SSHユーザーのキー認証

シナリオ：サーバーにSSHを必要とするユーザーが複数います。私が従うものはSSHユーザーのキー認証

AWS EC2で動作するサーバ（Ubuntu 14.04 Server）にuser1、user2を作成します。
サーバー/home/$USER/.ssh/authorized_keysファイルへ

を各ユーザの公開鍵（id_rsa.pub）を追加します。問題は、私が唯一のユーザの秘密鍵でubuntu（デフォルトユーザー）へのSSHことができます。しかし、私はuser1ユーザーにSSHできません。

サンプル

[[email protected] Downloads]# ssh -i ~/.ssh/id_rsa [email protected] -v 
OpenSSH_7.5p1, OpenSSL 1.1.0e 16 Feb 2017 
debug1: Reading configuration data /etc/ssh/ssh_config 
debug1: Connecting to DummyIP [DummyIP] port 22. 
debug1: Connection established. 
debug1: permanently_set_uid: 0/0 
debug1: identity file /root/.ssh/id_rsa type 1 
debug1: key_load_public: No such file or directory 
debug1: identity file /root/.ssh/id_rsa-cert type -1 
debug1: Enabling compatibility mode for protocol 2.0 
debug1: Local version string SSH-2.0-OpenSSH_7.5 
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.8 
debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.8 pat OpenSSH_6.6.1* compat 0x04000000 
debug1: Authenticating to DummyIP:22 as 'ruwan' 
debug1: SSH2_MSG_KEXINIT sent 
debug1: SSH2_MSG_KEXINIT received 
debug1: kex: algorithm: [email protected] 
debug1: kex: host key algorithm: ecdsa-sha2-nistp256 
debug1: kex: server->client cipher: [email protected] MAC: <implicit> compression: none 
debug1: kex: client->server cipher: [email protected] MAC: <implicit> compression: none 
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY 
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:0kqNAt37ILO4cCP8ioiLN9j9x9badLOGL+etIvSYzfs 
debug1: Host 'DummyIP' is known and matches the ECDSA host key. 
debug1: Found key in /root/.ssh/known_hosts:2 
debug1: rekey after 134217728 blocks 
debug1: SSH2_MSG_NEWKEYS sent 
debug1: expecting SSH2_MSG_NEWKEYS 
debug1: SSH2_MSG_NEWKEYS received 
debug1: rekey after 134217728 blocks 
debug1: SSH2_MSG_SERVICE_ACCEPT received 
debug1: Authentications that can continue: publickey 
debug1: Next authentication method: publickey 
debug1: Offering RSA public key: /root/.ssh/id_rsa 
debug1: Authentications that can continue: publickey 
debug1: No more authentication methods to try. 
Permission denied (publickey).

任意の詳細がrequeiredなら、私に知らせてください。

出典

2017-06-05 Ruwan Ranganath