wif.go

// Copyright (c) 2013-2016 The btcsuite developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package btcutil

import (
	"bytes"
	"errors"

	"github.com/btcsuite/btcd/btcec"
	"github.com/btcsuite/btcd/chaincfg"
	"github.com/btcsuite/btcd/chaincfg/chainhash"
	"github.com/btcsuite/btcutil/base58"
)

// ErrMalformedPrivateKey describes an error where a WIF-encoded private
// key cannot be decoded due to being improperly formatted.  This may occur
// if the byte length is incorrect or an unexpected magic number was
// encountered.
var ErrMalformedPrivateKey = errors.New("malformed private key")

// compressMagic is the magic byte used to identify a WIF encoding for
// an address created from a compressed serialized public key.
const compressMagic byte = 0x01

// WIF contains the individual components described by the Wallet Import Format
// (WIF).  A WIF string is typically used to represent a private key and its
// associated address in a way that  may be easily copied and imported into or
// exported from wallet software.  WIF strings may be decoded into this
// structure by calling DecodeWIF or created with a user-provided private key
// by calling NewWIF.
type WIF struct {
	// PrivKey is the private key being imported or exported.
	PrivKey *btcec.PrivateKey

	// CompressPubKey specifies whether the address controlled by the
	// imported or exported private key was created by hashing a
	// compressed (33-byte) serialized public key, rather than an
	// uncompressed (65-byte) one.
	CompressPubKey bool

	// netID is the bitcoin network identifier byte used when
	// WIF encoding the private key.
	netID byte
}

// NewWIF creates a new WIF structure to export an address and its private key
// as a string encoded in the Wallet Import Format.  The compress argument
// specifies whether the address intended to be imported or exported was created
// by serializing the public key compressed rather than uncompressed.
func NewWIF(privKey *btcec.PrivateKey, net *chaincfg.Params, compress bool) (*WIF, error) {
	if net == nil {
		return nil, errors.New("no network")
	}
	return &WIF{privKey, compress, net.PrivateKeyID}, nil
}

// IsForNet returns whether or not the decoded WIF structure is associated
// with the passed bitcoin network.
func (w *WIF) IsForNet(net *chaincfg.Params) bool {
	return w.netID == net.PrivateKeyID
}

// DecodeWIF creates a new WIF structure by decoding the string encoding of
// the import format.
//
// The WIF string must be a base58-encoded string of the following byte
// sequence:
//
//  * 1 byte to identify the network, must be 0x80 for mainnet or 0xef for
//    either testnet3 or the regression test network
//  * 32 bytes of a binary-encoded, big-endian, zero-padded private key
//  * Optional 1 byte (equal to 0x01) if the address being imported or exported
//    was created by taking the RIPEMD160 after SHA256 hash of a serialized
//    compressed (33-byte) public key
//  * 4 bytes of checksum, must equal the first four bytes of the double SHA256
//    of every byte before the checksum in this sequence
//
// If the base58-decoded byte sequence does not match this, DecodeWIF will
// return a non-nil error.  ErrMalformedPrivateKey is returned when the WIF
// is of an impossible length or the expected compressed pubkey magic number
// does not equal the expected value of 0x01.  ErrChecksumMismatch is returned
// if the expected WIF checksum does not match the calculated checksum.
func DecodeWIF(wif string) (*WIF, error) {
	decoded := base58.Decode(wif)
	decodedLen := len(decoded)
	var compress bool

	// Length of base58 decoded WIF must be 32 bytes + an optional 1 byte
	// (0x01) if compressed, plus 1 byte for netID + 4 bytes of checksum.
	switch decodedLen {
	case 1 + btcec.PrivKeyBytesLen + 1 + 4:
		if decoded[33] != compressMagic {
			return nil, ErrMalformedPrivateKey
		}
		compress = true
	case 1 + btcec.PrivKeyBytesLen + 4:
		compress = false
	default:
		return nil, ErrMalformedPrivateKey
	}

	// Checksum is first four bytes of double SHA256 of the identifier byte
	// and privKey.  Verify this matches the final 4 bytes of the decoded
	// private key.
	var tosum []byte
	if compress {
		tosum = decoded[:1+btcec.PrivKeyBytesLen+1]
	} else {
		tosum = decoded[:1+btcec.PrivKeyBytesLen]
	}
	cksum := chainhash.DoubleHashB(tosum)[:4]
	if !bytes.Equal(cksum, decoded[decodedLen-4:]) {
		return nil, ErrChecksumMismatch
	}

	netID := decoded[0]
	privKeyBytes := decoded[1 : 1+btcec.PrivKeyBytesLen]
	privKey, _ := btcec.PrivKeyFromBytes(btcec.S256(), privKeyBytes)
	return &WIF{privKey, compress, netID}, nil
}

// String creates the Wallet Import Format string encoding of a WIF structure.
// See DecodeWIF for a detailed breakdown of the format and requirements of
// a valid WIF string.
func (w *WIF) String() string {
	// Precalculate size.  Maximum number of bytes before base58 encoding
	// is one byte for the network, 32 bytes of private key, possibly one
	// extra byte if the pubkey is to be compressed, and finally four
	// bytes of checksum.
	encodeLen := 1 + btcec.PrivKeyBytesLen + 4
	if w.CompressPubKey {
		encodeLen++
	}

	a := make([]byte, 0, encodeLen)
	a = append(a, w.netID)
	// Pad and append bytes manually, instead of using Serialize, to
	// avoid another call to make.
	a = paddedAppend(btcec.PrivKeyBytesLen, a, w.PrivKey.D.Bytes())
	if w.CompressPubKey {
		a = append(a, compressMagic)
	}
	cksum := chainhash.DoubleHashB(a)[:4]
	a = append(a, cksum...)
	return base58.Encode(a)
}

// SerializePubKey serializes the associated public key of the imported or
// exported private key in either a compressed or uncompressed format.  The
// serialization format chosen depends on the value of w.CompressPubKey.
func (w *WIF) SerializePubKey() []byte {
	pk := (*btcec.PublicKey)(&w.PrivKey.PublicKey)
	if w.CompressPubKey {
		return pk.SerializeCompressed()
	}
	return pk.SerializeUncompressed()
}

// paddedAppend appends the src byte slice to dst, returning the new slice.
// If the length of the source is smaller than the passed size, leading zero
// bytes are appended to the dst slice before appending src.
func paddedAppend(size uint, dst, src []byte) []byte {
	for i := 0; i < int(size)-len(src); i++ {
		dst = append(dst, 0)
	}
	return append(dst, src...)
}