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Versions: (draft-richer-oauth-signed-http-request) 00 01 02 03

OAuth Working Group                                       J. Richer, Ed.
Internet-Draft
Intended status: Standards Track                              J. Bradley
Expires: February 9, 2017                                  Ping Identity
                                                           H. Tschofenig
                                                             ARM Limited
                                                         August 08, 2016


              A Method for Signing HTTP Requests for OAuth
                draft-ietf-oauth-signed-http-request-03

Abstract

   This document a method for offering data origin authentication and
   integrity protection of HTTP requests.  To convey the relevant data
   items in the request a JSON-based encapsulation is used and the JSON
   Web Signature (JWS) technique is re-used.  JWS offers integrity
   protection using symmetric as well as asymmetric cryptography.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 9, 2017.

Copyright Notice

   Copyright (c) 2016 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
   (http://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.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Generating a JSON Object from an HTTP Request . . . . . . . .   3
     3.1.  Calculating the query parameter list and hash . . . . . .   4
     3.2.  Calculating the header list and hash  . . . . . . . . . .   5
   4.  Sending the signed object . . . . . . . . . . . . . . . . . .   6
     4.1.  HTTP Authorization header . . . . . . . . . . . . . . . .   6
     4.2.  HTTP Form body  . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  HTTP Query parameter  . . . . . . . . . . . . . . . . . .   7
   5.  Validating the request  . . . . . . . . . . . . . . . . . . .   7
     5.1.  Validating the query parameter list and hash  . . . . . .   7
     5.2.  Validating the header list and hash . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  The 'pop' OAuth Access Token Type . . . . . . . . . . . .   9
     6.2.  JSON Web Signature and Encryption Type Values
           Registration  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     7.1.  Offering Confidentiality Protection for Access to
           Protected       Resources . . . . . . . . . . . . . . . .   9
     7.2.  Plaintext Storage of Credentials  . . . . . . . . . . . .  10
     7.3.  Entropy of Keys . . . . . . . . . . . . . . . . . . . . .  10
     7.4.  Denial of Service . . . . . . . . . . . . . . . . . . . .  10
     7.5.  Validating the integrity of HTTP message  . . . . . . . .  11
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   In order to prove possession of an access token and its associated
   key, an OAuth 2.0 client needs to compute some cryptographic function
   and present the results to the protected resource as a signature.
   The protected resource then needs to verify the signature and compare
   that to the expected keys associated with the access token.  This is
   in addition to the normal token protections provided by a bearer
   token [RFC6750] and transport layer security (TLS).

   Furthermore, it is desirable to bind the signature to the HTTP
   request.  Ideally, this should be done without replicating the
   information already present in the HTTP request more than required.
   However, many HTTP application frameworks insert extra headers, query



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   parameters, and otherwise manipulate the HTTP request on its way from
   the web server into the application code itself.  It is the goal of
   this draft to have a signature protection mechanism that is
   sufficiently robust against such deployment constraints while still
   providing sufficient security benefits.

   The key required for this signature calculation is distributed via
   mechanisms described in companion documents (see
   [I-D.ietf-oauth-pop-key-distribution] and
   [I-D.ietf-oauth-pop-architecture]).  The JSON Web Signature (JWS)
   specification [RFC7515] is used for computing a digital signature
   (which uses asymmetric cryptography) or a keyed message digest (in
   case of symmetric cryptography).

   The mechanism described in this document assumes that a client is in
   possession of an access token and asociated key.  That client then
   creates a JSON object including the access token, signs the JSON
   object using JWS, and issues an request to a resource server for
   access to a protected resource using the signed object as its
   authorization.  The protected resource validates the JWS signature
   and parses the JSON object to obtain token information.

2.  Terminology

   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 RFC
   2119 [RFC2119].

   Other terms such as "client", "authorization server", "access token",
   and "protected resource" are inherited from OAuth 2.0 [RFC6749].

   We use the term 'sign' (or 'signature') to denote both a keyed
   message digest and a digital signature operation.

3.  Generating a JSON Object from an HTTP Request

   This specification uses JSON Web Signatures [RFC7515] to protect the
   access token and, optionally, parts of the request.

   This section describes how to generate a JSON [RFC7159] object from
   the HTTP request.  Each value below is included as a member of the
   JSON object at the top level.

   at REQUIRED.  The access token value.  This string is assumed to have
      no particular format or structure and remains opaque to the
      client.




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   ts RECOMMENDED.  The timestamp.  This integer provides replay
      protection of the signed JSON object.  Its value MUST be a number
      containing an integer value representing number of whole integer
      seconds from midnight, January 1, 1970 GMT.

   m  OPTIONAL.  The HTTP Method used to make this request.  This MUST
      be the uppercase HTTP verb as a JSON string.

   u  OPTIONAL.  The HTTP URL host component as a JSON string.  This MAY
      include the port separated from the host by a colon in host:port
      format.

   p  OPTIONAL.  The HTTP URL path component of the request as an HTTP
      string.

   q  OPTIONAL.  The hashed HTTP URL query parameter map of the request
      as a two-part JSON array.  The first part of this array is a JSON
      array listing all query parameters that were used in the
      calculation of the hash in the order that they were added to the
      hashed value as described below.  The second part of this array is
      a JSON string containing the Base64URL encoded hash itself,
      calculated as described below.

   h  OPTIONAL.  The hashed HTTP request headers as a two-part JSON
      array.  The first part of this array is a JSON array listing all
      headers that were used in the calculation of the hash in the order
      that they were added to the hashed value as described below.  The
      second part of this array is a JSON string containing the
      Base64URL encoded hash itself, calculated as described below.

   b  OPTIONAL.  The base64URL encoded hash of the HTTP Request body,
      calculated as the SHA256 of the byte array of the body

   All hashes SHALL be calculated using the SHA256 algorithm. [[ Note to
   WG: do we want crypto agility here?  If so how do we signal this ]]

   The JSON object is signed using the algorithm appropriate to the
   associated access token key, usually communicated as part of key
   distribution [I-D.ietf-oauth-pop-key-distribution].

3.1.  Calculating the query parameter list and hash

   To generate the query parameter list and hash, the client creates two
   data objects: an ordered list of strings to hold the query parameter
   names and a string buffer to hold the data to be hashed.

   The client iterates through all query parameters in whatever order it
   chooses and for each query parameter it does the following:



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   1.  Adds the name of the query parameter to the end of the list.

   2.  Percent-encodes the name and value of the parameter as specified
       in [RFC3986].  Note that if the name and value have already been
       percent-encoded for transit, they are not re-encoded for this
       step.

   3.  Encodes the name and value of the query parameter as "name=value"
       and appends it to the string buffer separated by the ampersand
       "&" character.

   Repeated parameter names are processed separately with no special
   handling.  Parameters MAY be skipped by the client if they are not
   required (or desired) to be covered by the signature.

   The client then calculates the hash over the resulting string buffer.
   The list and the hash result are added to a list as the value of the
   "q" member.

   For example, the query parameter set of "b=bar", "a=foo", "c=duck" is
   concatenated into the string:

   b=bar&a=foo&c=duck

   When added to the JSON structure using this process, the results are:

   "q": [["b", "a", "c"], "u4LgkGUWhP9MsKrEjA4dizIllDXluDku6ZqCeyuR-JY"]

3.2.  Calculating the header list and hash

   To generate the header list and hash, the client creates two data
   objects: an ordered list of strings to hold the header names and a
   string buffer to hold the data to be hashed.

   The client iterates through all query parameters in whatever order it
   chooses and for each query parameter it does the following:

   1.  Lowercases the header name.

   2.  Adds the name of the header to the end of the list.

   3.  Encodes the name and value of the header as "name: value" and
       appends it to the string buffer separated by a newline "\n"
       character.

   Repeated header names are processed separately with no special
   handling.  Headers MAY be skipped by the client if they are not
   required (or desired) to be covered by the signature.



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   The client then calculates the hash over the resulting string buffer.
   The list and the hash result are added to a list as the value of the
   "h" member.

   For example, the headers "Content-Type: application/json" and "Etag:
   742-3u8f34-3r2nvv3" are concatenated into the string:

   content-type: application/json
   etag: 742-3u8f34-3r2nvv3

   "h": [["content-type", "etag"],
     "bZA981YJBrPlIzOvplbu3e7ueREXXr38vSkxIBYOaxI"]

4.  Sending the signed object

   In order to send the signed object to the protected resource, the
   client includes it in one of the following three places.

4.1.  HTTP Authorization header

   The client SHOULD send the signed object to the protected resource in
   the Authorization header.  The value of the signed object in JWS
   compact form is appended to the Authorization header as a PoP value.
   This is the preferred method.  Note that if this method is used, the
   Authorization header MUST NOT be included in the protected elements
   of the signed object.

   GET /resource/foo
   Authorization: PoP eyJ....omitted for brevity...

4.2.  HTTP Form body

   If the client is sending the request as a form-encoded HTTP message
   with parameters in the body, the client MAY send the signed object as
   part of that form body.  The value of the signed object in JWS
   compact form is sent as the form parameter pop_access_token.  Note
   that if this method is used, the body hash cannot be included in the
   protected elements of the signed object.

   POST /resource
   Content-type: application/www-form-encoded

   pop_access_token=eyJ....omitted for brevity...








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4.3.  HTTP Query parameter

   If neither the Authorization header nor the form-encoded body
   parameter are available to the client, the client MAY send the signed
   object as a query parameter.  The value of the signed object in JWS
   compact form is sent as the query parameter pop_access_token.  Note
   that if this method is used, the pop_access_token parameter MUST NOT
   be included in the protected elements of the signed object.

   GET /resource?pop_access_token=eyJ....

5.  Validating the request

   Just like with a bearer token [RFC6750], while the access token value
   included in the signed object is opaque to the client, it MUST be
   understood by the protected resource in order to fulfill the request.
   Also like a bearer token, the protected resource traditionally has
   several methods at its disposal for understanding the access token.
   It can look up the token locally (such as in a database), it can
   parse a structured token (such as JWT [RFC7519]), or it can use a
   service to look up token information (such as introspection
   [RFC7662]).  Whatever method is used to look up token information,
   the protected resource MUST have access to the key associated with
   the access token, as this key is required to validate the signature
   of the incoming request.  Validation of the signature is done using
   normal JWS validation for the signature and key type.

   Additionally, in order to trust any of the hashed components of the
   HTTP request, the protected resource MUST re-create and verify a hash
   for each component as described below.  This process is a mirror of
   the process used to create the hashes in the first place, with a mind
   toward the fact that order may have changed and that elements may
   have been added or deleted.  The protected resource MUST similarly
   compare the replicated values included in various JSON fields with
   the corresponding actual values from the request.  Failure to do so
   will allow an attacker to modify the underlying request while at the
   same time having the application layer verify the signature
   correctly.

5.1.  Validating the query parameter list and hash

   The client has at its disposal a map that indexes the query parameter
   names to the values given.  The client creates a string buffer for
   calculating the hash.  The client then iterates through the "list"
   portion of the "p" parameter.  For each item in the list (in the
   order of the list) it does the following:





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   1.  Fetch the value of the parameter from the HTTP request query
       parameter map.  If a parameter is found in the list of signed
       parameters but not in the map, the validation fails.

   2.  Percent-encodes the name and value of the parameter as specified
       in [RFC3986].  Note that if the name and value have already been
       percent-encoded for transit, they are not re-encoded for this
       step.

   3.  Encode the parameter as "name=value" and concatenate it to the
       end of the string buffer, separated by an ampersand character.

   The client calculates the hash of the string buffer and base64url
   encodes it.  The protected resource compares that string to the
   string passed in as the hash.  If the two match, the hash validates,
   and all named parameters and their values are considered covered by
   the signature.

   There MAY be additional query parameters that are not listed in the
   list and are therefore not covered by the signature.  The client MUST
   decide whether or not to accept a request with these uncovered
   parameters.

5.2.  Validating the header list and hash

   The client has at its disposal a map that indexes the header names to
   the values given.  The client creates a string buffer for calculating
   the hash.  The client then iterates through the "list" portion of the
   "h" parameter.  For each item in the list (in the order of the list)
   it does the following:

   1.  Fetch the value of the header from the HTTP request header map.
       If a header is found in the list of signed parameters but not in
       the map, the validation fails.

   2.  Encode the parameter as "name: value" and concatenate it to the
       end of the string buffer, separated by a newline character.

   The client calculates the hash of the string buffer and base64url
   encodes it.  The protected resource compares that string to the
   string passed in as the hash.  If the two match, the hash validates,
   and all named headers and their values are considered covered by the
   signature.

   There MAY be additional headers that are not listed in the list and
   are therefore not covered by the signature.  The client MUST decide
   whether or not to accept a request with these uncovered headers.




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6.  IANA Considerations

6.1.  The 'pop' OAuth Access Token Type

   Section 11.1 of [RFC6749] defines the OAuth Access Token Type
   Registry and this document adds another token type to this registry.

   Type name:  pop

   Additional Token Endpoint Response Parameters:  (none)

   HTTP Authentication Scheme(s):  Proof-of-possession access token for
      use with OAuth 2.0

   Change controller:  IETF

   Specification document(s):  [[ this document ]]

6.2.  JSON Web Signature and Encryption Type Values Registration

   This specification registers the "pop" type value in the IANA JSON
   Web Signature and Encryption Type Values registry [RFC7515]:

   o  "typ" Header Parameter Value: "pop"

   o  Abbreviation for MIME Type: None

   o  Change Controller: IETF

   o  Specification Document(s): [[ this document ]]

7.  Security Considerations

7.1.  Offering Confidentiality Protection for Access to Protected
      Resources

   This specification can be used with and without Transport Layer
   Security (TLS).

   Without TLS this protocol provides a mechanism for verifying the
   integrity of requests, it provides no confidentiality protection.
   Consequently, eavesdroppers will have full access to communication
   content and any further messages exchanged between the client and the
   resource server.  This could be problematic when data is exchanged
   that requires care, such as personal data.

   When TLS is used then confidentiality of the transmission can be
   ensured between endpoints, including both the request and the



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   response.  The use of TLS in combination with the signed HTTP request
   mechanism is highly recommended to ensure the confidentiality of the
   data returned from the protected resource.

7.2.  Plaintext Storage of Credentials

   The mechanism described in this document works in a similar way to
   many three-party authentication and key exchange mechanisms.  In
   order to compute the signature over the HTTP request, the client must
   have access to a key bound to the access token in plaintext form.  If
   an attacker were to gain access to these stored secrets at the client
   or (in case of symmetric keys) at the resource server they would be
   able to perform any action on behalf of any client just as if they
   had stolen a bearer token.

   It is therefore paramount to the security of the protocol that the
   private keys associated with the access tokens are protected from
   unauthorized access.

7.3.  Entropy of Keys

   Unless TLS is used between the client and the resource server,
   eavesdroppers will have full access to requests sent by the client.
   They will thus be able to mount off-line brute-force attacks to
   attempt recovery of the session key or private key used to compute
   the keyed message digest or digital signature, respectively.

   This specification assumes that the key used herein has been
   distributed via other mechanisms, such as
   [I-D.ietf-oauth-pop-key-distribution].  Hence, it is the
   responsibility of the authorization server and or the client to be
   careful when generating fresh and unique keys with sufficient entropy
   to resist such attacks for at least the length of time that the
   session keys (and the access tokens) are valid.

   For example, if the key bound to the access token is valid for one
   day, authorization servers must ensure that it is not possible to
   mount a brute force attack that recovers that key in less than one
   day.  Of course, servers are urged to err on the side of caution, and
   use the longest key length possible within reason.

7.4.  Denial of Service

   This specification includes a number of features which may make
   resource exhaustion attacks against resource servers possible.  For
   example, a resource server may need to process the incoming request,
   verify the access token, perform signature verification, and might
   (in certain circumstances) have to consult back-end databases or the



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   authorization server before granting access to the protected
   resource.  Many of these actions are shared with bearer tokens, but
   the additional cryptographic overhead of validating the signed
   request needs to be taken into consideration with deployment of this
   specification.

   An attacker may exploit this to perform a denial of service attack by
   sending a large number of invalid requests to the server.  The
   computational overhead of verifying the keyed message digest alone is
   not likely sufficient to mount a denial of service attack.  To help
   combat this, it is RECOMMENDED that the protected resource validate
   the access token (contained in the "at" member of the signed
   structure) before performing any cryptographic verification
   calculations.

7.5.  Validating the integrity of HTTP message

   This specification provides flexibility for selectively validating
   the integrity of the HTTP request, including header fields, query
   parameters, and message bodies.  Since all components of the HTTP
   request are only optionally validated by this method, and even some
   components may be validated only in part (e.g., some headers but not
   others) it is up to protected resource developers to verify that any
   vital parameters in a request are actually covered by the signature.
   Failure to do so could allow an attacker to inject vital parameters
   or headers into the request, ouside of the protection of the
   signature.

   The application verifying this signature MUST NOT assume that any
   particular parameter is appropriately covered by the signature unless
   it is included in the signed structure and the hash is verified.  Any
   applications that are sensitive of header or query parameter order
   MUST verify the order of the parameters on their own.  The
   application MUST also compare the values in the JSON container with
   the actual parameters received with the HTTP request (using a direct
   comparison or a hash calculation, as appropriate).  Failure to make
   this comparison will render the signature mechanism useless for
   protecting these elements.

   The behavior of repeated query parameters or repeated HTTP headers is
   undefined by this specification.  If a header or query parameter is
   repeated on either the outgoing request from the client or the
   incoming request to the protected resource, that query parameter or
   header name MUST NOT be covered by the hash and signature.

   This specification records the order in which query parameters and
   headers are hashed, but it does not guarantee that order is preserved
   between the client and protected resource.  If the order of



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   parameters or headers are significant to the underlying application,
   it MUST confirm their order on its own, apart from the signature and
   HTTP message validation.

8.  Privacy Considerations

   This specification addresses machine to machine communications and
   raises no privacy considerations beyond existing OAuth transactions.

9.  Acknowledgements

   The authors thank the OAuth Working Group for input into this work.

10.  Normative References

   [I-D.ietf-oauth-pop-architecture]
              Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
              Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
              Architecture", draft-ietf-oauth-pop-architecture-08 (work
              in progress), July 2016.

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", draft-ietf-oauth-pop-key-
              distribution-02 (work in progress), October 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://www.rfc-editor.org/info/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <http://www.rfc-editor.org/info/rfc6750>.






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   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://www.rfc-editor.org/info/rfc7519>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <http://www.rfc-editor.org/info/rfc7662>.

Authors' Addresses

   Justin Richer (editor)

   Email: ietf@justin.richer.org


   John Bradley
   Ping Identity

   Email: ve7jtb@ve7jtb.com
   URI:   http://www.thread-safe.com/


   Hannes Tschofenig
   ARM Limited
   Austria

   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at















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