Add salt to password hash and store it in an extra chunk

Add a salt that is appended to the input password and is stored in the `saLt` chunk

cryptpng.go

@@ -25,6 +25,7 @@ func check(err error) {
 	}
 }
 
+const saltChunkName = "saLt"
 const chunkName = "crPt"
 const chunkSize = 0x100000
 
@@ -69,8 +70,10 @@ func EncryptDataPng(f *os.File, fin *os.File, fout *os.File) {
 	check(err)
 	inputData, err := ioutil.ReadAll(fin)
 	check(err)
-	inputData, err = encryptData(inputData)
+	inputData, salt := encryptData(inputData)
 	check(err)
+	saltChunk := CreateChunk(salt, saltChunkName)
+	png.AddMetaChunk(saltChunk)
 	chunkCount := int(math.Ceil(float64(len(inputData)) / chunkSize))
 	for i := 0; i < chunkCount; i++ {
 		dataStart := i * chunkSize
@@ -87,12 +90,17 @@ func DecryptDataPng(f *os.File, fout *os.File) {
 	png := PngData{}
 	err := png.Read(f)
 	check(err)
+	salt := make([]byte, 0)
+	saltChunk := png.GetChunk(saltChunkName)
+	if saltChunk != nil {
+		salt = append(salt, saltChunk.data...)
+	}
 	var data []byte
 	for _, cryptChunk := range png.GetChunksByName(chunkName) {
 		data = append(data, cryptChunk.data...)
 	}
 	if len(data) > 0 {
-		data, err = decryptData(data)
+		data, err = decryptData(data, salt)
 		if err != nil {
 			log.Println("\nThe provided password is probably incorrect.")
 		}
@@ -105,26 +113,36 @@ func DecryptDataPng(f *os.File, fout *os.File) {
 }
 
 // creates an encrypted png chunk
-func encryptData(data []byte) ([]byte, error) {
-	key := readPassword()
-	return encrypt(key, data)
+func encryptData(data []byte) ([]byte, []byte) {
+	key, salt := readPassword(nil)
+	encData, err := encrypt(key, data)
+	check(err)
+	return encData, salt
 }
 
 // decrypts the data of a png chunk
-func decryptData(data []byte) ([]byte, error) {
-	key := readPassword()
+func decryptData(data []byte, salt []byte) ([]byte, error) {
+	key, _ := readPassword(&salt)
 	return decrypt(key, data)
 }
 
 // reads a password from the terminal
 // turns off the input for the typing of the password
-func readPassword() []byte {
+func readPassword(passwordSalt *[]byte) ([]byte, []byte) {
 	fmt.Print("Password: ")
 	bytePw, err := terminal.ReadPassword(int(syscall.Stdin))
 	check(err)
 	hash := sha512.New512_256()
-	hash.Write(bytePw)
-	return hash.Sum(nil)
+	if passwordSalt != nil {
+		hash.Write(append(*passwordSalt, bytePw...))
+		return hash.Sum(nil), *passwordSalt
+	} else {
+		salt := make([]byte, 32)
+		_, err = io.ReadFull(rand.Reader, salt)
+		check(err)
+		hash.Write(append(salt, bytePw...))
+		return hash.Sum(nil), salt
+	}
 }
 
 // encrypt and decrypt functions taken from