AWS:Cloudformation : Authentication to AWS::CloudFormation::Init(AWS::CloudFormation::Authentication)

AWS::CloudFormation::Authentication

Use the AWS::CloudFormation::Authentication type to specify authentication credentials for files or sources that you specify with the AWS::CloudFormation::Init type.

{
   "Type" : "AWS::CloudFormation::Authentication" {
      "String" : {
         "accessKeyId" : String,
         "buckets" : [ String, ... ],
         "password" : String,
         "secretKey" : String,
         "type" : String,
         "uris" : [ String, ... ],
         "username" : String,
         "roleName" : String
      },
      ...
   }
}     

...
            "files" : {
              "/etc/chef/chef-validator.pem" : {
                "source" : "https://s3.amazonaws.com/chefServer/web-validator.pem",
                "authentication" : "S3AccessCreds"
              },

To download file from S3, you need authentication.

1. Using "AWS::IAM::AccessKey"

....
"HostKeys" : {
      "Type" : "AWS::IAM::AccessKey",
      "Properties" : {
        "UserName" : { "Ref": "User" }
     }
    },
-----

        "AWS::CloudFormation::Authentication" : {
          "S3AccessCreds" : {
            "type" : "S3",
            "accessKeyId" : { "Ref" : "HostKeys" },
            "secretKey" : {"Fn::GetAtt": ["HostKeys", "SecretAccessKey"]},
            "buckets" : [ "myBucket" ]
          }
        }

To get accesskey and secretyKey provided by "AWS::IAM::AccessKey"  Or   rolename

     2. Using rolename

 "AWS::CloudFormation::Authentication": {
        "default" : {
            "type": "s3",
            "buckets": [ "myBucket" ],
            "roleName": { "Ref": "myRole" }
        }
    }

Reference:

1. http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-authentication.html#cfn-cloudformation-authentication-rolename
2. http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-authentication.html