~bzr-pqm/bzr/bzr.dev

Authentication ring

===================

When accessing a remote branch (specified as an URL), it may occur that the

server requests an authentication.

This authentication can be provided in different ways:

1. Embedding the user and password

in the URL::

  bzr branch <scheme>://<user>:<password>@host:port/path

* ``scheme``: Any transport protocol requiring authentication.

* ``user``: The login used to authenticate.

* ``password``: The associated password.

* ``host``: The address of the server.

* ``port``: The port the server is listening to.

* ``path``: The path on the server.

2. Embedding the user in the URL and let bzr find the right password or prompt

for one::

  bzr branch <scheme>://<user>@host/path

3. Embedding nothing in the URL and let bzr find user and password or prompt

for user and/or password::

  bzr branch <scheme>://host/path

This specification proposes a mechanism that will allow users to

just use ``bzr branch <scheme>://host/path`` or ``bzr branch

<scheme>://<user>@host/path`` and leaves bzr find the ``user``

and ``password`` in its configuration files.

When no user is specified for ``FTP``, ``SFTP`` or ``SSH``, the actual behavior

of ``bzr`` is to default to ``getpass.get_user()``.

Any implementation of this specification should respect that behaviour.

This specification also proposes a way to describe credentials so that several

remote branches can use the same definition. This is particularily important

for users handling a lot of passwords and who need to update them on a regular

basis.

Rationale

---------

Embedding user and passwords in the command line is a security

hazard (see `bug #34685

<https://launchpad.net/products/bzr/+bug/34685>`_).

Storing passwords in ``~/.bazaar/bazaar.conf`` or ``~/.bazaar/locations.conf``

is also a security risk.

Typing user and passwords is error-prone and boring.

Yet, a safe way to store passwords, while allowing bzr to retrieve them, when

needed, could improve the bzr user experience.

This specification describes a way to provide user and passwords to bzr while

storing them in a relatively safe way.

Note that ssh servers can be configured to use keys instead of (``user``,

``password``) and, when used with appropriate agents, provide the same kind of

comfort this specification aims to provide for all other schemes. Since ssh

agents provide a safer way to secure the passwords, this specification is

restricted to providing ``user`` but does not provide ``password`` when used

for ssh.

Authentication definitions

--------------------------

There are two kinds of authentication used by the various schemes supported by

bzr:

1. user and password

``FTP`` and ``SFTP`` needs a (``user``, ``password``) to authenticate against a

``host`` (SFTP can use ssh keys too, but we don't talk about that in this

specification as ssh agents provide a better solution).

2. user, realm and password

``HTTP`` and ``HTTPS`` needs a (``user, realm, password``) to authenticate

against a host. But, by using ``.htaccess`` files, for example, it is possible

to define several (``user, realm, password``) for a given ``host``. So what is

really needed is (``user``, ``password``, ``host``, ``path``). The ``realm``

can be ignored [#ignored_realm]_ as long as it is still presented to the user

when prompting for the password (unless someone found a way to declare two

different realms for the same path). 

``HTTP proxy`` can be handled as ``HTTP`` (or ``HTTPS``) by explicitly

specifying the appropriate port.

.. [#ignored_realm] The true purpose of realms is to allow the same credentials

   to be reused for disjoint hierarchies. Ignoring them in this specification

   aims to simplify the user experience while still allowing to share the same

   credentials for a whole hierarchy.

To take all schemes into account, the password will be deduced from a set of

authentication definitions (``scheme``, ``host``, ``port``, ``path``, ``user``,

``password``).

  * ``scheme``: can be empty (meaning the rest of the definition can be used

    for any scheme), ``SFTP`` and ``bzr+ssh`` should not be used here, ``ssh``

    should be used instead since this is the real scheme regarding

    authentication,

  * ``host``: can be empty (to act as a default for any host),

  * ``port`` can be empty (useful when an host provides several servers for the

    same scheme), only numerical values are allowed, this should be used only

    when the server uses a port different than the scheme standard port,

  * ``path``: can be empty (FTP or SFTP will never use it),

  * ``user``: can be empty (``bzr`` will defaults to python's

    ``getpass.get_user()`` for FTP, SFTP and ssh),

  * ``password``: can be empty (for security reasons, a user may use the

    definitions without storing the passwords but want to be prompted ; or the

    password will be provided by an external plugin via the

    ``password_encoding`` mechanism decribed below). Must be left empty for

    ``ssh``.

  * ``password_encoding``: can be empty (default is ``plaintext``).

Also note that an optional ``verify_certificates=no`` field will allow the

connection to ``HTTPS`` hosts that provides a self certified certificate (the

default should be to refuse the connection and inform the user). (Not

implemented yet)

Multiple definitions can be provided and, for a given URL, bzr will select a

(``user`` [, ``password``]) based on the following rules :

 1. the first match wins,

 2. empty fields match everything,

 3. ``scheme`` matches even if decorators are used in the requested URL,

 4. ``host`` matches exactly or act as a domain if it starts with '.'

    (``project.bzr.sf.net`` will match ``.bzr.sf.net`` but ``projectbzr.sf.net``

    will not match ``bzr.sf.net``).

 5. ``port`` matches if included in the requested URL (exact matches only)

 6. ``path`` matches if included in the requested URL (and by rule #2 above,

    empty paths will match any provided path).

An optional ``password_encoding`` field may specify how the password is encoded

but has no impact on the definition selection.

Possible values are ``plaintext`` (no encoding at all) and ``base64``. When the

field is absent, ``plaintext`` is assumed. Additional encodings may be added in

future versions.

Encoding passwords in ``base64``, while weak, provides protection against

accidental reading (if an administrator have to look into the file, he will not

see the passwords in clear).(Not implemented yet).

This specification intends to ease the authentication providing, not to secure

it in the best possible way.

Plugins can provide additional password encodings. The provided

``netrc_credential_store`` plugin can be used as an example implementation.

Future versions of this specification may provide additional

encodings [#password_encoding]_.

.. [#password_encoding] Additional password encoding methods may be defined

   that will rely on external means to store the password which, in these

   cases, will not appear anymore in the definition. It is assumed that

   additional password encodings will provide a storage outside of the file

   described here. The ``netrc`` encoding, for example, provides passwords by

   retrieving them from the ``.netrc`` file.

File format

-----------

Even if ``~/.bazaar/bazaar.conf`` and ``~/.bazaar/locations.conf`` seems to

provide most of the needed infrastructure, we choose to use a dedicated file

for the authentication info ``~/.bazaar/authentication.conf`` for the following

reasons:

  * allow the user to protect the content of one file only, relaxing security

    constraints on the others,

  * while ``locations.conf`` is organized around *local* branches,

    ``authentication.conf`` is organized around *remote* branches or more

    generally servers. The same authentification definition can even be used

    for several schemes for servers providing those schemes.

``~/.bazaar/authentication.conf`` will use the same file format as

``~/.bazaar/bazaar.conf``.

Each section describes an authentication definition.

The section name is an arbitrary string, only the ``DEFAULT`` value is reserved

and should appear as the *last* section.

Each section should define:

  * ``user``: the login to be used,

Each section could define:

  * ``host``: the remote server,

  * ``port``: the port the server is listening,

  * ``verify_certificates``: to control certificate verification (useful

    for self certified hosts). This applies to HTTPS only. Accepted values

    are yes and no, default to yes.

  * ``path``: the branch location,

  * ``password``: the password,

  * ``password_encoding``: the method used to encode the password if any,

The default content of the file will be::

    [DEFAULT]

This section could define:

  * ``user``: default user to be used (if not defined the usual

    bzr way applies, see below).

  * ``password_encoding``: default password encoding.

Use Cases

---------

The use cases described below use the file format defined above.

  * all FTP connections to the foo.net domain are done with the same (``user``,

    ``password``)::

        # Identity on foo.net

        [foo.net]

        scheme=ftp

        host=foo.net

        user=joe

        password=secret-pass

    will provide ('joe', 'secret-pass') for::

        bzr branch ftp://foo.net/bzr/branch

        bzr pull ftp://bzr.foo.net/bzr/product/branch/trunk

  * all connections are done with the same ``user`` (the remote one for which

    the default bzr one is not appropriate) and the password is always prompted

    with some exceptions::

        # Pet projects on hobby.net

        [hobby]

        host=r.hobby.net

        user=jim

        password=obvious1234

        # Home server

        [home]

        scheme=https

        host=home.net

        user=joe

        password='c2VjcmV0LXBhc3M='

        password_encoding=base64

        verify_certificates=no # Still searching a free certificate provider

        [DEFAULT]

        # Our local user is barbaz, on all remote sites we're known as foobar

        user=foobar

  * an HTTP server and a proxy::

        # development branches on dev server

        [dev]

        scheme=https

        host=dev.company.com

        path=/dev

        user=user1

        password=pass1

        # toy branches

        [localhost]

        scheme=http

        host=dev.company.com

        path=/

        user=user2

        password=pass2

        # proxy

        [proxy]

        scheme=http

        host=proxy.company.com

        port=3128

        user=proxyuser1

        password=proxypass1

  * source hosting provider declaring sub-domains for each project::

        [sfnet domain]

        # we use sftp, but ssh is the scheme used for authentication

        scheme=ssh

        # The leading '.' ensures that 'sf.net' alone doesn't match

        host=.sf.net

        user=georges

        password=ben...son

UI Changes

----------

Depending on the info provided in the URL, bzr will interact with the user in

different ways:

1. ``user`` and ``password`` given in the URL.

  Nothing to do.

2. ``user`` given in the URL.

  Get a password from ``~/.bazaar/authentication.conf`` or prompt

  for one if none is found.

3. No ``user`` given in the URL (and no ``password``).

  Get a user from ``~/.bazaar/authentication.conf`` or prompt for one if none is

  found. Continue as 2. (Not implemented yet)

Note: A user will be queried only if the server requires it for ``HTTP`` or

``HTTPS``, other protocols always require a user.

In any case, if the server refuses the authentication, bzr reports to the user

and terminates.

Implementation constraints

--------------------------

* bzr should be able to prompt for a ``user`` for a given (``scheme``, ``host``

  [, ``realm``]). Note that ``realm`` is available only after a first

  connection attempt to the server.

* No assumptions should be made about the clients of this service

  (i.e. Transport is the primary target but plugins must be able to use it as

  well, the definitions used: (``scheme, host, [port,] path``) are general

  enough to described credentials for ``svn`` servers or LaunchPad xmlrpc

  calls).

* Policies regarding default users may be taken into account by the

  implementations, there is no good way to represent that in this specification

  and stays flexible enough to accommodate various needs (default user policies

  may differ for different schemes and that may be easier to handle in the code

  than in the authentication file itself).

* If no user can be found by the mechanism described above, bzr should still

  default to ``getpass.get_user()`` and may attempt a second matching to obtain

  a password.

* As this specification proposes a matching between some credentials

  definitions and real urls, the implementation provides an optional UI

  feedback about which credential definition is used. Using ``-Dauth`` will

  output some traces in the ``.bzr.log`` file metionning the sections

  used. This allows the user to validate his definitions.

Questions and Answers

---------------------

  * What if a ``.authinfo`` file exists ?

    * It will be ignored,

    * Automatic (one-time) conversions may be proposed if sufficient demand

      exists,

  * What if a ``.netrc`` file exists ?

    * It is honored if the definition specifies

      ``password_encoding=netrc``.

  * What mode should the authentication file use ?

    * 600 read/write for owner only by default, if another mode (more

      permissive) is used, a warning will be issued to inform the users of the

      potential risks.(Not implemented yet)

  * What about using ``seahorse`` on Ubuntu or ``KeyChain Access`` on Mac OS X ?

    * plugins can be written and registered to handle the associated

      ``password_encoding``.

  * Could it be possible to encode the whole authentication file with a ssh key

    ?

    * yes and if the user configure a ssh-agent it will not be queried for

      pass-phrase every time we want to query the file for a password. But

      that seems a bit extreme for a first version.(Not implemented yet and

      may be never)

  * Why can't bzr update the authentication file when it queried the user for a

    password ?

    * a future version may address that but:

      1. The user may want to decide which passwords are stored in the file and

      which aren't.

      2. The user should decide if the passwords are encoded (and how) or not

      (but we may default to base64).

      3. The right definition may be hard to get right, but reducing it to

      (``scheme, host, [port,] user, password``) may be a good start. I.e. no

      path so that all paths on the host will match. The user will have to

      modify it for more complex configurations anyway.