secret/internal/cli/crypto.go

package cli

import (
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"sync/atomic"

	"filippo.io/age"
	"github.com/awnumar/memguard"
	"github.com/spf13/afero"
	"github.com/spf13/cobra"
	"sneak.berlin/go/secret/internal/secret"
	"sneak.berlin/go/secret/internal/vault"
)

// Sentinel errors for encrypt/decrypt operations
var (
	errNotAgeSecretKey = errors.New(
		"does not contain a valid age secret key")
)

// newCryptoCmd builds an encrypt/decrypt command with input/output flags
func newCryptoCmd(
	use, short, long string,
	run func(cli *Instance, secretName, inputFile, outputFile string) error,
) *cobra.Command {
	cmd := &cobra.Command{
		Use:   use,
		Short: short,
		Long:  long,
		Args:  cobra.ExactArgs(1),
		RunE: func(cmd *cobra.Command, args []string) error {
			inputFile, _ := cmd.Flags().GetString("input")
			outputFile, _ := cmd.Flags().GetString("output")

			cli, err := NewCLIInstance()
			if err != nil {
				return fmt.Errorf("failed to initialize CLI: %w", err)
			}

			cli.cmd = cmd

			destroySecrets := cli.readSecretEnv()
			defer destroySecrets()

			return run(cli, args[0], inputFile, outputFile)
		},
	}

	cmd.Flags().StringP("input", "i", "", "Input file (default: stdin)")
	cmd.Flags().StringP("output", "o", "", "Output file (default: stdout)")

	return cmd
}

func newEncryptCmd() *cobra.Command {
	return newCryptoCmd(
		"encrypt <secret-name>",
		"Encrypt data using an age secret key stored in a secret",
		"Encrypt data using an age secret key. If the secret doesn't "+
			"exist, a new age key is generated and stored.",
		(*Instance).Encrypt,
	)
}

func newDecryptCmd() *cobra.Command {
	return newCryptoCmd(
		"decrypt <secret-name>",
		"Decrypt data using an age secret key stored in a secret",
		"Decrypt data using an age secret key stored in the specified secret.",
		(*Instance).Decrypt,
	)
}

// storeNewEncryptionKey generates an age secret key and stores it as the
// named secret, holding the state directory lock while it does. It fails
// with vault.ErrSecretExists if another command stored the secret first.
// The caller must destroy the returned buffer.
func (cli *Instance) storeNewEncryptionKey(
	vlt *vault.Vault, secretName string,
) (*memguard.LockedBuffer, error) {
	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
	if err != nil {
		return nil, err
	}
	defer release()

	identity, err := age.GenerateX25519Identity()
	if err != nil {
		return nil, fmt.Errorf("failed to generate age key: %w", err)
	}

	secureBuffer := secret.IdentityToLockedBuffer(identity)

	err = vlt.AddSecret(secretName, secureBuffer, false)
	if err != nil {
		secureBuffer.Destroy()

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

	return secureBuffer, nil
}

// resolveEncryptionKey returns a secure buffer holding the age secret key
// for the named secret, generating and storing a new key if the secret
// does not exist. The caller must destroy the returned buffer. Only storing
// a new key takes the state directory lock, so that reading an existing key
// works on a read-only state directory and keeps no other command waiting
// at the passphrase prompt, and Encrypt streams its input and output
// unlocked.
func (cli *Instance) resolveEncryptionKey(
	vlt *vault.Vault, secretName string,
) (*memguard.LockedBuffer, error) {
	// Check if secret exists
	secretObj := secret.NewSecret(vlt, secretName)

	exists, err := secretObj.Exists()
	if err != nil {
		return nil, fmt.Errorf("failed to check if secret exists: %w", err)
	}

	if !exists {
		key, err := cli.storeNewEncryptionKey(vlt, secretName)
		if !errors.Is(err, vault.ErrSecretExists) {
			return key, err
		}
		// Another command stored the key since the check above: read it
	}

	// Secret exists, get the age secret key from it
	secretBuffer, err := vlt.GetSecret(secretName)
	if err != nil {
		return nil, fmt.Errorf("failed to get secret value: %w", err)
	}

	// Validate that it's a valid age secret key
	if !isValidAgeSecretKey(secretBuffer.String()) {
		secretBuffer.Destroy()

		return nil, fmt.Errorf("secret '%s' %w", secretName, errNotAgeSecretKey)
	}

	return secretBuffer, nil
}

// Encrypt encrypts data using an age secret key stored in a secret
func (cli *Instance) Encrypt(secretName, inputFile, outputFile string) error {
	err := vault.ValidateSecretName(secretName)
	if err != nil {
		return err
	}

	// Get current vault
	vlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
	if err != nil {
		return err
	}

	vlt.Mnemonic, vlt.UnlockPassphrase = cli.Mnemonic, cli.UnlockPassphrase

	// Get or create the age secret key for this secret
	keyBuffer, err := cli.resolveEncryptionKey(vlt, secretName)
	if err != nil {
		return err
	}
	defer keyBuffer.Destroy()

	// Parse the secret key
	identity, err := age.ParseX25519Identity(keyBuffer.String())
	if err != nil {
		return fmt.Errorf("failed to parse age secret key: %w", err)
	}

	// Get recipient from identity
	recipient := identity.Recipient()

	// Set up input reader
	var input io.Reader = os.Stdin

	if inputFile != "" {
		file, err := cli.fs.Open(inputFile)
		if err != nil {
			return fmt.Errorf("failed to open input file: %w", err)
		}
		defer func() { _ = file.Close() }()

		input = file
	}

	return cli.writeOutput(outputFile, func(output io.Writer) error {
		encryptor, err := age.Encrypt(output, recipient)
		if err != nil {
			return fmt.Errorf("failed to create age encryptor: %w", err)
		}

		_, err = io.Copy(encryptor, input)
		if err != nil {
			return fmt.Errorf("failed to encrypt data: %w", err)
		}

		err = encryptor.Close()
		if err != nil {
			return fmt.Errorf("failed to finalize encryption: %w", err)
		}

		return nil
	})
}

// Decrypt decrypts data using an age secret key stored in a secret
func (cli *Instance) Decrypt(secretName, inputFile, outputFile string) error {
	err := vault.ValidateSecretName(secretName)
	if err != nil {
		return err
	}

	// Get current vault
	vlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
	if err != nil {
		return err
	}

	vlt.Mnemonic, vlt.UnlockPassphrase = cli.Mnemonic, cli.UnlockPassphrase

	// Check if secret exists
	secretObj := secret.NewSecret(vlt, secretName)

	exists, err := secretObj.Exists()
	if err != nil {
		return fmt.Errorf("failed to check if secret exists: %w", err)
	}

	if !exists {
		return fmt.Errorf("secret '%s' %w", secretName, vault.ErrSecretNotFound)
	}

	// Get the age secret key from the secret
	secretBuffer, err := vlt.GetSecret(secretName)
	if err != nil {
		return fmt.Errorf("failed to get secret value: %w", err)
	}
	defer secretBuffer.Destroy()

	// Validate that it's a valid age secret key
	if !isValidAgeSecretKey(secretBuffer.String()) {
		return fmt.Errorf("secret '%s' %w", secretName, errNotAgeSecretKey)
	}

	// Parse the age secret key to get the identity
	identity, err := age.ParseX25519Identity(secretBuffer.String())
	if err != nil {
		return fmt.Errorf("failed to parse age secret key: %w", err)
	}

	// Set up input reader
	var input io.Reader = os.Stdin

	if inputFile != "" {
		file, err := cli.fs.Open(inputFile)
		if err != nil {
			return fmt.Errorf("failed to open input file: %w", err)
		}
		defer func() { _ = file.Close() }()

		input = file
	}

	return cli.writeOutput(outputFile, func(output io.Writer) error {
		decryptor, err := age.Decrypt(input, identity)
		if err != nil {
			return fmt.Errorf("failed to create age decryptor: %w", err)
		}

		_, err = io.Copy(output, decryptor)
		if err != nil {
			return fmt.Errorf("failed to decrypt data: %w", err)
		}

		return nil
	})
}

// temporaryOutput is the name of the temporary file writeOutput is filling,
// until it is renamed into place. The signal handler in Entry removes it,
// since for decrypt it holds part of the secret.
//
//nolint:gochecknoglobals // read by the signal handler, which has no Instance
var temporaryOutput atomic.Pointer[string]

// writeOutput calls write with stdout when outputFile is empty, and with
// outputFile itself when it is a device or a pipe, such as /dev/null.
// Otherwise write fills a new file of mode 0600 that replaces outputFile,
// or the file it is a symbolic link to, only once write has succeeded, so a
// failure leaves that file as it was, and it may be the input file too.
func (cli *Instance) writeOutput(
	outputFile string, write func(output io.Writer) error,
) error {
	if outputFile == "" {
		return write(cli.cmd.OutOrStdout())
	}

	info, err := cli.fs.Stat(outputFile)
	if err == nil && !info.Mode().IsRegular() {
		file, err := cli.fs.OpenFile(outputFile, os.O_WRONLY, 0)
		if err != nil {
			return fmt.Errorf("failed to open output file: %w", err)
		}
		defer func() { _ = file.Close() }()

		return write(file)
	}

	// Replace the file a symbolic link points to and keep the link, as
	// writing through the link would; /dev/stdout with stdout going to a
	// file is such a link. A path that does not exist yet is used as given.
	path, err := filepath.EvalSymlinks(outputFile)
	if err != nil {
		path = outputFile
	}

	err = secret.WriteFileAtomicFunc(cli.fs, path, func(file afero.File) error {
		name := file.Name()
		temporaryOutput.Store(&name)

		return write(file)
	})

	temporaryOutput.Store(nil)

	return err
}

// isValidAgeSecretKey checks if a string is a valid age secret key by
// attempting to parse it
func isValidAgeSecretKey(key string) bool {
	_, err := age.ParseX25519Identity(key)

	return err == nil
}