[cs615asa] An error has occurred when I try to connect to ec2 instance

[zwang60]

Hello, Everyone.

I have encountered a problem when use linux-lab to connect to 
ec2-instance. The details are as follows:

1. here is my aws configure information:
AWS Access Key ID [****************IDSA]:
AWS Secret Access Key [****************u0wO]:
Default region name [us-east-1]:
Default output format [json]:
2. create an instance use image-id: ami-3b361952
zwang60 at avatar:~$ aws ec2 run-instances --image-id ami-3b361952 --count 
1 --instance-type t1.micro --key-name mykeypair --security-groups 
MySecurityGroup
{
     "OwnerId": "916872542441",
     "ReservationId": "r-a95aac89",
     "Groups": [
         {
             "GroupName": "MySecurityGroup",
             "GroupId": "sg-521ef538"
         }
     ],
     "Instances": [
         {
             "Monitoring": {
                 "State": "disabled"
             },
             "PublicDnsName": null,
             "KernelId": "aki-407d9529",
             "State": {
                 "Code": 0,
                 "Name": "pending"
             },
             "EbsOptimized": false,
             "LaunchTime": "2014-04-14T15:49:13.000Z",
             "ProductCodes": [],
             "StateTransitionReason": null,
             "InstanceId": "i-68201938",
             "ImageId": "ami-3b361952",
             "PrivateDnsName": null,
             "KeyName": "mykeypair",
             "SecurityGroups": [
                 {
                     "GroupName": "MySecurityGroup",
                     "GroupId": "sg-521ef538"
                 }
             ],
             "ClientToken": null,
             "InstanceType": "t1.micro",
             "NetworkInterfaces": [],
             "Placement": {
                 "Tenancy": "default",
                 "GroupName": null,
                 "AvailabilityZone": "us-east-1c"
             },
             "Hypervisor": "xen",
             "BlockDeviceMappings": [],
             "Architecture": "i386",
             "StateReason": {
                 "Message": "pending",
                 "Code": "pending"
             },
             "RootDeviceName": "/dev/sda1",
             "VirtualizationType": "paravirtual",
             "RootDeviceType": "ebs",
             "AmiLaunchIndex": 0
         }
     ]
}
3. ssh to this instance:
zwang60 at avatar:~$ ssh -i mykeypair.pem 
ec2-user at ec2-54-81-79-177.compute-1.amazonaws.com
The authenticity of host 'ec2-54-81-79-177.compute-1.amazonaws.com 
(54.81.79.177)' can't be established.
RSA key fingerprint is 3a:86:5a:d0:51:ed:72:df:5e:e9:9c:c5:69:25:19:05.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 
'ec2-54-81-79-177.compute-1.amazonaws.com,54.81.79.177' (RSA) to the 
list of known hosts.
Permission denied (publickey,gssapi-keyex,gssapi-with-mic).
zwang60 at avatar:~$ ssh -i mykeypair.pem 
root at ec2-54-81-79-177.compute-1.amazonaws.com
Permission denied (publickey,gssapi-keyex,gssapi-with-mic).
zwang60 at avatar:~$ ssh -i mykeypair.pem 
ec2-user at ec2-54-81-79-177.compute-1.amazonaws.com
Permission denied (publickey,gssapi-keyex,gssapi-with-mic).
zwang60 at avatar:~$ ssh -i mykeypair.pem 
fedora at ec2-54-81-79-177.compute-1.amazonaws.com
Permission denied (publickey,gssapi-keyex,gssapi-with-mic).




What may cause this happen and is there any solution to  this problem?
Looking for your help to solve this problem.Thank you.
Zhe