Quick Start

Kopia is a simple command-line tool for managing encrypted backups in the cloud.

Key Concepts

Repository is a Content-Addressable Store of files and directories (known as Objects) identified by their Object IDs.
- Object ID is comprised of the type, the identifier of the block (Block ID) where the contents are stored and the encryption key. Example block ID:
```
D7ce4f067c179664...e746337031644a.715b50351785a6...439637a2c8e50c7
|\------------------------------/ \------------------------------/
type          block id                      encryption key
```
- Block IDs are derived from the contents of objects by using cryptographic hash, typically as one of SHA-2 hash functions.
- Encryption key is also derived from the contents of the object - this technique is known as Convergent Encryption
- Identical objects will have the same ID and thus will be stored only once with the same encryption key.
- A person with access to the object can easily compute its Object ID (including encryption key), but the knowledge of block id is not enough to be able to retrieve the content.
- Repository can be shared among many users as long as they all can compute the same object IDs
Vault securely stores Object IDs, encrypted with user-specific password or passphrase
Blob Storage stores unstructured, blocks of binary large objects (BLOBs). Supported backends include:
- Google Cloud Storage (GCS)
- Local or remote filesystem

There are three types of object IDs:

Data or Direct object where the entire object is stored in a single block
List objects, that store list of object IDs
Inline Text and Inline Binary objects, that represent the contents of the very short files directly in the object IDs, encoded either as text or base64

Some examples (note that block IDs and encryption keys have been shortened for illustration purposes):

D7ce4f067c.715b503c8 - is a Data object stored in block 7ce4f067c and encrypted with key 715b503c8
L43637a2c8.715b50351 - is a List object stored in block 43637a2c8 and encrypted with key 715b50351
Tquick brown fox - is an Inline Text object representing the text quick brown fox
BAQIDBA - is an Inline Binary object representing base-64 encoded bytes: 01 02 03 04

File contents are store as binary
Directory contents are stored as JSON-encoded objects storing file or subdirectory metadata and Object IDs of file/subdirectory contents.