Add typo detector test, fix panics

keys/wordcodec.go

@@ -122,6 +122,11 @@ func (c WordCodec) BytesToWords(raw []byte) (words []string, err error) {
 
 func (c WordCodec) WordsToBytes(words []string) ([]byte, error) {
 	l := len(words)
+
+	if l == 0 {
+		return nil, errors.New("Didn't provide any words")
+	}
+
 	n2048 := big.NewInt(2048)
 	nData := big.NewInt(0)
 	// since we output words based on the remainder, the first word has the lowest
@@ -143,6 +148,9 @@ func (c WordCodec) WordsToBytes(words []string) ([]byte, error) {
 	// copy into the container we have with the expected size
 	outLen, flex := bytelenFromWords(len(words))
 	toCheck := make([]byte, outLen)
+	if len(dataBytes) > outLen {
+		return nil, errors.New("Invalid data, could not have been generated by this codec")
+	}
 	copy(toCheck[outLen-len(dataBytes):], dataBytes)
 
 	// validate the checksum...