# Internals

The internals chapter describes and analyzes most of the inner workings of Borg.

Borg uses a low-level, key-value store, the Repository, and implements a more complex data structure on top of it, which is made up of the manifest, archives, items and data Chunks.

Each repository can hold multiple archives, which represent individual backups that contain a full archive of the files specified when the backup was performed.

Deduplication is performed globally across all data in the repository (multiple backups and even multiple hosts), both on data and file metadata, using Chunks created by the chunker using the Buzhash algorithm (“buzhash” and “buzhash64” chunker) or a simpler fixed block size algorithm (“fixed” chunker).

To perform the repository-wide deduplication, a hash of each chunk is checked against the chunks cache, which is a hash table of all chunks that already exist.

Layers in Borg. At the very top, commands are implemented, using a data access layer provided by the Archive and Item classes. The “key” object provides both compression and authenticated encryption used by the data access layer. The “key” object represents the sole trust boundary in Borg. The lowest layer is the repository, either accessed directly (Repository) or remotely (RemoteRepository).

Internals contents

- Security
  - Cryptography in Borg
    - Attack model
    - Structural Authentication
    - Passphrase notes
    - Encryption
      - AEAD modes
      - Legacy modes
    - Offline key security
    - Implementations used
  - Remote RPC protocol security
  - Using OpenSSL
  - Compression and Encryption
  - Fingerprinting
    - Stored chunk sizes
      - buzhash and buzhash64 chunker
      - fixed chunker
    - Secret key usage against fingerprinting
    - Stored chunk proximity
- Data structures and file formats
  - Repository
    - Keys
    - Repository objects
    - Repo object metadata
    - Compaction
  - The object graph
    - The manifest
      - Feature flags
    - Archives
    - Items
    - Chunks
      - “fixed” chunker
      - “buzhash” chunker
      - “buzhash64” chunker
  - The cache
  - Indexes / Caches memory usage
  - HashIndex
  - Encryption
    - AEAD modes
    - Legacy modes
    - Both modes
  - Key files
  - Compression
  - Lock files (fslocking)
  - Locks (storelocking)
  - Breaking the locks
  - Checksumming data structures
    - Lower layer — file_integrity
    - Upper layer
  - HardLinkManager and the hlid concept
- All about JSON: How to develop frontends
  - Dealing with non-unicode byte sequences and JSON limitations
  - Logging
  - Prompts
  - Passphrase prompts
  - Standard output
    - Archive formats
    - File listings
    - Archive Differencing
  - Message IDs