git/Documentation/git-credential.txt
M Hickford a5c76569e7 credential: new attribute oauth_refresh_token
Git authentication with OAuth access token is supported by every popular
Git host including GitHub, GitLab and BitBucket [1][2][3]. Credential
helpers Git Credential Manager (GCM) and git-credential-oauth generate
OAuth credentials [4][5]. Following RFC 6749, the application prints a
link for the user to authorize access in browser. A loopback redirect
communicates the response including access token to the application.

For security, RFC 6749 recommends that OAuth response also includes
expiry date and refresh token [6]. After expiry, applications can use
the refresh token to generate a new access token without user
reauthorization in browser. GitLab and BitBucket set the expiry at two
hours [2][3]. (GitHub doesn't populate expiry or refresh token.)

However the Git credential protocol has no attribute to store the OAuth
refresh token (unrecognised attributes are silently discarded). This
means that the user has to regularly reauthorize the helper in browser.
On a browserless system, this is particularly intrusive, requiring a
second device.

Introduce a new attribute oauth_refresh_token. This is especially
useful when a storage helper and a read-only OAuth helper are configured
together. Recall that `credential fill` calls each helper until it has a
non-expired password.

```
[credential]
	helper = storage  # eg. cache or osxkeychain
	helper = oauth
```

The OAuth helper can use the stored refresh token forwarded by
`credential fill` to generate a fresh access token without opening the
browser. See
https://github.com/hickford/git-credential-oauth/pull/3/files
for an implementation tested with this patch.

Add support for the new attribute to credential-cache. Eventually, I
hope to see support in other popular storage helpers.

Alternatives considered: ask helpers to store all unrecognised
attributes. This seems excessively complex for no obvious gain.
Helpers would also need extra information to distinguish between
confidential and non-confidential attributes.

Workarounds: GCM abuses the helper get/store/erase contract to store the
refresh token during credential *get* as the password for a fictitious
host [7] (I wrote this hack). This workaround is only feasible for a
monolithic helper with its own storage.

[1] https://github.blog/2012-09-21-easier-builds-and-deployments-using-git-over-https-and-oauth/
[2] https://docs.gitlab.com/ee/api/oauth2.html#access-git-over-https-with-access-token
[3] https://support.atlassian.com/bitbucket-cloud/docs/use-oauth-on-bitbucket-cloud/#Cloning-a-repository-with-an-access-token
[4] https://github.com/GitCredentialManager/git-credential-manager
[5] https://github.com/hickford/git-credential-oauth
[6] https://datatracker.ietf.org/doc/html/rfc6749#section-5.1
[7] 66b94e489a/src/shared/GitLab/GitLabHostProvider.cs (L207)

Signed-off-by: M Hickford <mirth.hickford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-04-21 09:38:30 -07:00

180 lines
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git-credential(1)
=================
NAME
----
git-credential - Retrieve and store user credentials
SYNOPSIS
--------
------------------
'git credential' (fill|approve|reject)
------------------
DESCRIPTION
-----------
Git has an internal interface for storing and retrieving credentials
from system-specific helpers, as well as prompting the user for
usernames and passwords. The git-credential command exposes this
interface to scripts which may want to retrieve, store, or prompt for
credentials in the same manner as Git. The design of this scriptable
interface models the internal C API; see credential.h for more
background on the concepts.
git-credential takes an "action" option on the command-line (one of
`fill`, `approve`, or `reject`) and reads a credential description
on stdin (see <<IOFMT,INPUT/OUTPUT FORMAT>>).
If the action is `fill`, git-credential will attempt to add "username"
and "password" attributes to the description by reading config files,
by contacting any configured credential helpers, or by prompting the
user. The username and password attributes of the credential
description are then printed to stdout together with the attributes
already provided.
If the action is `approve`, git-credential will send the description
to any configured credential helpers, which may store the credential
for later use.
If the action is `reject`, git-credential will send the description to
any configured credential helpers, which may erase any stored
credential matching the description.
If the action is `approve` or `reject`, no output should be emitted.
TYPICAL USE OF GIT CREDENTIAL
-----------------------------
An application using git-credential will typically use `git
credential` following these steps:
1. Generate a credential description based on the context.
+
For example, if we want a password for
`https://example.com/foo.git`, we might generate the following
credential description (don't forget the blank line at the end; it
tells `git credential` that the application finished feeding all the
information it has):
protocol=https
host=example.com
path=foo.git
2. Ask git-credential to give us a username and password for this
description. This is done by running `git credential fill`,
feeding the description from step (1) to its standard input. The complete
credential description (including the credential per se, i.e. the
login and password) will be produced on standard output, like:
protocol=https
host=example.com
username=bob
password=secr3t
+
In most cases, this means the attributes given in the input will be
repeated in the output, but Git may also modify the credential
description, for example by removing the `path` attribute when the
protocol is HTTP(s) and `credential.useHttpPath` is false.
+
If the `git credential` knew about the password, this step may
not have involved the user actually typing this password (the
user may have typed a password to unlock the keychain instead,
or no user interaction was done if the keychain was already
unlocked) before it returned `password=secr3t`.
3. Use the credential (e.g., access the URL with the username and
password from step (2)), and see if it's accepted.
4. Report on the success or failure of the password. If the
credential allowed the operation to complete successfully, then
it can be marked with an "approve" action to tell `git
credential` to reuse it in its next invocation. If the credential
was rejected during the operation, use the "reject" action so
that `git credential` will ask for a new password in its next
invocation. In either case, `git credential` should be fed with
the credential description obtained from step (2) (which also
contain the ones provided in step (1)).
[[IOFMT]]
INPUT/OUTPUT FORMAT
-------------------
`git credential` reads and/or writes (depending on the action used)
credential information in its standard input/output. This information
can correspond either to keys for which `git credential` will obtain
the login information (e.g. host, protocol, path), or to the actual
credential data to be obtained (username/password).
The credential is split into a set of named attributes, with one
attribute per line. Each attribute is specified by a key-value pair,
separated by an `=` (equals) sign, followed by a newline.
The key may contain any bytes except `=`, newline, or NUL. The value may
contain any bytes except newline or NUL.
In both cases, all bytes are treated as-is (i.e., there is no quoting,
and one cannot transmit a value with newline or NUL in it). The list of
attributes is terminated by a blank line or end-of-file.
Git understands the following attributes:
`protocol`::
The protocol over which the credential will be used (e.g.,
`https`).
`host`::
The remote hostname for a network credential. This includes
the port number if one was specified (e.g., "example.com:8088").
`path`::
The path with which the credential will be used. E.g., for
accessing a remote https repository, this will be the
repository's path on the server.
`username`::
The credential's username, if we already have one (e.g., from a
URL, the configuration, the user, or from a previously run helper).
`password`::
The credential's password, if we are asking it to be stored.
`password_expiry_utc`::
Generated passwords such as an OAuth access token may have an expiry date.
When reading credentials from helpers, `git credential fill` ignores expired
passwords. Represented as Unix time UTC, seconds since 1970.
`oauth_refresh_token`::
An OAuth refresh token may accompany a password that is an OAuth access
token. Helpers must treat this attribute as confidential like the password
attribute. Git itself has no special behaviour for this attribute.
`url`::
When this special attribute is read by `git credential`, the
value is parsed as a URL and treated as if its constituent parts
were read (e.g., `url=https://example.com` would behave as if
`protocol=https` and `host=example.com` had been provided). This
can help callers avoid parsing URLs themselves.
+
Note that specifying a protocol is mandatory and if the URL
doesn't specify a hostname (e.g., "cert:///path/to/file") the
credential will contain a hostname attribute whose value is an
empty string.
+
Components which are missing from the URL (e.g., there is no
username in the example above) will be left unset.
Unrecognised attributes are silently discarded.
GIT
---
Part of the linkgit:git[1] suite