Saving Data on Android

Decryp/ng the file
Locate 
decrypt
 and add the following code:
private
 
fun
 
decrypt
(map: HashMap)
: ByteArray? 
{ 
  
var
 decrypted: ByteArray? = 
null
 
  
try
 { 
    
// 1
 
    
val
 salt = map[
"salt"
] 
    
val
 iv = map[
"iv"
] 
    
val
 encrypted = map[
"encrypted"
] 
    
// 2 
 
    
val
 passwordChar = passwordString.toCharArray() 
    
val
 pbKeySpec = PBEKeySpec(passwordChar, salt, 
1324
, 
256
) 
    
val
 secretKeyFactory = 
SecretKeyFactory.getInstance(
"PBKDF2WithHmacSHA1"
) 
    
val
 keyBytes = 
secretKeyFactory.generateSecret(pbKeySpec).encoded 
    
val
 keySpec = SecretKeySpec(keyBytes, 
"AES"
) 
    
// 3 
 
    
val
 cipher = Cipher.getInstance(
"AES/CBC/PKCS7Padding"
) 
    
val
 ivSpec = IvParameterSpec(iv) 
    cipher.init(Cipher.DECRYPT_MODE, keySpec, ivSpec) 
    decrypted = cipher.doFinal(encrypted) 
  } 
catch
 (e: Exception) { 
    Log.e(
"SIMPLENOTE"
, 
"decryption exception"
, e) 
  } 
  
// 4
 
  
return
 decrypted 
}
Saving Data on Android
Chapter 1: Using Files
raywenderlich.com
36

---

Here’s what’s happening:
1. Retrieve the 
salt
, 
iv
 and 
encrypted
 fields from the 
HashMap
.
2. Regenerate the key from the password.
3. Decrypt the encrypted data.
4. Return the decrypted data.
Call the decrypt function after reading the file in 
getNote
.
val
 note = Note(fileName, 
""
) 
if
 (isExternalStorageReadable()) { 
  
// 1
 
  ObjectInputStream(noteFileInputStream(note.fileName)).use 
{ stream -> 
    
// 2
 
    
val
 mapFromFile = stream.readObject() 
as
 HashMapByteArray> 
    
// 3
 
    
val
 decrypted = decrypt(mapFromFile) 
    
if
 (decrypted != 
null
) { 
      note.noteText = String(decrypted) 
    } 
  } 
} 
return
 note
Here’s how it works:
1. Open an 
ObjectInputStream
 with 
use
 for reading data.
2. Read the data and store it into the 
HashMap
.
3. Decrypt the file with 
decrypt
; if it was successful, assign the decrypt text to 
note.noteText
.
Build and run the code now. Notice the weird, encrypted, obfuscated data is now 
decrypted and understandable again! :]
Saving Data on Android
Chapter 1: Using Files
raywenderlich.com
37

---

Understanding Parceliza/on and 
Serializa/on
In computer science, ** marshaling** or 
marshaling
 is the process of transforming 
an object into a format that is suitable for transmission or storage. Android apps 
often transfer data from one activity to another, where 
Parcelization
 and 
Serialization
 are the means of marshaling objects.
By default — and because your app utilized 
ObjectOutputStream
 to write the file — 
the encrypted data, iv and salt values were all