# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   kms_key_rotation_enabled_check
# 
# Description:
#   This control checks whether key rotation is enabled for AWS KMS customer-managed keys.
# 
# Reports on:
#   AWS::KMS::Key
# 
# 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 KMS key resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a KMS key resource
#       And: 'KeySpec' is provided and is a value other than 'SYMMETRIC_DEFAULT'
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a KMS key resource
#       And: 'KeySpec' is not provided or is provided and is set to 'SYMMETRIC_DEFAULT'
#       And: 'EnableKeyRotation' is not provided
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a KMS key resource
#       And: 'KeySpec' is not provided or is provided and is set to 'SYMMETRIC_DEFAULT'
#       And: 'EnableKeyRotation' is provided and is 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 a KMS key resource
#       And: 'KeySpec' is not provided or is provided and is set to 'SYMMETRIC_DEFAULT'
#       And: 'EnableKeyRotation' is provided and is set to bool(true)
#      Then: PASS

#
# Constants
#
let KMS_KEY_TYPE = "AWS::KMS::Key"
let INPUT_DOCUMENT = this

#
# Assignments
#
let kms_keys = Resources.*[ Type == %KMS_KEY_TYPE ]

#
# Primary Rules
#
rule kms_key_rotation_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                         %kms_keys not empty {
    check(%kms_keys.Properties)
        <<
        [CT.KMS.PR.1]: Require any AWS KMS key to have rotation configured
        [FIX]: Set 'EnableKeyRotation' to 'true' for AWS KMS symmetric-encryption keys.
        >>
}

rule kms_key_rotation_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %KMS_KEY_TYPE) {
    check(%INPUT_DOCUMENT.%KMS_KEY_TYPE.resourceProperties)
        <<
        [CT.KMS.PR.1]: Require any AWS KMS key to have rotation configured
        [FIX]: Set 'EnableKeyRotation' to 'true' for AWS KMS symmetric-encryption keys.
        >>
}

#
# Parameterized Rules
#
rule check(kms_keys) {
    %kms_keys[
        # Scenario 2
        filter_is_kms_cmk_symmetric_key(this)
    ] {
        # Scenario 3, 4 and 5
        EnableKeyRotation exists
        EnableKeyRotation == true
    }
}

rule filter_is_kms_cmk_symmetric_key(kms_key) {
    %kms_key {
        KeySpec not exists or
        KeySpec == "SYMMETRIC_DEFAULT"
    }
}

#
# 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
}