borg/docs/internals/security.rst

.. somewhat surprisingly the "bash" highlighter gives nice results with
   the pseudo-code notation used in the "Encryption" section.

.. highlight:: bash

========
Security
========

.. _borgcrypto:

Cryptography in Borg
====================

Attack model
------------

The attack model of Borg is that the environment of the client process
(e.g. ``borg create``) is trusted and the repository (server) is not. The
attacker has any and all access to the repository, including interactive
manipulation (man-in-the-middle) for remote repositories.

Furthermore the client environment is assumed to be persistent across
attacks (practically this means that the security database cannot be
deleted between attacks).

Under these circumstances Borg guarantees that the attacker cannot

1. modify the data of any archive without the client detecting the change
2. rename, remove or add an archive without the client detecting the change
3. recover plain-text data
4. recover definite (heuristics based on access patterns are possible)
   structural information such as the object graph (which archives
   refer to what chunks)

The attacker can always impose a denial of service per definition (he could
forbid connections to the repository, or delete it entirely).

Structural Authentication
-------------------------

Borg is fundamentally based on an object graph structure (see :ref:`internals`),
where the root object is called the manifest.

Borg follows the `Horton principle`_, which states that
not only the message must be authenticated, but also its meaning (often
expressed through context), because every object used is referenced by a
parent object through its object ID up to the manifest. The object ID in
Borg is a MAC of the object's plaintext, therefore this ensures that
an attacker cannot change the context of an object without forging the MAC.

In other words, the object ID itself only authenticates the plaintext of the
object and not its context or meaning. The latter is established by a different
object referring to an object ID, thereby assigning a particular meaning to
an object. For example, an archive item contains a list of object IDs that
represent packed file metadata. On their own it's not clear that these objects
would represent what they do, but by the archive item referring to them
in a particular part of its own data structure assigns this meaning.

This results in a directed acyclic graph of authentication from the manifest
to the data chunks of individual files.

.. rubric:: Authenticating the manifest

Since the manifest has a fixed ID (000...000) the aforementioned authentication
does not apply to it, indeed, cannot apply to it; it is impossible to authenticate
the root node of a DAG through its edges, since the root node has no incoming edges.

With the scheme as described so far an attacker could easily replace the manifest,
therefore Borg includes a tertiary authentication mechanism (TAM) that is applied
to the manifest since version 1.0.9 (see :ref:`tam_vuln`).

TAM works by deriving a separate key through HKDF_ from the other encryption and
authentication keys and calculating the HMAC of the metadata to authenticate [#]_::

    # RANDOM(n) returns n random bytes
    salt = RANDOM(64)

    ikm = id_key || enc_key || enc_hmac_key
    # *context* depends on the operation, for manifest authentication it is
    # the ASCII string "borg-metadata-authentication-manifest".
    tam_key = HKDF-SHA-512(ikm, salt, context)

    # *data* is a dict-like structure
    data[hmac] = zeroes
    packed = pack(data)
    data[hmac] = HMAC(tam_key, packed)
    packed_authenticated = pack(data)

Since an attacker cannot gain access to this key and also cannot make the
client authenticate arbitrary data using this mechanism, the attacker is unable
to forge the authentication.

This effectively 'anchors' the manifest to the key, which is controlled by the
client, thereby anchoring the entire DAG, making it impossible for an attacker
to add, remove or modify any part of the DAG without Borg being able to detect
the tampering.

Note that when using BORG_PASSPHRASE the attacker cannot swap the *entire*
repository against a new repository with e.g. repokey mode and no passphrase,
because Borg will abort access when BORG_PASSPRHASE is incorrect.

However, interactively a user might not notice this kind of attack
immediately, if she assumes that the reason for the absent passphrase
prompt is a set BORG_PASSPHRASE. See issue :issue:`2169` for details.

.. [#] The reason why the authentication tag is stored in the packed
       data itself is that older Borg versions can still read the
       manifest this way, while a changed layout would have broken
       compatibility.

Encryption
----------

Encryption is currently based on the Encrypt-then-MAC construction,
which is generally seen as the most robust way to create an authenticated
encryption scheme from encryption and message authentication primitives.

Every operation (encryption, MAC / authentication, chunk ID derivation)
uses independent, random keys generated by `os.urandom`_ [#]_.

Borg does not support unauthenticated encryption -- only authenticated encryption
schemes are supported. No unauthenticated encryption schemes will be added
in the future.

Depending on the chosen mode (see :ref:`borg_init`) different primitives are used:

- The actual encryption is currently always AES-256 in CTR mode. The
  counter is added in plaintext, since it is needed for decryption,
  and is also tracked locally on the client to avoid counter reuse.

- The authentication primitive is either HMAC-SHA-256 or BLAKE2b-256
  in a keyed mode. HMAC-SHA-256 uses 256 bit keys, while BLAKE2b-256
  uses 512 bit keys.

  The latter is secure not only because BLAKE2b itself is not
  susceptible to `length extension`_, but also since it truncates the
  hash output from 512 bits to 256 bits, which would make the
  construction safe even if BLAKE2b were broken regarding length
  extension or similar attacks.

- The primitive used for authentication is always the same primitive
  that is used for deriving the chunk ID, but they are always
  used with independent keys.

Encryption::

    id = AUTHENTICATOR(id_key, data)
    compressed = compress(data)

    iv = reserve_iv()
    encrypted = AES-256-CTR(enc_key, 8-null-bytes || iv, compressed)
    authenticated = type-byte || AUTHENTICATOR(enc_hmac_key, encrypted) || iv || encrypted


Decryption::

    # Given: input *authenticated* data, possibly a *chunk-id* to assert
    type-byte, mac, iv, encrypted = SPLIT(authenticated)

    ASSERT(type-byte is correct)
    ASSERT( CONSTANT-TIME-COMPARISON( mac, AUTHENTICATOR(enc_hmac_key, encrypted) ) )

    decrypted = AES-256-CTR(enc_key, 8-null-bytes || iv, encrypted)
    decompressed = decompress(decrypted)

    ASSERT( CONSTANT-TIME-COMPARISON( chunk-id, AUTHENTICATOR(id_key, decompressed) ) )

The client needs to track which counter values have been used, since
encrypting a chunk requires a starting counter value and no two chunks
may have overlapping counter ranges (otherwise the bitwise XOR of the
overlapping plaintexts is revealed).

The client does not directly track the counter value, because it
changes often (with each encrypted chunk), instead it commits a
"reservation" to the security database and the repository by taking
the current counter value and adding 4 GiB / 16 bytes (the block size)
to the counter. Thus the client only needs to commit a new reservation
every few gigabytes of encrypted data.

This mechanism also avoids reusing counter values in case the client
crashes or the connection to the repository is severed, since any
reservation would have been committed to both the security database
and the repository before any data is encrypted. Borg uses its
standard mechanism (SaveFile) to ensure that reservations are durable
(on most hardware / storage systems), therefore a crash of the
client's host would not impact tracking of reservations.

However, this design is not infallible, and requires synchronization
between clients, which is handled through the repository. Therefore in
a multiple-client scenario a repository can trick a client into
reusing counter values by ignoring counter reservations and replaying
the manifest (which will fail if the client has seen a more recent
manifest or has a more recent nonce reservation). If the repository is
untrusted, but a trusted synchronization channel exists between
clients, the security database could be synchronized between them over
said trusted channel. This is not part of Borgs functionality.

.. [#] Using the :ref:`borg key migrate-to-repokey <borg_key_migrate-to-repokey>`
       command a user can convert repositories created using Attic in "passphrase"
       mode to "repokey" mode. In this case the keys were directly derived from
       the user's passphrase at some point using PBKDF2.

       Borg does not support "passphrase" mode otherwise any more.

.. _key_encryption:

Offline key security
--------------------

Borg cannot secure the key material while it is running, because the keys
are needed in plain to decrypt/encrypt repository objects.

For offline storage of the encryption keys they are encrypted with a
user-chosen passphrase.

A 256 bit key encryption key (KEK) is derived from the passphrase
using PBKDF2-HMAC-SHA256 with a random 256 bit salt which is then used
to Encrypt-*and*-MAC (unlike the Encrypt-*then*-MAC approach used
otherwise) a packed representation of the keys with AES-256-CTR with a
constant initialization vector of 0. A HMAC-SHA256 of the plaintext is
generated using the same KEK and is stored alongside the ciphertext,
which is converted to base64 in its entirety.

This base64 blob (commonly referred to as *keyblob*) is then stored in
the key file or in the repository config (keyfile and repokey modes
respectively).

This scheme, and specifically the use of a constant IV with the CTR
mode, is secure because an identical passphrase will result in a
different derived KEK for every key encryption due to the salt.

The use of Encrypt-and-MAC instead of Encrypt-then-MAC is seen as
uncritical (but not ideal) here, since it is combined with AES-CTR mode,
which is not vulnerable to padding attacks.


.. seealso::

   Refer to the :ref:`key_files` section for details on the format.

   Refer to issue :issue:`747` for suggested improvements of the encryption
   scheme and password-based key derivation.

Implementations used
--------------------

We do not implement cryptographic primitives ourselves, but rely
on widely used libraries providing them:

- AES-CTR and HMAC-SHA-256 from OpenSSL 1.0 / 1.1 are used,
  which is also linked into the static binaries we provide.
  We think this is not an additional risk, since we don't ever
  use OpenSSL's networking, TLS or X.509 code, but only their
  primitives implemented in libcrypto.
- SHA-256 and SHA-512 from Python's hashlib_ standard library module are used
- HMAC, PBKDF2 and a constant-time comparison from Python's hmac_ standard
  library module is used.
- BLAKE2b is either provided by the system's libb2, an official implementation,
  or a bundled copy of the BLAKE2 reference implementation (written in C).

Implemented cryptographic constructions are:

- Encrypt-then-MAC based on AES-256-CTR and either HMAC-SHA-256
  or keyed BLAKE2b256 as described above under Encryption_.
- Encrypt-and-MAC based on AES-256-CTR and HMAC-SHA-256
  as described above under `Offline key security`_.
- HKDF_-SHA-512

.. _Horton principle: https://en.wikipedia.org/wiki/Horton_Principle
.. _HKDF: https://tools.ietf.org/html/rfc5869
.. _length extension: https://en.wikipedia.org/wiki/Length_extension_attack
.. _hashlib: https://docs.python.org/3/library/hashlib.html
.. _hmac: https://docs.python.org/3/library/hmac.html
.. _os.urandom: https://docs.python.org/3/library/os.html#os.urandom

Remote RPC protocol security
============================

.. note:: This section could be further expanded / detailed.

The RPC protocol is fundamentally based on msgpack'd messages exchanged
over an encrypted SSH channel (the system's SSH client is used for this
by piping data from/to it).

This means that the authorization and transport security properties
are inherited from SSH and the configuration of the SSH client and the
SSH server -- Borg RPC does not contain *any* networking
code. Networking is done by the SSH client running in a separate
process, Borg only communicates over the standard pipes (stdout,
stderr and stdin) with this process. This also means that Borg doesn't
have to directly use a SSH client (or SSH at all). For example,
``sudo`` or ``qrexec`` could be used as an intermediary.

By using the system's SSH client and not implementing a
(cryptographic) network protocol Borg sidesteps many security issues
that would normally impact distributing statically linked / standalone
binaries.

The remainder of this section will focus on the security of the RPC
protocol within Borg.

The assumed worst-case a server can inflict to a client is a
denial of repository service.

The situation were a server can create a general DoS on the client
should be avoided, but might be possible by e.g. forcing the client to
allocate large amounts of memory to decode large messages (or messages
that merely indicate a large amount of data follows). The RPC protocol
code uses a limited msgpack Unpacker to prohibit this.

We believe that other kinds of attacks, especially critical vulnerabilities
like remote code execution are inhibited by the design of the protocol:

1. The server cannot send requests to the client on its own accord,
   it only can send responses. This avoids "unexpected inversion of control"
   issues.
2. msgpack serialization does not allow embedding or referencing code that
   is automatically executed. Incoming messages are unpacked by the msgpack
   unpacker into native Python data structures (like tuples and dictionaries),
   which are then passed to the rest of the program.

   Additional verification of the correct form of the responses could be implemented.
3. Remote errors are presented in two forms:

   1. A simple plain-text *stderr* channel. A prefix string indicates the kind of message
      (e.g. WARNING, INFO, ERROR), which is used to suppress it according to the
      log level selected in the client.

      A server can send arbitrary log messages, which may confuse a user. However,
      log messages are only processed when server requests are in progress, therefore
      the server cannot interfere / confuse with security critical dialogue like
      the password prompt.
   2. Server-side exceptions passed over the main data channel. These follow the
      general pattern of server-sent responses and are sent instead of response data
      for a request.