credentials API

The credentials API provides an abstracted way of gathering username and password credentials from the user (even though credentials in the wider world can take many forms, in this document the word "credential" always refers to a username and password pair).

This document describes two interfaces: the C API that the credential subsystem provides to the rest of Git, and the protocol that Git uses to communicate with system-specific "credential helpers". If you are writing Git code that wants to look up or prompt for credentials, see the section "C API" below. If you want to write your own helper, see the section on "Credential Helpers" below.

Typical setup

+-----------------------+
| Git code (C)          |--- to server requiring --->
|                       |        authentication
|.......................|
| C credential API      |--- prompt ---> User
+-----------------------+
        ^      |
        | pipe |
        |      v
+-----------------------+
| Git credential helper |
+-----------------------+

The Git code (typically a remote-helper) will call the C API to obtain credential data like a login/password pair (credential_fill). The API will itself call a remote helper (e.g. "git credential-cache" or "git credential-store") that may retrieve credential data from a store. If the credential helper cannot find the information, the C API will prompt the user. Then, the caller of the API takes care of contacting the server, and does the actual authentication.

C API

The credential C API is meant to be called by Git code which needs to acquire or store a credential. It is centered around an object representing a single credential and provides three basic operations: fill (acquire credentials by calling helpers and/or prompting the user), approve (mark a credential as successfully used so that it can be stored for later use), and reject (mark a credential as unsuccessful so that it can be erased from any persistent storage).

Data Structures

struct credential: This struct represents a single username/password combination along with any associated context. All string fields should be heap-allocated (or NULL if they are not known or not applicable). The meaning of the individual context fields is the same as their counterparts in the helper protocol; see the section below for a description of each field.

The helpers member of the struct is a string_list of helpers. Each string specifies an external helper which will be run, in order, to either acquire or store credentials. See the section on credential helpers below. This list is filled-in by the API functions according to the corresponding configuration variables before consulting helpers, so there usually is no need for a caller to modify the helpers field at all.

This struct should always be initialized with CREDENTIAL_INIT or credential_init.

Functions

credential_init: Initialize a credential structure, setting all fields to empty.
credential_clear: Free any resources associated with the credential structure, returning it to a pristine initialized state.
credential_fill: Instruct the credential subsystem to fill the username and password fields of the passed credential struct by first consulting helpers, then asking the user. After this function returns, the username and password fields of the credential are guaranteed to be non-NULL. If an error occurs, the function will die().
credential_reject: Inform the credential subsystem that the provided credentials have been rejected. This will cause the credential subsystem to notify any helpers of the rejection (which allows them, for example, to purge the invalid credentials from storage). It will also free() the username and password fields of the credential and set them to NULL (readying the credential for another call to credential_fill). Any errors from helpers are ignored.
credential_approve: Inform the credential subsystem that the provided credentials were successfully used for authentication. This will cause the credential subsystem to notify any helpers of the approval, so that they may store the result to be used again. Any errors from helpers are ignored.
credential_from_url: Parse a URL into broken-down credential fields.

Example

The example below shows how the functions of the credential API could be used to login to a fictitious "foo" service on a remote host:

int foo_login(struct foo_connection *f)
{
        int status;
        /*
         * Create a credential with some context; we don't yet know the
         * username or password.
         */

        struct credential c = CREDENTIAL_INIT;
        c.protocol = xstrdup("foo");
        c.host = xstrdup(f->hostname);

        /*
         * Fill in the username and password fields by contacting
         * helpers and/or asking the user. The function will die if it
         * fails.
         */
        credential_fill(&c);

        /*
         * Otherwise, we have a username and password. Try to use it.
         */
        status = send_foo_login(f, c.username, c.password);
        switch (status) {
        case FOO_OK:
                /* It worked. Store the credential for later use. */
                credential_accept(&c);
                break;
        case FOO_BAD_LOGIN:
                /* Erase the credential from storage so we don't try it
                 * again. */
                credential_reject(&c);
                break;
        default:
                /*
                 * Some other error occurred. We don't know if the
                 * credential is good or bad, so report nothing to the
                 * credential subsystem.
                 */
        }

        /* Free any associated resources. */
        credential_clear(&c);

        return status;
}

Credential Helpers

Credential helpers are programs executed by Git to fetch or save credentials from and to long-term storage (where "long-term" is simply longer than a single Git process; e.g., credentials may be stored in-memory for a few minutes, or indefinitely on disk).

Each helper is specified by a single string in the configuration variable credential.helper (and others, see git-config(1)). The string is transformed by Git into a command to be executed using these rules:

If the helper string begins with "!", it is considered a shell snippet, and everything after the "!" becomes the command.
Otherwise, if the helper string begins with an absolute path, the verbatim helper string becomes the command.
Otherwise, the string "git credential-" is prepended to the helper string, and the result becomes the command.

The resulting command then has an "operation" argument appended to it (see below for details), and the result is executed by the shell.

Here are some example specifications:

# run "git credential-foo"
foo

# same as above, but pass an argument to the helper
foo --bar=baz

# the arguments are parsed by the shell, so use shell
# quoting if necessary
foo --bar="whitespace arg"

# you can also use an absolute path, which will not use the git wrapper
/path/to/my/helper --with-arguments

# or you can specify your own shell snippet
!f() { echo "password=`cat $HOME/.secret`"; }; f

Generally speaking, rule (3) above is the simplest for users to specify. Authors of credential helpers should make an effort to assist their users by naming their program "git-credential-$NAME", and putting it in the $PATH or $GIT_EXEC_PATH during installation, which will allow a user to enable it with git config credential.helper $NAME.

When a helper is executed, it will have one "operation" argument appended to its command line, which is one of:

get: Return a matching credential, if any exists.
store: Store the credential, if applicable to the helper.
erase: Remove a matching credential, if any, from the helper’s storage.

The details of the credential will be provided on the helper’s stdin stream. The exact format is the same as the input/output format of the git credential plumbing command (see the section INPUT/OUTPUT FORMAT in git-credential(1) for a detailed specification).

For a get operation, the helper should produce a list of attributes on stdout in the same format. A helper is free to produce a subset, or even no values at all if it has nothing useful to provide. Any provided attributes will overwrite those already known about by Git. If a helper outputs a quit attribute with a value of true or 1, no further helpers will be consulted, nor will the user be prompted (if no credential has been provided, the operation will then fail).

For a store or erase operation, the helper’s output is ignored. If it fails to perform the requested operation, it may complain to stderr to inform the user. If it does not support the requested operation (e.g., a read-only store), it should silently ignore the request.

If a helper receives any other operation, it should silently ignore the request. This leaves room for future operations to be added (older helpers will just ignore the new requests).