keyfunc/internal/mnemonic/mnemonic.go

// Package mnemonic finds the mnemonic the keys are derived from.
package mnemonic

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"slices"
	"strings"

	"golang.org/x/term"
	"sneak.berlin/go/keyfunc/internal/bip39"
)

const (
	// CommandVariable holds a shell command whose output is the
	// mnemonic.
	CommandVariable = "KEYFUNC_MNEMONIC_COMMAND"

	// Variable holds the mnemonic itself.
	Variable = "KEYFUNC_MNEMONIC"
)

// ErrMissing is returned when there is nowhere left to look and
// standard input is not a terminal, so there is nobody to ask.
var ErrMissing = errors.New(
	"no mnemonic given and standard input is not a terminal",
)

// The lengths a BIP-39 mnemonic may have.
const (
	twelve     = 12
	fifteen    = 15
	eighteen   = 18
	twentyOne  = 21
	twentyFour = 24
)

// ErrEmpty is returned for a mnemonic with no words in it, such as the
// output of a mnemonic command that printed nothing.
var ErrEmpty = errors.New("the mnemonic is empty")

// ErrWordCount is returned for a mnemonic with a number of words that
// BIP-39 does not allow.
var ErrWordCount = errors.New("a mnemonic has 12, 15, 18, 21 or 24 words")

// ErrUnknownWord is returned for a mnemonic with a word that is not on
// the BIP-39 English word list. The error gives the word's position,
// never the word, so the mnemonic stays out of the message.
var ErrUnknownWord = errors.New("not on the BIP-39 word list")

// ErrChecksum is returned for a mnemonic that fails the BIP-39
// checksum.
var ErrChecksum = errors.New("the mnemonic fails its BIP-39 checksum")

// Read returns the mnemonic. The command given on the command line is
// used first; then the command in KEYFUNC_MNEMONIC_COMMAND; then the
// mnemonic in KEYFUNC_MNEMONIC; then a prompt on the terminal with
// echo turned off. The first source that has one wins.
func Read(ctx context.Context, command string) (string, error) {
	if command == "" {
		command = os.Getenv(CommandVariable)
	}

	if command != "" {
		words, err := run(ctx, command)
		if err != nil {
			return "", err
		}

		return checked(words)
	}

	if words := os.Getenv(Variable); words != "" {
		return checked(words)
	}

	return ask()
}

// run executes the command with sh and returns its standard output.
// If the command fails, its standard error becomes part of the error.
func run(ctx context.Context, command string) (string, error) {
	//nolint:gosec // running the user's own command is the point
	shell := exec.CommandContext(ctx, "sh", "-c", command)
	shell.Env = ChildEnv()

	var complaint bytes.Buffer

	shell.Stderr = &complaint

	out, err := shell.Output()
	if err != nil {
		said := strings.TrimSpace(complaint.String())
		if said == "" {
			return "", fmt.Errorf("the mnemonic command failed: %w", err)
		}

		return "", fmt.Errorf(
			"the mnemonic command failed: %w: %s", err, said,
		)
	}

	return string(out), nil
}

// ChildEnv is the tool's environment with the mnemonic variables taken
// out, for the programs the tool starts: the mnemonic command, ssh and
// sftp. The mnemonic, from either variable, never leaves the tool.
func ChildEnv() []string {
	environ := os.Environ()
	kept := make([]string, 0, len(environ))

	for _, entry := range environ {
		name, _, _ := strings.Cut(entry, "=")
		if name == Variable || name == CommandVariable {
			continue
		}

		kept = append(kept, entry)
	}

	return kept
}

// ask prompts on the terminal with echo turned off.
func ask() (string, error) {
	fd := int(os.Stdin.Fd())

	if !term.IsTerminal(fd) {
		return "", ErrMissing
	}

	fmt.Fprint(os.Stderr, "mnemonic: ")

	typed, err := term.ReadPassword(fd)

	fmt.Fprintln(os.Stderr)

	if err != nil {
		return "", fmt.Errorf("reading the mnemonic: %w", err)
	}

	return checked(string(typed))
}

// checked joins the words with single spaces, whatever whitespace
// separated them, since the seed is computed over the string itself,
// and refuses a mnemonic that is not a valid BIP-39 mnemonic, with an
// error that says what is wrong with it.
func checked(text string) (string, error) {
	words := strings.Fields(text)

	switch len(words) {
	case 0:
		return "", ErrEmpty
	case twelve, fifteen, eighteen, twentyOne, twentyFour:
	default:
		return "", fmt.Errorf("%w, not %d", ErrWordCount, len(words))
	}

	wordList := bip39.English()

	for i, word := range words {
		if !slices.Contains(wordList, word) {
			return "", fmt.Errorf(
				"word %d of the mnemonic is %w", i+1, ErrUnknownWord,
			)
		}
	}

	joined := strings.Join(words, " ")

	if !bip39.IsMnemonicValid(joined) {
		return "", ErrChecksum
	}

	return joined, nil
}