// Copyright 2015 Matthew Holt and The Caddy 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
//
//     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 caddypki

import (
	"bytes"
	"crypto"
	"crypto/ecdsa"
	"crypto/ed25519"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"errors"
	"fmt"
	"os"

	"github.com/caddyserver/certmagic"
	"go.step.sm/crypto/pemutil"
)

func pemDecodeCertificate(pemDER []byte) (*x509.Certificate, error) {
	pemBlock, remaining := pem.Decode(pemDER)
	if pemBlock == nil {
		return nil, fmt.Errorf("no PEM block found")
	}
	if len(remaining) > 0 {
		return nil, fmt.Errorf("input contained more than a single PEM block")
	}
	if pemBlock.Type != "CERTIFICATE" {
		return nil, fmt.Errorf("expected PEM block type to be CERTIFICATE, but got '%s'", pemBlock.Type)
	}
	return x509.ParseCertificate(pemBlock.Bytes)
}

func pemDecodeCertificateChain(pemDER []byte) ([]*x509.Certificate, error) {
	chain, err := pemutil.ParseCertificateBundle(pemDER)
	if err != nil {
		return nil, fmt.Errorf("failed parsing certificate chain: %w", err)
	}

	return chain, nil
}

func pemEncodeCert(der []byte) ([]byte, error) {
	return pemEncode("CERTIFICATE", der)
}

func pemEncode(blockType string, b []byte) ([]byte, error) {
	var buf bytes.Buffer
	err := pem.Encode(&buf, &pem.Block{Type: blockType, Bytes: b})
	return buf.Bytes(), err
}

func trusted(cert *x509.Certificate) bool {
	chains, err := cert.Verify(x509.VerifyOptions{})
	return len(chains) > 0 && err == nil
}

// KeyPair represents a public-private key pair, where the
// public key is also called a certificate.
type KeyPair struct {
	// The certificate. By default, this should be the path to
	// a PEM file unless format is something else.
	Certificate string `json:"certificate,omitempty"`

	// The private key. By default, this should be the path to
	// a PEM file unless format is something else.
	PrivateKey string `json:"private_key,omitempty"` //nolint:gosec // false positive: yes it's exported, since it needs to encode/decode as JSON; and is often just a filepath

	// The format in which the certificate and private
	// key are provided. Default: pem_file
	Format string `json:"format,omitempty"`
}

// Load loads the certificate chain and (optional) private key from
// the corresponding files, using the configured format. If a
// private key is read, it will be verified to belong to the first
// certificate in the chain.
func (kp KeyPair) Load() ([]*x509.Certificate, crypto.Signer, error) {
	switch kp.Format {
	case "", "pem_file":
		certData, err := os.ReadFile(kp.Certificate)
		if err != nil {
			return nil, nil, err
		}
		chain, err := pemDecodeCertificateChain(certData)
		if err != nil {
			return nil, nil, err
		}

		var key crypto.Signer
		if kp.PrivateKey != "" {
			keyData, err := os.ReadFile(kp.PrivateKey)
			if err != nil {
				return nil, nil, err
			}
			key, err = certmagic.PEMDecodePrivateKey(keyData)
			if err != nil {
				return nil, nil, err
			}
			if err := verifyKeysMatch(chain[0], key); err != nil {
				return nil, nil, err
			}
		}

		return chain, key, nil

	default:
		return nil, nil, fmt.Errorf("unsupported format: %s", kp.Format)
	}
}

// verifyKeysMatch verifies that the public key in the [x509.Certificate] matches
// the public key of the [crypto.Signer].
func verifyKeysMatch(crt *x509.Certificate, signer crypto.Signer) error {
	switch pub := crt.PublicKey.(type) {
	case *rsa.PublicKey:
		pk, ok := signer.Public().(*rsa.PublicKey)
		if !ok {
			return fmt.Errorf("private key type %T does not match issuer public key type %T", signer.Public(), pub)
		}
		if !pub.Equal(pk) {
			return errors.New("private key does not match issuer public key")
		}
	case *ecdsa.PublicKey:
		pk, ok := signer.Public().(*ecdsa.PublicKey)
		if !ok {
			return fmt.Errorf("private key type %T does not match issuer public key type %T", signer.Public(), pub)
		}
		if !pub.Equal(pk) {
			return errors.New("private key does not match issuer public key")
		}
	case ed25519.PublicKey:
		pk, ok := signer.Public().(ed25519.PublicKey)
		if !ok {
			return fmt.Errorf("private key type %T does not match issuer public key type %T", signer.Public(), pub)
		}
		if !pub.Equal(pk) {
			return errors.New("private key does not match issuer public key")
		}
	default:
		return fmt.Errorf("unsupported key type: %T", pub)
	}

	return nil
}