// Package nebula contains helper functions and types which are useful for
// setting up nebula configs, processes, and deployments.
package nebula

import (
	"crypto"
	"crypto/ed25519"
	"crypto/rand"
	"encoding/pem"
	"errors"
	"fmt"
	"io"
	"net"
	"time"

	"github.com/slackhq/nebula/cert"
	"golang.org/x/crypto/curve25519"
)

// ErrInvalidSignature is returned from functions when a signature validation
// fails.
var ErrInvalidSignature = errors.New("invalid signature")

// HostCredentials contains the certificate and private key files which will
// need to be present on a particular host. Each file is PEM encoded.
type HostCredentials struct {
	CACertPEM   string
	HostKeyPEM  string
	HostCertPEM string
}

// CACredentials contains the certificate and private files which can be used to
// create and validate HostCredentials. Each file is PEM encoded.
type CACredentials struct {
	CACertPEM string
	CAKeyPEM  string
}

// NewHostCredentials generates a new key/cert for a nebula host using the CA
// key which will be found in the adminFS.
func NewHostCredentials(
	caCreds CACredentials, hostName string, ip net.IP,
) (
	HostCredentials, error,
) {

	// The logic here is largely based on
	// https://github.com/slackhq/nebula/blob/v1.4.0/cmd/nebula-cert/sign.go

	caKey, _, err := cert.UnmarshalEd25519PrivateKey([]byte(caCreds.CAKeyPEM))
	if err != nil {
		return HostCredentials{}, fmt.Errorf("unmarshaling ca.key: %w", err)
	}

	caCert, _, err := cert.UnmarshalNebulaCertificateFromPEM([]byte(caCreds.CACertPEM))
	if err != nil {
		return HostCredentials{}, fmt.Errorf("unmarshaling ca.crt: %w", err)
	}

	issuer, err := caCert.Sha256Sum()
	if err != nil {
		return HostCredentials{}, fmt.Errorf("getting ca.crt issuer: %w", err)
	}

	expireAt := caCert.Details.NotAfter.Add(-1 * time.Second)

	subnet := caCert.Details.Subnets[0]
	if !subnet.Contains(ip) {
		return HostCredentials{}, fmt.Errorf("invalid ip %q, not contained by network subnet %q", ip, subnet)
	}

	var hostPub, hostKey []byte
	{
		var pubkey, privkey [32]byte
		if _, err := io.ReadFull(rand.Reader, privkey[:]); err != nil {
			return HostCredentials{}, fmt.Errorf("reading random bytes to form private key: %w", err)
		}
		curve25519.ScalarBaseMult(&pubkey, &privkey)
		hostPub, hostKey = pubkey[:], privkey[:]
	}

	hostCert := cert.NebulaCertificate{
		Details: cert.NebulaCertificateDetails{
			Name: hostName,
			Ips: []*net.IPNet{{
				IP:   ip,
				Mask: subnet.Mask,
			}},
			NotBefore: time.Now(),
			NotAfter:  expireAt,
			PublicKey: hostPub,
			IsCA:      false,
			Issuer:    issuer,
		},
	}

	if err := hostCert.CheckRootConstrains(caCert); err != nil {
		return HostCredentials{}, fmt.Errorf("validating certificate constraints: %w", err)
	}

	if err := hostCert.Sign(caKey); err != nil {
		return HostCredentials{}, fmt.Errorf("signing host cert with ca.key: %w", err)
	}

	hostKeyPEM := cert.MarshalX25519PrivateKey(hostKey)

	hostCertPEM, err := hostCert.MarshalToPEM()
	if err != nil {
		return HostCredentials{}, fmt.Errorf("marshalling host.crt: %w", err)
	}

	return HostCredentials{
		CACertPEM:   caCreds.CACertPEM,
		HostKeyPEM:  string(hostKeyPEM),
		HostCertPEM: string(hostCertPEM),
	}, nil
}

// NewCACredentials generates a CACredentials. The domain should be the network's root domain,
// and is included in the signing certificate's Name field.
func NewCACredentials(domain string, subnet *net.IPNet) (CACredentials, error) {

	pubKey, privKey, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		panic(fmt.Errorf("generating ed25519 key: %w", err))
	}

	now := time.Now()
	expireAt := now.Add(2 * 365 * 24 * time.Hour)

	caCert := cert.NebulaCertificate{
		Details: cert.NebulaCertificateDetails{
			Name:      fmt.Sprintf("%s cryptic-net root cert", domain),
			Subnets:   []*net.IPNet{subnet},
			NotBefore: now,
			NotAfter:  expireAt,
			PublicKey: pubKey,
			IsCA:      true,
		},
	}

	if err := caCert.Sign(privKey); err != nil {
		return CACredentials{}, fmt.Errorf("signing caCert: %w", err)
	}

	caKeyPEM := cert.MarshalEd25519PrivateKey(privKey)

	caCertPEM, err := caCert.MarshalToPEM()
	if err != nil {
		return CACredentials{}, fmt.Errorf("marshaling caCert: %w", err)
	}

	return CACredentials{
		CACertPEM: string(caCertPEM),
		CAKeyPEM:  string(caKeyPEM),
	}, nil
}

// ValidateHostCertPEM checks if the given host certificate was signed by the
// given CA certificate, and returns ErrInvalidSignature if validation fails.
func ValidateHostCertPEM(caCertPEM, hostCertPEM string) error {

	caCert, _, err := cert.UnmarshalNebulaCertificateFromPEM([]byte(caCertPEM))
	if err != nil {
		return fmt.Errorf("unmarshaling CA certificate as PEM: %w", err)
	}

	hostCert, _, err := cert.UnmarshalNebulaCertificateFromPEM([]byte(hostCertPEM))
	if err != nil {
		return fmt.Errorf("unmarshaling host certificate as PEM: %w", err)
	}

	caPubKey := ed25519.PublicKey(caCert.Details.PublicKey)

	if !hostCert.CheckSignature(caPubKey) {
		return ErrInvalidSignature
	}

	return nil
}

// IPFromHostCertPEM is a convenience function for parsing the IP of a host out
// of its nebula cert.
func IPFromHostCertPEM(hostCertPEM string) (net.IP, error) {

	hostCert, _, err := cert.UnmarshalNebulaCertificateFromPEM([]byte(hostCertPEM))
	if err != nil {
		return nil, fmt.Errorf("unmarshaling host certificate as PEM: %w", err)
	}

	ips := hostCert.Details.Ips
	if len(ips) == 0 {
		return nil, fmt.Errorf("malformed nebula host cert: no IPs")
	}

	return ips[0].IP, nil
}

// SignAndWrap signs the given bytes using the keyPEM, and writes an
// encoded, versioned structure containing the signature and the given bytes.
func SignAndWrap(into io.Writer, keyPEM string, b []byte) error {

	key, _, err := cert.UnmarshalEd25519PrivateKey([]byte(keyPEM))
	if err != nil {
		return fmt.Errorf("unmarshaling private key: %w", err)
	}

	sig, err := key.Sign(rand.Reader, b, crypto.Hash(0))
	if err != nil {
		return fmt.Errorf("generating signature: %w", err)
	}

	if _, err := into.Write([]byte("0")); err != nil {
		return fmt.Errorf("writing version byte: %w", err)
	}

	err = pem.Encode(into, &pem.Block{
		Type:  "SIGNATURE",
		Bytes: sig,
	})

	if err != nil {
		return fmt.Errorf("writing PEM encoding of signature: %w", err)
	}

	if _, err := into.Write(b); err != nil {
		return fmt.Errorf("writing input bytes: %w", err)
	}

	return nil
}

// Unwrap reads a stream of bytes which was produced by SignAndWrap, and returns
// the original inpute to SignAndWrap as well as the signature which was
// created. ValidateSignature can be used to validate the signature.
func Unwrap(from io.Reader) (b, sig []byte, err error) {

	full, err := io.ReadAll(from)
	if err != nil {
		return nil, nil, fmt.Errorf("reading full input: %w", err)
	} else if len(full) < 3 {
		return nil, nil, fmt.Errorf("input too small")
	} else if full[0] != '0' {
		return nil, nil, fmt.Errorf("unexpected version byte: %d", full[0])
	}

	full = full[1:]

	pemBlock, rest := pem.Decode(full)
	if pemBlock == nil {
		return nil, nil, fmt.Errorf("PEM-encoded signature could not be decoded")
	}

	return rest, pemBlock.Bytes, nil
}

// ValidateSignature can be used to validate a signature produced by Unwrap.
func ValidateSignature(certPEM string, b, sig []byte) error {

	cert, _, err := cert.UnmarshalNebulaCertificateFromPEM([]byte(certPEM))
	if err != nil {
		return fmt.Errorf("unmarshaling certificate as PEM: %w", err)
	}

	pubKey := ed25519.PublicKey(cert.Details.PublicKey)

	if !ed25519.Verify(pubKey, b, sig) {
		return ErrInvalidSignature
	}

	return nil
}