And now I want to talk about how you can authenticate to newly created instances. And the process is slightly different for Linux and Windows Operating Systems. But the underlying method is fundamentally the same through the use of key pairs. So, when you create an EC2 instance, you are asked to select an existing or create an EC2 key pair. So, what is a key pair and what does it do?

A key pair as the name implies is made up of two components: a public key and a private key. These keys can either be ED25519 or 2048-bit SSH-2 RSA keys. However, please note that ED25519 keys are not supported for Windows instances, EC2 Instance Connect, or EC2 Serial Console. The function of key pairs is to encrypt the login information for Linux and Windows EC2 instances and then decrypt the same information allowing you to authenticate onto the instance. 

The public key uses public key cryptography to encrypt data such as the username and password. For Windows instances, the private key is used to decrypt the administrative password allowing you to gain access to the login credentials including the password using RDP. For Linux instances, the private key is used to SSH onto the instance, and the public key is held and kept by AWS. The private key is our responsibility to keep and to ensure that it is not lost. 

So, going back to when you create your EC2 instance, you're given the opportunity to download the key pair in the form of a pem file. Once you've done this, you must keep that file safe until you're ready to log into the associated instance. It's worth noting that you can use the same key pair on multiple instances. To understand the differences between connecting to a Windows instance and a Linux instance, please take a look at the following two labs where you will be shown how to create an instance and then connect to them using the key pairs.