# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   cloudwatch_log_group_encrypted_check
# 
# Description:
#   This control checks whether an Amazon CloudWatch log group is encrypted at rest with an AWS KMS key
# 
# Reports on:
#   AWS::Logs::LogGroup
# 
# 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 CloudWatch log group resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudWatch log group resource
#       And: 'KmsKeyId' has not been provided
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudWatch log group resource
#       And: 'KmsKeyId' has been provided as an empty string or invalid local reference to a KMS Key
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudWatch log group resource
#       And: 'KmsKeyId' has been provided as a non-empty string or valid local reference to a KMS Key
#      Then: PASS

#
# Constants
#
let CLOUDWATCH_LOGS_TYPE = "AWS::Logs::LogGroup"
let INPUT_DOCUMENT = this

#
# Assignments
#
let cloudwatch_log_groups = Resources.*[ Type == %CLOUDWATCH_LOGS_TYPE ]

#
# Primary Rules
#
rule cloudwatch_log_group_encrypted_check when is_cfn_template(%INPUT_DOCUMENT)
                                               %cloudwatch_log_groups not empty {
    check(%cloudwatch_log_groups.Properties)
        <<
        [CT.CLOUDWATCH.PR.3]: Require an Amazon CloudWatch log group to be encrypted at rest with an AWS KMS key
        [FIX]: Set 'KmsKeyId' to the ARN of an AWS KMS customer managed key configured with permissions that allow the CloudWatch service principal to use the key.
        >>
}

rule cloudwatch_log_group_encrypted_check when is_cfn_hook(%INPUT_DOCUMENT, %CLOUDWATCH_LOGS_TYPE) {
    check(%INPUT_DOCUMENT.%CLOUDWATCH_LOGS_TYPE.resourceProperties)
        <<
        [CT.CLOUDWATCH.PR.3]: Require an Amazon CloudWatch log group to be encrypted at rest with an AWS KMS key
        [FIX]: Set 'KmsKeyId' to the ARN of an AWS KMS customer managed key configured with permissions that allow the CloudWatch service principal to use the key.
        >>
}

#
# Parameterized Rules
#
rule check(cloudwatch_log_group){
    %cloudwatch_log_group {
        # Scenario 2
        KmsKeyId exists
        # Scenario 3 and 4
        check_is_string_and_not_empty(KmsKeyId) or
        check_local_references(%INPUT_DOCUMENT, KmsKeyId, "AWS::KMS::Key")
    }
}

#
# 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_is_string_and_not_empty(value) {
    %value {
        this is_string
        this != /\A\s*\z/
    }
}

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, resource_type) {
    let referenced_resource = %doc.Resources[ keys == %resource_key ]
    %referenced_resource not empty
    %referenced_resource {
        Type == %resource_type
    }
}