# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   cloud_trail_encryption_enabled_check
# 
# Description:
#   This control checks whether your AWS CloudTrail is configured to use the server-side encryption (SSE) AWS KMS key encryption.
# 
# Reports on:
#   AWS::CloudTrail::Trail
# 
# 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 AWS CloudTrail trails
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudTrail trail resource
#       And: 'KMSKeyId' is not present
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudTrail trail resource
#       And: 'KMSKeyId' has been provided and is set to an empty string or a non-valid local reference to a KMS Key or
#            Alias
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudTrail trail resource
#       And: 'KMSKeyId' has been provided and is a non-empty string or a valid reference to a KMS Key or Alias.
#      Then: PASS

#
# Constants
#
let CLOUDTRAIL_TRAIL_TYPE = "AWS::CloudTrail::Trail"
let INPUT_DOCUMENT = this

#
# Assignments
#
let cloudtrail_trails = Resources.*[ Type == %CLOUDTRAIL_TRAIL_TYPE ]

#
# Primary Rules
#
rule cloud_trail_encryption_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                               %cloudtrail_trails not empty {
    check_cloudtrail_kms_key_configuration(%cloudtrail_trails.Properties)
        <<
        [CT.CLOUDTRAIL.PR.1]: Require an AWS CloudTrail trail to have encryption at rest activated
        [FIX]: Set the 'KMSKeyId' property to a valid KMS key.
        >>
}

rule cloud_trail_encryption_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %CLOUDTRAIL_TRAIL_TYPE) {
    check_cloudtrail_kms_key_configuration(%INPUT_DOCUMENT.%CLOUDTRAIL_TRAIL_TYPE.resourceProperties)
        <<
        [CT.CLOUDTRAIL.PR.1]: Require an AWS CloudTrail trail to have encryption at rest activated
        [FIX]: Set the 'KMSKeyId' property to a valid KMS key.
        >>
}

#
# Parameterized Rules
#
rule check_cloudtrail_kms_key_configuration(cloudtrail_trail){
    %cloudtrail_trail {
        # Scenario 2
        KMSKeyId exists
        # Scenario 3 and 4
        check_is_string_and_not_empty(KMSKeyId) or
        check_kms_key_id_local_ref(KMSKeyId)
    }
}

rule check_kms_key_id_local_ref(key_ref) {
    %key_ref {
      check_local_references(%INPUT_DOCUMENT, this, "AWS::KMS::Key") or
      check_local_references(%INPUT_DOCUMENT, this, "AWS::KMS::Alias")
    }
 }

#
# Utility Rules
#
rule check_is_string_and_not_empty(value) {
    %value {
        this is_string
        this != /\A\s*\z/
    }
}

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
    }
}