AuthenticationData model cannot send required fields for UIA stages (e.g. m.login.recaptcha response)

[2001arman]

Preflight Checklist

• I could not find a solution in the existing issues, docs, nor discussions

Describe your problem

Problem

The AuthenticationData model currently only supports the following fields:

class AuthenticationData {
  String? type;
  String? session;
}

However, according to the Matrix Client-Server API specification, UIA authentication stages may require additional parameters depending on the stage.

For example, the m.login.recaptcha stage requires the response field containing the captcha token.

Spec reference:
https://spec.matrix.org/v1.17/client-server-api/#google-recaptcha

Example request from the spec:

{
  "auth": {
    "type": "m.login.recaptcha",
    "session": "SESSION_ID",
    "response": "CAPTCHA_TOKEN"
  }
}

Because AuthenticationData does not allow extra fields, it is currently impossible to send the required response value using the SDK.

This results in Synapse returning:
M_CAPTCHA_NEEDED: Captcha response is required

Describe your ideal solution

Proposed Solution

Allow arbitrary authentication parameters to be passed through the AuthenticationData model.

Example implementation:

class AuthenticationData {
  String? type;
  String? session;
  Map<String, Object?>? additionalFields;

  AuthenticationData({
    this.type,
    this.session,
    this.additionalFields,
  });

  Map<String, Object?> toJson() {
    final data = <String, Object?>{};
    if (type != null) data['type'] = type;
    if (session != null) data['session'] = session;
    if (additionalFields != null) data.addAll(additionalFields!);
    return data;
  }
}

Example Usage

auth: AuthenticationData(
  type: AuthenticationTypes.recaptcha,
  session: session,
  additionalFields: {
    "response": captchaToken,
  },
)

Why This Is Needed

Matrix UIA (User-Interactive Authentication) supports many authentication stages, each requiring different fields such as:

• response for m.login.recaptcha
• threepid_creds for m.login.email.identity
• identifier / password for m.login.password

Restricting the model to only type and session prevents the SDK from supporting the full UIA specification.

Environment

• matrix-dart-sdk version: (your version)
• homeserver: Synapse

Version

6.1.1

Security requirements

No response

Additional Context

Register using captcha