GitHub introduced a change last week that drops support for weak cryptographic keys. This has resulted in connectivity issues for some users who are connecting to GitHub using SSH and the built-in client SSH Executable.
ssh-keygen.exe -t rsa -C "メールアドレス"
Generating public/private rsa key pair.
Enter file in which to save the key (/c/Users/username/.ssh/id_rsa): #左のパスで良ければ[Enter]。
Created directory '/c/Users/username/.ssh'.
Enter passphrase (empty for no passphrase): #パスフレーズを打つ
Enter same passphrase again: #もう一度確認でパスフレーズ
eval `ssh-agent` ssh-add ~/.ssh/id_rsa
Enter passphrase for /c/Users/username/.ssh/id_rsa: #パスフレーズを入れる
Identity added: /c/Users/username/.ssh/id_rsa (/c/Users/username/.ssh/id_rsa)