Independent Submission                                           F. Dold
Request for Comments: 8905                              Taler Systems SA
Category: Informational                                      C. Grothoff
ISSN: 2070-1721                      Bern University of Applied Sciences
                                                            October 2020

The 'payto' URI Scheme for Payments

Abstract

This document defines the 'payto' Uniform Resource Identifier (URI) scheme for designating targets for payments.

A unified URI scheme for all payment target types allows applications to offer user interactions with URIs that represent payment targets, simplifying the introduction of new payment systems and applications.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not candidates for any level of Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8905.

Copyright Notice

Copyright © 2020 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.

Table of Contents

   1.  Introduction
     1.1.  Objective
     1.2.  Requirements Language
   2.  Syntax of a 'payto' URI
   3.  Semantics
   4.  Examples
   5.  Generic Options
   6.  Internationalization and Character Encoding
   7.  Tracking Payment Target Types
     7.1.  ACH Bank Account
     7.2.  Business Identifier Code
     7.3.  International Bank Account Number
     7.4.  Unified Payments Interface
     7.5.  Bitcoin Address
     7.6.  Interledger Protocol Address
     7.7.  Void Payment Target
   8.  Security Considerations
   9.  IANA Considerations
   10. Payto Payment Target Types
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Authors' Addresses

1. Introduction

This document defines the 'payto' Uniform Resource Identifier (URI) [RFC3986] scheme for designating transfer form data for payments.

1.1. Objective

A 'payto' URI always identifies the target of a payment. A 'payto' URI consists of a payment target type, a target identifier, and optional parameters such as an amount or a payment reference.

The interpretation of the target identifier is defined by the payment target type and typically represents either a bank account or an (unsettled) transaction.

A unified URI scheme for all payment target types allows applications to offer user interactions with URIs that represent payment targets, simplifying the introduction of new payment systems and applications.

1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Syntax of a 'payto' URI

This document uses the Augmented Backus-Naur Form (ABNF) of [RFC5234].

     payto-URI = "payto://" authority path-abempty [ "?" opts ]
     opts = opt *( "&" opt )
     opt-name = generic-opt / authority-specific-opt
     opt-value = *pchar
     opt = opt-name "=" opt-value
     generic-opt = "amount" / "receiver-name" / "sender-name" /
                   "message" / "instruction"
     authority-specific-opt = ALPHA *( ALPHA / DIGIT / "-" / "." )
     authority = ALPHA *( ALPHA / DIGIT / "-" / "." )

'path-abempty' is defined in Section 3.3 of [RFC3986]. 'pchar' is defined in Appendix A of [RFC3986].

3. Semantics

The authority component of a payment URI identifies the payment target type. The payment target types are defined in the "Payto Payment Target Types" registry (see Section 10). The path component of the URI identifies the target for a payment as interpreted by the respective payment target type. The query component of the URI can provide additional parameters for a payment. Every payment target type SHOULD accept the options defined in generic-opt. The default operation of applications that invoke a URI with the 'payto' scheme MUST be to launch an application (if available) associated with the payment target type that can initiate a payment. If multiple handlers are registered for the same payment target type, the user SHOULD be able to choose which application to launch. This allows users with multiple bank accounts (each accessed via the respective bank's banking application) to choose which account to pay with. An application SHOULD allow dereferencing a 'payto' URI even if the payment target type of that URI is not registered in the "Payto Payment Target Types" registry. Details of the payment MUST be taken from the path and options given in the URI. The user SHOULD be allowed to modify these details before confirming a payment.

4. Examples

Valid Example:

   payto://iban/DE75512108001245126199?amount=EUR:200.0&message=hello

Invalid Example (authority missing):

   payto:iban/12345

5. Generic Options

Applications MUST accept URIs with options in any order. The "amount" option MUST NOT occur more than once. Other options MAY be allowed multiple times, with further restrictions depending on the payment target type. The following options SHOULD be understood by every payment target type.

   amount:  The amount to transfer.  The format MUST be:

amount = currency ":" unit [ "." fraction ]
currency = 1*ALPHA
unit = 1*(DIGIT / ",")
fraction = 1*(DIGIT / ",")

If a 3-letter 'currency' is used, it MUST be an [ISO4217] alphabetic code. A payment target type MAY define semantics beyond ISO 4217 for currency codes that are not 3 characters. The 'unit' value MUST be smaller than 2^53. If present, the 'fraction' MUST consist of no more than 8 decimal digits. The use of commas is optional for readability, and they MUST be ignored.

   receiver-name:  Name of the entity that receives the payment
   
      (creditor).  The value of this option MAY be subject to lossy
      conversion, modification, and truncation (for example, due to line
      wrapping or character set conversion).
   
   sender-name:  Name of the entity that makes the payment (debtor).
      The value of this option MAY be subject to lossy conversion,
      modification, and truncation (for example, due to line wrapping or
      character set conversion).
   
   message:  A short message to identify the purpose of the payment.
      The value of this option MAY be subject to lossy conversion,
      modification, and truncation (for example, due to line wrapping or
      character set conversion).
   
   instruction:  A short message giving payment reconciliation
      instructions to the recipient.  An instruction that follows the
      character set and length limitation defined by the respective
      payment target type SHOULD NOT be subject to lossy conversion.

6. Internationalization and Character Encoding

Various payment systems use restricted character sets. An application that processes 'payto' URIs MUST convert characters that are not allowed by the respective payment systems into allowable characters using either an encoding or a replacement table. This conversion process MAY be lossy, except for the instruction field. If the value of the instruction field would be subject to lossy conversion, modification, or truncation, the application SHOULD refuse further processing of the payment until a different value for the instruction is provided.

To avoid special encoding rules for the payment target identifier, the userinfo component [RFC3986] is disallowed in 'payto' URIs. Instead, the payment target identifier is given as an option, where encoding rules are uniform for all options.

Defining a generic way of tagging the language of option fields containing natural language text (such as "receiver-name", "sender- name", and "message) is out of the scope of this document, as internationalization must accommodate the restrictions and requirements of the underlying banking system of the payment target type. The internationalization concerns SHOULD be individually defined by each payment target type.

7. Tracking Payment Target Types

A registry of "Payto Payment Target Types" is described in Section 10. The registration policy for this registry is "First Come First Served", as described in [RFC8126]. When requesting new entries, careful consideration of the following criteria is strongly advised:

1. The description clearly defines the syntax and semantics of the payment target and optional parameters if applicable.

2. Relevant references are provided if they are available.

3. The chosen name is appropriate for the payment target type, does not conflict with well-known payment systems, and avoids potential to confuse users.

4. The payment system underlying the payment target type is not fundamentally incompatible with the general options (such as positive decimal amounts) in this specification.

5. The payment target type is not a vendor-specific version of a payment target type that could be described more generally by a vendor-neutral payment target type.

6. The specification of the new payment target type remains within the scope of payment transfer form data. In particular, specifying complete invoices is not in scope. Neither are processing instructions to the payment processor or bank beyond a simple payment.

7. The payment target and the options do not contain the payment sender's account details.

Documents that support requests for new registry entries should provide the following information for each entry:

   Name:  The name of the payment target type (case-insensitive ASCII
      string, restricted to alphanumeric characters, dots, and dashes).
   
   Description:  A description of the payment target type, including the
      semantics of the path in the URI if applicable.
   
   Example:  At least one example URI to illustrate the payment target
      type.
   
   Contact:  The contact information of a person to contact for further
      information.
   
   References:  Optionally, references describing the payment target
      type (such as an RFC) and target-specific options or references
      describing the payment system underlying the payment target type.

This document populates the registry with seven entries as follows (see also Section 10).

7.1. ACH Bank Account

   Name:  ach
   
   Description:  Automated Clearing House (ACH).  The path consists of
      two components: the routing number and the account number.
      Limitations on the length and character set of option values are
      defined by the implementation of the handler.  Language tagging
      and internationalization of options are not supported.

Example:

      payto://ach/122000661/1234
   
   Contact:  N/A
   
   References:  [NACHA], RFC 8905

7.2. Business Identifier Code

   Name:  bic
   
   Description:  Business Identifier Code (BIC).  The path consists of
      just a BIC.  This is used for wire transfers between banks.  The
      registry for BICs is provided by the Society for Worldwide
      Interbank Financial Telecommunication (SWIFT).  The path does not
      allow specifying a bank account number.  Limitations on the length
      and character set of option values are defined by the
      implementation of the handler.  Language tagging and
      internationalization of options are not supported.

Example:

      payto://bic/SOGEDEFFXXX
   
   Contact:  N/A
   
   References:  [BIC], RFC 8905

7.3. International Bank Account Number

   Name:  iban
   
   Description:  International Bank Account Number (IBAN).  Generally,
      the IBAN allows to unambiguously derive the associated Business
      Identifier Code (BIC) using a lookup in the respective proprietary
      translation table.  However, some legacy applications process
      payments to the same IBAN differently based on the specified BIC.
      Thus, the path can consist of either a single component (the IBAN)
      or two components (BIC followed by IBAN).  The "message" option of
      the 'payto' URI corresponds to the "unstructured remittance
      information" of Single Euro Payments Area (SEPA) credit transfers
      and is thus limited to 140 characters with character set
      limitations that differ according to the countries of the banks
      and payment processors involved in the payment.  The "instruction"
      option of the 'payto' URI corresponds to the "end-to-end
      identifier" of SEPA credit transfers and is thus limited to, at
      most, 35 characters, which can be alphanumeric or from the allowed
      set of special characters, i.e., "+?/-:().,'".  Language tagging
      and internationalization of options are not supported.

Examples:

      payto://iban/DE75512108001245126199
      payto://iban/SOGEDEFFXXX/DE75512108001245126199
   
   Contact:  N/A
   
   References:  [ISO20022], RFC 8905

7.4. Unified Payments Interface

   Name:  upi
   
   Description:  Unified Payment Interface (UPI).  The path is an
      account alias.  The amount and receiver-name options are mandatory
      for this payment target.  Limitations on the length and character
      set of option values are defined by the implementation of the
      handler.  Language tags and internationalization of options are
      not supported.

Example:

      payto://upi/alice@example.com?receiver-name=Alice&amount=INR:200
   
   Contact:  N/A
   
   References:  [UPILinking], RFC 8905

7.5. Bitcoin Address

   Name:  bitcoin
   
   Description:  Bitcoin protocol.  The path is a "bitcoinaddress", as
      per [BIP0021].  Limitations on the length and character set of
      option values are defined by the implementation of the handler.
      Language tags and internationalization of options are not
      supported.

Example:

      payto://bitcoin/12A1MyfXbW6RhdRAZEqofac5jCQQjwEPBu
   
   Contact:  N/A
   
   References:  [BIP0021], RFC 8905

7.6. Interledger Protocol Address

   Name:  ilp
   
   Description:  Interledger protocol (ILP).  The path is an ILP
      address, as per [ILP-ADDR].  Limitations on the length and
      character set of option values are defined by the implementation
      of the handler.  Language tagging and internationalization of
      options are not supported.

Example:

      payto://ilp/g.acme.bob
   
   Contact:  N/A
   
   References:  [ILP-ADDR], RFC 8905

7.7. Void Payment Target

   Name:  void
   
   Description:  The "void" payment target type allows specifying the
      parameters of an out-of-band payment (such as cash or other types
      of in-person transactions).  The path is optional and interpreted
      as a comment.  Limitations on the length and character set of
      option values are defined by the implementation of the handler.
      Language tags and internationalization of options are not
      supported.

Example:

      payto://void/?amount=EUR:10.5
   
   Contact:  N/A
   
   References:  RFC 8905

8. Security Considerations

Interactive applications handling the 'payto' URI scheme MUST NOT initiate any financial transactions without prior review and confirmation from the user and MUST take measures to prevent clickjacking [HMW12].

Unless a 'payto' URI is received over a trusted, authenticated channel, a user might not be able to identify the target of a payment. In particular, due to homographs [unicode-tr36], a payment target type SHOULD NOT use human-readable names in combination with unicode in the target account specification, as it could give the user the illusion of being able to identify the target account from the URI.

The authentication/authorization mechanisms and transport security services used to process a payment encoded in a 'payto' URI are handled by the application and are not in scope of this document.

To avoid unnecessary data collection, payment target types SHOULD NOT include personally identifying information about the sender of a payment that is not essential for an application to conduct a payment.

9. IANA Considerations

IANA maintains the "Uniform Resource Identifier (URI) Schemes" registry, which contains an entry for the 'payto' URI scheme as follows. IANA has updated that entry to reference this document.

   Scheme name:  payto
   
   Status:  provisional
   
   URI scheme syntax:  See Section 2 of RFC 8905.
   
   URI scheme semantics:  See Section 3 of RFC 8905.
   
   Applications/protocols that use this scheme name:  payto URIs are
      mainly used by financial software.
   
   Contact:  Christian Grothoff <grothoff@gnu.org>
   
   Change controller:  Christian Grothoff <grothoff@gnu.org>
   
   References:  See Section 11 of RFC 8905.

10. Payto Payment Target Types

This document specifies a list of payment target types. It is possible that future work will need to specify additional payment target types. The GNUnet Assigned Numbers Authority (GANA) [GANA] operates the "Payto Payment Target Types" registry to track the following information for each payment target type:

   Name:  The name of the payment target type (case-insensitive ASCII
      string, restricted to alphanumeric characters, dots, and dashes)
   
   Contact:  The contact information of a person to contact for further
      information
   
   References:  Optionally, references describing the payment target
      type (such as an RFC) and target-specific options or references
      describing the payment system underlying the payment target type

The entries in the "Payto Payment Target Types" registry defined in this document are as follows:

                     +=========+=========+===========+
                     | Name    | Contact | Reference |
                     +=========+=========+===========+
                     | ach     | N/A     | RFC 8905  |
                     +---------+---------+-----------+
                     | bic     | N/A     | RFC 8905  |
                     +---------+---------+-----------+
                     | iban    | N/A     | RFC 8905  |
                     +---------+---------+-----------+
                     | upi     | N/A     | RFC 8905  |
                     +---------+---------+-----------+
                     | bitcoin | N/A     | RFC 8905  |
                     +---------+---------+-----------+
                     | ilp     | N/A     | RFC 8905  |
                     +---------+---------+-----------+
                     | void    | N/A     | RFC 8905  |
                     +---------+---------+-----------+

Table 1

11. References

11.1. Normative References

   [ISO20022] International Organization for Standardization, "Financial
              Services - Universal financial industry message scheme",
              ISO 20022, May 2013, <https://www.iso.org>.
   
   [ISO4217]  International Organization for Standardization, "Codes for
              the representation of currencies", ISO 4217, August 2015,
              <https://www.iso.org>.
   
   [NACHA]    Nacha, "2020 Nacha Operating Rules & Guidelines", 2019.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.
   
   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.
   
   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[unicode-tr36]

              Davis, M., Ed. and M. Suignard, Ed., "Unicode Technical
              Report #36: Unicode Security Considerations", September
              2014.

11.2. Informative References

   [BIC]      International Organization for Standardization, "Banking
              -- Banking telecommunication messages -- Business
              identifier code (BIC)", ISO 9362, December 2014,
              <https://www.iso.org>.
   
   [BIP0021]  Schneider, N. and M. Corallo, "Bitcoin Improvement
              Proposal 21", September 2019, <https://en.bitcoin.it/w/
              index.php?title=BIP_0021&oldid=66778>.
   
   [GANA]     GNUnet e.V., "GNUnet Assigned Numbers Authority (GANA)",
              April 2020, <https://gana.gnunet.org/>.
   
   [HMW12]    Huang, L., Moshchuk, A., Wang, H., Schecter, S., and C.
              Jackson, "Clickjacking: Attacks and Defenses", 2012,
              <https://www.usenix.org/system/files/conference/
              usenixsecurity12/sec12-final39.pdf>.

[ILP-ADDR] Interledger, "ILP Addresses - v2.0.0",

<https://interledger.org/rfcs/0015-ilp-addresses/>.

[UPILinking]

              National Payments Corporation of India, "Unified Payment
              Interface - Common URL Specifications For Deep Linking And
              Proximity Integration", November 2017,
              <https://www.npci.org.in/sites/default/files/
              UPI%20Linking%20Specs_ver%201.6.pdf>.

Authors' Addresses

Florian Dold
Taler Systems SA
7, rue de Mondorf
L-5421 Erpeldange
Luxembourg

Email:

          dold@taler.net

Christian Grothoff
Bern University of Applied Sciences
Quellgasse 21
CH-2501 Biel/Bienne
Switzerland

   Email: christian.grothoff@bfh.ch