/*
Copyright 2021 The Kubernetes 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 certs

// NB(directxman12): nothing has verified that this has good settings.  In fact,
// the setting generated here are probably terrible, but they're fine for integration
// tests.  These ABSOLUTELY SHOULD NOT ever be exposed in the public API.  They're
// ONLY for use with envtest's ability to configure webhook testing.
// If I didn't otherwise not want to add a dependency on cfssl, I'd just use that.

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/elliptic"
	crand "crypto/rand"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"fmt"
	"math/big"
	"net"
	"time"

	certutil "k8s.io/client-go/util/cert"
)

var (
	ellipticCurve = elliptic.P256()
	bigOne        = big.NewInt(1)
)

// CertPair is a private key and certificate for use for client auth, as a CA, or serving.
type CertPair struct {
	Key  crypto.Signer
	Cert *x509.Certificate
}

// CertBytes returns the PEM-encoded version of the certificate for this pair.
func (k CertPair) CertBytes() []byte {
	return pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: k.Cert.Raw,
	})
}

// AsBytes encodes keypair in the appropriate formats for on-disk storage (PEM and
// PKCS8, respectively).
func (k CertPair) AsBytes() (cert []byte, key []byte, err error) {
	cert = k.CertBytes()

	rawKeyData, err := x509.MarshalPKCS8PrivateKey(k.Key)
	if err != nil {
		return nil, nil, fmt.Errorf("unable to encode private key: %w", err)
	}

	key = pem.EncodeToMemory(&pem.Block{
		Type:  "PRIVATE KEY",
		Bytes: rawKeyData,
	})

	return cert, key, nil
}

// TinyCA supports signing serving certs and client-certs,
// and can be used as an auth mechanism with envtest.
type TinyCA struct {
	CA      CertPair
	orgName string

	nextSerial *big.Int
}

// newPrivateKey generates a new private key of a relatively sane size (see
// rsaKeySize).
func newPrivateKey() (crypto.Signer, error) {
	return ecdsa.GenerateKey(ellipticCurve, crand.Reader)
}

// NewTinyCA creates a new a tiny CA utility for provisioning serving certs and client certs FOR TESTING ONLY.
// Don't use this for anything else!
func NewTinyCA() (*TinyCA, error) {
	caPrivateKey, err := newPrivateKey()
	if err != nil {
		return nil, fmt.Errorf("unable to generate private key for CA: %w", err)
	}
	caCfg := certutil.Config{CommonName: "envtest-environment", Organization: []string{"envtest"}}
	caCert, err := certutil.NewSelfSignedCACert(caCfg, caPrivateKey)
	if err != nil {
		return nil, fmt.Errorf("unable to generate certificate for CA: %w", err)
	}

	return &TinyCA{
		CA:         CertPair{Key: caPrivateKey, Cert: caCert},
		orgName:    "envtest",
		nextSerial: big.NewInt(1),
	}, nil
}

func (c *TinyCA) makeCert(cfg certutil.Config) (CertPair, error) {
	now := time.Now()

	key, err := newPrivateKey()
	if err != nil {
		return CertPair{}, fmt.Errorf("unable to create private key: %w", err)
	}

	serial := new(big.Int).Set(c.nextSerial)
	c.nextSerial.Add(c.nextSerial, bigOne)

	template := x509.Certificate{
		Subject:      pkix.Name{CommonName: cfg.CommonName, Organization: cfg.Organization},
		DNSNames:     cfg.AltNames.DNSNames,
		IPAddresses:  cfg.AltNames.IPs,
		SerialNumber: serial,

		KeyUsage:    x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
		ExtKeyUsage: cfg.Usages,

		// technically not necessary for testing, but let's set anyway just in case.
		NotBefore: now.UTC(),
		// 1 week -- the default for cfssl, and just long enough for a
		// long-term test, but not too long that anyone would try to use this
		// seriously.
		NotAfter: now.Add(168 * time.Hour).UTC(),
	}

	certRaw, err := x509.CreateCertificate(crand.Reader, &template, c.CA.Cert, key.Public(), c.CA.Key)
	if err != nil {
		return CertPair{}, fmt.Errorf("unable to create certificate: %w", err)
	}

	cert, err := x509.ParseCertificate(certRaw)
	if err != nil {
		return CertPair{}, fmt.Errorf("generated invalid certificate, could not parse: %w", err)
	}

	return CertPair{
		Key:  key,
		Cert: cert,
	}, nil
}

// NewServingCert returns a new CertPair for a serving HTTPS on localhost (or other specified names).
func (c *TinyCA) NewServingCert(names ...string) (CertPair, error) {
	if len(names) == 0 {
		names = []string{"localhost"}
	}
	dnsNames, ips, err := resolveNames(names)
	if err != nil {
		return CertPair{}, err
	}

	return c.makeCert(certutil.Config{
		CommonName:   "localhost",
		Organization: []string{c.orgName},
		AltNames: certutil.AltNames{
			DNSNames: dnsNames,
			IPs:      ips,
		},
		Usages: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
	})
}

// ClientInfo describes some Kubernetes user for the purposes of creating
// client certificates.
type ClientInfo struct {
	// Name is the user name (embedded as the cert's CommonName)
	Name string
	// Groups are the groups to which this user belongs (embedded as the cert's
	// Organization)
	Groups []string
}

// NewClientCert produces a new CertPair suitable for use with Kubernetes
// client cert auth with an API server validating based on this CA.
func (c *TinyCA) NewClientCert(user ClientInfo) (CertPair, error) {
	return c.makeCert(certutil.Config{
		CommonName:   user.Name,
		Organization: user.Groups,
		Usages:       []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
	})
}

func resolveNames(names []string) ([]string, []net.IP, error) {
	dnsNames := []string{}
	ips := []net.IP{}
	for _, name := range names {
		if name == "" {
			continue
		}
		ip := net.ParseIP(name)
		if ip == nil {
			dnsNames = append(dnsNames, name)
			// Also resolve to IPs.
			nameIPs, err := net.LookupHost(name)
			if err != nil {
				return nil, nil, err
			}
			for _, nameIP := range nameIPs {
				ip = net.ParseIP(nameIP)
				if ip != nil {
					ips = append(ips, ip)
				}
			}
		} else {
			ips = append(ips, ip)
		}
	}
	return dnsNames, ips, nil
}