---
description: >-
  Credentials helpers are external programs that know how to store and retrieve
  API tokens for remote OpenTofu services.
---

# Credentials Helpers

For OpenTofu-specific features that interact with remote network services,
such as module registries, OpenTofu by default looks for
API credentials to use in these calls in
[the CLI configuration](../cli/config/config-file.mdx).

Credentials helpers offer an alternative approach that allows you to customize
how OpenTofu obtains credentials using an external program, which can then
directly access an existing secrets management system in your organization.

This page is about how to write and install a credentials helper. To learn
how to configure a credentials helper that was already installed, see
[the CLI config Credentials Helpers section](../cli/config/config-file.mdx#credentials-helpers).

## How OpenTofu finds Credentials Helpers

A credentials helper is a normal executable program that is installed in a
particular location and whose name follows a specific naming convention.

A credentials helper called "credstore", for example, would be implemented as
an executable program named `terraform-credentials-credstore` (with an `.exe`
extension on Windows only), and installed in one of the
[default plugin search locations](../cli/config/config-file.mdx#provider-installation).

## How OpenTofu runs Credentials Helpers

Once OpenTofu has located the configured credentials helper, it will execute
it once for each credentials request that cannot be satisfied by a `credentials`
block in the CLI configuration.

For the following examples, we'll assume a "credstore" credentials helper
configured as follows:

```
credentials_helper "credstore" {
  args = ["--host=credstore.example.com"]
}
```

OpenTofu runs the helper program with each of the arguments given in `args`,
followed by an _verb_ and then the hostname that the verb will apply to.
The current set of verbs are:

* `get`: retrieve the credentials for the given hostname
* `store`: store new credentials for the given hostname
* `forget`: delete any stored credentials for the given hostname

To represent credentials, the credentials helper protocol uses a JSON object
whose contents correspond with the contents of
[`credentials` blocks in the CLI configuration](../cli/config/config-file.mdx#credentials).
To represent an API token, the object contains a property called "token" whose
value is the token string:

```json
{
  "token": "example-token-value"
}
```

The following sections describe the specific expected behaviors for each of the
three verbs.

## `get`: retrieve the credentials for the given hostname

To retrieve credentials for `app.example.io`, OpenTofu would run the
"credstore" helper as follows:

```
terraform-credentials-credstore --host=credstore.example.com get app.example.io
```

If the credentials helper is able to provide credentials for the given host
then it must print a JSON credentials object to its stdout stream and then
exit with status code zero to indicate success.

If the credentials helper definitively has no credentials for the given host,
then it must print an empty JSON object to stdout and exit with status zero.

If the credentials helper is unable to provide the requested credentials for
any other reason, it must print an end-user-oriented plain text error message
to its stderr stream and then exit with a _non-zero_ status code.

## `store`: store new credentials for the given hostname

To store new credentials for `app.example.io`, OpenTofu would run the
"credstore" helper as follows:

```
terraform-credentials-credstore --host=credstore.example.com store app.example.io
```

OpenTofu then writes a JSON credentials object to the helper program's stdin
stream. If the helper is able to store the given credentials then it must do
so and then exit with status code zero and no output on stdout or stderr to
indicate success.

If it is unable to store the given credentials for any reason, it _must_ still
fully read its stdin until EOF and then print an end-user-oriented plain text
error message to its stderr stream before exiting with a non-zero status
code.

The new credentials must fully replace any existing credentials stored for the
given hostname.

## `forget`: delete any stored credentials for the given hostname

To forget any existing credentials for `app.example.io`, OpenTofu would run
the "credstore" helper as follows:

```
terraform-credentials-credstore --host=credstore.example.com forget app.example.io
```

No JSON credentials objects are used for the `forget` verb.

If the helper program is able to delete its stored credentials for the given
hostname or if there are no such credentials stored already then it must
exist with status code zero and produce no output on stdout or stderr.

If it is unable to forget the stored credentials for any reason, particularly
if the helper cannot be sure that the credentials are no longer available for
retrieval, the helper program must print an end-user-oriented plain text error
message to its stderr stream and then exit with a non-zero status code.

## Handling Other Commands

The credentials helper protocol may be extended with additional verbs in future,
so for forward-compatibility a credentials helper must react to any unsupported
verb by printing an end-user-oriented plain text error message to its stderr
stream and then exiting with a non-zero status code.

## Handling Unsupported Credentials Object Properties

OpenTofu defines only the `token` property within JSON credentials
objects.

If a credentials helper is asked to store an object that has any properties
other than `token` and if it is not able to faithfully retain them then it
must behave as if the object is unstorable, returning an error. It must _not_
store the `token` value in isolation and silently drop other properties, as
that might change the meaning of the credentials object.

If technically possible within the constraints of the target system, a
credentials helper should prefer to store the whole JSON object as-is for
later retrieval. For systems that are more constrained, it's acceptable to
store only the `token` string so long as the program rejects objects containing
other properties as described above.

## Installing a Credentials Helper

OpenTofu does not have any automatic installation mechanism for credentials
helpers. Instead, the user must extract the helper program executable into
one of the [default plugin search locations](../cli/config/config-file.mdx#provider-installation).

If you are packaging a credentials helper for distribution, place it in an
named with the expected naming scheme (`terraform-credentials-example`) and,
if the containing archive format supports it and it's meaningful for the
target operating system, mark the file as executable to increase the chances
that it will work immediately after extraction.

OpenTofu does _not_ honor the `-plugin-dir` argument to `tofu init` when
searching for credentials helpers, because credentials are also used by other
commands that can be run prior to `tofu init`. Only the default search
locations are supported.
