/*
 * MinIO Cloud Storage, (C) 2020 MinIO, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package cmd

import (
	"encoding/xml"
	"errors"
	"io"
)

const (
	// AES256 is used with SSE-S3
	AES256 SSEAlgorithm = "AES256"
	// AWSKms is used with SSE-KMS
	AWSKms SSEAlgorithm = "aws:kms"
)

// SSEAlgorithm - represents valid SSE algorithms supported; currently only AES256 is supported
type SSEAlgorithm string

// UnmarshalXML - Unmarshals XML tag to valid SSE algorithm
func (alg *SSEAlgorithm) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	var s string
	if err := d.DecodeElement(&s, &start); err != nil {
		return err
	}

	switch s {
	case string(AES256):
		*alg = AES256
	case string(AWSKms):
		*alg = AWSKms
	default:
		return errors.New("Unknown SSE algorithm")
	}

	return nil
}

// MarshalXML - Marshals given SSE algorithm to valid XML
func (alg *SSEAlgorithm) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
	return e.EncodeElement(string(*alg), start)
}

// EncryptionAction - for ApplyServerSideEncryptionByDefault XML tag
type EncryptionAction struct {
	Algorithm   SSEAlgorithm `xml:"SSEAlgorithm,omitempty"`
	MasterKeyID string       `xml:"KMSMasterKeyID,omitempty"`
}

// SSERule - for ServerSideEncryptionConfiguration XML tag
type SSERule struct {
	DefaultEncryptionAction EncryptionAction `xml:"ApplyServerSideEncryptionByDefault"`
}

const xmlNS = "http://s3.amazonaws.com/doc/2006-03-01/"

// BucketSSEConfig - represents default bucket encryption configuration
type BucketSSEConfig struct {
	XMLNS   string    `xml:"xmlns,attr,omitempty"`
	XMLName xml.Name  `xml:"ServerSideEncryptionConfiguration"`
	Rules   []SSERule `xml:"Rule"`
}

// ParseBucketSSEConfig - Decodes given XML to a valid default bucket encryption config
func ParseBucketSSEConfig(r io.Reader) (*BucketSSEConfig, error) {
	var config BucketSSEConfig
	err := xml.NewDecoder(r).Decode(&config)
	if err != nil {
		return nil, err
	}

	// Validates server-side encryption config rules
	// Only one rule is allowed on AWS S3
	if len(config.Rules) != 1 {
		return nil, errors.New("Only one server-side encryption rule is allowed")
	}

	for _, rule := range config.Rules {
		switch rule.DefaultEncryptionAction.Algorithm {
		case AES256:
			if rule.DefaultEncryptionAction.MasterKeyID != "" {
				return nil, errors.New("MasterKeyID is allowed with aws:kms only")
			}
		case AWSKms:
			if rule.DefaultEncryptionAction.MasterKeyID == "" {
				return nil, errors.New("MasterKeyID is missing")
			}
		}
	}

	if config.XMLNS == "" {
		config.XMLNS = xmlNS
	}

	return &config, nil
}