Credential Provider Protocol

This document describes information for building a Cargo credential provider. For information on setting up or using a credential provider, see Registry Authentication.

When using an external credential provider, Cargo communicates with the credential provider using stdin/stdout messages passed as single lines of JSON.

Cargo will always execute the credential provider with the --cargo-plugin argument. This enables a credential provider executable to have additional functionality beyond what Cargo needs. Additional arguments are included in the JSON via the args field.

JSON messages

The JSON messages in this document have newlines added for readability. Actual messages must not contain newlines.

Credential hello

{
    "v":[1]
}

Requests sent by Cargo will include a v field set to one of the versions listed here. If Cargo does not support any of the versions offered by the credential provider, it will issue an error and shut down the credential process.

Registry information

{
    // Index URL of the registry
    "index-url":"https://github.com/rust-lang/crates.io-index",
    // Name of the registry in configuration (optional)
    "name": "crates-io",
    // HTTP headers received from attempting to access an authenticated registry (optional)
    "headers": ["WWW-Authenticate: cargo"]
}

Login request

{
    // Protocol version
    "v":1,
    // Action to perform: login
    "kind":"login",
    // Registry information (see Registry information)
    "registry":{"index-url":"sparse+https://registry-url/index/", "name": "my-registry"},
    // User-specified token from stdin or command line (optional)
    "token": "<the token value>",
    // URL that the user could visit to get a token (optional)
    "login-url": "http://registry-url/login",
    // Additional command-line args (optional)
    "args":[]
}

If the token field is set, then the credential provider should use the token provided. If the token is not set, then the credential provider should prompt the user for a token.

In addition to the arguments that may be passed to the credential provider in configuration, cargo login also supports passing additional command line args via cargo login -- <additional args>. These additional arguments will be included in the args field after any args from Cargo configuration.

Read request

{
    // Protocol version
    "v":1,
    // Request kind: get credentials
    "kind":"get",
    // Action to perform: read crate information
    "operation":"read",
    // Registry information (see Registry information)
    "registry":{"index-url":"sparse+https://registry-url/index/", "name": "my-registry"},
    // Additional command-line args (optional)
    "args":[]
}

Publish request

{
    // Protocol version
    "v":1,
    // Request kind: get credentials
    "kind":"get",
    // Action to perform: publish crate
    "operation":"publish",
    // Crate name
    "name":"sample",
    // Crate version
    "vers":"0.1.0",
    // Crate checksum
    "cksum":"...",
    // Registry information (see Registry information)
    "registry":{"index-url":"sparse+https://registry-url/index/", "name": "my-registry"},
    // Additional command-line args (optional)
    "args":[]
}

Get success response

{"Ok":{
    // Response kind: this was a get request
    "kind":"get",
    // Token to send to the registry
    "token":"...",
    // Cache control. Can be one of the following:
    // * "never": do not cache
    // * "session": cache for the current cargo session
    // * "expires": cache for the current cargo session until expiration
    "cache":"expires",
    // Unix timestamp (only for "cache": "expires")
    "expiration":1693942857,
    // Is the token operation independent?
    "operation_independent":true
}}

The token will be sent to the registry as the value of the Authorization HTTP header.

operation_independent indicates whether the token can be cached across different operations (such as publishing or fetching). In general, this should be true unless the provider wants to generate tokens that are scoped to specific operations.

Login success response

{"Ok":{
    // Response kind: this was a login request
    "kind":"login"
}}

Logout success response

{"Ok":{
    // Response kind: this was a logout request
    "kind":"logout"
}}

Failure response (URL not supported)

{"Err":{
    "kind":"url-not-supported"
}}

Sent if the credential provider is designed to only handle specific registry URLs and the given URL is not supported. Cargo will attempt another provider if available.

Failure response (not found)

{"Err":{
    // Error: The credential could not be found in the provider.
    "kind":"not-found"
}}

Sent if the credential could not be found. This is expected for get requests where the credential is not available, or logout requests where there is nothing found to erase.

Failure response (operation not supported)

{"Err":{
    // Error: The credential could not be found in the provider.
    "kind":"operation-not-supported"
}}

Sent if the credential provider does not support the requested operation. If a provider only supports get and a login is requested, the provider should respond with this error.

Failure response (other)

{"Err":{
    // Error: something else has failed
    "kind":"other",
    // Error message string to be displayed
    "message": "free form string error message",
    // Detailed cause chain for the error (optional)
    "caused-by": ["cause 1", "cause 2"]
}}

Example communication to request a token for reading:

  1. Cargo spawns the credential process, capturing stdin and stdout.
  2. Credential process sends the Hello message to Cargo 
    { "v": [1] }
  3. Cargo sends the CredentialRequest message to the credential process (newlines added for readability). 
    {
    "v": 1,
    "kind": "get",
    "operation": "read",
    "registry":{"index-url":"sparse+https://registry-url/index/"}
}
  4. Credential process sends the CredentialResponse to Cargo (newlines added for readability). 
    {
    "token": "...",
    "cache": "session",
    "operation_independent": true
}
  5. Cargo closes the stdin pipe to the credential provider and it exits.
  6. Cargo uses the token for the remainder of the session (until Cargo exits) when interacting with this registry.