/*
Copyright CAcert Inc.
SPDX-License-Identifier: Apache-2.0

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 revoking takes care of handling certificate revocation requests.
package revoking

import (
	"crypto"
	"crypto/rand"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/asn1"
	"fmt"
	"math/big"
	"strings"
	"time"

	"github.com/balacode/go-delta"
	"github.com/sirupsen/logrus"
)

var OidCRLReason = asn1.ObjectIdentifier{2, 5, 29, 21}

const defaultCRLValidity = 24 * time.Hour

type CRLReason int

// CRL reason codes as defined in RFC 5280 section 5.3.1
const (
	CRLReasonUnspecified          CRLReason = 0
	CRLReasonKeyCompromise        CRLReason = 1
	CRLReasonCACompromise         CRLReason = 2
	CRLReasonAffiliationChanged   CRLReason = 3
	CRLReasonSuperseded           CRLReason = 4
	CRLReasonCessationOfOperation CRLReason = 5
	CRLReasonCertificateHold      CRLReason = 6
	CRLReasonRemoveFromCRL        CRLReason = 8
	CRLReasonPrivilegeWithdrawn   CRLReason = 9
	CRLReasonAACompromise         CRLReason = 10
)

var crlReasonNames = map[CRLReason]string{
	CRLReasonUnspecified:          "unspecified",
	CRLReasonKeyCompromise:        "keyCompromise",
	CRLReasonCACompromise:         "CACompromise",
	CRLReasonAffiliationChanged:   "affiliationChanged",
	CRLReasonSuperseded:           "superseded",
	CRLReasonCessationOfOperation: "cessationOfOperation",
	CRLReasonCertificateHold:      "certificateHold",
	CRLReasonRemoveFromCRL:        "removeFromCRL",
	CRLReasonPrivilegeWithdrawn:   "privilegeWithdrawn",
	CRLReasonAACompromise:         "AACompromise",
}

func (r CRLReason) String() string {
	if reason, ok := crlReasonNames[r]; ok {
		return reason
	}

	return crlReasonNames[CRLReasonUnspecified]
}

func (r CRLReason) BuildExtension() pkix.Extension {
	extBytes, _ := asn1.Marshal(r)

	return pkix.Extension{Id: OidCRLReason, Value: extBytes}
}

// ParseReason takes a reason string and performs a case-insensitive match to a reason code
func ParseReason(rs string) CRLReason {
	for key, name := range crlReasonNames {
		if strings.EqualFold(name, rs) {
			return key
		}
	}

	return CRLReasonUnspecified
}

type X509Revoking struct {
	repository   Repository
	crlAlgorithm x509.SignatureAlgorithm
	crlIssuer    *x509.Certificate
	signer       crypto.Signer
	logger       *logrus.Logger
}

type RevokeCertificate struct {
	serialNumber *big.Int
	reason       CRLReason
}

func NewRevokeCertificate(serialNumber *big.Int, reason CRLReason) *RevokeCertificate {
	return &RevokeCertificate{
		serialNumber: serialNumber,
		reason:       reason,
	}
}

type CRLInformation struct {
	CRL        []byte // DER encoded CRL
	Number     *big.Int
	NextUpdate time.Time
}

func (r *X509Revoking) Revoke(revokeCertificate *RevokeCertificate) (*x509.RevocationListEntry, error) {
	revoked := &x509.RevocationListEntry{
		SerialNumber:   revokeCertificate.serialNumber,
		RevocationTime: time.Now(),
		ReasonCode:     int(revokeCertificate.reason),
	}

	if err := r.repository.StoreRevocation(revoked); err != nil {
		return nil, fmt.Errorf("could not store revocation %w", err)
	}

	return revoked, nil
}

func (r *X509Revoking) createCRL() (*CRLInformation, error) {
	revoked, err := r.repository.RevokedCertificates()
	if err != nil {
		return nil, fmt.Errorf("could not get revocation information: %w", err)
	}

	nextNumber, err := r.repository.NextCRLNumber()
	if err != nil {
		return nil, fmt.Errorf("could not get next CRL number: %w", err)
	}

	nextUpdate := time.Now().UTC().Add(defaultCRLValidity)

	list, err := x509.CreateRevocationList(rand.Reader, &x509.RevocationList{
		SignatureAlgorithm:        r.crlAlgorithm,
		RevokedCertificateEntries: revoked,
		Number:                    nextNumber,
		ThisUpdate:                time.Now().UTC(),
		NextUpdate:                nextUpdate,
	}, r.crlIssuer, r.signer)
	if err != nil {
		return nil, fmt.Errorf("could not sign revocation list: %w", err)
	}

	err = r.repository.StoreCRL(nextNumber, list)
	if err != nil {
		return nil, fmt.Errorf("could not store new CRL: %w", err)
	}

	return &CRLInformation{CRL: list, Number: nextNumber, NextUpdate: nextUpdate}, nil
}

func (r *X509Revoking) GetCurrentCRL() (*CRLInformation, error) {
	return r.GetCRL(nil)
}

const maximumCRLLifetime = 6 * time.Hour

func (r *X509Revoking) GetCRL(number *big.Int) (*CRLInformation, error) {
	var (
		crl  []byte
		err  error
		list *x509.RevocationList
	)

	crl, err = r.repository.LoadCRL(number)
	if err != nil {
		r.logger.WithError(err).Warn("could not load CRL")

		if number != nil {
			return nil, fmt.Errorf("could not load crl number 0x%0x", number)
		}
	}

	if crl != nil {
		list, err = x509.ParseRevocationList(crl)
		if err != nil {
			r.logger.WithError(err).Warn("could not parse CRL")
		}

		if list != nil {
			stillValid := list.ThisUpdate.Add(maximumCRLLifetime).After(time.Now().UTC())

			if number != nil || stillValid {
				return &CRLInformation{CRL: crl, Number: list.Number, NextUpdate: list.NextUpdate}, nil
			}
		}
	}

	return r.createCRL()
}

func NewX509Revoking(
	repo Repository,
	crlAlgorithm x509.SignatureAlgorithm,
	issuer *x509.Certificate,
	signer crypto.Signer,
	logger *logrus.Logger,
) *X509Revoking {
	return &X509Revoking{
		repository:   repo,
		crlAlgorithm: crlAlgorithm,
		crlIssuer:    issuer,
		signer:       signer,
		logger:       logger,
	}
}

type Result struct {
	CRLData []byte
	IsDelta bool
	Number  *big.Int
}

type FetchCRLHandler struct {
	repositories map[string]*X509Revoking
}

func NewFetchCRLHandler(repositories map[string]*X509Revoking) *FetchCRLHandler {
	return &FetchCRLHandler{repositories: repositories}
}

func (h *FetchCRLHandler) FetchCRL(issuerID string, crlNumber *big.Int) (*Result, error) {
	var (
		currentCRL, oldCRL *CRLInformation
		err                error
	)

	repo, ok := h.repositories[issuerID]
	if !ok {
		return nil, fmt.Errorf("unknown issuer ID %s", issuerID)
	}

	if repo.signer == nil {
		return nil, fmt.Errorf("key for issuer ID %s is not available", issuerID)
	}

	currentCRL, err = repo.GetCurrentCRL()
	if err != nil {
		return nil, fmt.Errorf("could not get CRL for issuer ID %s: %w", issuerID, err)
	}

	if crlNumber == nil {
		return &Result{CRLData: currentCRL.CRL, Number: currentCRL.Number, IsDelta: false}, nil
	}

	if crlNumber.Cmp(currentCRL.Number) == 0 {
		return &Result{CRLData: nil, Number: currentCRL.Number, IsDelta: false}, nil
	}

	oldCRL, err = repo.GetCRL(crlNumber)
	if err != nil {
		return &Result{CRLData: currentCRL.CRL, Number: currentCRL.Number, IsDelta: false}, nil
	}

	diff := delta.Make(oldCRL.CRL, currentCRL.CRL)
	patchBytes := diff.Bytes()

	if len(patchBytes) > len(currentCRL.CRL) {
		return &Result{CRLData: currentCRL.CRL, Number: currentCRL.Number, IsDelta: false}, nil
	}

	return &Result{CRLData: patchBytes, Number: currentCRL.Number, IsDelta: true}, nil
}