远程主机用自己的私钥，解密登录密码，如果密码正确，就同意用户登录。

这个过程本身是安全的，但是实施的时候存在一个风险：如果有人截获了登录请求，然后冒充远程主机，将伪造的公钥发给用户，那么用户很难辨别真伪。因为不像https协议，SSH协议的公钥是没有证书中心（CA）公证的，也就是说，都是自己签发的。

可以设想，如果攻击者插在用户与远程主机之间（比如在公共的wifi区域），用伪造的公钥，获取用户的登录密码。再用这个密码登录远程主机，那么SSH的安全机制就荡然无存了。这种风险就是著名的"中间人攻击"（Man-in-the-middle attack）。

SSH协议是如何应对的呢？

四、口令登录

如果你是第一次登录对方主机，系统会出现下面的提示：

　　The authenticity of host 'host (12.18.429.21)' can't be established.

　　RSA key fingerprint is 98:2e:d7:e0:de:9f:ac:67:28:c2:42:2d:37:16:58:4d.

　　Are you sure you want to continue connecting (yes/no)?

这段话的意思是，无法确认host主机的真实性，只知道它的公钥指纹，问你还想继续连接吗？

所谓"公钥指纹"，是指公钥长度较长（这里采用RSA算法，长达1024位），很难比对，所以对其进行MD5计算，将它变成一个128位的指纹。上例中是98:2e:d7:e0:de:9f:ac:67:28:c2:42:2d:37:16:58:4d，再进行比较，就容易多了。

很自然的一个问题就是，用户怎么知道远程主机的公钥指纹应该是多少？回答是没有好办法，远程主机必须在自己的网站上贴出公钥指纹，以便用户自行核对。

假定经过风险衡量以后，用户决定接受这个远程主机的公钥。

　　Are you sure you want to continue connecting (yes/no)? yes

系统会出现一句提示，表示host主机已经得到认可。

　　Warning: Permanently added 'host,12.18.429.21' (RSA) to the list of known hosts.

然后，会要求输入密码。

　　Password: (enter password)

如果密码正确，就可以登录了。

当远程主机的公钥被接受以后，它就会被保存在文件$HOME/.ssh/known_hosts之中。下次再连接这台主机，系统就会认出它的公钥已经保存在本地了，从而跳过警告部分，直接提示输入密码。

每个SSH用户都有自己的known_hosts文件，此外系统也有一个这样的文件，通常是/etc/ssh/ssh_known_hosts，保存一些对所有用户都可信赖的远程主机的公钥。

五、公钥登录

使用密码登录，每次都必须输入密码，非常麻烦。好在SSH还提供了公钥登录，可以省去输入密码的步骤。

所谓"公钥登录"，原理很简单，就是用户将自己的公钥储存在远程主机上。登录的时候，远程主机会向用户发送一段随机字符串，用户用自己的私钥加密后，再发回来。远程主机用事先储存的公钥进行解密，如果成功，就证明用户是可信的，直接允许登录shell，不再要求密码。

这种方法要求用户必须提供自己的公钥。如果没有现成的，可以直接用ssh-keygen生成一个：

　　$ ssh-keygen

运行上面的命令以后，系统会出现一系列提示，可以一路回车。其中有一个问题是，要不要对私钥设置口令（passphrase），如果担心私钥的安全，这里可以设置一个。

运行结束以后，在$HOME/.ssh/目录下，会新生成两个文件：id_rsa.pub和id_rsa。前者是你的公钥，后者是你的私钥。

这时再输入下面的命令，将公钥传送到远程主机host上面：

　　$ ssh-copy-id user@host

好了，从此你再登录，就不需要输入密码了。

如果还是不行，就打开远程主机的/etc/ssh/sshd_config这个文件，检查下面几行前面"#"注释是否取掉。

　　RSAAuthentication yes

　　PubkeyAuthentication yes

　　AuthorizedKeysFile .ssh/authorized_keys

然后，重启远程主机的ssh服务。

　　// ubuntu系统

　　service ssh restart

　　// debian系统

　　/etc/init.d/ssh restart

六、authorized_keys文件

远程主机将用户的公钥，保存在登录后的用户主目录的$HOME/.ssh/authorized_keys文件中。公钥就是一段字符串，只要把它追加在authorized_keys文件的末尾就行了。

这里不使用上面的ssh-copy-id命令，改用下面的命令，解释公钥的保存过程：

　　$ ssh user@host 'mkdir -p .ssh && cat >> .ssh/authorized_keys' < ~/.ssh/id_rsa.pub

这条命令由多个语句组成，依次分解开来看：（1）"$ ssh user@host"，表示登录远程主机；（2）单引号中的mkdir .ssh && cat >> .ssh/authorized_keys，表示登录后在远程shell上执行的命令：（3）"$ mkdir -p .ssh"的作用是，如果用户主目录中的.ssh目录不存在，就创建一个；（4）'cat >> .ssh/authorized_keys' < ~/.ssh/id_rsa.pub的作用是，将本地的公钥文件~/.ssh/id_rsa.pub，重定向追加到远程文件authorized_keys的末尾。

写入authorized_keys文件后，公钥登录的设置就完成了。

==============================================

关于shell远程登录的部分就写到这里，下一次接着介绍《远程操作和端口转发》。

接着前一次的文章，继续介绍SSH的用法。

SSH原理与运用（二）：远程操作与端口转发

七、远程操作

SSH不仅可以用于远程主机登录，还可以直接在远程主机上执行操作。

上一节的操作，就是一个例子：

单引号中间的部分，表示在远程主机上执行的操作；后面的输入重定向，表示数据通过SSH传向远程主机。

这就是说，SSH可以在用户和远程主机之间，建立命令和数据的传输通道，因此很多事情都可以通过SSH来完成。

下面看几个例子。

【例1】

将$HOME/src/目录下面的所有文件，复制到远程主机的$HOME/src/目录。

　　$ cd && tar czv src | ssh user@host 'tar xz'

【例2】

将远程主机$HOME/src/目录下面的所有文件，复制到用户的当前目录。

　　$ ssh user@host 'tar cz src' | tar xzv

【例3】

查看远程主机是否运行进程httpd。

　　$ ssh user@host 'ps ax | grep [h]ttpd'

八、绑定本地端口

既然SSH可以传送数据，那么我们可以让那些不加密的网络连接，全部改走SSH连接，从而提高安全性。

假定我们要让8080端口的数据，都通过SSH传向远程主机，命令就这样写：

　　$ ssh -D 8080 user@host

SSH会建立一个socket，去监听本地的8080端口。一旦有数据传向那个端口，就自动把它转移到SSH连接上面，发往远程主机。可以想象，如果8080端口原来是一个不加密端口，现在将变成一个加密端口。

九、本地端口转发

有时，绑定本地端口还不够，还必须指定数据传送的目标主机，从而形成点对点的"端口转发"。为了区别后文的"远程端口转发"，我们把这种情况称为"本地端口转发"（Local forwarding）。

假定host1是本地主机，host2是远程主机。由于种种原因，这两台主机之间无法连通。但是，另外还有一台host3，可以同时连通前面两台主机。因此，很自然的想法就是，通过host3，将host1连上host2。

我们在host1执行下面的命令：

　　$ ssh -L 2121:host2:21 host3

命令中的L参数一共接受三个值，分别是"本地端口:目标主机:目标主机端口"，它们之间用冒号分隔。这条命令的意思，就是指定SSH绑定本地端口2121，然后指定host3将所有的数据，转发到目标主机host2的21端口（假定host2运行FTP，默认端口为21）。

这样一来，我们只要连接host1的2121端口，就等于连上了host2的21端口。

　　$ ftp localhost:2121

"本地端口转发"使得host1和host3之间仿佛形成一个数据传输的秘密隧道，因此又被称为"SSH隧道"。

下面是一个比较有趣的例子。

　　$ ssh -L 5900:localhost:5900 host3

它表示将本机的5900端口绑定host3的5900端口（这里的localhost指的是host3，因为目标主机是相对host3而言的）。

另一个例子是通过host3的端口转发，ssh登录host2。

　　$ ssh -L 9001:host2:22 host3

这时，只要ssh登录本机的9001端口，就相当于登录host2了。

　　$ ssh -p 9001 localhost

上面的-p参数表示指定登录端口。

十、远程端口转发

既然"本地端口转发"是指绑定本地端口的转发，那么"远程端口转发"（remote forwarding）当然是指绑定远程端口的转发。

还是接着看上面那个例子，host1与host2之间无法连通，必须借助host3转发。但是，特殊情况出现了，host3是一台内网机器，它可以连接外网的host1，但是反过来就不行，外网的host1连不上内网的host3。这时，"本地端口转发"就不能用了，怎么办？

解决办法是，既然host3可以连host1，那么就从host3上建立与host1的SSH连接，然后在host1上使用这条连接就可以了。

我们在host3执行下面的命令：

　　$ ssh -R 2121:host2:21 host1

R参数也是接受三个值，分别是"远程主机端口:目标主机:目标主机端口"。这条命令的意思，就是让host1监听它自己的2121端口，然后将所有数据经由host3，转发到host2的21端口。由于对于host3来说，host1是远程主机，所以这种情况就被称为"远程端口绑定"。

绑定之后，我们在host1就可以连接host2了：

这里必须指出，"远程端口转发"的前提条件是，host1和host3两台主机都有sshD和ssh客户端。

十一、SSH的其他参数

SSH还有一些别的参数，也值得介绍。

N参数，表示只连接远程主机，不打开远程shell；T参数，表示不为这个连接分配TTY。这个两个参数可以放在一起用，代表这个SSH连接只用来传数据，不执行远程操作。

　　$ ssh -NT -D 8080 host

f参数，表示SSH连接成功后，转入后台运行。这样一来，你就可以在不中断SSH连接的情况下，在本地shell中执行其他操作。

　　$ ssh -f -D 8080 host

要关闭这个后台连接，就只有用kill命令去杀掉进程。

十二、参考文献

　　* SSH, The Secure Shell: The Definitive Guide: 2.4. Authentication by Cryptographic Key, O'reilly

　　* SSH, The Secure Shell: The Definitive Guide: 9.2. Port Forwarding, O'reilly

　　* Shebang: Tips for Remote Unix Work (SSH, screen, and VNC)

　　* brihatch: SSH Host Key Protection

　　* brihatch: SSH User Identities

　　* IBM developerWorks: 实战 SSH 端口转发

　　* Jianing YANG：ssh隧道技术简介

　　* WikiBooks: Internet Technologies/SSH

　　* Buddhika Chamith: SSH Tunneling Explained

（完）

远程电脑上通过命令行将80端口的访问全部转发给本机的8080端口，ubuntu测试有效： 参考资料url：  https://thomas-barthelemy.github.io/2016/05/02/permanent-ssh-tunnel/

nohup sudo ssh -fnL 80:127.0.0.1:8080 hzh@localhost -N      

其中 hzh是 当前用户。如果要停用，只有采用 kill -9 去杀掉该进程。

DH密钥交换（Diffie–Hellman key exchange）算法笔记

DH密钥交换算法的作用是获得 forward security，不知道什么是forward security？百度下。顺带百度下 “ECDHE” 或DHE。

注意：只是个人理解，可能有不正确的地方

下文中^代表乘方运算，例如2^3=2*2*2=6，参考：http://zh.wikipedia.org/wiki/%E5%86%AA

%代表模运算，例如5%3=2，参考：http://zh.wikipedia.org/wiki/%E6%A8%A1%E9%99%A4

DH密钥交换算法的作用是使通信双方可以在不安全的通道中建立一个相同的密钥，用于加密通信。

基本原理示例：

1、通信方A和通信方B约定一个初始数g，g是公开的，如g=5

2、A生成一个随机数a，a是保密的，如a=6

3、A计算g^a发送给B，g^a=5^6

4、B生成一个随机数b，b是保密的，如b=15

5、B计算g^b发送给A，g^b=5^15

6、A接收到g^b后，再使用保密的a，计算(g^b)^a=g^(a*b)=5^(6*15)

7、B接收到g^a后，再使用保密的b，计算(g^a)^b=g^(a*b)=5^(6*15)

8、这样通信方A和B得到一个相同的“密钥”g^(a*b)=5^(6*15)

整个通信过程中g、g^a、g^b是公开的，但由于g、a、b都是整数，通过g和g^a得到a还是比较容易的，b也是如此，所以最终的“密钥”g^(a*b)还是可以被计算出来的。所以实际的过程还需要在基本原理上加入新的计算——模运算：

1、通信方A和通信方B约定一个初始数g，如g=5，一个质数p，如p=23，g和p是公开的

3、A计算g^a%p发送给B，g^a%p=5^6%23=8

5、B计算g^b%p发送给A，g^b%p=5^15%23=19

6、A接收到g^b%p后，再使用保密的a，计算(g^b%p)^a%p=19^6%23=2

7、B接收到g^a%p后，再使用保密的b，计算(g^a%p)^b%p=8^15%23=2

8、这样通信方A和B得到一个相同的密钥：2

(g^b%p)^a%p=(g^a%p)^b%p的证明：

如果a=2：

(g^b%p)^a%p=(g^b%p)^2%p=(g^b-n*p)^2%p=(g^(2*b)-2*g^b*n*p+(n*p)^2)%p=g^(2*b)%p

可以看出(g^b-n*p)^2展开后除g^(2*b)外，其它都是p的倍数，所以整个算式的结果是g^(2*b)%p

同理对(g^b-n*p)^a展开后除g^(a*b)外，其它都是p的倍数，所以整个算式的结果是g^(a*b)%p

同样可以得出(g^a%p)^b%p=g^(a*b)%p

所以(g^b%p)^a%p=(g^a%p)^b%p

整个通信过程中g、p、g^a%p、g^b%p是公开的，这时通过g、p、g^a%p得到a比较难，同样通过g、p、g^b%p得到b比较难，所以最终的密钥是比较安全的。

以g=5、p=23、g^a%p=8计算a为例，a=log(5, (8+23*n))，这个只能将n的可能值逐个带入公式试验才能得到a的值。如果a、p是比较大的数那么计算更加困难。

如果注意的是，为了防止应用优化算法计算上述问题，质数p不是随便选择的，需要符合一定的条件。随机数a、b的生成算法也必需注意，应使结果尽可能随机，不能出现可预测的规律，否则会使破解变的容易。

通过上述计算过程也可以看出DH算法不仅可以应用在2方通信的情况，如果多方通信，也可以使用该算法。

DH密钥交换算法无法验证对方身份，所以DH密钥交换算法不能抵御中间人攻击（MITM，Man-in-the-middle attack）。

参考：

wiki： http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange

The host key（服务器端发送给client端的） is used to sign the Diffie-Hellman parameters. It is used during the key exchange;  这句非常重要，因为该 host key 其实是用来产生 g和p两个素数的，即generator 和 a large prime number。

https://security.stackexchange.com/questions/76894/how-does-ssh-use-both-rsa-and-diffie-hellman

Understanding the SSH Encryption and Connection Process

https://www.digitalocean.com/community/tutorials/understanding-the-ssh-encryption-and-connection-process

Introduction

SSH, or secure shell, is a secure protocol and the most common way of safely administering remote servers. Using a number of encryption technologies, SSH provides a mechanism for establishing a cryptographically secured connection between two parties, authenticating each side to the other, and passing commands and output back and forth.

In other guides, we have discussed how to configure SSH key-based access, how to connect using SSH, and some SSH tips and tricks.

In this guide, we will be examining the underlying encryption techniques that SSH employs and the methods it uses to establish secure connections. This information can be useful for understanding the various layers of encryption and the different steps needed to form a connection and authenticate both parties.

Symmetric Encryption, Asymmetric Encryption, and Hashes

In order to secure the transmission of information, SSH employs a number of different types of data manipulation techniques at various points in the transaction. These include forms of symmetrical encryption, asymmetrical encryption, and hashing.

Symmetrical Encryption

The relationship of the components that encrypt and decrypt data determine whether an encryption scheme is symmetrical or asymmetrical.

Symmetrical encryption is a type of encryption where one key can be used to encrypt messages to the opposite party, and also to decrypt the messages received from the other participant. This means that anyone who holds the key can encrypt and decrypt messages to anyone else holding the key.

This type of encryption scheme is often called "shared secret" encryption, or "secret key" encryption. There is typically only a single key that is used for all operations, or a pair of keys where the relationship is easy to discover and it is trivial to derive the opposite key.

Symmetric keys are used by SSH in order to encrypt the entire connection. Contrary to what some users assume, public/private asymmetrical key pairs that can be created are only used for authentication, not the encrypting the connection. The symmetrical encryption allows even password authentication to be protected against snooping.

The client and server both contribute toward establishing this key, and the resulting secret is never known to outside parties. The secret key is created through a process known as a key exchange algorithm. This exchange results in the server and client both arriving at the same key independently by sharing certain pieces of public data and manipulating them with certain secret data. This process is explained in greater detail later on.

The symmetrical encryption key created by this procedure is session-based and constitutes the actual encryption for the data sent between server and client. Once this is established, the rest of the data must be encrypted with this shared secret. This is done prior to authenticating a client.

SSH can be configured to utilize a variety of different symmetrical cipher systems, including AES, Blowfish, 3DES, CAST128, and Arcfour. The server and client can both decide on a list of their supported ciphers, ordered by preference. The first option from the client's list that is available on the server is used as the cipher algorithm in both directions.

On Ubuntu 14.04, both the client and the server are defaulted like this: 

aes128-ctr

, 

aes192-ctr

aes256-ctr

,

arcfour256

arcfour128

[email protected]

[email protected]

[email protected]

aes128-cbc

blowfish-cbc

cast128-cbc

aes192-cbc

aes256-cbc

arcfour

.

This means that if two Ubuntu 14.04 machines are connecting to each other (without overriding the default ciphers through configuration options), they will always use the 

aes128-ctr

 cipher to encrypt their connection.

Asymmetrical Encryption

Asymmetrical encryption is different from symmetrical encryption in that to send data in a single direction, two associated keys are needed. One of these keys is known as the private key, while the other is called the public key.

The public key can be freely shared with any party. It is associated with its paired key, but the private keycannot be derived from the public key. The mathematical relationship between the public key and the private key allows the public key to encrypt messages that can only be decrypted by the private key. This is a one-way ability, meaning that the public key has no ability to decrypt the messages it writes, nor can it decrypt anything the private key may send it.

The private key should be kept entirely secret and should never be shared with another party. This is a key requirement for the public key paradigm to work. The private key is the only component capable of decrypting messages that were encrypted using the associated public key. By virtue of this fact, any entity capable decrypting these messages has demonstrated that they are in control of the private key.

SSH utilizes asymmetric encryption in a few different places. During the initial key exchange process used to set up the symmetrical encryption (used to encrypt the session), asymmetrical encryption is used. In this stage, both parties produce temporary key pairs and exchange the public key in order to produce the shared secret that will be used for symmetrical encryption.

The more well-discussed use of asymmetrical encryption with SSH comes from SSH key-based authentication. SSH key pairs can be used to authenticate a client to a server. The client creates a key pair and then uploads the public key to any remote server it wishes to access. This is placed in a file called

authorized_keys

 within the 

~/.ssh

 directory in the user account's home directory on the remote server.

After the symmetrical encryption is established to secure communications between the server and client, the client must authenticate to be allowed access. The server can use the public key in this file to encrypt a challenge message to the client. If the client can prove that it was able to decrypt this message, it has demonstrated that it owns the associated private key. The server then can set up the environment for the client.

Hashing

Another form of data manipulation that SSH takes advantage of is cryptographic hashing. Cryptographic hash functions are methods of creating a succinct "signature" or summary of a set of information. Their main distinguishing attributes are that they are never meant to be reversed, they are virtually impossible to influence predictably, and they are practically unique.

Using the same hashing function and message should produce the same hash; modifying any portion of the data should produce an entirely different hash. A user should not be able to produce the original message from a given hash, but they should be able to tell if a given message produced a given hash.

Given these properties, hashes are mainly used for data integrity purposes and to verify the authenticity of communication. The main use in SSH is with HMAC, or hash-based message authentication codes. These are used to ensure that the received message text is intact and unmodified.

As part of the symmetrical encryption negotiation outlined above, a message authentication code (MAC) algorithm is selected. The algorithm is chosen by working through the client's list of acceptable MAC choices. The first one out of this list that the server supports will be used.

Each message that is sent after the encryption is negotiated must contain a MAC so that the other party can verify the packet integrity. The MAC is calculated from the symmetrical shared secret, the packet sequence number of the message, and the actual message content.

The MAC itself is sent outside of the symmetrically encrypted area as the final part of the packet. Researchers generally recommend this method of encrypting the data first, and then calculating the MAC.

How Does SSH Work?

You probably already have a basic understanding of how SSH works. The SSH protocol employs a client-server model to authenticate two parties and encrypt the data between them.

The server component listens on a designated port for connections. It is responsible for negotiating the secure connection, authenticating the connecting party, and spawning the correct environment if the credentials are accepted.

The client is responsible for beginning the initial TCP handshake with the server, negotiating the secure connection, verifying that the server's identity matches previously recorded information, and providing credentials to authenticate.

An SSH session is established in two separate stages. The first is to agree upon and establish encryption to protect future communication. The second stage is to authenticate the user and discover whether access to the server should be granted.

 首先，client端发起连接后，第一件事情是，client和server通过已知（公开的）的g和p两个大素数，然后通过自己的私有素数pc(client端)和ps(服务器端)，通过 Diffie-Hellman algorithm key exchange 算法，产生一个client端和server端都相同但却外界不可知的私有数据，作为该次会话session从头至尾的对称加密密钥，以后所有数据都使用该对称加密密钥，服务器端使用它加密数据，客户端使用它来解密，反之亦然。 

然后，在上一步后，有了相同的对称加密公共密钥之后，马上进行client的认证，可以通过账户、密码认证，也可以通过rsa的公私钥来认证，视情况而定。如果采用账户、密码认证，则整个ssh从发起连接开始，至始至终都和rsa非对称加密算法没什么关系，至始至终都派不上用场。

Diffie-Hellman algorithm key exchange 步骤中产生的key在该次会话session结束后，会立即销毁，下个会话再重新进行key exchange生成新key，因此做到了 forward secure。

The only thing the long-lasting keypair （也即private RSA SSH key for authentication）is used for is authentication（长期不变的keypair只是在认证的时候用一下）。

Negotiating Encryption for the Session（也叫 key exchange）

When a TCP connection is made by a client, the server responds with the protocol versions it supports. If the client can match one of the acceptable protocol versions, the connection continues. The server also provides its public host key, which the client can use to check whether this was the intended host.

At this point, both parties negotiate a session key using a version of something called the Diffie-Hellman algorithm. This algorithm (and its variants) make it possible for each party to combine their own private data with public data from the other system to arrive at an identical secret session key.

The session key will be used to encrypt the entire session. The public and private key pairs used for this part of the procedure are completely separate from the SSH keys used to authenticate a client to the server.

The basis of this procedure for classic Diffie-Hellman is:

1. Both parties agree on a large prime number（大素数p）, which will serve as a seed value.
2. Both parties agree on an encryption generator (typically AES)（大素数g）, which will be used to manipulate the values in a predefined way.
3. Independently, each party comes up with another prime number which is kept secret from the other party（上一篇文章的：A生成一个随机数a，a是保密的）. This number is used as the private key for this interaction (different than the private SSH key used for authentication，如果使用账户密码认证，则private SSH key for authentication至始至终都派不上用场).
4. The generated private key, the encryption generator, and the shared prime number are used to generate a public key that is derived from the private key, but which can be shared with the other party.
5. Both participants then exchange their generated public keys.   （就是通过g^a%p计算的）
6. The receiving entity uses their own private key, the other party's public key, and the original shared prime number to compute a shared secret key. Although this is independently computed by each party, using opposite private and public keys, it will result in the same shared secret key.
7. The shared secret is then used to encrypt all communication that follows.

The shared secret encryption that is used for the rest of the connection is called binary packet protocol. The above process allows each party to equally participate in generating the shared secret, which does not allow one end to control the secret. It also accomplishes the task of generating an identical shared secret without ever having to send that information over insecure channels.

The generated secret is a symmetric key, meaning that the same key used to encrypt a message can be used to decrypt it on the other side. The purpose of this is to wrap all further communication in an encrypted tunnel that cannot be deciphered by outsiders.

After the session encryption is established, the user authentication stage begins.

Authenticating the User's Access to the Server

The next stage involves authenticating the user and deciding access. There are a few different methods that can be used for authentication, based on what the server accepts.

The simplest is probably password authentication, in which the server simply prompts the client for the password of the account they are attempting to login with. The password is sent through the negotiated encryption, so it is secure from outside parties.

Even though the password will be encrypted, this method is not generally recommended due to the limitations on the complexity of the password. Automated scripts can break passwords of normal lengths very easily compared to other authentication methods.

The most popular and recommended alternative is the use of SSH key pairs. SSH key pairs are asymmetric keys, meaning that the two associated keys serve different functions.

The public key is used to encrypt data that can only be decrypted with the private key. The public key can be freely shared, because, although it can encrypt for the private key, there is no method of deriving the private key from the public key.

Authentication using SSH key pairs begins after the symmetric encryption has been established as described in the last section. The procedure happens like this:

1. The client begins by sending an ID for the key pair it would like to authenticate with to the server.
2. The server check's the 

   authorized_keys

    file of the account that the client is attempting to log into for the key ID.
3. If a public key with matching ID is found in the file, the server generates a random number and uses the public key to encrypt the number.
4. The server sends the client this encrypted message.
5. If the client actually has the associated private key, it will be able to decrypt the message using that key, revealing the original number.
6. The client combines the decrypted number with the shared session key that is being used to encrypt the communication, and calculates the MD5 hash of this value.
7. The client then sends this MD5 hash back to the server as an answer to the encrypted number message.
8. The server uses the same shared session key and the original number that it sent to the client to calculate the MD5 value on its own. It compares its own calculation to the one that the client sent back. If these two values match, it proves that the client was in possession of the private key and the client is authenticated.

As you can see, the asymmetry of the keys allows the server to encrypt messages to the client using the public key. The client can then prove that it holds the private key by decrypting the message correctly. The two types of encryption that are used (symmetric shared secret, and asymmetric public-private keys) are each able to leverage their specific strengths in this model.

Conclusion

Learning about the connection negotiation steps and the layers of encryption at work in SSH can help you better understand what is happening when you login to a remote server. Hopefully, you now have a better idea of relationship between various components and algorithms, and understand how all of these pieces fit together.

RSA, DSA和ECDSA的区别：

加密和签名是一回事，详见 这篇文章。

常见的非对称加密算法如下：

RSA：由 RSA 公司发明，是一个支持变长密钥的公共密钥算法，需要加密的文件块的长度也是可变的；

DSA（Digital Signature Algorithm）：数字签名算法，是一种标准的 DSS（数字签名标准）；

ECC（Elliptic Curves Cryptography）：椭圆曲线密码编码学。

ECDSA算法用于数字签名，是ECC与DSA的结合，整个签名过程与DSA类似，所不一样的是签名中采取的算法为ECC，最后签名出来的值也是分为r,s。

数字签名加密算法（RSA、DSA、ECDSA）分别的例子：

RSA的例子：

import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Signature;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import javax.crypto.Cipher;
import com.sun.org.apache.xerces.internal.impl.dv.util.Base64;


/**
 * 非对称加密算法
 * @author sunx
 *
 */
public class RSA {
    private static String src = "securtity RSA";

    public static void main(String[] args) {
        jdkRSA();
    }

    public static void jdkRSA() {
        try {
            //1、初始化密钥
            KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
            keyPairGenerator.initialize(512);
            KeyPair keyPair = keyPairGenerator.generateKeyPair();
            RSAPublicKey rsaPublicKey = (RSAPublicKey) keyPair.getPublic();// 甲方公钥
            RSAPrivateKey rsaPrivateKey = (RSAPrivateKey) keyPair.getPrivate();// 甲方私钥

            //2、执行签名
            PKCS8EncodedKeySpec pkcs8EncodedKeySpec = new PKCS8EncodedKeySpec(rsaPrivateKey.getEncoded());
            KeyFactory keyFactory = KeyFactory.getInstance("RSA");
            PrivateKey privateKey = keyFactory.generatePrivate(pkcs8EncodedKeySpec);// 生成私钥
            Signature signature = Signature.getInstance("MD5withRSA");
            signature.initSign(privateKey);
            signature.update(src.getBytes());
            byte[] result = signature.sign();// 得到签名
            System.out.println("jdk rsa sign:" + result.toString());

            //对明文加密
            String beforeText = encrypt(src, privateKey);
            System.out.println("解密前内容：" + beforeText);

            //3、验证签名
            X509EncodedKeySpec x509EncodedKeySpec = new X509EncodedKeySpec(rsaPublicKey.getEncoded());
            keyFactory = KeyFactory.getInstance("RSA");
            PublicKey publicKey = keyFactory.generatePublic(x509EncodedKeySpec);// 得到公钥
            signature = Signature.getInstance("MD5withRSA");
            signature.initVerify(publicKey);
            signature.update(src.getBytes());
            boolean bool = signature.verify(result);
            System.out.println(bool);

            //解密密文
            String afterText = decrypt(beforeText, publicKey);
            System.out.println("解密后内容：" + afterText);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 私钥对明文数据进行加密
     * @param plainText
     * @param privateKey
     * @return
     */
    public static String encrypt(String plainText, PrivateKey privateKey) {
        String result = null;
        try {
            Cipher cipher = Cipher.getInstance("RSA");
            cipher.init(Cipher.ENCRYPT_MODE, privateKey);
            byte[] output = cipher.doFinal(plainText.getBytes());
            result = Base64.encode(output);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return result;
    }

    /**
     * 公钥对密文数据进行解密
     * @param cipherText
     * @param PublicKey
     * @return
     */
    public static String decrypt(String cipherText, PublicKey publicKey) {
        String result = null;
        try {
            Cipher cipher = Cipher.getInstance("RSA");
            cipher.init(Cipher.DECRYPT_MODE, publicKey);
            result = new String(cipher.doFinal(Base64.decode(cipherText)));
        } catch (Exception e) {
            e.printStackTrace();
        }
        return result;

    }
}      

DSA的例子：

import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Signature;
import java.security.interfaces.DSAPrivateKey;
import java.security.interfaces.DSAPublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;


/**
 * DSA数字加密算法
 * @author sunx
 *
 */
public class DSA {

    private static String src = "securtity DSA";
    public static void main(String[] args) {
        jdkDSA();
    }

    public static void jdkDSA(){
        //1、初始化密钥
        try {
            KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("DSA");
            keyPairGenerator.initialize(512);
            KeyPair keyPair = keyPairGenerator.generateKeyPair();
            DSAPublicKey dsaPublicKey = (DSAPublicKey) keyPair.getPublic();
            DSAPrivateKey dsaPrivateKey = (DSAPrivateKey) keyPair.getPrivate();

            //2、执行签名
            PKCS8EncodedKeySpec pkcs8EncodedKeySpec = new PKCS8EncodedKeySpec(dsaPrivateKey.getEncoded());
            KeyFactory keyFactory = KeyFactory.getInstance("DSA");
            PrivateKey privateKey = keyFactory.generatePrivate(pkcs8EncodedKeySpec);
            Signature signature = Signature.getInstance("SHA1withDSA");
            signature.initSign(privateKey);
            signature.update(src.getBytes());
            byte[] result = signature.sign();
            System.out.println("jdk Dsa sign:" + result.toString());

            //3、验证签名
            X509EncodedKeySpec x509EncodedKeySpec = new X509EncodedKeySpec(dsaPublicKey.getEncoded());
            keyFactory = KeyFactory.getInstance("DSA");
            PublicKey publicKey = keyFactory.generatePublic(x509EncodedKeySpec);
            signature = Signature.getInstance("SHA1withDSA");
            signature.initVerify(publicKey);
            signature.update(src.getBytes());
            boolean bool = signature.verify(result);
            System.out.println(bool);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}      

ECDSA的例子：

import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Signature;
import java.security.interfaces.ECPrivateKey;
import java.security.interfaces.ECPublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

/**
 * 椭圆曲线数字签名算法
 * @author sunx
 *
 */
public class ECDSA {

    private static String src = "securtity ECDSA";

    public static void main(String[] args) {
        jdkECDSA();
    }

    public static void jdkECDSA(){
        //1、初始化密钥
        try {
            KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("EC");
            keyPairGenerator.initialize(256);
            KeyPair keyPair = keyPairGenerator.generateKeyPair();
            ECPublicKey ecPublicKey = (ECPublicKey) keyPair.getPublic();
            ECPrivateKey ecPrivateKey = (ECPrivateKey) keyPair.getPrivate();

            //2、执行签名
            PKCS8EncodedKeySpec pkcs8EncodedKeySpec = new PKCS8EncodedKeySpec(ecPrivateKey.getEncoded());
            KeyFactory keyFactory = KeyFactory.getInstance("EC");
            PrivateKey privateKey = keyFactory.generatePrivate(pkcs8EncodedKeySpec);
            Signature signature = Signature.getInstance("SHA1withECDSA");
            signature.initSign(privateKey);
            signature.update(src.getBytes());
            byte[] result = signature.sign();
            System.out.println("jdk ecdsa sign:" + result.toString());

            //3、验证签名
            X509EncodedKeySpec x509EncodedKeySpec = new X509EncodedKeySpec(ecPublicKey.getEncoded());
            keyFactory = KeyFactory.getInstance("EC");
            PublicKey publicKey = keyFactory.generatePublic(x509EncodedKeySpec);
            signature = Signature.getInstance("SHA1withECDSA");
            signature.initVerify(publicKey);
            signature.update(src.getBytes());
            boolean bool = signature.verify(result);
            System.out.println(bool);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}      

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