# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   rds_cluster_storage_encrypted_check
# 
# Description:
#   This control checks whether the storage encryption is configured on Amazon Relational Database Service (RDS) database (DB) clusters that are not being restored from an existing cluster.
# 
# Reports on:
#   AWS::RDS::DBCluster
# 
# 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 RDS DB cluster resources
#       Then: SKIP
#   Scenario: 2
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an RDS DB cluster resource
#        And: 'SourceDBClusterIdentifier' or 'ReplicationSourceIdentifier' has been provided
#       Then: SKIP
#   Scenario: 3
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an RDS DB cluster resource
#        And: 'SourceDBClusterIdentifier' or 'ReplicationSourceIdentifier' has not been provided
#        And: 'StorageEncrypted' has not been provided
#       Then: FAIL
#   Scenario: 4
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an RDS DB cluster resource
#        And: 'SourceDBClusterIdentifier' or 'ReplicationSourceIdentifier' has not been provided
#        And: 'StorageEncrypted' has been provided and set to bool(false)
#       Then: FAIL
#   Scenario: 5
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an RDS DB cluster resource
#        And: 'SourceDBClusterIdentifier' or 'ReplicationSourceIdentifier' has not been provided
#        And: 'StorageEncrypted' has been provided and set to bool(true)
#       Then: PASS

#
# Constants
#
let RDS_CLUSTER_TYPE = "AWS::RDS::DBCluster"
let INPUT_DOCUMENT = this

#
# Assignments
#
let rds_cluster = Resources.*[ Type == %RDS_CLUSTER_TYPE ]

#
# Primary Rules
#
rule rds_cluster_storage_encrypted_check when is_cfn_template(%INPUT_DOCUMENT)
                                              %rds_cluster not empty {
    check(%rds_cluster.Properties)
        <<
        [CT.RDS.PR.16]: Require an Amazon RDS database cluster to have encryption at rest configured
        [FIX]: Set 'StorageEncrypted' to 'true'.
        >>
}

rule rds_cluster_storage_encrypted_check when is_cfn_hook(%INPUT_DOCUMENT, %RDS_CLUSTER_TYPE) {
    check(%INPUT_DOCUMENT.%RDS_CLUSTER_TYPE.resourceProperties)
        <<
        [CT.RDS.PR.16]: Require an Amazon RDS database cluster to have encryption at rest configured
        [FIX]: Set 'StorageEncrypted' to 'true'.
        >>
}

#
# Parameterized Rules
#
rule check(rds_cluster) {
    %rds_cluster [
        # Scenario 2
        filter_sources(this)
    ] {
        # Scenario 3
        StorageEncrypted exists

        # Scenario 4 and 5
        StorageEncrypted == true
    }
}

rule filter_sources(rds_cluster) {
    %rds_cluster {
        # Scenario 2
        SourceDBClusterIdentifier not exists or
        filter_property_is_empty_string(SourceDBClusterIdentifier) or
        filter_is_not_valid_local_reference(%INPUT_DOCUMENT, SourceDBClusterIdentifier, "AWS::RDS::DBCluster")

        ReplicationSourceIdentifier not exists or
        filter_property_is_empty_string(ReplicationSourceIdentifier) or
        filter_replication_source_identifier(ReplicationSourceIdentifier)
    }
}

rule filter_property_is_empty_string(value) {
    %value {
        this is_string
        this == /\A\s*\z/
    }
}

rule filter_is_not_valid_local_reference(doc, reference_properties, referenced_resource_type) {
    %reference_properties {
        this not is_string
        this is_struct

        when this.'Ref' exists {
            'Ref' {
                when query_for_resource(%doc, this, %referenced_resource_type) {
                    this not exists
                }
                this exists
            }
        }
        when this.'Ref' not exists {
            this exists
        }
    }
}

rule filter_replication_source_identifier(reference_properties) {
    filter_is_not_valid_local_reference_via_join(%INPUT_DOCUMENT, %reference_properties, "AWS::RDS::DBCluster")
    filter_is_not_valid_local_reference_via_join(%INPUT_DOCUMENT, %reference_properties, "AWS::RDS::DBInstance")
}

rule filter_is_not_valid_local_reference_via_join(doc, reference_properties, referenced_resource_type) {
    %reference_properties {
        this not is_string
        this is_struct

        when this.'Fn::Join' exists {
            'Fn::Join' {
                when filter_list_contains_valid_local_reference(this[1], %doc, %referenced_resource_type) {
                    this not exists
                }
                this exists
            }
        }
        when this.'Fn::Join' not exists {
            this exists
        }
    }
}

rule filter_list_contains_valid_local_reference(list, doc, referenced_resource_type) {
     some %list.* {
         check_local_references(%doc, this, %referenced_resource_type)
     }
 }

#
# 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
}

rule check_local_references(doc, reference_properties, referenced_resource_type) {
    %reference_properties {
        'Fn::GetAtt' {
            query_for_resource(%doc, this[0], %referenced_resource_type)
                <<Local Stack reference was invalid>>
        } or Ref {
            query_for_resource(%doc, this, %referenced_resource_type)
                <<Local Stack reference was invalid>>
        }
    }
}

rule query_for_resource(doc, resource_key, referenced_resource_type) {
    let referenced_resource = %doc.Resources[ keys == %resource_key ]
    %referenced_resource not empty
    %referenced_resource {
        Type == %referenced_resource_type
    }
}