by 
Each AMI publisher on EC2 decides what user should have ssh access enabled by default and what ssh credentials should allow you to gain access as that user.
For the second part, most AMIs allow you to ssh into the system with the ssh keypair you specified at launch time. This is so common, users often assume that it is built into EC2 even though it must be enabled by each AMI provider.
Unfortunately, there is no standard ssh username that is used to access EC2 instances across operating systems, distros, and AMI providers.
Default ssh Usernames For AWS EC2:
Here are some of the ssh usernames that I am aware of at this time:
| OS | Username |
| Amazon Linux | ec2-user |
| Ubuntu | ubuntu |
| Debian | admin |
| RHEL 6.4 and later | ec2-user |
| RHEL 6.3 and earlier | root |
| Fedora | fedora |
| Centos | centos |
| SUSE | root |
| Bitnami | bitnami |
| Turnkey | root |
| NanoStack | ubuntu |
| FreeBSD | ec2-user |
| OmniOS | root |
Even though the above list will get you in to most official AMIs, there may still be situations where you aren’t quite sure how the AMI was built or what user should be used for ssh.
Also, read How to Resize EBS Volumes with Linux EC2 Instances in AWS
