Permission denied (publickey)

Question

ssh -i [full path to keypair file] ec2-user@[EC2 instance hostname or IP address]

I did this and it worked before and suddenly I am getting Permission denied (publickey) error.

mac-pro:aws me$ ssh -i key.pem ubuntu@elastic_ip_address -v
OpenSSH_5.6p1, OpenSSL 0.9.8r 8 Feb 2011
debug1: Reading configuration data /etc/ssh_config
debug1: Applying options for *
debug1: Connecting to elastic_ip_address [elastic_ip_address] port 22.
debug1: Connection established.
debug1: identity file key.pem type -1
debug1: identity file key.pem-cert type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.5p1 Debian-4ubuntu4
debug1: match: OpenSSH_5.5p1 Debian-4ubuntu4 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.6
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr 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 'elastic_ip_address' is known and matches the RSA host key.
debug1: Found key in /Users/me/.ssh/known_hosts:8
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
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: key.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).

Incidentally, I am using the same key for three different EC2 instances. I am not sure this is the reason why I am getting this error. I can access two other EC2 instances over ssh using the same key. BUT I can not access only one instance.

Answer 1

correct user? ec2-user@ or root@