最经典的非对称加密算法:RSA 算法。

Hash 算法

input: 需要 hash 的对象

output: 一个唯一的确定的字符串 ID

比较经典的 Hash 算法:SHA-256(生成一个 64 位 16 进制字符串), SHA-1



SSH key

ssh key


Ssh-keygen is a tool for creating new authentication key pairs for SSH. Such key pairs are used for automating logins, single sign-on, and for authenticating hosts.

SSH Keys and Public Key Authentication

The SSH protocol uses public key cryptography for authenticating hosts and users. The authentication keys, called SSH keys, are created using the keygen program.

SSH introduced public key authentication as a more secure alternative to the older .rhosts authentication. It improved security by avoiding the need to have password stored in files, and eliminated the possibility of a compromised server stealing the user's password.

However, SSH keys are authentication credentials just like passwords. Thus, they must be managed somewhat analogously(类似) to user names and passwords. They should have a proper termination process so that keys are removed when no longer needed.

Creating an SSH Key Pair for User Authentication

The simplest way to generate a key pair is to run ssh-keygen without arguments. In this case, it will prompt(提示) for the file in which to store keys. Here's an example:

klar (11:39) ~>ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/ylo/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/ylo/.ssh/id_rsa.
Your public key has been saved in /home/ylo/.ssh/
The key fingerprint is:
SHA256:Up6KjbnEV4Hgfo75YM393QdQsK3Z0aTNBz0DoirrW+c [email protected]
The key's randomart image is:
+---[RSA 2048]----+
| . ..oo..|
| . . . . .o.X.|
| . . o. ..+ B|
| . o.o .+ ..|
| ..o.S o.. |
| . %o= . |
| @.B... . |
| o.=. o. . . .|
| .oo E. . .. |
klar (11:40) ~>

First, the tool asked where to save the file. SSH keys for user authentication are usually stored in the user's .ssh directory under the home directory. However, in enterprise environments, the location is often different. The default key file name depends on the algorithm, in this case id_rsa when using the default RSA algorithm. It could also be, for example, id_dsa or id_ecdsa.

Then it asks to enter a passphrase. The passphrase(密码短语) is used for encrypting the key, so that it cannot be used even if someone obtains the private key file. The passphrase should be cryptographically strong. Our online random password generator is one possible tool for generating strong passphrases.

Choosing an Algorithm and Key Size

SSH supports several public key algorithms for authentication keys. These include:

  • rsa - an old algorithm based on the difficulty of factoring large numbers. A key size of at least 2048 bits is recommended for RSA; 4096 bits is better. RSA is getting old and significant advances are being made in factoring. Choosing a different algorithm may be advisable. It is quite possible the RSA algorithm will become practically breakable in the foreseeable future. All SSH clients support this algorithm.

  • dsa - an old US government Digital Signature Algorithm. It is based on the difficulty of computing discrete logarithms. A key size of 1024 would normally be used with it. DSA in its original form is no longer recommended.

  • ecdsa - a new Digital Signature Algorithm standarized by the US government, using elliptic curves. This is probably a good algorithm for current applications. Only three key sizes are supported: 256, 384, and 521 (sic!) bits. We would recommend always using it with 521 bits, since the keys are still small and probably more secure than the smaller keys (even though they should be safe as well). Most SSH clients now support this algorithm.

  • ed25519 - this is a new algorithm added in OpenSSH. Support for it in clients is not yet universal. Thus its use in general purpose applications may not yet be advisable.

The algorithm is selected using the -t option and key size using the -b option. The following commands illustrate:

ssh-keygen -t rsa -b 4096
ssh-keygen -t dsa
ssh-keygen -t ecdsa -b 521
ssh-keygen -t ed25519

Specifying the File Name

Normally, the tool prompts for the file in which to store the key. However, it can also be specified on the command line using the -f <filename> option.

ssh-keygen -f ~/tatu-key-ecdsa -t ecdsa -b 521

Copying the Public Key to the Server

创建公钥后,把公钥配置到 GitHub 服务器上,就可以访问 GitHub。

To use public key authentication, the public key must be copied to a server and installed in an authorized_keys file. This can be conveniently done using the ssh-copy-id tool. Like this:

ssh-copy-id -i ~/.ssh/tatu-key-ecdsa [email protected]

Once the public key has been configured on the server, the server will allow any connecting user that has the private key to log in. During the login process, the client proves possession of the private key by digitally signing the key exchange.


SSH provides a secure channel over an unsecured network by using a client–server architecture, connecting an SSH client application with an SSH server.

ssh 是一个远程登陆服务器的命令:


Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
Please contact your system administrator.
Add correct host key in /Users/isaacalves/.ssh/known_hosts to get rid of this message.
Offending RSA key in /Users/isaacalves/.ssh/known_hosts:12
RSA host key for has changed and you have requested strict checking.
Host key verification failed.



ssh-keygen -R remote-IP # Remove all keys belonging to a hostname from a known_hosts file.

记得改之前备份 know_hosts 文件: ~/.ssh/known_hosts

可能的原因:服务器中的配置文件中,IP 地址和公钥是配对的,现在 IP 地址没改,你公钥改了(可能是换了一台客户端),那么就无法访问服务器了。