secret/internal/secret/pgpunlocker.go

package secret

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"strings"
	"time"

	"filippo.io/age"
	"github.com/awnumar/memguard"
	"github.com/spf13/afero"
)

// gpgNoPublicKeyStatus is the status line gpg writes when it has no key for
// the ID it was asked to list: 9 is gpg's error code for "No public key".
const gpgNoPublicKeyStatus = "[GNUPG:] ERROR keylist.getkey 9\n"

var (
	errGPGKeyIDEmpty    = errors.New("GPG key ID cannot be empty")
	errInvalidGPGKeyID  = errors.New("invalid GPG key ID format")
	errNoGPGFingerprint = errors.New("could not find fingerprint for GPG key")
	errNilDataBuffer    = errors.New("data buffer is nil")
)

// ErrGPGKeyNotFound is returned by ResolveGPGKeyFingerprint for a key ID
// that matches no key in the GPG keyring.
var ErrGPGKeyNotFound = errors.New("GPG key not found")

// Variables to allow overriding in tests
var (
	// GPGEncryptFunc is the function used for GPG encryption
	// Can be overridden in tests to provide a non-interactive implementation
	//nolint:gochecknoglobals // Required for test mocking
	GPGEncryptFunc func(
		data *memguard.LockedBuffer, keyID string,
	) ([]byte, error) = gpgEncryptDefault

	// GPGDecryptFunc is the function used for GPG decryption
	// Can be overridden in tests to provide a non-interactive implementation
	//nolint:gochecknoglobals // Required for test mocking
	GPGDecryptFunc func(
		encryptedData []byte,
	) (*memguard.LockedBuffer, error) = gpgDecryptDefault

	// gpgKeyIDRegex validates GPG key IDs
	// Allows either:
	// 1. Email addresses (user@domain.tld format)
	// 2. Short key IDs (8 hex characters)
	// 3. Long key IDs (16 hex characters)
	// 4. Full fingerprints (40 hex characters)
	gpgKeyIDRegex = regexp.MustCompile(
		`^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$|` +
			`^[A-Fa-f0-9]{8}$|` +
			`^[A-Fa-f0-9]{16}$|` +
			`^[A-Fa-f0-9]{40}$`,
	)
)

// PGPUnlockerMetadata extends UnlockerMetadata with PGP-specific data
type PGPUnlockerMetadata struct {
	UnlockerMetadata

	// GPG key ID used for encryption
	GPGKeyID string `json:"gpgKeyId"`
}

// PGPUnlocker represents a PGP-protected unlocker
type PGPUnlocker struct {
	Directory string
	Metadata  UnlockerMetadata
	fs        afero.Fs
}

// NewPGPUnlocker creates a new PGPUnlocker instance
func NewPGPUnlocker(
	fs afero.Fs, directory string, metadata UnlockerMetadata,
) *PGPUnlocker {
	return &PGPUnlocker{
		Directory: directory,
		Metadata:  metadata,
		fs:        fs,
	}
}

// GetIdentity implements Unlocker interface for PGP-based unlockers
func (p *PGPUnlocker) GetIdentity() (*age.X25519Identity, error) {
	DebugWith("Getting PGP unlocker identity",
		slog.String("unlocker_id", p.GetID()),
		slog.String("unlocker_type", p.GetType()),
	)

	// Step 1: Read the encrypted age private key from filesystem
	agePrivKeyPath := filepath.Join(p.Directory, "priv.age.gpg")
	Debug("Reading PGP-encrypted age private key", "path", agePrivKeyPath)

	encryptedAgePrivKeyData, err := afero.ReadFile(p.fs, agePrivKeyPath)
	if err != nil {
		Debug("Failed to read PGP-encrypted age private key",
			"error", err, "path", agePrivKeyPath)

		return nil, fmt.Errorf("failed to read encrypted age private key: %w", err)
	}

	DebugWith("Read PGP-encrypted age private key",
		slog.String("unlocker_id", p.GetID()),
		slog.Int("encrypted_length", len(encryptedAgePrivKeyData)),
	)

	// Step 2: Decrypt the age private key using GPG
	Debug("Decrypting age private key with GPG", "unlocker_id", p.GetID())

	agePrivKeyBuffer, err := GPGDecryptFunc(encryptedAgePrivKeyData)
	if err != nil {
		Debug("Failed to decrypt age private key with GPG",
			"error", err, "unlocker_id", p.GetID())

		return nil, fmt.Errorf("failed to decrypt age private key with GPG: %w", err)
	}
	defer agePrivKeyBuffer.Destroy()

	DebugWith("Successfully decrypted age private key with GPG",
		slog.String("unlocker_id", p.GetID()),
		slog.Int("decrypted_length", agePrivKeyBuffer.Size()),
	)

	// Step 3: Parse the decrypted age private key
	Debug("Parsing decrypted age private key", "unlocker_id", p.GetID())

	ageIdentity, err := age.ParseX25519Identity(agePrivKeyBuffer.String())
	if err != nil {
		Debug("Failed to parse age private key", "error", err, "unlocker_id", p.GetID())

		return nil, fmt.Errorf("failed to parse age private key: %w", err)
	}

	DebugWith("Successfully parsed PGP age identity",
		slog.String("unlocker_id", p.GetID()),
		slog.String("public_key", ageIdentity.Recipient().String()),
	)

	return ageIdentity, nil
}

// GetType implements Unlocker interface
func (p *PGPUnlocker) GetType() string {
	return "pgp"
}

// GetMetadata implements Unlocker interface
func (p *PGPUnlocker) GetMetadata() UnlockerMetadata {
	return p.Metadata
}

// GetDirectory implements Unlocker interface
func (p *PGPUnlocker) GetDirectory() string {
	return p.Directory
}

// GetID implements Unlocker interface: the name of the unlocker's directory
func (p *PGPUnlocker) GetID() string {
	return filepath.Base(p.Directory)
}

// Remove implements Unlocker interface - removes the PGP unlocker
func (p *PGPUnlocker) Remove() error {
	// For PGP unlockers, we just need to remove the directory
	err := RemoveDirAtomic(p.fs, p.Directory)
	if err != nil {
		return fmt.Errorf("failed to remove PGP unlocker directory: %w", err)
	}

	return nil
}

// generatePGPUnlockerName generates a unique name for the PGP unlocker
// based on hostname and time
func generatePGPUnlockerName() (string, error) {
	hostname, err := os.Hostname()
	if err != nil {
		return "", fmt.Errorf("failed to get hostname: %w", err)
	}

	enrollmentTime := time.Now().UTC().Format(UnlockerTimeFormat)

	return fmt.Sprintf("%s-pgp-%s", hostname, enrollmentTime), nil
}

// pgpUnlockerDir returns the directory in vault for a new PGP unlocker,
// named after the host and the time.
func pgpUnlockerDir(vault VaultInterface) (string, error) {
	// Generate the unlocker name based on hostname and time
	unlockerName, err := generatePGPUnlockerName()
	if err != nil {
		return "", fmt.Errorf("failed to generate unlocker name: %w", err)
	}

	vaultDir, err := vault.GetDirectory()
	if err != nil {
		return "", fmt.Errorf("failed to get vault directory: %w", err)
	}

	return filepath.Join(vaultDir, "unlockers.d", unlockerName), nil
}

// CreatePGPUnlocker creates a new PGP unlocker and stores it in vault.
// It encrypts to the GPG key gpgKeyID and records fingerprint, that key's
// fingerprint as ResolveGPGKeyFingerprint returns it, in the metadata.
// Everything that can fail short of writing a file is done before anything
// is written, and the files are written through WriteDir, so a failure
// leaves no partial unlocker. The long-term key comes from vault's
// GetOrDeriveLongTermKey: the key it holds when it is unlocked, else the
// key from its mnemonic or its current unlocker.
func CreatePGPUnlocker(
	vault VaultInterface, gpgKeyID, fingerprint string,
) (*PGPUnlocker, error) {
	err := checkGPGAvailable()
	if err != nil {
		return nil, err
	}

	unlockerDir, err := pgpUnlockerDir(vault)
	if err != nil {
		return nil, err
	}

	fs := vault.GetFilesystem()

	// Step 1: Generate a new age keypair for the PGP unlocker
	ageIdentity, err := age.GenerateX25519Identity()
	if err != nil {
		return nil, fmt.Errorf("failed to generate age keypair: %w", err)
	}

	// Step 2: Encrypt the long-term private key to the new keypair, and the
	// keypair's private key to the GPG key
	encryptedLtPrivKey, encryptedAgePrivKey, err := encryptPGPUnlockerKeys(
		vault, ageIdentity, gpgKeyID)
	if err != nil {
		return nil, err
	}

	pgpMetadata := PGPUnlockerMetadata{
		UnlockerMetadata: UnlockerMetadata{
			Type:      "pgp",
			CreatedAt: time.Now(),
			Flags:     []string{"gpg", "encrypted"},
		},
		GPGKeyID: fingerprint,
	}

	metadataBytes, err := json.MarshalIndent(pgpMetadata, "", "  ")
	if err != nil {
		return nil, fmt.Errorf("failed to marshal unlocker metadata: %w", err)
	}

	// Step 3: Write the unlocker's files, the metadata last
	err = WriteDir(fs, unlockerDir, func(dir string) error {
		return writePGPUnlockerFiles(fs, dir, ageIdentity.Recipient(),
			encryptedLtPrivKey, encryptedAgePrivKey, metadataBytes)
	})
	if err != nil {
		return nil, err
	}

	return &PGPUnlocker{
		Directory: unlockerDir,
		Metadata:  pgpMetadata.UnlockerMetadata,
		fs:        fs,
	}, nil
}

// encryptPGPUnlockerKeys returns the vault's long-term private key encrypted
// to the new PGP unlocker's age keypair, and that keypair's private key
// encrypted to the GPG key gpgKeyID.
func encryptPGPUnlockerKeys(
	vault VaultInterface, ageIdentity *age.X25519Identity, gpgKeyID string,
) ([]byte, []byte, error) {
	// The key vault holds, or else from its mnemonic or current unlocker
	ltIdentity, err := vault.GetOrDeriveLongTermKey()
	if err != nil {
		return nil, nil, fmt.Errorf("failed to get long-term key: %w", err)
	}

	ltPrivKeyData := IdentityToLockedBuffer(ltIdentity)
	defer ltPrivKeyData.Destroy()

	encryptedLtPrivKey, err := EncryptToRecipient(
		ltPrivKeyData, ageIdentity.Recipient())
	if err != nil {
		return nil, nil, fmt.Errorf(
			"failed to encrypt long-term private key to age unlocker: %w", err)
	}

	agePrivateKeyBuffer := IdentityToLockedBuffer(ageIdentity)
	defer agePrivateKeyBuffer.Destroy()

	encryptedAgePrivKey, err := GPGEncryptFunc(agePrivateKeyBuffer, gpgKeyID)
	if err != nil {
		return nil, nil, fmt.Errorf(
			"failed to encrypt age private key with GPG: %w", err)
	}

	return encryptedLtPrivKey, encryptedAgePrivKey, nil
}

// writePGPUnlockerFiles writes the files of a PGP unlocker into dir, the
// metadata last.
func writePGPUnlockerFiles(
	fs afero.Fs, dir string, ageRecipient *age.X25519Recipient,
	encryptedLtPrivKey, encryptedAgePrivKey, metadataBytes []byte,
) error {
	err := WriteFileAtomic(fs, filepath.Join(dir, "pub.txt"),
		[]byte(ageRecipient.String()))
	if err != nil {
		return fmt.Errorf("failed to write age recipient: %w", err)
	}

	err = WriteFileAtomic(fs, filepath.Join(dir, "longterm.age"), encryptedLtPrivKey)
	if err != nil {
		return fmt.Errorf("failed to write encrypted long-term private key: %w", err)
	}

	err = WriteFileAtomic(fs, filepath.Join(dir, "priv.age.gpg"), encryptedAgePrivKey)
	if err != nil {
		return fmt.Errorf("failed to write encrypted age private key: %w", err)
	}

	err = WriteFileAtomic(fs,
		filepath.Join(dir, "unlocker-metadata.json"), metadataBytes)
	if err != nil {
		return fmt.Errorf("failed to write unlocker metadata: %w", err)
	}

	return nil
}

// validateGPGKeyID validates that a GPG key ID is safe for command execution
func validateGPGKeyID(keyID string) error {
	if keyID == "" {
		return errGPGKeyIDEmpty
	}

	if !gpgKeyIDRegex.MatchString(keyID) {
		return fmt.Errorf("%w: %s", errInvalidGPGKeyID, keyID)
	}

	return nil
}

// ResolveGPGKeyFingerprint resolves any GPG key identifier to its full fingerprint
func ResolveGPGKeyFingerprint(keyID string) (string, error) {
	err := validateGPGKeyID(keyID)
	if err != nil {
		return "", fmt.Errorf("invalid GPG key ID: %w", err)
	}

	// Use GPG to get the full fingerprint for the key. --status-fd 1 adds
	// gpg's status lines to the output.
	cmd := exec.CommandContext( //nolint:gosec // G204: keyID validated above
		context.Background(),
		"gpg", "--status-fd", "1",
		"--list-keys", "--with-colons", "--fingerprint", keyID,
	)

	output, err := cmd.Output()
	if err != nil {
		if strings.Contains(string(output), gpgNoPublicKeyStatus) {
			return "", fmt.Errorf("%w: %s", ErrGPGKeyNotFound, keyID)
		}

		return "", fmt.Errorf(
			"failed to resolve GPG key fingerprint: %w", withGPGStderr(err))
	}

	// Parse the output to extract the fingerprint
	for line := range strings.SplitSeq(string(output), "\n") {
		if strings.HasPrefix(line, "fpr:") {
			fields := strings.Split(line, ":")
			if len(fields) >= 10 && fields[9] != "" {
				return fields[9], nil
			}
		}
	}

	return "", fmt.Errorf("%w: %s", errNoGPGFingerprint, keyID)
}

// checkGPGAvailable verifies that GPG is available
func checkGPGAvailable() error {
	cmd := exec.CommandContext(context.Background(), "gpg", "--version")

	err := cmd.Run()
	if err != nil {
		return fmt.Errorf(
			"GPG not available: %w (make sure 'gpg' command is installed and in PATH)",
			err)
	}

	return nil
}

// withGPGStderr appends gpg's stderr, which says why gpg failed, to err
// from cmd.Output. cmd.Output keeps that stderr in the *exec.ExitError.
// err is returned unchanged when gpg wrote nothing to stderr.
func withGPGStderr(err error) error {
	var exitErr *exec.ExitError

	if !errors.As(err, &exitErr) {
		return err
	}

	stderr := strings.TrimSpace(string(exitErr.Stderr))
	if stderr == "" {
		return err
	}

	return fmt.Errorf("%w: %s", err, stderr)
}

// gpgEncryptDefault is the default implementation of GPG encryption
func gpgEncryptDefault(data *memguard.LockedBuffer, keyID string) ([]byte, error) {
	if data == nil {
		return nil, errNilDataBuffer
	}

	err := validateGPGKeyID(keyID)
	if err != nil {
		return nil, fmt.Errorf("invalid GPG key ID: %w", err)
	}

	cmd := exec.CommandContext( //nolint:gosec // G204: keyID validated above
		context.Background(),
		"gpg", "--trust-model", "always", "--armor", "--encrypt", "-r", keyID,
	)
	cmd.Stdin = strings.NewReader(data.String())

	output, err := cmd.Output()
	if err != nil {
		return nil, fmt.Errorf("GPG encryption failed: %w", withGPGStderr(err))
	}

	return output, nil
}

// gpgDecryptDefault is the default implementation of GPG decryption
func gpgDecryptDefault(encryptedData []byte) (*memguard.LockedBuffer, error) {
	cmd := exec.CommandContext(context.Background(), "gpg", "--quiet", "--decrypt")
	cmd.Stdin = strings.NewReader(string(encryptedData))

	output, err := cmd.Output()
	if err != nil {
		return nil, fmt.Errorf("GPG decryption failed: %w", withGPGStderr(err))
	}

	// Create a secure buffer for the decrypted data
	outputBuffer := memguard.NewBufferFromBytes(output)

	return outputBuffer, nil
}