Added new logger.

Added a new logger that persists logs for a longer duration to the
user's cache directory. Logs are encrypted. The new logs are sent
in addition to the user's logcat output.

src/org/thoughtcrime/securesms/logging/LogSecretProvider.java

@@ -0,0 +1,56 @@
+package org.thoughtcrime.securesms.logging;
+
+import android.content.Context;
+import android.os.Build;
+import android.support.annotation.NonNull;
+
+import org.thoughtcrime.securesms.crypto.KeyStoreHelper;
+import org.thoughtcrime.securesms.util.Base64;
+import org.thoughtcrime.securesms.util.TextSecurePreferences;
+
+import java.io.IOException;
+import java.security.SecureRandom;
+
+class LogSecretProvider {
+
+  static byte[] getOrCreateAttachmentSecret(@NonNull Context context) {
+    String unencryptedSecret = TextSecurePreferences.getLogUnencryptedSecret(context);
+    String encryptedSecret   = TextSecurePreferences.getLogEncryptedSecret(context);
+
+    if      (unencryptedSecret != null) return parseUnencryptedSecret(unencryptedSecret);
+    else if (encryptedSecret != null)   return parseEncryptedSecret(encryptedSecret);
+    else                                return createAndStoreSecret(context);
+  }
+
+  private static byte[] parseUnencryptedSecret(String secret) {
+    try {
+      return Base64.decode(secret);
+    } catch (IOException e) {
+      throw new AssertionError("Failed to decode the unecrypted secret.");
+    }
+  }
+
+  private static byte[] parseEncryptedSecret(String secret) {
+    if (Build.VERSION.SDK_INT > Build.VERSION_CODES.M) {
+      KeyStoreHelper.SealedData encryptedSecret = KeyStoreHelper.SealedData.fromString(secret);
+      return KeyStoreHelper.unseal(encryptedSecret);
+    } else {
+      throw new AssertionError("OS downgrade not supported. KeyStore sealed data exists on platform < M!");
+    }
+  }
+
+  private static byte[] createAndStoreSecret(@NonNull Context context) {
+    SecureRandom random = new SecureRandom();
+    byte[]       secret = new byte[32];
+    random.nextBytes(secret);
+
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
+      KeyStoreHelper.SealedData encryptedSecret = KeyStoreHelper.seal(secret);
+      TextSecurePreferences.setLogEncryptedSecret(context, encryptedSecret.serialize());
+    } else {
+      TextSecurePreferences.setLogUnencryptedSecret(context, Base64.encodeBytes(secret));
+    }
+
+    return secret;
+  }
+}