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keyfunc mnemonic derives a 12, 18 or 24 word child mnemonic through BIP-85's own mnemonic application, with the specification's test vectors as tests. One review round, passed with no findings; the reviewer reproduced the vectors from an implementation written from the specification alone. Model: opus-5 (implementation and review); fable-5-1 (landing)
95 lines
2.6 KiB
Go
95 lines
2.6 KiB
Go
// Package derive turns a mnemonic into the bytes a key is made from.
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package derive
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import (
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"errors"
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"fmt"
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"git.eeqj.de/sneak/secret/pkg/bip85"
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"github.com/btcsuite/btcd/btcutil/hdkeychain"
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"github.com/btcsuite/btcd/chaincfg"
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bip39 "github.com/tyler-smith/go-bip39"
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)
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const (
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// purpose is the number BIP-85 reserves for itself.
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purpose = 83696968
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// Size is how many bytes every key type is given.
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Size = 32
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// MaxIndex is the largest key index there is. Every element of
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// the path is hardened, and a hardened BIP-32 child index stops
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// here.
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MaxIndex = 1<<31 - 1
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)
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// ErrIndexTooLarge is returned for a key index above MaxIndex. Such an
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// index has no hardened child to derive, so there is no key to give
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// back rather than a key nobody else would reproduce.
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var ErrIndexTooLarge = errors.New("the key index is too large")
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// Path returns the derivation path for an application number and a key
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// index.
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func Path(application, index uint32) string {
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return fmt.Sprintf("m/%d'/%d'/%d'", purpose, application, index)
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}
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// CheckIndex refuses a key index above MaxIndex, so that every key
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// type turns such an index down before deriving anything.
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func CheckIndex(index uint32) error {
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if index > MaxIndex {
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return fmt.Errorf(
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"%w: %d is above %d",
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ErrIndexTooLarge, index, MaxIndex,
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)
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}
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return nil
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}
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// Master returns the BIP-32 master key a mnemonic stands for: the
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// mnemonic becomes a seed with an empty passphrase, and the seed
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// becomes the key.
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func Master(words string) (*hdkeychain.ExtendedKey, error) {
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seed := bip39.NewSeed(words, "")
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master, err := hdkeychain.NewMaster(seed, &chaincfg.MainNetParams)
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if err != nil {
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return nil, fmt.Errorf("making the master key: %w", err)
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}
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return master, nil
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}
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// Bytes returns the bytes for an application number and a key index.
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// The mnemonic becomes a seed with an empty passphrase, the seed
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// becomes a master key, the master key gives BIP-85 entropy at the
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// path, and the entropy seeds the generator the bytes are read from.
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// An index above MaxIndex is refused before any of that happens.
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func Bytes(words string, application, index uint32) ([]byte, error) {
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err := CheckIndex(index)
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if err != nil {
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return nil, err
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}
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master, err := Master(words)
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if err != nil {
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return nil, err
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}
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entropy, err := bip85.DeriveBIP85Entropy(master, Path(application, index))
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if err != nil {
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return nil, fmt.Errorf("deriving entropy: %w", err)
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}
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out := make([]byte, Size)
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_, err = bip85.NewBIP85DRNG(entropy).Read(out)
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if err != nil {
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return nil, fmt.Errorf("reading derived bytes: %w", err)
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}
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return out, nil
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}
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