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Network Working Group                                    E. Hammer-Lahav
Internet-Draft                                                    Yahoo!
Intended status: Standards Track                         January 9, 2011
Expires: July 13, 2011


                 OAuth 2.0 MAC Token and Authentication
                   draft-hammer-oauth-v2-mac-token-00

Abstract

   This document specifies the OAuth 2.0 MAC token type and
   authentication scheme.

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 July 13, 2011.

Copyright Notice

   Copyright (c) 2011 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
   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.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Notational Conventions . . . . . . . . . . . . . . . . . .  5
   2.  Issuing MAC-Type Access Tokens . . . . . . . . . . . . . . . .  5
   3.  Making Requests  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  The Authorization Request Header . . . . . . . . . . . . .  6
     3.2.  Signature  . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.  Normalized Request String  . . . . . . . . . . . . . .  7
       3.2.2.  hmac-sha-1 . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.3.  hmac-sha-256 . . . . . . . . . . . . . . . . . . . . . 12
   4.  Verifying Requests . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Scheme Extensions  . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     6.1.  Secrets Transmission . . . . . . . . . . . . . . . . . . . 13
     6.2.  Confidentiality of Requests  . . . . . . . . . . . . . . . 13
     6.3.  Spoofing by Counterfeit Servers  . . . . . . . . . . . . . 13
     6.4.  Plaintext Storage of Credentials . . . . . . . . . . . . . 13
     6.5.  Entropy of Secrets . . . . . . . . . . . . . . . . . . . . 14
     6.6.  Denial of Service / Resource Exhaustion Attacks  . . . . . 14
     6.7.  Coverage Limitations . . . . . . . . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  The "secret" OAuth Parameter . . . . . . . . . . . . . . . 15
     7.2.  The "secret" OAuth Parameter . . . . . . . . . . . . . . . 15
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Document History  . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17




















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1.  Introduction

   OAuth 2.0 ([I-D.ietf-oauth-v2]) defines a token-based authentication
   framework in which third-party applications (clients) access
   protected resources using access tokens.  Access tokens are obtained
   via the resource owners' authorization from an authorization server.

   This specification defines the MAC token type for use with the OAuth
   2.0 framework.  It defines type-specific token attributes and
   provides a method for making authenticated HTTP requests with partial
   cryptographic verification of the request - covering the HTTP method,
   request URI, host, and in some cases the request body.

   This specification does not define methods for the client to
   specifically request a MAC-type token from the authorization server.
   Additionally, it does not include any discovery facilities for
   identifying which token types are supported by a resource server or
   how the client may go about obtaining access tokens.  This
   specification assumes that the authorization server has issued the
   client a MAC-type token and describes how the client authenticates
   using that access token.

   The MAC token type is not compatible with the "HMAC-SHA1" signature
   method defined in OAuth 1.0 [RFC5849].

   This specification is an extension of [I-D.ietf-oauth-v2] and uses
   its terminology.

   Please discuss this draft on the oauth@ietf.org [1] mailing list.

1.1.  Example

   The client attempts to access a protected resource without
   authentication, making the following HTTP request to the resource
   server:


     GET /resource/1?b=1&a=2 HTTP/1.1
     Host: example.com


   The resource server returns the following authentication challenge:


     HTTP/1.1 401 Unauthorized
     WWW-Authenticate: OAuth2
     Date: Thu, 02 Dec 2010 21:39:45 GMT




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   The client has previously obtained a set of token credentials for
   accessing resources on the "http://example.com/" resource server.
   The credentials issued to the client by the authorization server
   included the following attributes:

   Access token:  h480djs93hd8

   Token type:  mac

   MAC algorithm:  hmac-sha-1

   Token secret:  489dks293j39

   The client attempts the HTTP request again, this time using the token
   credentials issued by the authorization server earlier to
   authenticate.  To construct the authentication header, the client
   calculates the current timestamp and a nonce.  The nonce is unique to
   the timestamp used, typically a random string:

   Timestamp:  137131200

   Nonce:  dj83hs9s

   The client normalizes the request and constructs the signature base
   string (the new line separator character is represented by "\n" for
   display purposes only):


     h480djs93hd8\n
     137131200\n
     dj83hs9s\n
     GET\n
     example.com\n
     80\n
     /resource/1\n
     a=2\n
     b=1


   The signature base string is signed using the specified MAC token
   algorithm "hmac-sha-1" with the signature base string as text and the
   token secret as key.  The resulting digest is base64-encoded to
   produce the request signature:


     IdSrHQHTwCPWGrqzGGIR791ZJXE=





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   The client includes the access token, timestamp, nonce, and signature
   with the request using the "Authorization" request header field:


     GET /resource/1 HTTP/1.1
     Host: example.com
     Authorization: MAC token='h480djs93hd8',
                        timestamp='137131200',
                        nonce='dj83hs9s',
                        signature='IdSrHQHTwCPWGrqzGGIR791ZJXE='


   The resource server validates the request by calculating the
   signature again based on the request received and verifies the
   validity and scope of the access token.  If valid, the resource
   server responds with the requested protected resource representation.

1.2.  Notational Conventions

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
   document are to be interpreted as described in [RFC2119].

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [I-D.ietf-httpbis-p1-messaging].


2.  Issuing MAC-Type Access Tokens

   Authorization servers issuing MAC-type access tokens MUST include the
   following parameters whenever a response includes the "access_token"
   parameter:

   secret
         REQUIRED.  The token shared secret used as the MAC algorithm
         key.

   algorithm
         REQUIRED.  The MAC algorithm used to calculate the request
         signature.  Value MUST be one of "hmac-sha-1", "hmac-sha-256",
         or a registered extension algorithm name as described in
         Section 5.


3.  Making Requests

   To make authenticated requests, the client must be in possession of a
   valid MAC-type access token, issued by an authorization server



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   accepted by the resource server.  The client constructs the request
   by calculating of a set of attributes, and adding them to the HTTP
   request using the Authorization header field (Section 3.1).
   Authenticated request can be sent in response to an authentication
   challenge or directly.

3.1.  The Authorization Request Header

   The "Authorization" request header field uses the framework defined
   by [RFC2617] as follows:


     credentials    = 'MAC' [ RWS 1#param ]

     param          = access-token /
                      timestamp /
                      nonce /
                      signature

     access-token   = 'token' '=' quoted-string
     timestamp      = 'timestamp' '=' <"> 1*DIGIT <">
     nonce          = 'nonce' '=' quoted-string
     signature      = 'signature' '=' quoted-string


   The "token" attribute value is set to the access token received from
   the authorization server.

   The "timestamp" attribute value is set to the current time expressed
   in the number of seconds since January 1, 1970 00:00:00 GMT, and MUST
   be a positive integer.

   The "nonce" attribute value is set to a random string, uniquely
   generated by the client to allow the resource server to verify that a
   request has never been made before and helps prevent replay attacks
   when requests are made over an insecure channel.  The nonce value
   MUST be unique across all requests with the same timestamp and access
   token combination.

   To avoid the need to retain an infinite number of nonce values for
   future checks, servers MAY choose to restrict the time period after
   which a request with an old timestamp is rejected.  Such a
   restriction implies a level of synchronization between the client's
   and server's clocks.  The client MAY use the "Date" response header
   field to synchronize its clock after a failed request.

   The "signature" attribute value is set as described in Section 3.2.




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   Each of the four attributes MUST appear once, and only once.

3.2.  Signature

   The client uses the MAC token algorithm and the access token secret -
   both provided by the authorization server - to calculate the request
   signature.  This specification defines two algorithms: "hmac-sha-1"
   and "hmac-sha-256", and provides an extension registry for additional
   algorithms.

3.2.1.  Normalized Request String

   The normalized request string is a consistent, reproducible
   concatenation of several of the HTTP request elements into a single
   string.  By normalizing the request into a reproducible string, the
   client and resource server can both sign the same string.  The string
   is constructed by concatenating together, in order, the following
   HTTP request elements:

   1.   The access token.

   2.   A new line character (ASCII code 10).

   3.   The timestamp value calculated for the request.

   4.   A new line character (ASCII code 10).

   5.   The nonce value generated for the request.

   6.   A new line character (ASCII code 10).

   7.   The HTTP request method in upper case.  For example: "HEAD",
        "GET", "POST", etc.

   8.   A new line character (ASCII code 10).

   9.   The hostname included in the HTTP request using the "Host"
        request header field in lower case.

   10.  A new line character (ASCII code 10).

   11.  The port as included in the HTTP request using the "Host"
        request header field.  If the header field does not include a
        port, the default value for the scheme MUST be used (e.g. 80 for
        HTTP and 443 for HTTPS).

   12.  A new line character (ASCII code 10).




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   13.  The path component of the HTTP request URI as defined by
        [RFC3986] section 3.3.

   14.  A new line character (ASCII code 10).

   15.  The query component of the HTTP request URI as defined by
        [RFC3986] section 3.4, normalized as described in
        Section 3.2.1.1.

   For example, the HTTP request:


       GET /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q HTTP/1.1
       Host: example.com


   using access token "kkk9d7dh3k39sjv7", timestamp "137131201", and
   nonce "7d8f3e4a" is normalized into the following string (the new
   line Separator character is represented by "\n" for display purposes
   only):


     kkk9d7dh3k39sjv7\n
     137131201\n
     7d8f3e4a\n
     GET\n
     example.com\n
     80\n
     /request\n
     a2=r%20b\n
     a3=2%20q\n
     a3=a\n
     b5=%3D%253D\n
     c%40=\n
     c2=


3.2.1.1.  Parameters Normalization

   The query component is parsed into a list of name/value parameter
   pairs by treating it as an "application/x-www-form-urlencoded"
   string, separating the names and values and decoding them as defined
   by [W3C.REC-html401-19991224] section 17.13.4.

   Once separated and decoded, the parameters are concatenated back
   together as follows:





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   1.  First, the name and value of each parameter are escaped using the
       [RFC3986] percent-encoding (%XX) mechanism.  Characters in the
       unreserved character set as defined by [RFC3986] section 2.3
       (ALPHA, DIGIT, "-", ".", "_", "~") MUST NOT be encoded.  All
       other characters MUST be encoded.  The two hexadecimal characters
       used to represent encoded characters MUST be upper case.

   2.  The parameters are sorted by name, using ascending byte value
       ordering.  If two or more parameters share the same name, they
       are sorted by their value.

   3.  The name of each parameter is concatenated to its corresponding
       value using an "=" character (ASCII code 61) as separator, even
       if the value is empty.

   4.  The sorted name/value pairs are concatenated together into a
       single string by using an new line character (ASCII code 10) as
       separator.

   Note that the percent-encoding method described is different from the
   encoding scheme used by the "application/x-www-form-urlencoded"
   content-type (for example, it encodes space characters as "%20"
   instead of the "+" character).  It MAY be different from the percent-
   encoding functions provided by web development frameworks (e.g.
   encode different characters, use lower case hexadecimal characters).

   For example, the HTTP request URI:


     /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q


   Contains the following (fully decoded) parameters used in the
   signature base sting:

                             +------+-------+
                             | Name | Value |
                             +------+-------+
                             |  b5  |  =%3D |
                             |  a3  |   a   |
                             |  c@  |       |
                             |  a2  |  r b  |
                             |  c2  |       |
                             |  a3  |  2 q  |
                             +------+-------+

   Note that the value of "b5" is "=%3D" and not "==".  Both "c@" and
   "c2" have empty values.  While the encoding rules specified in this



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   specification for the purpose of constructing the signature base
   string exclude the use of a "+" character (ASCII code 43) to
   represent an encoded space character (ASCII code 32), this practice
   is widely used in "application/x-www-form-urlencoded" encoded values,
   and MUST be properly decoded, as demonstrated by one of the "a3"
   parameter instances (the "a3" parameter is used twice in this
   request).

   The parsed parameters are normalized as follows:

                                 Encoded:

                            +------+----------+
                            | Name |   Value  |
                            +------+----------+
                            |  b5  | %3D%253D |
                            |  a3  |     a    |
                            | c%40 |          |
                            |  a2  |   r%20b  |
                            |  c2  |          |
                            |  a3  |   2%20q  |
                            +------+----------+

                                  Sorted:

                            +------+----------+
                            | Name |   Value  |
                            +------+----------+
                            |  a2  |   r%20b  |
                            |  a3  |   2%20q  |
                            |  a3  |     a    |
                            |  b5  | %3D%253D |
                            | c%40 |          |
                            |  c2  |          |
                            +------+----------+
















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                            Concatenated Pairs:

                              +-------------+
                              |  Name=Value |
                              +-------------+
                              |   a2=r%20b  |
                              |   a3=2%20q  |
                              |     a3=a    |
                              | b5=%3D%253D |
                              |    c%40=    |
                              |     c2=     |
                              +-------------+

   And concatenated together into a single string (the new line
   separator character is represented by "\n" for display purposes
   only):


     a2=r%20b\n
     a3=2%20q\n
     a3=a\n
     b5=%3D%253D\n
     c%40=\n
     c2=


3.2.2.  hmac-sha-1

   "hmac-sha-1" uses the HMAC-SHA1 algorithm as defined in [RFC2104]:


     digest = HMAC-SHA1 (key, text)


   Where:

   text
         is set to the value of the normalize request string as
         described in Section 3.2.1.

   key
         is set to the access token shared-secret provided by the
         authorization server.

   digest
         is used to set the value of the "signature" attribute, after
         the result octet string is base64-encoded per [RFC2045] section
         6.8.



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3.2.3.  hmac-sha-256

   "hmac-sha-1" uses the HMAC algorithm as defined in [RFC2104] together
   with the SHA-256 hash function defined in [NIST FIPS-180-3]:


     digest = HMAC-SHA256 (key, text)


   Where:

   text
         is set to the value of the normalize request string as
         described in Section 3.2.1.

   key
         is set to the access token shared-secret provided by the
         authorization server.

   digest
         is used to set the value of the "signature" attribute, after
         the result octet string is base64-encoded per [RFC2045] section
         6.8.


4.  Verifying Requests

   A servers receiving an authenticated request validates it by
   performing the following REQUIRED steps:

   1.  Recalculate the request signature as described in Section 3.2 and
       compare it to the value received from the client via the
       "signature" attribute.

   2.  Ensure that the combination of nonce, timestamp, and access token
       received from the client has not been used before in a previous
       request (the server MAY reject requests with stale timestamps;
       the determination of staleness is left up to the server to
       define).

   3.  Verify the scope and status of the access token.

   If the request fails verification, the server SHOULD respond with an
   HTTP 401 (unauthorized) status code, and SHOULD include a token
   scheme authentication challenge using the WWW-Authenticate header
   field.  The server MAY include further details about why the request
   was rejected using the error attribute.




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5.  Scheme Extensions

   [[ TBD ]]


6.  Security Considerations

   As stated in [RFC2617], the greatest sources of risks are usually
   found not in the core protocol itself but in policies and procedures
   surrounding its use.  Implementers are strongly encouraged to assess
   how this protocol addresses their security requirements.

6.1.  Secrets Transmission

   This specification does not describe any mechanism for obtaining or
   transmitting access token secrets.  Methods used to obtain tokens
   should ensure that these transmissions are protected using transport-
   layer mechanisms such as TLS or SSL.

6.2.  Confidentiality of Requests

   While this protocol provides a mechanism for verifying the integrity
   of requests, it provides no guarantee of request confidentiality.
   Unless further precautions are taken, eavesdroppers will have full
   access to request content.  Servers should carefully consider the
   kinds of data likely to be sent as part of such requests, and should
   employ transport-layer security mechanisms to protect sensitive
   resources.

6.3.  Spoofing by Counterfeit Servers

   This protocol makes no attempt to verify the authenticity of the
   resource server.  A hostile party could take advantage of this by
   intercepting the client's requests and returning misleading or
   otherwise incorrect responses.  Service providers should consider
   such attacks when developing services using this protocol, and should
   require transport-layer security for any requests where the
   authenticity of the resource server or of request responses is an
   issue.

6.4.  Plaintext Storage of Credentials

   The access token shared-secret functions the same way passwords do in
   traditional authentication systems.  In order to compute the
   signature, the server must have access to the secret in plaintext
   form.  This is in contrast, for example, to modern operating systems,
   which store only a one-way hash of user credentials.




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   If an attacker were to gain access to these secrets - or worse, to
   the server's database of all such secrets - he or she would be able
   to perform any action on behalf of any resource owner.  Accordingly,
   it is critical that servers protect these secrets from unauthorized
   access.

6.5.  Entropy of Secrets

   Unless a transport-layer security protocol is used, eavesdroppers
   will have full access to authenticated requests and signatures, and
   will thus be able to mount offline brute-force attacks to recover the
   secret used.  Authorization servers should be careful to assign
   shared-secrets which are long enough, and random enough, to resist
   such attacks for at least the length of time that the shared-secrets
   are valid.

   For example, if shared-secrets are valid for two weeks, authorization
   servers should ensure that it is not possible to mount a brute force
   attack that recovers the shared-secret in less than two weeks.  Of
   course, authorization servers are urged to err on the side of
   caution, and use the longest secrets reasonable.

   It is equally important that the pseudo-random number generator
   (PRNG) used to generate these secrets be of sufficiently high
   quality.  Many PRNG implementations generate number sequences that
   may appear to be random, but which nevertheless exhibit patterns or
   other weaknesses which make cryptanalysis or brute force attacks
   easier.  Implementers should be careful to use cryptographically
   secure PRNGs to avoid these problems.

6.6.  Denial of Service / Resource Exhaustion Attacks

   This specification includes a number of features which may make
   resource exhaustion attacks against servers possible.  For example,
   this protocol requires servers to track used nonces.  If an attacker
   is able to use many nonces quickly, the resources required to track
   them may exhaust available capacity.  And again, this protocol can
   require servers to perform potentially expensive computations in
   order to verify the signature on incoming requests.  An attacker may
   exploit this to perform a denial of service attack by sending a large
   number of invalid requests to the server.

   Resource Exhaustion attacks are by no means specific to this
   specification.  However, implementers should be careful to consider
   the additional avenues of attack that this protocol exposes, and
   design their implementations accordingly.  For example, entropy
   starvation typically results in either a complete denial of service
   while the system waits for new entropy or else in weak (easily



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   guessable) secrets.  When implementing this protocol, servers should
   consider which of these presents a more serious risk for their
   application and design accordingly.

6.7.  Coverage Limitations

   The normalized request string has been designed to support the
   authentication methods defined in this specification.  Those
   designing additional methods, should evaluated the compatibility of
   the normalized request string with their security requirements.
   Since the normalized request string does not cover the entire HTTP
   request, servers should employ additional mechanisms to protect such
   elements.


7.  IANA Considerations

7.1.  The "secret" OAuth Parameter

   Parameter name:  secret

   Parameter usage location:  The end-user authorization endpoint
      response and the token endpoint response.

   Change controller:  IETF

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

   Related information:  None

7.2.  The "secret" OAuth Parameter

   Parameter name:  secret

   Parameter usage location:  The end-user authorization endpoint
      response and the token endpoint response.

   Change controller:  IETF

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

   Related information:  None


8.  Acknowledgments

   The author would like to thank [[ some people ]] for their
   suggestions, feedback, and continued support.



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Appendix A.  Document History

   [[ To be removed by the RFC editor before publication as an RFC. ]]

   -00

   o  Initial draft.


9.  References

9.1.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
              "HTTP/1.1, part 1: URIs, Connections, and Message
              Parsing", draft-ietf-httpbis-p1-messaging-08 (work in
              progress), October 2009.

   [I-D.ietf-oauth-v2]
              Hammer-Lahav, E., Recordon, D., and D. Hardt, "The OAuth
              2.0 Protocol Framework", draft-ietf-oauth-v2-11 (work in
              progress), November 2010.

   [NIST FIPS-180-3]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS). FIPS PUB 180-3, October 2008".

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.



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Internet-Draft             OAuth 2.0 MAC Token              January 2011


   [W3C.REC-html401-19991224]
              Hors, A., Jacobs, I., and D. Raggett, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

9.2.  Informative References

   [RFC5849]  Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
              April 2010.

URIs

   [1]  <https://www.ietf.org/mailman/listinfo/oauth>


Author's Address

   Eran Hammer-Lahav
   Yahoo!

   Email: eran@hueniverse.com
   URI:   http://hueniverse.com




























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