Add BIP39 key derivation

Author: guggero

bip39/bip39.go

@@ -0,0 +1,137 @@
+// Package bip39 is the Golang implementation of the BIP39 spec.
+// This code was copied from https://github.com/tyler-smith/go-bip39 which is
+// also MIT licensed.
+//
+// The official BIP39 spec can be found at
+// https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki
+package bip39
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"math/big"
+	"strings"
+)
+
+var (
+	// Some bitwise operands for working with big.Ints
+	shift11BitsMask = big.NewInt(2048)
+	bigOne          = big.NewInt(1)
+
+	// used to isolate the checksum bits from the entropy+checksum byte array
+	wordLengthChecksumMasksMapping = map[int]*big.Int{
+		12: big.NewInt(15),
+		15: big.NewInt(31),
+		18: big.NewInt(63),
+		21: big.NewInt(127),
+		24: big.NewInt(255),
+	}
+	// used to use only the desired x of 8 available checksum bits.
+	// 256 bit (word length 24) requires all 8 bits of the checksum,
+	// and thus no shifting is needed for it (we would get a divByZero crash
+	// if we did)
+	wordLengthChecksumShiftMapping = map[int]*big.Int{
+		12: big.NewInt(16),
+		15: big.NewInt(8),
+		18: big.NewInt(4),
+		21: big.NewInt(2),
+	}
+)
+
+var (
+	// ErrInvalidMnemonic is returned when trying to use a malformed mnemonic.
+	ErrInvalidMnemonic = errors.New("invalid mnenomic")
+
+	// ErrChecksumIncorrect is returned when entropy has the incorrect checksum.
+	ErrChecksumIncorrect = errors.New("checksum incorrect")
+)
+
+// EntropyFromMnemonic takes a mnemonic generated by this library,
+// and returns the input entropy used to generate the given mnemonic.
+// An error is returned if the given mnemonic is invalid.
+func EntropyFromMnemonic(mnemonic string) ([]byte, error) {
+	mnemonicSlice, isValid := splitMnemonicWords(mnemonic)
+	if !isValid {
+		return nil, ErrInvalidMnemonic
+	}
+
+	wordMap := make(map[string]int)
+	for i, v := range English {
+		wordMap[v] = i
+	}
+	
+	// Decode the words into a big.Int.
+	b := big.NewInt(0)
+	for _, v := range mnemonicSlice {
+		index, found := wordMap[v]
+		if found == false {
+			return nil, fmt.Errorf("word `%v` not found in " +
+				"reverse map", v)
+		}
+		var wordBytes [2]byte
+		binary.BigEndian.PutUint16(wordBytes[:], uint16(index))
+		b = b.Mul(b, shift11BitsMask)
+		b = b.Or(b, big.NewInt(0).SetBytes(wordBytes[:]))
+	}
+
+	// Build and add the checksum to the big.Int.
+	checksum := big.NewInt(0)
+	checksumMask := wordLengthChecksumMasksMapping[len(mnemonicSlice)]
+	checksum = checksum.And(b, checksumMask)
+
+	b.Div(b, big.NewInt(0).Add(checksumMask, bigOne))
+
+	// The entropy is the underlying bytes of the big.Int. Any upper bytes
+	// of all 0's are not returned so we pad the beginning of the slice with
+	// empty bytes if necessary.
+	entropy := b.Bytes()
+	entropy = padByteSlice(entropy, len(mnemonicSlice)/3*4)
+
+	// Generate the checksum and compare with the one we got from the mneomnic.
+	entropyChecksumBytes := computeChecksum(entropy)
+	entropyChecksum := big.NewInt(int64(entropyChecksumBytes[0]))
+	if l := len(mnemonicSlice); l != 24 {
+		checksumShift := wordLengthChecksumShiftMapping[l]
+		entropyChecksum.Div(entropyChecksum, checksumShift)
+	}
+
+	if checksum.Cmp(entropyChecksum) != 0 {
+		return nil, ErrChecksumIncorrect
+	}
+
+	return entropy, nil
+}
+
+func computeChecksum(data []byte) []byte {
+	hasher := sha256.New()
+	hasher.Write(data)
+	return hasher.Sum(nil)
+}
+
+// padByteSlice returns a byte slice of the given size with contents of the
+// given slice left padded and any empty spaces filled with 0's.
+func padByteSlice(slice []byte, length int) []byte {
+	offset := length - len(slice)
+	if offset <= 0 {
+		return slice
+	}
+	newSlice := make([]byte, length)
+	copy(newSlice[offset:], slice)
+	return newSlice
+}
+
+func splitMnemonicWords(mnemonic string) ([]string, bool) {
+	// Create a list of all the words in the mnemonic sentence
+	words := strings.Fields(mnemonic)
+
+	// Get num of words
+	numOfWords := len(words)
+
+	// The number of words should be 12, 15, 18, 21 or 24
+	if numOfWords%3 != 0 || numOfWords < 12 || numOfWords > 24 {
+		return nil, false
+	}
+	return words, true
+}