A key pair is used to access your instances via SSH. Key pairs are an integral part of asymmetrical cryptography and are an encryption technique that makes use of two non-identical keys to encrypt and decrypt data. A user's public key can be shared freely, and data that is encrypted with the public key can only be decrypted with the private key. When you create a key pair with EC2, the key can be associated with any number of instances, and you download a .pem file, which is similar to a .ppk file in Putty or an id_rsa file on Linux machines.