adds fernet key decryption analogous to python

- first fragment currently auto-starts process
  to decrypt static message
- token parsed from message correctly
- token validation not yet sucessfull

app/build.gradle

@@ -48,6 +48,7 @@ dependencies {
     implementation "androidx.camera:camera-lifecycle:$camerax_version"
     implementation 'com.google.zxing:core:3.3.0'
     implementation "androidx.camera:camera-view:1.0.0-alpha31"
+    implementation 'com.macasaet.fernet:fernet-java8:1.4.2'
     testImplementation 'junit:junit:4.+'
     androidTestImplementation 'androidx.test.ext:junit:1.1.2'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'

app/src/main/java/com/example/bump/FirstFragment.kt

@@ -6,6 +6,7 @@ import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
 import androidx.navigation.fragment.findNavController
+import com.example.bump.MessageProcessor
 import com.maenle.bump.databinding.FragmentFirstBinding
 
 /**
@@ -35,6 +36,19 @@ class FirstFragment : Fragment() {
         binding.buttonFirst.setOnClickListener {
             findNavController().navigate(R.id.action_FirstFragment_to_SecondFragment)
         }
+
+        testDecryption()
+    }
+
+    fun testDecryption() {
+        val code = "dydoes-unknowledgeable-indiscretion-househusbands-pot-walloper-indiscretion-discophorous-transcriptions-dydoes-poodle-faker-transcriptions-budlike"
+        var mp = MessageProcessor()
+        if(mp.codeValid(code)) {
+            mp.codeSave(code)
+        }
+
+        mp.decrypt("M1dEAxKZ5HUHCJoRkgGOvAABhqCAAAAAAGG2eKTSlKXWLDQx5B_wssZsNwsanzQID2UyUm4KKuKYKgfwH5MG2N-qzt6K4mg3pfZmWPaiDB9PiqlX236k6zo9Yvvq")
+
     }
 
     override fun onDestroyView() {

app/src/main/java/com/example/bump/MessageProcessor.kt

@@ -0,0 +1,73 @@
+package com.example.bump
+
+import android.util.Log
+import com.google.zxing.common.StringUtils
+import java.security.SecureRandom
+import java.util.Base64
+import javax.crypto.spec.PBEKeySpec
+import javax.crypto.SecretKeyFactory
+import com.macasaet.fernet.Key
+import com.macasaet.fernet.Token
+import com.macasaet.fernet.StringValidator
+import com.macasaet.fernet.Validator
+import com.maenle.bump.MainActivity
+import java.time.Duration
+import java.time.temporal.TemporalAmount
+
+class MessageProcessor {
+    lateinit var sender:String
+    lateinit var key: String
+
+    fun codeValid(code: String): Boolean {
+        return code.split("-").size >= KEY_LENGTH + SENDER_LENGTH
+    }
+
+    fun codeSave(new_code: String) {
+        var code: List<String> = new_code.split("-")
+        sender = code.subList(0, SENDER_LENGTH).joinToString("-")
+        key = code.subList(SENDER_LENGTH, code.size).joinToString("-")
+        Log.d(TAG, sender)
+        Log.d(TAG, key)
+    }
+
+    fun decrypt(message : String) {
+        // Data from encryption
+        val decoded : ByteArray = Base64.getUrlDecoder().decode(message)
+        val salt = decoded.copyOfRange(0, 16)
+        val iter = decoded.copyOfRange(16, 20)
+        val str_token = String(Base64.getUrlEncoder().encode(decoded.copyOfRange(20, decoded.size)))
+
+        // Derive Fernet key
+        val saltedKey = deriveKey(key, salt, 100_000)
+        val fernetKey = Key(saltedKey)
+
+        val token =
+            Token.fromString(str_token);
+
+
+        // Decrypt
+        val validator: Validator<String> = object : StringValidator {
+            override fun getTimeToLive(): TemporalAmount {
+                return Duration.ofHours(24)
+            }
+        }
+        val data = token.validateAndDecrypt(fernetKey, validator)
+        Log.d(TAG, data )
+    }
+
+    fun deriveKey(password: String, salt: ByteArray, iterations : Int): String {
+        val derivedKeyLength = 256
+        val spec = PBEKeySpec(password.toCharArray(), salt, iterations, derivedKeyLength)
+        val secretKeyFactory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256")
+        val key = secretKeyFactory.generateSecret(spec).encoded
+        return Base64.getUrlEncoder().encodeToString(key)
+    }
+
+    companion object {
+        private val TAG = MainActivity::class.java.simpleName
+        val KEY_LENGTH = 8
+        val SENDER_LENGTH = 4
+    }
+
+
+}