# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   cloudfront_viewer_policy_https_check
# 
# Description:
#   This control checks whether your Amazon CloudFront distributions use HTTPS, either directly or through a redirection.
# 
# Reports on:
#   AWS::CloudFront::Distribution
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document does not contain any CloudFront distribution resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'DistributionConfig.DefaultCacheBehavior' is missing on the CloudFront distribution resource
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'DistributionConfig.DefaultCacheBehavior' is present on the CloudFront distribution resource
#       And: 'ViewerProtocolPolicy' in 'DefaultCacheBehavior' is missing or set to a value other than 'https-only' or
#            'redirect-to-https' (e.g. 'allow-all')
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'DistributionConfig.CacheBehavior' is provided on the CloudFront distribution resource
#       And: 'ViewerProtocolPolicy' in the 'CacheBehavior' is  is missing or set to a value other than 'https-only' or
#            'redirect-to-https' (e.g. 'allow-all')
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'DistributionConfig.DefaultCacheBehavior' is present on the CloudFront distribution resource
#       And: 'ViewerProtocolPolicy' in 'DefaultCacheBehavior' is set to 'https-only' or 'redirect-to-https'
#      Then: PASS
#   Scenario: 6
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'DistributionConfig.CacheBehavior' are provided on the CloudFront distribution resource as a non-empty list
#       And: 'ViewerProtocolPolicy' in the 'CacheBehavior' is set to 'https-only' or 'redirect-to-https'
#      Then: PASS

#
# Constants
#
let CLOUDFRONT_DISTRIBUTION_TYPE = "AWS::CloudFront::Distribution"
let ALLOWED_VIEWER_PROTOCOL_POLICIES = [ "https-only", "redirect-to-https" ]
let INPUT_DOCUMENT = this

#
# Assignments
#
let cloudfront_distributions = Resources.*[ Type == %CLOUDFRONT_DISTRIBUTION_TYPE ]

#
# Primary Rules
#
rule cloudfront_viewer_policy_https_check when is_cfn_template(%INPUT_DOCUMENT)
                                               %cloudfront_distributions not empty {
    check(%cloudfront_distributions.Properties)
        <<
        [CT.CLOUDFRONT.PR.3]: Require an Amazon CloudFront distribution to have encryption in transit configured
        [FIX]: Set 'ViewerProtocolPolicy' in 'DefaultCacheBehavior' and 'CacheBehavior' to 'https-only' or 'redirect-to-https'.
        >>
}

rule cloudfront_viewer_policy_https_check when is_cfn_hook(%INPUT_DOCUMENT, %CLOUDFRONT_DISTRIBUTION_TYPE) {
    check(%INPUT_DOCUMENT.%CLOUDFRONT_DISTRIBUTION_TYPE.resourceProperties)
        <<
        [CT.CLOUDFRONT.PR.3]: Require an Amazon CloudFront distribution to have encryption in transit configured
        [FIX]: Set 'ViewerProtocolPolicy' in 'DefaultCacheBehavior' and 'CacheBehavior' to 'https-only' or 'redirect-to-https'.
        >>
}

#
# Parameterized Rules
#
rule check(cloudfront_distribution) {
    %cloudfront_distribution {
        DistributionConfig exists
        DistributionConfig is_struct

        DistributionConfig {
            DefaultCacheBehavior exists
            DefaultCacheBehavior is_struct

            DefaultCacheBehavior {
                # Scenarios 2 and 4
                check_viewer_protocol_policy(this)
            }

            when CacheBehaviors exists
                 CacheBehaviors is_list
                 CacheBehaviors not empty {

                    CacheBehaviors[*] {
                        # Scenarios 3 and 5
                        check_viewer_protocol_policy(this)
                    }
            }
        }
    }
}

rule check_viewer_protocol_policy(cache_behaviour) {
    %cache_behaviour {
        ViewerProtocolPolicy exists
        ViewerProtocolPolicy in %ALLOWED_VIEWER_PROTOCOL_POLICIES
    }
}

#
# Utility Rules
#
rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists  or
        Resources exists
    }
}

rule is_cfn_hook(doc, RESOURCE_TYPE) {
    %doc.%RESOURCE_TYPE.resourceProperties exists
}