#
#####################################
##           Gherkin               ##
#####################################
# Rule Identifier:
#    S3_BUCKET_SERVER_SIDE_ENCRYPTION_ENABLED
#
# Description:
#   Checks if your Amazon S3 bucket either has the Amazon S3 default encryption enabled or that the Amazon S3 bucket policy
#   explicitly denies put-object requests without server side encryption that uses AES-256 or AWS Key Management Service.
#
# Reports on:
#    AWS::S3::Bucket
#
# Evaluates:
#    AWS CloudFormation
#
# Rule Parameters:
#    NA
#
# Scenarios:
# a) SKIP: when there are no S3 resource present
# b) PASS: when all S3 resources Bucket Encryption ServerSideEncryptionByDefault is set to either "aws:kms" or "AES256"
# c) FAIL: when all S3 resources have Bucket Encryption ServerSideEncryptionByDefault is not set or does not have "aws:kms" or "AES256" configurations
# d) SKIP: when metadata includes the suppression for rule S3_BUCKET_SERVER_SIDE_ENCRYPTION_ENABLED

#
# Select all S3 resources from incoming template (payload)
#

let s3_buckets_server_side_encryption = Resources.*[ Type == 'AWS::S3::Bucket'
  Metadata.guard.SuppressedRules not exists or
  Metadata.guard.SuppressedRules.* != "S3_BUCKET_SERVER_SIDE_ENCRYPTION_ENABLED"
]

rule S3_BUCKET_SERVER_SIDE_ENCRYPTION_ENABLED when %s3_buckets_server_side_encryption !empty {
  %s3_buckets_server_side_encryption.Properties.BucketEncryption exists
  %s3_buckets_server_side_encryption.Properties.BucketEncryption.ServerSideEncryptionConfiguration[*].ServerSideEncryptionByDefault.SSEAlgorithm in ["aws:kms","AES256"]
  <<
    Violation: S3 Bucket must enable server-side encryption.
    Fix: Set the S3 Bucket property BucketEncryption.ServerSideEncryptionConfiguration.ServerSideEncryptionByDefault.SSEAlgorithm to either "aws:kms" or "AES256"
  >>
}