Factor getting secure random bytes into Crypto interface

app/src/main/java/com/stevesoltys/seedvault/crypto/Crypto.kt

@@ -13,6 +13,7 @@ import java.io.IOException
 import java.io.InputStream
 import java.io.OutputStream
 import java.security.GeneralSecurityException
+import java.security.SecureRandom
 import javax.crypto.spec.SecretKeySpec
 
 /**
@@ -33,6 +34,11 @@ import javax.crypto.spec.SecretKeySpec
  */
 internal interface Crypto {
 
+    /**
+     * Returns a ByteArray with bytes retrieved from [SecureRandom].
+     */
+    fun getRandomBytes(size: Int): ByteArray
+
     /**
      * Returns a [AesGcmHkdfStreaming] encrypting stream
      * that gets encrypted and authenticated the given associated data.
@@ -107,6 +113,11 @@ internal class CryptoImpl(
     private val key: ByteArray by lazy {
         deriveStreamKey(keyManager.getMainKey(), "app data key".toByteArray())
     }
+    private val secureRandom: SecureRandom by lazy { SecureRandom() }
+
+    override fun getRandomBytes(size: Int) = ByteArray(size).apply {
+        secureRandom.nextBytes(this)
+    }
 
     @Throws(IOException::class, GeneralSecurityException::class)
     override fun newEncryptingStream(

app/src/main/java/com/stevesoltys/seedvault/ui/recoverycode/RecoveryCodeViewModel.kt

@@ -22,7 +22,6 @@ import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.launch
 import org.calyxos.backup.storage.api.StorageBackup
 import java.io.IOException
-import java.security.SecureRandom
 
 internal const val WORD_NUM = 12
 
@@ -40,8 +39,7 @@ internal class RecoveryCodeViewModel(
 
     internal val wordList: List<CharArray> by lazy {
         // we use our own entropy to not having to trust the library to use SecureRandom
-        val entropy = ByteArray(Mnemonics.WordCount.COUNT_12.bitLength / 8)
+        val entropy = crypto.getRandomBytes(Mnemonics.WordCount.COUNT_12.bitLength / 8)
-        SecureRandom().nextBytes(entropy)
         // create the words from the entropy
         Mnemonics.MnemonicCode(entropy).words
     }

app/src/test/java/com/stevesoltys/seedvault/crypto/CryptoIntegrationTest.kt

@@ -2,6 +2,9 @@ package com.stevesoltys.seedvault.crypto
 
 import com.stevesoltys.seedvault.assertReadEquals
 import com.stevesoltys.seedvault.header.HeaderReaderImpl
+import org.hamcrest.MatcherAssert.assertThat
+import org.hamcrest.Matchers.equalTo
+import org.hamcrest.Matchers.not
 import org.junit.jupiter.api.Assertions.assertThrows
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.TestInstance
@@ -21,6 +24,14 @@ class CryptoIntegrationTest {
 
     private val cleartext = Random.nextBytes(Random.nextInt(1, 422300))
 
+    @Test
+    fun `sanity check for getRandomBytes()`() {
+        assertThat(crypto.getRandomBytes(42), not(equalTo(crypto.getRandomBytes(42))))
+        assertThat(crypto.getRandomBytes(42), not(equalTo(crypto.getRandomBytes(42))))
+        assertThat(crypto.getRandomBytes(42), not(equalTo(crypto.getRandomBytes(42))))
+        assertThat(crypto.getRandomBytes(42), not(equalTo(crypto.getRandomBytes(42))))
+    }
+
     @Test
     fun `decrypting encrypted cleartext works`() {
         val ad = Random.nextBytes(42)