---
title: "Application Data Storage"
sidebar:
  label: Application Data Storage
---

GpgFrontend employs a robust and secure approach for storing application data,
especially for sensitive or dynamic information such as key server settings, key
generation profiles, and other internal objects.

> Note: Prior to version 2.1.9, application data encryption was handled
> differently: The reference for each data object was generated by simply
> computing the SHA-256 hash of the master application key combined with the
> object name. All objects were encrypted directly with the master key, without
> per-object key derivation, HMAC, or authenticated encryption modes. Advanced
> techniques such as HKDF-based key derivation, per-object HMAC, and
> authenticated encryption (e.g., AES-GCM) were introduced in v2.1.9 for
> significantly improved security.

## Data Object Structure

- Unique Reference: Each piece of application data is associated with a unique
  reference, generated using an HMAC-SHA256 digest of the object name (or a
  random value for unnamed objects) and a legacy key. This ensures both
  uniqueness and integrity of the data mapping.
- Per-Object Encryption: Every data object is encrypted individually using a key
  derived from the current active application secure key and the object’s
  reference. Key derivation utilizes HKDF-SHA256 for high security and
  resistance to key reuse attacks.
- Key Identification: The first part of each stored object file contains an
  identifier for the key used to encrypt it, allowing for seamless key rotation
  and backward compatibility.
- Encryption Algorithms: Actual data encryption uses lightweight, authenticated
  cryptography (such as AES-GCM or an equivalent mode), ensuring both
  confidentiality and integrity of application objects.

## File Layout and Access

- Storage Location: All encrypted data objects are stored in the `data_objs/`
  subdirectory within the application’s data directory.
- File Naming: Encrypted objects are stored as files named after the hexadecimal
  encoding of their reference value. This obfuscates file content and prevents
  direct association with plaintext object names.
- Data Separation: Data created under different SecureLevels or key environments
  is logically separated, providing clear isolation between different security
  contexts.
- Secure Key Retrieval: On access, the application retrieves the correct
  encryption key for each data object using its embedded key identifier, and
  securely derives the per-object key to decrypt or update the content.
- Atomic Updates: Data updates are handled in a manner that prevents corruption
  or partial writes, ensuring data integrity even in the event of application or
  system interruptions.

## Security Considerations

- Strong Isolation: Per-object encryption and unique key derivation mean that
  compromise of one data object does not impact the security of others.
- Key Rotation Support: When application keys are rotated, existing objects
  remain accessible through their associated key identifiers, supporting smooth
  transitions and automated data migration.
- User Transparency: All encryption, decryption, and data migration processes
  are handled transparently by GpgFrontend, requiring no manual intervention
  from users in regular workflows.