master_key.go

package bip32

import (
	"crypto/hmac"
	"crypto/sha512"
	"io"
)

// GenerateMasterKey is a more convinient implemenation w.r.t to
// NewMasterKey. The seed needed is read from the given entropy source
// rand, and the seed length is optional which would be RecommendedSeedLen
// if omitted.
func GenerateMasterKey(rand io.Reader, keyID Magic,
	strength ...int) (*PrivateKey, error) {
	seedLen := RecommendedSeedLen
	if len(strength) > 0 && strength[0] >= MinSeedBytes &&
		strength[0] <= MaxSeedBytes {
		seedLen = strength[0]
	}

	seed := make([]byte, seedLen)
	if _, err := io.ReadFull(rand, seed); nil != err {
		return nil, ErrNoEnoughEntropy
	}

	// delegate the common derivation routine
	return newMaster(seed, keyID)
}

// NewMasterKey creates a new master node for use in creating a hierarchical
// deterministic key chain.  The seed must be between 128 and 512 bits and
// should be generated by a cryptographically secure random source.
//
// NOTE: There is an extremely small chance (< 1 in 2^127) the provided seed
// will derive to an unusable secret key.  The ErrUnusable error will be
// returned if this should occur, so the caller must check for it and generate a
// new seed accordingly.
func NewMasterKey(seed []byte, keyID Magic) (*PrivateKey, error) {
	// Per [BIP32], the seed must be in range [MinSeedBytes, MaxSeedBytes].
	if len(seed) < MinSeedBytes || len(seed) > MaxSeedBytes {
		return nil, ErrInvalidSeedLen
	}

	return newMaster(seed, keyID)
}

// newMaster is helper function to derives a extended private key based on the
// given valid seed and the provided keyID.
//
// Official specification goes as https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#master-key-generation
func newMaster(seed []byte, keyID Magic) (*PrivateKey, error) {
	// I = HMAC-SHA512(Key = "Bitcoin seed", Data = S)
	hmac512 := hmac.New(sha512.New, masterHMACKey)
	hmac512.Write(seed)
	I := hmac512.Sum(nil)

	secretKey, chainCode := I[:len(I)/2], I[len(I)/2:]
	// Ensure the key in usable.
	if _, ok := ToUsableScalar(secretKey); !ok {
		return nil, ErrUnusableSeed
	}

	// fingerprint of parent
	parentFP := []byte{0x00, 0x00, 0x00, 0x00}

	return NewPrivateKey(keyID[:], 0, parentFP, 0, chainCode, secretKey), nil
}