Sometimes there might be a need for an application to sign a data form (Data Forms (XEP-0004) ) using other credentials than provided by the XMPP account or connection. An example can be automatic account creation using In-Band Registration (XEP-0077) . Many server operators disable the in-band registration feature since it makes it possible for malicious users or robots to freely create accounts on the server. One way to combat robots, has been through the use of CAPTCHA Forms (XEP-0158) . But in some cases, like in the Internet of Things, it is not robots that are the problem, but malicious users.
This document describes a method whereby forms can be signed using other credentials. This can be used in an In-band registration form to sign the form with the credentials of a special account on the server with permissions to create new XMPP accounts, with perhaps a limit on number of accounts that can be created. This method can be used by manufacturers of devices for Internet of Things, so that devices can create accounts automatically on XMPP servers in an orderly fashion, and manufacturers are allowed to administer and control their automatically created accounts separately. It also provides a mechanism whereby server operators can monitor who is responsible for account creation and to what extent.
A fixed algorithm (OAuth 1.0 Protocol ) has been chosen in favor of a method where the user can select an authentication method from a list of available methods, modelled in the likeness of SASL. The main reason is to avoid multiple callbacks during form signature. The idea is to make form signature possible without having to do any intermediate server callbacks, or having to change the original request returning the form. The method is still extensible, allowing possible future extensions. The form signing algorithm to use is defined by the FORM_TYPE parameter in the form being signed.
A form that needs to be signed by the client using external credentials, shows this by including a hidden FORM_TYPE field valued urn:xmpp:xdata:signature:oauth1. The sub-namespace :oauth1 identifies the algorithm to be used, in this case OAUTH v1.0.
The form contains a set of invisible parameters that the underlying software signing the form and supporting this specification must understand. These are:
|oauth_version||Must be 1.0. Is not changed by the client performing the signing.|
|oauth_signature_method||Specifies the signature method, or hash function, to use when signing the form. This can be changed by the client. Possible values are: HMAC-SHA1, RSA-SHA1 and PLAINTEXT.|
|oauth_token||This is a token provided by the server to the client. This parameter might not be available if the server has provided the client with this token earlier during the session.|
|oauth_token_secret||This is a temporary secret shared between the server and client, and is related to the token. This parameter might not be available if the server has provided the client with this token earlier during the session.|
|oauth_nonce||A nonce value that the client has to set. Can be a random alphanumerical string.|
|oauth_timestamp||Number of seconds since 1st of January 1970, 00:00:00 GMT. The client has to set this at the time of signature.|
|oauth_consumer_key||A key identifying the account doing the signing of the form. The client has to set this to identify who performs the signature.|
|oauth_signature||The signature, signing the form. The client has to set this with the signature of the form, as calculated and described below.|
Before calculating the signature, the following functions need to be defined:
Converts the sequence of octets x into a Base-64 encoded string.
The string s are escaped using the RFC 3986  percent-encoding (%xx) mechanism. Characters not in the unreserved character set (§ 2.3) MUST be encoded. Characters in the unreserved character set MUST NOT be encoded. Hexadecimal characters in encodings MUST be upper case. Text names and values MUST first be normalized using Normalization Form C (NFC) as defined in Unicode Standard Annex #15, Unicode Normalization Forms  and then encoded as UTF-8 octets before percent-encoding them per RFC 3629 .
Unreserved characters are alphanumeric characters (a-z, A-Z, 0-9), hyphen ('-'), period ('.'), underscore ('_') and tilde ('~').
Corresponds to the Hash-function to use, according to the parameter oauth_signature_method. It computes the hash value of the octets x, and returns a sequence of octets.
To calculate the signature of the form, follow the steps in the following subsections, in the same order.
You choose Hash function by setting the parameter oauth_signature_method to either HMAC-SHA1, RSA-SHA1 or PLAINTEXT.
First, all form parameters to be signed, including hidden parameters and the OAUTH parameters except oauth_token_secret and oauth_signature, are sorted by variable name (var).
Each (name, value) pair in the list of sorted parameters are first transformed into pairs of Escape(name)=Escape(value) segments, and then concatenated into one string, where each segment is delimited using an ampersand ('&') character.
The Signature Base String (BStr) is then formed concatenating Escape(type) (the form type used when submitting the form), Escape(to) (the full destination address, including resource, if any) and Escape(PStr), using ampersands ('&') as delimiter.
Here, the client signing the form must have the oauth_consumer_secret available to be able to sign the form correctly.
In this example, the oauth_consumer_secret must be an RSA private key.
If PLAINTEXT is chosen, no Hash function is used. Instead, the signature is calculated as follows:
Once the signature has been calculated, the corresponding parameter oauth_signature has to be set in the form before sending it to the destination address.
When the recipient receives the signed formm it performs the same calculations as above, based on the parameters received, and knowledge of the shared secret which it can look up by using the parameter oauth_consumer_key.
As mentioned above, a major use case for signing forms, is in-band registration for creating new accounts on an XMPP Server, as defined in XEP-0077.
In case a form signature is not value, the server MUST respond with a bad-request error message, as follows:
If an entity supports signing forms as specified herein, it MUST advertise that fact by returning a feature of "urn:xmpp:xdata:signature:oauth1" in response to Service Discovery (XEP-0030)  information requests.
In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) . However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
This document only specifies signatures using OAUTH v1.0. Each entity that supports form signatures using this method, reports this by including the namespace urn:xmpp:xdata:signature:oauth1 in its features. The specification is extensible, in that it can allow for future signature methods to be used, by defining new namespaces replacing oauth1 by another string corresponding to the desired method, but leaving the base part of the namespace urn:xmpp:xdata:signature: intact.
The PLAINTEXT signature method should only be used if SSL/TLS is used by both the entity signing the form as well as the creator of the form. If the creator of the form is a server, this later part can be ignored. If unsure, PLAINTEXT should only be used in development & debugging cycles of an application, and not in production environments.
If the server provides information to be used in signing a form, it must also verify that the client only changes values it is allowed to change. An alternative is to not use the values provided by the client for the corresponding server-side parameters when calculating the signature on the server side. This makes sure a client cannot willfully alter server-side parameters when it returns the signed form.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The XMPP Registrar  includes the following information in its registries.
Field Standardization for Data Forms (XEP-0068)  defines a process for standardizing the fields used within Data Forms qualified by a particular namespace, and XEP-0128 describes how to use field standardization in the context of service discovery. This section registers fields for server information scoped by the "urn:xmpp:xdata:signature:oauth1" FORM_TYPE.
Thanks to Kevin Smith, Lance Stout, Matthew Wild, Philipp Hancke and Tobias Markmann for all valuable feedback.
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