// Package pin lets agents trust the master without a certificate authority.
// The master serves a self-signed certificate for its own key pair; agents
// are configured with the fingerprint of that public key and accept the
// connection only if it matches. The TLS handshake proves the master holds
// the private key, the same way SSH host keys work.
package pin

import (
	"crypto"
	"crypto/ed25519"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/hex"
	"errors"
	"fmt"
	"math/big"
	"time"
)

// GenerateKey creates a new master key pair.
func GenerateKey() (ed25519.PrivateKey, error) {
	_, priv, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		return nil, fmt.Errorf("generate key: %w", err)
	}
	return priv, nil
}

// Certificate returns a self-signed TLS certificate for key. Its validity is
// effectively unlimited because trust comes from the pinned fingerprint, not
// from the certificate's dates or issuer.
func Certificate(key ed25519.PrivateKey) (tls.Certificate, error) {
	tmpl := &x509.Certificate{
		SerialNumber: big.NewInt(1),
		Subject:      pkix.Name{CommonName: "package-updater-master"},
		NotBefore:    time.Now().Add(-time.Hour),
		NotAfter:     time.Now().AddDate(100, 0, 0),
		KeyUsage:     x509.KeyUsageDigitalSignature,
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
	}
	der, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, key.Public(), key)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("create certificate: %w", err)
	}
	return tls.Certificate{Certificate: [][]byte{der}, PrivateKey: key}, nil
}

// Fingerprint returns "sha256:" + hex SHA-256 of pub's PKIX DER encoding.
// This is the MASTER_KEY value agents are configured with.
func Fingerprint(pub crypto.PublicKey) (string, error) {
	der, err := x509.MarshalPKIXPublicKey(pub)
	if err != nil {
		return "", fmt.Errorf("marshal public key: %w", err)
	}
	sum := sha256.Sum256(der)
	return "sha256:" + hex.EncodeToString(sum[:]), nil
}

// ClientConfig returns a TLS config that accepts only a server whose
// certificate carries the public key with the given fingerprint.
func ClientConfig(fingerprint string) *tls.Config {
	return &tls.Config{
		MinVersion: tls.VersionTLS13,
		// Chain and hostname verification are replaced by the fingerprint
		// check below: the master's certificate is self-signed by design.
		// The handshake still verifies the server's signature with the
		// certificate's key, so presenting a copied certificate fails.
		InsecureSkipVerify: true, //nolint:gosec // see comment above
		VerifyPeerCertificate: func(raw [][]byte, _ [][]*x509.Certificate) error {
			if len(raw) == 0 {
				return errors.New("pin: server sent no certificate")
			}
			cert, err := x509.ParseCertificate(raw[0])
			if err != nil {
				return fmt.Errorf("pin: parse certificate: %w", err)
			}
			got, err := Fingerprint(cert.PublicKey)
			if err != nil {
				return err
			}
			if subtle.ConstantTimeCompare([]byte(got), []byte(fingerprint)) != 1 {
				return fmt.Errorf("pin: fingerprint mismatch: server presented %s", got)
			}
			return nil
		},
	}
}