XEP-0348: Signing Forms

Abstract:This specification describes a method whereby a client can sign a form using credentials not related to the current connection.
Author:Peter Waher
Copyright:© 1999 – 2017 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status:Experimental
Type:Standards Track
Version:0.2
Last Updated:2015-11-09

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.

Table of Contents


1. Introduction
2. Signing a form
    2.1. Choose Hash function
    2.2. Sort parameters
    2.3. Calculating Parameter String (PStr)
    2.4. Calculating Signature Base String (BStr)
    2.5. Generating Signature
    2.6. Sending Form
    2.7. Validating Form
3. Use Cases
    3.1. Securing In-Band Registration of new accounts
4. Determining Support
5. Implementation Notes
    5.1. Signature algorithms and namespaces
6. Security Considerations
    6.1. PLAINTEXT
    6.2. Session parameters
7. IANA Considerations
8. XMPP Registrar Considerations
    8.1. Field Standardization
9. Acknowledgements

Appendices
    A: Document Information
    B: Author Information
    C: Legal Notices
    D: Relation to XMPP
    E: Discussion Venue
    F: Requirements Conformance
    G: Notes
    H: Revision History

1. Introduction

Sometimes there might be a need for an application to sign a data form (Data Forms (XEP-0004) [1]) using other credentials than provided by the XMPP account or connection. An example can be automatic account creation using In-Band Registration (XEP-0077) [2]. 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) [3]. 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 [4]) 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.

2. Signing a form

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.

Example 1. Form requiring signature

			
    <x xmlns='jabber:x:data' type='form'>
      <title>Create Account</title>
      <field type='hidden' var='FORM_TYPE'>
        <value>urn:xmpp:xdata:signature:oauth1</value>
      </field>
      <field type='hidden' var='oauth_version'>
        <value>1.0</value>
      </field>
      <field type='hidden' var='oauth_signature_method'>
        <value>HMAC-SHA1</value>
      </field>
      <field type='hidden' var='oauth_token'>
        <value>...<value>
      </field>
      <field type='hidden' var='oauth_token_secret'>
        <value>...<value>
      </field>
      <field type='hidden' var='oauth_nonce'>
        <value/>
      </field>
      <field type='hidden' var='oauth_timestamp'>
        <value/>
      </field>
      <field type='hidden' var='oauth_consumer_key'>
        <value/>
      </field>
      <field type='hidden' var='oauth_signature'>
        <value/>
      </field>
      <field type='text-single' label='User Name' var='UserName'>
        <required/>
      </field>
      <field type='text-single' label='Password' var='Password'>
        <required/>
      </field>
    </x>

The form contains a set of invisible parameters that the underlying software signing the form and supporting this specification must understand. These are:

Table 1: OAUTH parameters

Name Description
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:

Base64(x)

Converts the sequence of octets x into a Base-64 encoded string.

Escape(s)

The string s are escaped using the RFC 3986 [5] 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 [6] and then encoded as UTF-8 octets before percent-encoding them per RFC 3629 [7].

Unreserved characters are alphanumeric characters (a-z, A-Z, 0-9), hyphen ('-'), period ('.'), underscore ('_') and tilde ('~').

H(x)

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.

2.1 Choose Hash function

You choose Hash function by setting the parameter oauth_signature_method to either HMAC-SHA1, RSA-SHA1 or PLAINTEXT.

2.2 Sort parameters

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).

2.3 Calculating Parameter String (PStr)

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.

Example 2. Calculating Parameter String

				
    PStr:=Escape(name_1)+'='+Escape(value_1)+'&'+...+Escape(name_N)+'='+Escape(value_N)

2.4 Calculating Signature Base String (BStr)

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.

Example 3. Calculating Signature Base String

				
    BStr:=Escape(type)+'&'+Escape(to)+'&'+Escape(PStr)

2.5 Generating Signature

Depending on which signature method is chosen, different Hash functions are used. If HMAC-SHA1 is chosen, then the Hash function HMAC(text, key) according to RFC 2104 [8] is used as follows:

Example 4. HMAC-SHA1

				
    H(x):=HMAC-SHA1(x,Escape(oauth_consumer_secret)+'&'+Escape(oauth_token_secret))
    oauth_signature:=Escape(Base64(H(BStr)))

Here, the client signing the form must have the oauth_consumer_secret available to be able to sign the form correctly.

If RSA-SHA1 is chosen, then the signature method RSASSA-PKCS1-v1_5(K, M) according to RFC 3447 [9] is used as follows:

Example 5. RSA-SHA1

				
    H(x):=RSASSA-PKCS1-v1_5(oauth_consumer_secret,x)
    oauth_signature:=Escape(Base64(H(BStr)))

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:

Example 6. RSA-SHA1

				
    oauth_signature:=Escape(oauth_consumer_secret)+Escape(oauth_token_secret)

2.6 Sending Form

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.

2.7 Validating Form

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.

3. Use Cases

3.1 Securing In-Band Registration of new accounts

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.

Example 7. Entity Requests Registration Fields from Host

				
    <iq type='get'
        from='juliet@capulet.com/balcony'
        to='contests.shakespeare.lit'
        id='reg3'>
      <query xmlns='jabber:iq:register'/>
    </iq>

Example 8. Host Returns Registration Form to Entity requiring signature

				
    <iq type='result'
        from='contests.shakespeare.lit'
        to='juliet@capulet.com/balcony'
        id='reg3'>
      <query xmlns='jabber:iq:register'>
        <instructions>
          Use the enclosed form to register. If your Jabber client does not
          support Data Forms, visit http://www.shakespeare.lit/contests.php
        </instructions>
        <x xmlns='jabber:x:data' type='form'>
          <title>Contest Registration</title>
          <instructions>
            Please provide the following information
            to sign up for our special contests!
          </instructions>
          <field type='hidden' var='FORM_TYPE'>
            <value>urn:xmpp:xdata:signature:oauth1</value>
          </field>
          <field type='text-single' label='Given Name' var='first'>
            <required/>
          </field>
          <field type='text-single' label='Family Name' var='last'>
            <required/>
          </field>
          <field type='text-single' label='Email Address' var='email'>
            <required/>
          </field>
          <field type='list-single' label='Gender' var='x-gender'>
            <option label='Male'><value>M</value></option>
            <option label='Female'><value>F</value></option>
          </field>
          <field type='hidden' var='oauth_version'>
            <value>1.0</value>
          </field>
          <field type='hidden' var='oauth_signature_method'>
            <value>HMAC-SHA1</value>
          </field>
          <field type='hidden' var='oauth_token'>
            <value>...<value>
          </field>
          <field type='hidden' var='oauth_token_secret'>
            <value>...<value>
          </field>
          <field type='hidden' var='oauth_nonce'>
            <value/>
          </field>
          <field type='hidden' var='oauth_timestamp'>
            <value/>
          </field>
          <field type='hidden' var='oauth_consumer_key'>
            <value/>
          </field>
          <field type='hidden' var='oauth_signature'>
            <value/>
          </field>
        </x>
      </query>
    </iq>

Example 9. User Submits Signed Registration Form

				
    <iq type='set'
        from='juliet@capulet.com/balcony'
        to='contests.shakespeare.lit'
        id='reg4'>
      <query xmlns='jabber:iq:register'>
        <x xmlns='jabber:x:data' type='submit'>
          <field type='hidden' var='FORM_TYPE'>
            <value>urn:xmpp:xdata:signature:oauth1</value>
          </field>
          <field type='text-single' label='Given Name' var='first'>
            <value>Juliet</value>
          </field>
          <field type='text-single' label='Family Name' var='last'>
            <value>Capulet</value>
          </field>
          <field type='text-single' label='Email Address' var='email'>
            <value>juliet@capulet.com</value>
          </field>
          <field type='list-single' label='Gender' var='x-gender'>
            <value>F</value>
          </field>
          <field type='hidden' var='oauth_version'>
            <value>1.0</value>
          </field>
          <field type='hidden' var='oauth_signature_method'>
            <value>HMAC-SHA1</value>
          </field>
          <field type='hidden' var='oauth_token'>
            <value>...<value>
          </field>
          <field type='hidden' var='oauth_token_secret'>
            <value>...<value>
          </field>
          <field type='hidden' var='oauth_nonce'>
            <value>...<value>
          </field>
          <field type='hidden' var='oauth_timestamp'>
            <value>...<value>
          </field>
          <field type='hidden' var='oauth_consumer_key'>
            <value>...<value>
          </field>
          <field type='hidden' var='oauth_signature'>
            <value>...<value>
          </field>
        </x>
      </query>
    </iq>

In case a form signature is not value, the server MUST respond with a bad-request error message, as follows:

Example 10. Error message when form signature is invalid

				
    <iq type='error'
        from='contests.shakespeare.lit'
        to='juliet@capulet.com/balcony'
        id='reg4'>
      <error code='400' type='modify'>
        <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
      </error>
    </iq>

4. Determining Support

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) [10] information requests.

Example 11. Service discovery information request

			
    <iq type='get'
        from='example.org'
        to='device@example.org'
        id='disco1'>
      <query xmlns='http://jabber.org/protocol/disco#info'/>
    </iq>

Example 12. Service discovery information response

			
    <iq type='result'
        from='device@example.org'
        to='example.org'
        id='disco1'>
      <query xmlns='http://jabber.org/protocol/disco#info'>
        ...
        <feature var='urn:xmpp:xdata:signature:oauth1'/>
        ...
      </query>
    </iq>

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) [11]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

5. Implementation Notes

5.1 Signature algorithms and namespaces

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.

6. Security Considerations

6.1 PLAINTEXT

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.

6.2 Session parameters

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.

7. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [12].

8. XMPP Registrar Considerations

The XMPP Registrar [13] includes the following information in its registries.

8.1 Field Standardization

Field Standardization for Data Forms (XEP-0068) [14] 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.

Example 13. Registry submission

				
<form_type>
  <name>urn:xmpp:xdata:signature:oauth1</name>
  <doc>XEP-xxxx</doc>
  <desc>
    Forms that require signatures using OAuth v1.0 signature algorithm.
  </desc>
  <field
      var='oauth_version'
      type='hidden'
      label='OAuth version. Must be 1.0.'/>
  <field
      var='oauth_signature_method'
      type='hidden'
      label='OAuth signature method. Can be "HMAC-SHA1", "RSA-SHA1" or "PLAINTEXT".'/>
  <field
      var='oauth_token'
      type='hidden'
      label='OAuth token. Provided by the creator of the form.'/>
  <field
      var='oauth_token_secret'
      type='hidden'
      label='OAuth token secret. Provided by the creator of the form.'/>
  <field
      var='oauth_nonce'
      type='hidden'
      label='Nonce value. Random string, created by the entity signing the form.'/>
  <field
      var='oauth_timestamp'
      type='hidden'
      label='Seconds since 1st of January 1970, 00:00:00 GMT. Value provided by the entity signing the form.'/>
  <field
      var='oauth_consumer_key'
      type='hidden'
      label='Consumer Key, identifying the entity signing the form using an external set of credentials. Value provided by the entity signing the form.'/>
  <field
      var='oauth_signature'
      type='hidden'
      label='Form signature. Value provided by the entity signing the form.'/>
</form_type>

9. Acknowledgements

Thanks to Kevin Smith, Lance Stout, Matthew Wild, Philipp Hancke and Tobias Markmann for all valuable feedback.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0348
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Standards Track
Version: 0.2
Last Updated: 2015-11-09
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0004, XEP-0030, XEP-0068, XEP-0077
Supersedes: None
Superseded By: None
Short Name: signing-forms
Source Control: HTML
This document in other formats: XML  PDF

Appendix B: Author Information

Peter Waher

Email: peterwaher@hotmail.com
JabberID: peter.waher@jabber.org
URI: http://www.linkedin.com/in/peterwaher

Appendix C: Legal Notices

Copyright

This XMPP Extension Protocol is copyright © 1999 – 2017 by the XMPP Standards Foundation (XSF).

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

1. XEP-0004: Data Forms <https://xmpp.org/extensions/xep-0004.html>.

2. XEP-0077: In-Band Registration <https://xmpp.org/extensions/xep-0077.html>.

3. XEP-0158: CAPTCHA Forms <https://xmpp.org/extensions/xep-0158.html>.

4. RFC-5849: The OAuth 1.0 Protocol <http://tools.ietf.org/html/rfc5849>.

5. RFC 3986: Uniform Resource Identifiers (URI): Generic Syntax <http://tools.ietf.org/html/rfc3986>.

6. Unicode Standard Annex #15, Unicode Normalization Forms <http://unicode.org/reports/tr15/#Norm_Forms>.

7. RFC 3629: UTF-8, a transformation format of ISO 10646 <http://tools.ietf.org/html/rfc3629>.

8. RFC 2104: HMAC: Keyed-Hashing for Message Authentication <http://tools.ietf.org/html/rfc2104>.

9. RFC 3447: Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1 <http://tools.ietf.org/html/rfc3447>.

10. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.

11. XEP-0115: Entity Capabilities <https://xmpp.org/extensions/xep-0115.html>.

12. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

13. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.

14. XEP-0068: Field Data Standardization for Data Forms <https://xmpp.org/extensions/xep-0068.html>.

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

Version 0.2 (2015-11-09)

Updated contact information.

Updated example JIDs to example.org

(pw)

Version 0.1 (2014-05-28)

Initial published version approved by the XMPP Council.

(editor (mam))

Version 0.0.3 (2014-05-13)

Added acknowledgements section.

(pw)

Version 0.0.2 (2014-05-09)

Removed links to articles expression opinions.

Reformulated the reference to SASL in the introduction.

A reference to Unicode Standard Annex #15, Unicode Normalization Forms, and NFC normalization has been added.

(pw)

Version 0.0.1 (2014-04-16)

First draft.

(pw)

END