我按照以下步骤解决了这个问题：

步骤1：检查SSH密钥

$ cd ~/.ssh
# Checks to see if there is a directory named ".ssh" in your user directory
# If it says "No such file or directory" skip to step 3. Otherwise continue to step 2.

步骤2：备份和删除现有SSH密钥

$ ls
# Lists all the subdirectories in the current directory
# config  id_rsa  id_rsa.pub  known_hosts

$ mkdir key_backup
# Makes a subdirectory called "key_backup" in the current directory

$ cp id_rsa* key_backup
# Copies the id_rsa keypair into key_backup

$ rm id_rsa*
# Deletes the id_rsa keypair

步骤3*生成一个新的SSH密钥

$ ssh-keygen -t rsa -C "your_email@youremail.com"
# Creates a new ssh key using the provided email

# Generating public/private rsa key pair.
# Enter file in which to save the key (/home/you/.ssh/id_rsa):    
# Enter passphrase (empty for no passphrase): [Type a passphrase]
# Enter same passphrase again: [Type passphrase again]    
# Your identification has been saved in /home/you/.ssh/id_rsa.
# Your public key has been saved in /home/you/.ssh/id_rsa.pub.
# The key fingerprint is:
# 01:0f:f4:3b:ca:85:d6:17:a1:7d:f0:68:9d:f0:a2:db your_email@youremail.com

步骤4：：将SSH密钥添加到GitHub

$ sudo apt-get install xclip
# Downloads and installs xclip

$ xclip -sel clip < ~/.ssh/id_rsa.pub
# Copies the contents of the id_rsa.pub file to your clipboard

然后，到histub去做：

1. 转到帐户设置
2. 单击左侧边栏中的“SSH键”
3. 单击“添加SSH键”
4. 将你的密钥粘贴到“键”字段中。
5. 单击“添加键”
6. 通过输入GitHub密码确认操作

步骤5：：把所有的东西都测试出来

$ ssh -T git@github.com
# Attempts to ssh to github

如果没问题，你会发现

Hi username! You've successfully authenticated, but GitHub does not
# provide shell access.

否则(在我身上)，你会看到

Agent admitted failure to sign using the key.
# debug1: No more authentication methods to try.
# Permission denied (publickey).

解决这个问题

$ ssh-add
# Enter passphrase for /home/you/.ssh/id_rsa: [tippy tap]
# Identity added: /home/you/.ssh/id_rsa (/home/you/.ssh/id_rsa)

如果已经在~/.ssh中有一个公钥(并且已经将该密钥添加到您的GitHub帐户)，可能只需要再次将你的密钥加载到SSH代理中。

若要测试ssh代理是否具有密钥，请键入ssh-add -l如果结果是：

The agent has no identities.

然后简单地将你的密钥加载到SSH代理中，如下所示：

ssh-add ~/.ssh/github_rsa

(GitHub)_RSA是我的机器上存储的SSH密钥的名称。除其他外，此文件还可以命名为：id_(RSA)

在此之后，你必须输入密码作为密钥(这可能是登录到GitHub的密码)。如果你收到这样的信息：

Identity added: /Users/name/.ssh/github_rsa (/Users/cpotzinger/.ssh/github_rsa)