Add crypto method to get salted names for package data and APKs

This will only hide installed apps from naive attackers as the APKs are still not encrypted and even then other attacks would be possible.

However, it allows us to simplify our storage plugin API.

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

@@ -1,6 +1,7 @@
 package com.stevesoltys.seedvault.crypto
 
 import com.google.crypto.tink.subtle.AesGcmHkdfStreaming
+import com.stevesoltys.seedvault.encodeBase64
 import com.stevesoltys.seedvault.header.HeaderReader
 import com.stevesoltys.seedvault.header.MAX_SEGMENT_LENGTH
 import com.stevesoltys.seedvault.header.MAX_VERSION_HEADER_SIZE
@@ -13,6 +14,8 @@ import java.io.IOException
 import java.io.InputStream
 import java.io.OutputStream
 import java.security.GeneralSecurityException
+import java.security.MessageDigest
+import java.security.NoSuchAlgorithmException
 import java.security.SecureRandom
 import javax.crypto.spec.SecretKeySpec
 
@@ -39,6 +42,10 @@ internal interface Crypto {
      */
     fun getRandomBytes(size: Int): ByteArray
 
+    fun getNameForPackage(salt: String, packageName: String): String
+
+    fun getNameForApk(salt: String, packageName: String, suffix: String = ""): String
+
     /**
      * Returns a [AesGcmHkdfStreaming] encrypting stream
      * that gets encrypted and authenticated the given associated data.
@@ -119,6 +126,24 @@ internal class CryptoImpl(
         secureRandom.nextBytes(this)
     }
 
+    override fun getNameForPackage(salt: String, packageName: String): String {
+        return sha256("$salt$packageName".toByteArray()).encodeBase64()
+    }
+
+    override fun getNameForApk(salt: String, packageName: String, suffix: String): String {
+        return sha256("${salt}APK$packageName$suffix".toByteArray()).encodeBase64()
+    }
+
+    private fun sha256(bytes: ByteArray): ByteArray {
+        val messageDigest: MessageDigest = try {
+            MessageDigest.getInstance("SHA-256")
+        } catch (e: NoSuchAlgorithmException) {
+            throw AssertionError(e)
+        }
+        messageDigest.update(bytes)
+        return messageDigest.digest()
+    }
+
     @Throws(IOException::class, GeneralSecurityException::class)
     override fun newEncryptingStream(
         outputStream: OutputStream,

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

@@ -1,7 +1,12 @@
 package com.stevesoltys.seedvault.crypto
 
+import com.stevesoltys.seedvault.getRandomBase64
+import com.stevesoltys.seedvault.getRandomString
 import com.stevesoltys.seedvault.header.HeaderReaderImpl
+import com.stevesoltys.seedvault.metadata.METADATA_SALT_SIZE
 import io.mockk.mockk
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertNotEquals
 import org.junit.jupiter.api.Assertions.assertThrows
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.TestInstance
@@ -26,4 +31,64 @@ class CryptoImplTest {
         }
     }
 
+    @Test
+    fun `getNameForPackage() and getNameForApk() return deterministic results`() {
+        val salt = getRandomBase64(METADATA_SALT_SIZE)
+        val packageName = getRandomString(32)
+        assertEquals(
+            crypto.getNameForPackage(salt, packageName),
+            crypto.getNameForPackage(salt, packageName)
+        )
+        assertEquals(
+            crypto.getNameForApk(salt, packageName),
+            crypto.getNameForApk(salt, packageName)
+        )
+    }
+
+    @Test
+    fun `getNameForPackage() and getNameForApk() return different results`() {
+        val salt = getRandomBase64(METADATA_SALT_SIZE)
+        val packageName = getRandomString(32)
+        assertNotEquals(
+            crypto.getNameForPackage(salt, packageName),
+            crypto.getNameForApk(salt, packageName)
+        )
+    }
+
+    @Test
+    fun `getNameForPackage() and getNameForApk() return different results for different salts`() {
+        val packageName = getRandomString(32)
+        assertNotEquals(
+            crypto.getNameForPackage(getRandomBase64(METADATA_SALT_SIZE), packageName),
+            crypto.getNameForPackage(getRandomBase64(METADATA_SALT_SIZE), packageName)
+        )
+        assertNotEquals(
+            crypto.getNameForApk(getRandomBase64(METADATA_SALT_SIZE), packageName),
+            crypto.getNameForApk(getRandomBase64(METADATA_SALT_SIZE), packageName)
+        )
+    }
+
+    @Test
+    fun `getNameForPackage() and getNameForApk() return different for different package names`() {
+        val salt = getRandomBase64(METADATA_SALT_SIZE)
+        assertNotEquals(
+            crypto.getNameForPackage(salt, getRandomString(32)),
+            crypto.getNameForPackage(salt, getRandomString(32))
+        )
+        assertNotEquals(
+            crypto.getNameForApk(salt, getRandomString(32)),
+            crypto.getNameForApk(salt, getRandomString(32))
+        )
+    }
+
+    @Test
+    fun `getNameForApk() return different results for different suffixes`() {
+        val salt = getRandomBase64(METADATA_SALT_SIZE)
+        val packageName = getRandomString(32)
+        assertNotEquals(
+            crypto.getNameForApk(salt, packageName, getRandomString(4)),
+            crypto.getNameForApk(salt, packageName, getRandomString(5))
+        )
+    }
+
 }