如何通过 ssh 连接到 AWS 实例？

我有：

1. 在 AWS 注册；
2. 在 AWS 网站上创建公钥和证书并将它们保存到磁盘；

3. 转到我的控制台并创建环境变量：

   $ export JAVA_HOME=/usr/lib/jvm/java-6-openjdk/
   

   $ export EC2_CERT=/home/default/aws/cert-EBAINCRNWHDSCWWIHSOKON2YWGJZ5LSQ.pem
   $ export EC2_PRIVATE_KEY=/home/default/aws/pk-EBAINCRNWHDSCWWIHSOKON2YWGJZ5LSQ.pem

4. 告诉 AWS API 使用此密钥对并将密钥对保存到文件中：

   $ ec2-add-keypair ec2-keypair > ec2-keypair.pem
   

5. 使用此密钥对启动了 AWS Ubuntu 9 实例：

   $ ec2-run-instances ami-ed46a784 -k ec2-keypair
   

6. 尝试建立与实例的 ssh 连接：

   $ ssh -v -i ec2-keypair.pem ubuntu@ec2-174-129-185-190.compute-1.amazonaws.com
   

   OpenSSH_5.1p1 Debian-5ubuntu1, OpenSSL 0.9.8g 19 Oct 2007
   debug1: Reading configuration data /etc/ssh/ssh_config
   debug1: Applying options for * debug1: Connecting to ec2-174-129-185-190.compute-1.amazonaws.com [174.129.185.190] port 22.
   debug1: Connection established.
   debug1: identity file ec2-keypair.pem type -1
   debug1: Remote protocol version 2.0, remote software version OpenSSH_5.1p1 Debian-5ubuntu1
   debug1: match: OpenSSH_5.1p1 Debian-5ubuntu1 pat OpenSSH*
   debug1: Enabling compatibility mode for protocol 2.0
   debug1: Local version string SSH-2.0-OpenSSH_5.1p1 Debian-5ubuntu1
   debug1: SSH2_MSG_KEXINIT sent
   debug1: SSH2_MSG_KEXINIT received
   debug1: kex: server->client aes128-cbc hmac-md5 none
   debug1: kex: client->server aes128-cbc hmac-md5 none
   debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
   debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
   debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
   debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
   debug1: Host 'ec2-174-129-185-190.compute-1.amazonaws.com‘ is known and matches the RSA host key.
   debug1: Found key in /home/default/.ssh/known_hosts:11
   debug1: ssh_rsa_verify: signature correct
   debug1: SSH2_MSG_NEWKEYS sent
   debug1: expecting SSH2_MSG_NEWKEYS
   debug1: SSH2_MSG_NEWKEYS received
   debug1: SSH2_MSG_SERVICE_REQUEST sent
   debug1: SSH2_MSG_SERVICE_ACCEPT received
   debug1: Authentications that can continue: publickey
   debug1: Next authentication method: publickey
   debug1: Trying private key: ec2-keypair.pem
   debug1: read PEM private key done: type RSA
   debug1: Authentications that can continue: publickey
   debug1: No more authentication methods to try.
   Permission denied (publickey).

可能是什么问题以及如何使其发挥作用？