secret/internal/secret/crypto.go

package secret

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"os"
	"syscall"
	"unsafe"

	"filippo.io/age"
	"github.com/awnumar/memguard"
	"golang.org/x/term"
)

var (
	errNilPassphraseBuffer = errors.New("passphrase buffer is nil")
	errStdinNotTerminal    = errors.New(
		"cannot read passphrase from non-terminal stdin " +
			"(piped input or script). Please set the SB_UNLOCK_PASSPHRASE " +
			"environment variable or run interactively")
	errStderrNotTerminal = errors.New(
		"cannot prompt for passphrase: stderr is not a terminal " +
			"(running in non-interactive mode). Please set the " +
			"SB_UNLOCK_PASSPHRASE environment variable")
	errEmptyPassphrase = errors.New("passphrase cannot be empty")
)

// EncryptToRecipient encrypts data to a recipient using age
// The data parameter should be a LockedBuffer for secure memory handling
func EncryptToRecipient(
	data *memguard.LockedBuffer, recipient age.Recipient,
) ([]byte, error) {
	if data == nil {
		return nil, errNilDataBuffer
	}

	Debug("EncryptToRecipient starting", "data_length", data.Size())

	var buf bytes.Buffer

	Debug("Creating age encryptor")

	w, err := age.Encrypt(&buf, recipient)
	if err != nil {
		Debug("Failed to create encryptor", "error", err)

		return nil, fmt.Errorf("failed to create encryptor: %w", err)
	}

	Debug("Created age encryptor successfully")
	Debug("Writing data to encryptor")

	_, err = w.Write(data.Bytes())
	if err != nil {
		Debug("Failed to write data to encryptor", "error", err)

		return nil, fmt.Errorf("failed to write data: %w", err)
	}

	Debug("Wrote data to encryptor successfully")
	Debug("Closing encryptor")

	err = w.Close()
	if err != nil {
		Debug("Failed to close encryptor", "error", err)

		return nil, fmt.Errorf("failed to close encryptor: %w", err)
	}

	Debug("Closed encryptor successfully")

	result := buf.Bytes()
	Debug("EncryptToRecipient completed successfully", "result_length", len(result))

	return result, nil
}

// DecryptWithIdentity decrypts data with an identity using age
func DecryptWithIdentity(
	data []byte, identity age.Identity,
) (*memguard.LockedBuffer, error) {
	r, err := age.Decrypt(bytes.NewReader(data), identity)
	if err != nil {
		return nil, fmt.Errorf("failed to create decryptor: %w", err)
	}

	result, err := io.ReadAll(r)
	if err != nil {
		return nil, fmt.Errorf("failed to read decrypted data: %w", err)
	}

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

	// Zero out the original slice to prevent plaintext from lingering
	// in unprotected memory
	for i := range result {
		result[i] = 0
	}

	return resultBuffer, nil
}

// IdentityToLockedBuffer returns the private key of id, in age's text form, in
// a new locked buffer. The caller must destroy it.
//
// This is best effort. age gives the key only as a string in ordinary memory.
// The bytes of that string are moved into the buffer, which overwrites them,
// although Go otherwise never changes a string; nothing else holds this one.
// The copies age makes while building the string are left in ordinary memory.
// Avoiding those would mean encoding the key here, straight into the buffer.
func IdentityToLockedBuffer(id *age.X25519Identity) *memguard.LockedBuffer {
	key := id.String()

	//nolint:gosec // G103: the string's own bytes, which NewBufferFromBytes wipes
	keyBytes := unsafe.Slice(unsafe.StringData(key), len(key))

	return memguard.NewBufferFromBytes(keyBytes)
}

// EncryptWithPassphrase encrypts data using a passphrase with age's
// scrypt-based encryption. Both data and passphrase parameters should
// be LockedBuffers for secure memory handling
func EncryptWithPassphrase(
	data *memguard.LockedBuffer, passphrase *memguard.LockedBuffer,
) ([]byte, error) {
	if data == nil {
		return nil, errNilDataBuffer
	}

	if passphrase == nil {
		return nil, errNilPassphraseBuffer
	}

	// Create recipient directly from passphrase - unavoidable string
	// conversion due to age API
	recipient, err := age.NewScryptRecipient(passphrase.String())
	if err != nil {
		return nil, fmt.Errorf("failed to create scrypt recipient: %w", err)
	}

	return EncryptToRecipient(data, recipient)
}

// DecryptWithPassphrase decrypts data using a passphrase with age's
// scrypt-based decryption. The passphrase parameter should be a
// LockedBuffer for secure memory handling
func DecryptWithPassphrase(
	encryptedData []byte, passphrase *memguard.LockedBuffer,
) (*memguard.LockedBuffer, error) {
	if passphrase == nil {
		return nil, errNilPassphraseBuffer
	}

	// Create identity directly from passphrase - unavoidable string
	// conversion due to age API
	identity, err := age.NewScryptIdentity(passphrase.String())
	if err != nil {
		return nil, fmt.Errorf("failed to create scrypt identity: %w", err)
	}

	return DecryptWithIdentity(encryptedData, identity)
}

// ReadPassphrase reads a passphrase securely from the terminal without echoing
// This version is for unlocking and doesn't require confirmation
// Returns a LockedBuffer containing the passphrase for secure memory handling
func ReadPassphrase(prompt string) (*memguard.LockedBuffer, error) {
	// Check if stdin is a terminal
	if !term.IsTerminal(syscall.Stdin) {
		// Not a terminal - never read passphrases from piped input
		// for security reasons
		return nil, errStdinNotTerminal
	}

	// stdin is a terminal, check if stderr is also a terminal for
	// interactive prompting
	if !term.IsTerminal(syscall.Stderr) {
		return nil, errStderrNotTerminal
	}

	// Both stdin and stderr are terminals - use secure password reading
	fmt.Fprint(os.Stderr, prompt) // Write prompt to stderr, not stdout

	passphrase, err := term.ReadPassword(syscall.Stdin)
	if err != nil {
		return nil, fmt.Errorf("failed to read passphrase: %w", err)
	}

	// Print newline to stderr since ReadPassword doesn't echo
	fmt.Fprintln(os.Stderr)

	if len(passphrase) == 0 {
		return nil, errEmptyPassphrase
	}

	// Create a secure buffer and copy the passphrase
	secureBuffer := memguard.NewBufferFromBytes(passphrase)

	// Clear the original passphrase slice
	for i := range passphrase {
		passphrase[i] = 0
	}

	return secureBuffer, nil
}