/*
Copyright © The ESO Authors

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

    https://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 providercerts manages TLS certificates for external secrets providers.
package providercerts

import (
	"context"
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"fmt"
	"math/big"
	"slices"
	"time"

	corev1 "k8s.io/api/core/v1"
	apierrors "k8s.io/apimachinery/pkg/api/errors"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/types"
)

const (
	caCertName     = "ca.crt"
	caKeyName      = "ca.key"
	certName       = "tls.crt"
	keyName        = "tls.key"
	clientCertName = "client.crt"
	clientKeyName  = "client.key"

	// we're using a fixed secret name for the provider certificates
	// otherwise we would need to introduce a lot of complexity to the core controller
	// because we would have to maintain a mapping of provider names (aws) to provider kinds (parameterstore, generators etc.)
	providerSecretName = "external-secrets-provider-tls"

	// certValidityDuration is the validity period for generated certificates (10 years).
	certValidityDuration = 87600 * time.Hour

	errGeneratePrivateKey = "failed to generate private key: %w"
)

// ProviderCertConfig defines configuration for a single provider's certificates.
type ProviderCertConfig struct {
	Namespace    string   // Namespace where provider is deployed
	ServiceNames []string // Service names for DNS SANs
}

// ProviderCertificates holds the generated certificates for a provider.
type ProviderCertificates struct {
	ServerCert []byte
	ServerKey  []byte
	ClientCert []byte
	ClientKey  []byte
}

// KeyPairArtifacts holds a certificate and private key.
type KeyPairArtifacts struct {
	Cert    *x509.Certificate
	Key     *rsa.PrivateKey
	CertPEM []byte
	KeyPEM  []byte
}

// ReconcileProviderCert ensures provider certificates exist and are valid.
func (r *ProviderCertReconciler) ReconcileProviderCert(ctx context.Context, config *ProviderCertConfig) error {
	if config == nil {
		return nil
	}
	log := r.Log.WithValues("secret", providerSecretName, "namespace", config.Namespace)

	// Get or create the secret
	secretName := types.NamespacedName{
		Name:      providerSecretName,
		Namespace: config.Namespace,
	}

	var secret corev1.Secret
	err := r.Get(ctx, secretName, &secret)
	if err != nil {
		if !apierrors.IsNotFound(err) {
			return fmt.Errorf("failed to get provider secret: %w", err)
		}

		// Secret doesn't exist, create it
		log.Info("creating provider certificate secret")
		secret = corev1.Secret{
			ObjectMeta: metav1.ObjectMeta{
				Name:      providerSecretName,
				Namespace: config.Namespace,
				Labels: map[string]string{
					"app.kubernetes.io/managed-by": "external-secrets",
					"app.kubernetes.io/component":  "provider-certificates",
				},
			},
			Type: corev1.SecretTypeTLS,
		}
	}

	// Check if certificates need refresh
	needRefresh := r.needProviderCertRefresh(&secret, config)

	if needRefresh {
		log.Info("refreshing provider certificates")
		if err := r.refreshProviderCerts(&secret, config); err != nil {
			return fmt.Errorf("failed to refresh provider certificates: %w", err)
		}

		// Create or update the secret
		if secret.UID == "" {
			if err := r.Create(ctx, &secret); err != nil {
				return fmt.Errorf("failed to create provider secret: %w", err)
			}
			log.Info("created provider certificate secret")
		} else {
			if err := r.Update(ctx, &secret); err != nil {
				return fmt.Errorf("failed to update provider secret: %w", err)
			}
			log.Info("updated provider certificate secret")
		}
	}

	return nil
}

// needProviderCertRefresh checks if provider certificates need to be refreshed.
func (r *ProviderCertReconciler) needProviderCertRefresh(secret *corev1.Secret, config *ProviderCertConfig) bool {
	// If secret has no data, we need to generate certificates
	if secret.Data == nil {
		return true
	}

	// Check if all required keys exist
	requiredKeys := []string{caCertName, caKeyName, certName, keyName, clientCertName, clientKeyName}
	for _, key := range requiredKeys {
		if _, ok := secret.Data[key]; !ok {
			return true
		}
	}

	// Validate CA certificate
	if !r.validCACert(secret.Data[caCertName], secret.Data[caKeyName]) {
		return true
	}

	// Validate server certificate
	dnsNames := r.getProviderDNSNames(config)
	if !r.validProviderCert(secret.Data[caCertName], secret.Data[certName], secret.Data[keyName], dnsNames) {
		return true
	}

	// Validate client certificate
	if !r.validProviderClientCert(secret.Data[caCertName], secret.Data[clientCertName], secret.Data[clientKeyName]) {
		return true
	}

	return false
}

// refreshProviderCerts generates new certificates for the provider.
func (r *ProviderCertReconciler) refreshProviderCerts(secret *corev1.Secret, config *ProviderCertConfig) error {
	now := time.Now()
	begin := now.Add(-1 * time.Hour)
	end := now.Add(certValidityDuration)

	// Check if we need to generate a new CA or reuse existing
	var caArtifacts *KeyPairArtifacts
	var err error

	if secret.Data != nil && r.validCACert(secret.Data[caCertName], secret.Data[caKeyName]) {
		// Reuse existing CA
		caArtifacts, err = buildArtifactsFromSecret(secret)
		if err != nil {
			return fmt.Errorf("failed to load existing CA: %w", err)
		}
	} else {
		// Generate new CA
		caArtifacts, err = r.createCACert(begin, end)
		if err != nil {
			return fmt.Errorf("failed to create CA certificate: %w", err)
		}
	}

	// Generate server certificate
	serverCert, serverKey, err := r.createProviderServerCert(caArtifacts, config, begin, end)
	if err != nil {
		return fmt.Errorf("failed to create server certificate: %w", err)
	}

	// Generate client certificate
	clientCert, clientKey, err := r.createProviderClientCert(caArtifacts, begin, end)
	if err != nil {
		return fmt.Errorf("failed to create client certificate: %w", err)
	}

	// Populate secret
	if secret.Data == nil {
		secret.Data = make(map[string][]byte)
	}
	secret.Data[caCertName] = caArtifacts.CertPEM
	secret.Data[caKeyName] = caArtifacts.KeyPEM
	secret.Data[certName] = serverCert
	secret.Data[keyName] = serverKey
	secret.Data[clientCertName] = clientCert
	secret.Data[clientKeyName] = clientKey

	return nil
}

// createCACert creates a new CA certificate.
func (r *ProviderCertReconciler) createCACert(begin, end time.Time) (*KeyPairArtifacts, error) {
	template := &x509.Certificate{
		SerialNumber: big.NewInt(0),
		Subject: pkix.Name{
			CommonName:   r.CAName,
			Organization: []string{r.CAOrganization},
		},
		DNSNames:              []string{r.CAName},
		NotBefore:             begin,
		NotAfter:              end,
		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment | x509.KeyUsageCertSign,
		BasicConstraintsValid: true,
		IsCA:                  true,
	}

	key, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		return nil, fmt.Errorf(errGeneratePrivateKey, err)
	}

	certDER, err := x509.CreateCertificate(rand.Reader, template, template, &key.PublicKey, key)
	if err != nil {
		return nil, fmt.Errorf("failed to create certificate: %w", err)
	}

	cert, err := x509.ParseCertificate(certDER)
	if err != nil {
		return nil, fmt.Errorf("failed to parse certificate: %w", err)
	}

	certPEM := pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: certDER,
	})

	keyPEM := pem.EncodeToMemory(&pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(key),
	})

	return &KeyPairArtifacts{
		Cert:    cert,
		Key:     key,
		CertPEM: certPEM,
		KeyPEM:  keyPEM,
	}, nil
}

// createProviderServerCert generates a server certificate for the provider.
func (r *ProviderCertReconciler) createProviderServerCert(
	caArtifacts *KeyPairArtifacts,
	config *ProviderCertConfig,
	begin, end time.Time,
) ([]byte, []byte, error) {
	dnsNames := r.getProviderDNSNames(config)

	// Create certificate template
	template := &x509.Certificate{
		SerialNumber: big.NewInt(time.Now().UnixNano()),
		Subject: pkix.Name{
			CommonName:   "external-secrets-provider",
			Organization: []string{r.CAOrganization},
		},
		DNSNames:    dnsNames,
		NotBefore:   begin,
		NotAfter:    end,
		KeyUsage:    x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
	}

	// Generate private key
	key, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		return nil, nil, fmt.Errorf(errGeneratePrivateKey, err)
	}

	// Create certificate
	certDER, err := x509.CreateCertificate(rand.Reader, template, caArtifacts.Cert, &key.PublicKey, caArtifacts.Key)
	if err != nil {
		return nil, nil, fmt.Errorf("failed to create certificate: %w", err)
	}

	// Encode certificate
	certPEM := pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: certDER,
	})

	// Encode private key
	keyPEM := pem.EncodeToMemory(&pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(key),
	})

	return certPEM, keyPEM, nil
}

// createProviderClientCert generates a client certificate for the ESO controller.
func (r *ProviderCertReconciler) createProviderClientCert(
	caArtifacts *KeyPairArtifacts,
	begin, end time.Time,
) ([]byte, []byte, error) {
	// Create certificate template
	template := &x509.Certificate{
		SerialNumber: big.NewInt(time.Now().UnixNano()),
		Subject: pkix.Name{
			CommonName:   "external-secrets-controller",
			Organization: []string{r.CAOrganization},
		},
		NotBefore:   begin,
		NotAfter:    end,
		KeyUsage:    x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
	}

	// Generate private key
	key, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		return nil, nil, fmt.Errorf(errGeneratePrivateKey, err)
	}

	// Create certificate
	certDER, err := x509.CreateCertificate(rand.Reader, template, caArtifacts.Cert, &key.PublicKey, caArtifacts.Key)
	if err != nil {
		return nil, nil, fmt.Errorf("failed to create certificate: %w", err)
	}

	// Encode certificate
	certPEM := pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: certDER,
	})

	// Encode private key
	keyPEM := pem.EncodeToMemory(&pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(key),
	})

	return certPEM, keyPEM, nil
}

// getProviderDNSNames returns the DNS names for the provider service.
func (r *ProviderCertReconciler) getProviderDNSNames(config *ProviderCertConfig) []string {
	dnsNames := make([]string, 0, len(config.ServiceNames)*4)
	for _, serviceName := range config.ServiceNames {
		dnsNames = append(dnsNames,
			serviceName,
			fmt.Sprintf("%s.%s", serviceName, config.Namespace),
			fmt.Sprintf("%s.%s.svc", serviceName, config.Namespace),
			fmt.Sprintf("%s.%s.svc.cluster.local", serviceName, config.Namespace),
		)
	}
	return dnsNames
}

// validCACert validates a CA certificate.
func (r *ProviderCertReconciler) validCACert(caCert, caKey []byte) bool {
	if len(caCert) == 0 || len(caKey) == 0 {
		return false
	}

	// Parse CA certificate
	caDer, _ := pem.Decode(caCert)
	if caDer == nil {
		return false
	}
	caCertParsed, err := x509.ParseCertificate(caDer.Bytes)
	if err != nil {
		return false
	}

	// Check if CA is still valid with lookahead
	if time.Now().After(lookaheadTime()) && lookaheadTime().After(caCertParsed.NotAfter) {
		return false
	}

	return true
}

// validProviderCert validates a provider server certificate.
func (r *ProviderCertReconciler) validProviderCert(caCert, cert, key []byte, dnsNames []string) bool {
	if len(caCert) == 0 || len(cert) == 0 || len(key) == 0 {
		return false
	}

	// Parse CA certificate
	pool := x509.NewCertPool()
	caDer, _ := pem.Decode(caCert)
	if caDer == nil {
		return false
	}
	caCertParsed, err := x509.ParseCertificate(caDer.Bytes)
	if err != nil {
		return false
	}
	pool.AddCert(caCertParsed)

	// Parse server certificate
	certDer, _ := pem.Decode(cert)
	if certDer == nil {
		return false
	}
	certParsed, err := x509.ParseCertificate(certDer.Bytes)
	if err != nil {
		return false
	}

	// Verify certificate is signed by CA
	opts := x509.VerifyOptions{
		Roots:       pool,
		CurrentTime: lookaheadTime(),
		KeyUsages:   []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
	}

	if _, err := certParsed.Verify(opts); err != nil {
		return false
	}

	// Check DNS names are present
	for _, dnsName := range dnsNames {
		if !slices.Contains(certParsed.DNSNames, dnsName) {
			return false
		}
	}

	return true
}

// validProviderClientCert validates a provider client certificate.
func (r *ProviderCertReconciler) validProviderClientCert(caCert, cert, key []byte) bool {
	if len(caCert) == 0 || len(cert) == 0 || len(key) == 0 {
		return false
	}

	// Parse CA certificate
	pool := x509.NewCertPool()
	caDer, _ := pem.Decode(caCert)
	if caDer == nil {
		return false
	}
	caCertParsed, err := x509.ParseCertificate(caDer.Bytes)
	if err != nil {
		return false
	}
	pool.AddCert(caCertParsed)

	// Parse client certificate
	certDer, _ := pem.Decode(cert)
	if certDer == nil {
		return false
	}
	certParsed, err := x509.ParseCertificate(certDer.Bytes)
	if err != nil {
		return false
	}

	// Verify certificate is signed by CA
	opts := x509.VerifyOptions{
		Roots:       pool,
		CurrentTime: lookaheadTime(),
		KeyUsages:   []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
	}

	if _, err := certParsed.Verify(opts); err != nil {
		return false
	}

	return true
}

// buildArtifactsFromSecret builds KeyPairArtifacts from a secret.
func buildArtifactsFromSecret(secret *corev1.Secret) (*KeyPairArtifacts, error) {
	caCertPEM := secret.Data[caCertName]
	caKeyPEM := secret.Data[caKeyName]

	caDer, _ := pem.Decode(caCertPEM)
	if caDer == nil {
		return nil, fmt.Errorf("failed to decode CA certificate")
	}
	caCert, err := x509.ParseCertificate(caDer.Bytes)
	if err != nil {
		return nil, fmt.Errorf("failed to parse CA certificate: %w", err)
	}

	keyDer, _ := pem.Decode(caKeyPEM)
	if keyDer == nil {
		return nil, fmt.Errorf("failed to decode CA key")
	}
	caKey, err := x509.ParsePKCS1PrivateKey(keyDer.Bytes)
	if err != nil {
		return nil, fmt.Errorf("failed to parse CA key: %w", err)
	}

	return &KeyPairArtifacts{
		Cert:    caCert,
		Key:     caKey,
		CertPEM: caCertPEM,
		KeyPEM:  caKeyPEM,
	}, nil
}

// lookaheadTime returns the current time plus a lookahead duration
// to ensure certificates are refreshed before expiration.
func lookaheadTime() time.Time {
	return time.Now().Add(365 * 24 * time.Hour) // 1 year lookahead
}