MSC4183: Additional Error Codes for submitToken endpoint

proposals/4183-submitToken-error-codes.md

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+# MSC4183: Additional Error Codes for submitToken endpoint
+
+The [`POST
+/_matrix/identity/v2/validate/email/submitToken`](https://spec.matrix.org/v1.11/identity-service-api/#post_matrixidentityv2validateemailsubmittoken)
+and [`POST
+/_matrix/identity/v2/validate/msisdn/submitToken`](https://spec.matrix.org/v1.11/identity-service-api/#post_matrixidentityv2validatemsisdnsubmittoken)
+endpoints do not specify any specific error codes, instead relying on the common error codes defined in the identity
+service API.
+
+However, these common error codes don't have any codes to signal many errors that can occur in these APIs: most
+obviously, that the token the user entered was incorrect.
+
+This MSC can be considered similar to [MSC4178](https://github.com/matrix-org/matrix-spec-proposals/pull/4178) although
+that MSC is for `requestToken` on the C-S API only.
+
+The [`POST
+/_matrix/client/v3/account/3pid/email/requestToken`](https://spec.matrix.org/v1.11/client-server-api/#post_matrixclientv3account3pidemailrequesttoken)
+endpoint in the C/S API also specifies a `submit_url` response parameter, defining its parameters to be the same as the
+Identity API's `submitToken` endpoints. Everything this MSC specifies applies to this endpoint in the same way.
+
+## Proposal
+
+Add the following specific error code as a code that can be returned by the two endpoints given above:
+ * `M_TOKEN_INCORRECT`: Indicates that the token that the user entered to validate the session is incorrect.
+
+HTTP status code 400 should be used for this error.
+
+Additionally specify that the following common error codes can be returned:
+ * `M_INVALID_PARAM`: One of the supplied parameters in not valid.
+ * `M_SESSION_EXPIRED`: The validation session is question has expired.
+
+HTTP status code 400 should also be used for both of these errors.
+
+Also change the C/S API's definition of [`POST
+/_matrix/client/v3/account/3pid/email/requestToken`](https://spec.matrix.org/v1.11/client-server-api/#post_matrixclientv3account3pidemailrequesttoken)
+to specify that the endpoint is the same as the I/S API version in all ways, including response / error codes, rather than just parameters.
+
+## Potential issues
+
+None foreseen.
+
+## Alternatives
+
+None considered.
+
+## Security considerations
+
+None foreseen.
+
+## Unstable prefix
+
+No unstable prefix is deemed necessary. Sydent already sends the common error codes and also sends
+`M_NO_VALID_SESSION` if the code is incorrect. Once an identity server (or homeserver) switches to
+use the new error code, clients (including homeservers proxying the IS API) may not recognise the
+error condition correctly until updated to support the new code. We say that this is acceptable in
+favour of avoiding the complexity of negotiating error codes with API versions. Since the identity
+server is generally used via the homeserver now, most users of this API will not currently receive
+a sensible error code in this situation anyway.
+
+## Dependencies
+
+None