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PROPOSED STANDARD
Errata Exist
Network Working Group                                         B. Sterman
Request for Comments: 5090                               Kayote Networks
Obsoletes: 4590                                            D. Sadolevsky
Category: Standards Track                                 SecureOL, Inc.
                                                             D. Schwartz
                                                         Kayote Networks
                                                             D. Williams
                                                           Cisco Systems
                                                                 W. Beck
                                                     Deutsche Telekom AG
                                                           February 2008


               RADIUS Extension for Digest Authentication

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document defines an extension to the Remote Authentication
   Dial-In User Service (RADIUS) protocol to enable support of Digest
   Authentication, for use with HTTP-style protocols like the Session
   Initiation Protocol (SIP) and HTTP.






















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Table of Contents

   1. Introduction ....................................................3
      1.1. Motivation .................................................3
      1.2. Terminology ................................................3
      1.3. Overview ...................................................4
   2. Detailed Description ............................................6
      2.1. RADIUS Client Behavior .....................................6
      2.2. RADIUS Server Behavior .....................................9
   3. New RADIUS Attributes ..........................................12
      3.1. Digest-Response Attribute .................................12
      3.2. Digest-Realm Attribute ....................................13
      3.3. Digest-Nonce Attribute ....................................13
      3.4. Digest-Response-Auth Attribute ............................14
      3.5. Digest-Nextnonce Attribute ................................14
      3.6. Digest-Method Attribute ...................................15
      3.7. Digest-URI Attribute ......................................15
      3.8. Digest-Qop Attribute ......................................15
      3.9. Digest-Algorithm Attribute ................................16
      3.10. Digest-Entity-Body-Hash Attribute ........................16
      3.11. Digest-CNonce Attribute ..................................17
      3.12. Digest-Nonce-Count Attribute .............................17
      3.13. Digest-Username Attribute ................................17
      3.14. Digest-Opaque Attribute ..................................18
      3.15. Digest-Auth-Param Attribute ..............................18
      3.16. Digest-AKA-Auts Attribute ................................19
      3.17. Digest-Domain Attribute ..................................19
      3.18. Digest-Stale Attribute ...................................20
      3.19. Digest-HA1 Attribute .....................................20
      3.20. SIP-AOR Attribute ........................................21
   4. Diameter Compatibility .........................................21
   5. Table of Attributes ............................................21
   6. Examples .......................................................23
   7. IANA Considerations ............................................27
   8. Security Considerations ........................................28
      8.1. Denial of Service .........................................28
      8.2. Confidentiality and Data Integrity ........................28
   9. References .....................................................29
      9.1. Normative References ......................................29
      9.2. Informative References ....................................30
   Appendix A - Changes from RFC 4590 ................................31
   Acknowledgements ..................................................31









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1.  Introduction

1.1.  Motivation

   The HTTP Digest Authentication mechanism, defined in [RFC2617], was
   subsequently adapted for use with SIP [RFC3261].  Due to the
   limitations and weaknesses of Digest Authentication (see [RFC2617],
   Section 4), additional authentication and encryption mechanisms are
   defined in SIP [RFC3261], including Transport Layer Security (TLS)
   [RFC4346] and Secure MIME (S/MIME) [RFC3851].  However, Digest
   Authentication support is mandatory in SIP implementations, and
   Digest Authentication is the preferred way for a SIP UA to
   authenticate itself to a proxy server.  Digest Authentication is used
   in other protocols as well.

   To simplify the provisioning of users, there is a need to support
   this authentication mechanism within Authentication, Authorization,
   and Accounting (AAA) protocols such as RADIUS [RFC2865] and Diameter
   [RFC3588].

   This document defines an extension to the RADIUS protocol to enable
   support of Digest Authentication for use with SIP, HTTP, and other
   HTTP-style protocols using this authentication method.  Support for
   Digest mechanisms such as Authentication and Key Agreement (AKA)
   [RFC3310] is also supported.  A companion document [RFC4740] defines
   support for Digest Authentication within Diameter.

1.2.  Terminology

   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].

   The use of normative requirement key words in this document shall
   apply only to RADIUS client and RADIUS server implementations that
   include the features described in this document.  This document
   creates no normative requirements for existing implementations.

   HTTP-style protocol
      The term "HTTP-style" denotes any protocol that uses HTTP-like
      headers and uses HTTP Digest Authentication as described in
      [RFC2617].  Examples are HTTP and the Session Initiation Protocol
      (SIP).

   NAS  (Network Access Server)
      The RADIUS client.





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   nonce
      An unpredictable value used to prevent replay attacks.  The nonce
      generator may use cryptographic mechanisms to produce nonces it
      can recognize without maintaining state.

   protection space
      HTTP-style protocols differ in their definition of the protection
      space.  For HTTP, it is defined as the combination of the realm
      and canonical root URL of the requested resource for which the use
      is authorized by the RADIUS server.  In the case of SIP, the realm
      string alone defines the protection space.

   SIP UA (SIP User Agent)
      An Internet endpoint that uses the Session Initiation Protocol.

   SIP UAS (SIP User Agent Server)
      A logical entity that generates a response to a SIP (Session
      Initiation Protocol) request.

1.3.  Overview

   HTTP Digest is a challenge-response protocol used to authenticate a
   client's request to access some resource on a server.  Figure 1 shows
   a single HTTP Digest transaction.

                              HTTP/SIP..
               +------------+  (1)     +------------+
               |            |--------->|            |
               | HTTP-style |  (2)     | HTTP-style |
               | client     |<---------| server     |
               |            |  (3)     |            |
               |            |--------->|            |
               |            |  (4)     |            |
               |            |<---------|            |
               +------------+          +------------+

               Figure 1: Digest Operation without RADIUS

   If the client sends a request without any credentials (1), the server
   will reply with an error response (2) containing a nonce.  The client
   creates a cryptographic digest from parts of the request, from the
   nonce it received from the server, and from a shared secret.  The
   client retransmits the request (3) to the server, but now includes
   the digest within the packet.  The server does the same digest
   calculation as the client and compares the result with the digest it
   received in (3).  If the digest values are identical, the server
   grants access to the resource and sends a positive response to the




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   client (4).  If the digest values differ, the server sends a negative
   response to the client (4).

   Instead of maintaining a local user database, the server could use
   RADIUS to access a centralized user database.  However, RADIUS
   [RFC2865] does not include support for HTTP Digest Authentication.
   The RADIUS client cannot use the User-Password Attribute, since it
   does not receive a password from the HTTP-style client.  The CHAP-
   Challenge and CHAP-Password attributes described in [RFC1994] are
   also not suitable since the Challenge Handshake Authentication
   Protocol (CHAP) algorithm is not compatible with HTTP Digest.

   This document defines new attributes that enable the RADIUS server to
   perform the digest calculation defined in [RFC2617], providing
   support for Digest Authentication as a native authentication
   mechanism within RADIUS.

   The nonces required by the digest algorithm are generated by the
   RADIUS server.  Generating them in the RADIUS client would save a
   round-trip, but introduce security and operational issues.  Some
   digest algorithms -- e.g., AKA [RFC3310] -- would not work.

   Figure 2 depicts a scenario in which the HTTP-style server defers
   authentication to a RADIUS server.  Entities A and B communicate
   using HTTP or SIP, while entities B and C communicate using RADIUS.

                       HTTP/SIP           RADIUS

               +-----+    (1)    +-----+           +-----+
               |     |==========>|     |    (2)    |     |
               |     |           |     |---------->|     |
               |     |           |     |    (3)    |     |
               |     |    (4)    |     |<----------|     |
               |     |<==========|     |           |     |
               |     |    (5)    |     |           |     |
               |     |==========>|     |           |     |
               |  A  |           |  B  |    (6)    |  C  |
               |     |           |     |---------->|     |
               |     |           |     |    (7)    |     |
               |     |           |     |<----------|     |
               |     |    (8)    |     |           |     |
               |     |<==========|     |           |     |
               +-----+           +-----+           +-----+

                ====> HTTP/SIP
                ----> RADIUS

                     Figure 2: HTTP Digest over RADIUS



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   The entities have the following roles:

   A: HTTP client / SIP UA

   B: {HTTP server / HTTP proxy server / SIP proxy server / SIP UAS}
      acting also as a RADIUS NAS

   C: RADIUS server

   The following messages are sent in this scenario:

   A sends B an HTTP/SIP request without an Authorization header (step
   1).  B sends an Access-Request packet with the newly defined Digest-
   Method and Digest-URI attributes but without a Digest-Nonce Attribute
   to the RADIUS server, C (step 2).  C chooses a nonce and responds
   with an Access-Challenge (step 3).  This Access-Challenge contains
   Digest attributes, from which B takes values to construct an HTTP/SIP
   "(Proxy) Authorization required" response.  B sends this response to
   A (step 4).  A resends its request with its credentials (step 5).  B
   sends an Access-Request to C (step 6).  C checks the credentials and
   replies with Access-Accept or Access-Reject (step 7).  Depending on
   C's result, B processes A's request or rejects it with a "(Proxy)
   Authorization required" response (step 8).

2.  Detailed Description

2.1.  RADIUS Client Behavior

   The attributes described in this document are sent in cleartext.
   Therefore, were a RADIUS client to accept secure connections (HTTPS
   or SIPS) from HTTP-style clients, this could result in information
   intentionally protected by HTTP-style clients being sent in the clear
   during RADIUS exchange.

2.1.1.  Credential Selection

   On reception of an HTTP-style request message, the RADIUS client
   checks whether it is authorized to authenticate the request.  Where
   an HTTP-style request traverses several proxies, and each of the
   proxies requests to authenticate the HTTP-style client, the request
   at the HTTP-style server may contain multiple credential sets.

   The RADIUS client can use the realm directive in HTTP to determine
   which credentials are applicable.  Where none of the realms are of
   interest, the RADIUS client MUST behave as though no relevant
   credentials were sent.  In all situations, the RADIUS client MUST
   send zero or exactly one credential to the RADIUS server.  The RADIUS




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   client MUST choose the credential of the (Proxy-)Authorization header
   if the realm directive matches its locally configured realm.

2.1.2.  Constructing an Access-Request

   If a matching (Proxy-)Authorization header is present and contains
   HTTP Digest information, the RADIUS client checks the nonce
   parameter.

   If the RADIUS client recognizes the nonce, it takes the header
   directives and puts them into a RADIUS Access-Request packet.  It
   puts the response directive into a Digest-Response Attribute and the
   realm, nonce, digest-uri, qop, algorithm, cnonce, nc, username, and
   opaque directives into the respective Digest-Realm, Digest-Nonce,
   Digest-URI, Digest-Qop, Digest-Algorithm, Digest-CNonce, Digest-
   Nonce-Count, Digest-Username, and Digest-Opaque attributes.  The
   RADIUS client puts the request method into the Digest-Method
   Attribute.

   Due to HTTP syntactic requirements, quoted strings found in HTTP
   Digest directives may contain escaped quote and backslash characters.
   When translating these directives into RADIUS attributes, the RADIUS
   client only removes the leading and trailing quote characters which
   surround the directive value, it does not unescape anything within
   the string.  See Section 3 for an example.

   If the Quality of Protection (qop) directive's value is 'auth-int',
   the RADIUS client calculates H(entity-body) as described in
   [RFC2617], Section 3.2.1, and puts the result in a Digest-Entity-
   Body-Hash Attribute.

   The RADIUS client adds a Message-Authenticator Attribute, defined in
   [RFC3579], and sends the Access-Request packet to the RADIUS server.

   The RADIUS server processes the packet and responds with an Access-
   Accept or an Access-Reject.

2.1.3.  Constructing an Authentication-Info Header

   After having received an Access-Accept from the RADIUS server, the
   RADIUS client constructs an Authentication-Info header:

   o  If the Access-Accept packet contains a Digest-Response-Auth
      Attribute, the RADIUS client checks the Digest-Qop Attribute:

      *  If the Digest-Qop Attribute's value is 'auth' or not specified,
         the RADIUS client puts the Digest-Response-Auth Attribute's




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         content into the Authentication-Info header's rspauth directive
         of the HTTP-style response.

      *  If the Digest-Qop Attribute's value is 'auth-int', the RADIUS
         client ignores the Access-Accept packet and behaves as if it
         had received an Access-Reject packet (Digest-Response-Auth
         can't be correct as the RADIUS server does not know the
         contents of the HTTP-style response's body).

   o  If the Access-Accept packet contains a Digest-HA1 Attribute, the
      RADIUS client checks the qop and algorithm directives in the
      Authorization header of the HTTP-style request it wants to
      authorize:

      *  If the qop directive is missing or its value is 'auth', the
         RADIUS client ignores the Digest-HA1 Attribute.  It does not
         include an Authentication-Info header in its HTTP-style
         response.

      *  If the qop directive's value is 'auth-int' and at least one of
         the following conditions is true, the RADIUS client calculates
         the contents of the HTTP-style response's rspauth directive:

         +  The algorithm directive's value is 'MD5-sess' or 'AKAv1-
            MD5-sess'.

         +  IP Security (IPsec) is configured to protect traffic between
            the RADIUS client and RADIUS server with IPsec (see Section
            8).

         The RADIUS client creates the HTTP-style response message and
         calculates the hash of this message's body.  It uses the result
         and the Digest-URI Attribute's value of the corresponding
         Access-Request packet to perform the H(A2) calculation.  It
         takes the Digest-Nonce, Digest-Nonce-Count, Digest-CNonce, and
         Digest-Qop values of the corresponding Access-Request and the
         Digest-HA1 Attribute's value to finish the computation of the
         rspauth value.

   o  If the Access-Accept packet contains neither a Digest-Response-
      Auth nor a Digest-HA1 Attribute, the RADIUS client will not create
      an Authentication-Info header for its HTTP-style response.

   When the RADIUS server provides a Digest-Nextnonce Attribute in the
   Access-Accept packet, the RADIUS client puts the contents of this
   attribute into a nextnonce directive.  Now it can send an HTTP-style
   response.




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2.1.4.  Failed Authentication

   If the RADIUS client did receive an HTTP-style request without a
   (Proxy-)Authorization header matching its locally configured realm
   value, it obtains a new nonce and sends an error response (401 or
   407) containing a (Proxy-)Authenticate header.

   If the RADIUS client receives an Access-Challenge packet in response
   to an Access-Request containing a Digest-Nonce Attribute, the RADIUS
   server did not accept the nonce.  If a Digest-Stale Attribute is
   present in the Access-Challenge and has a value of 'true' (without
   surrounding quotes), the RADIUS client sends an error response (401
   or 407) containing a WWW-/Proxy-Authenticate header with the stale
   directive set to 'true' and the digest directives derived from the
   Digest-* attributes.

   If the RADIUS client receives an Access-Reject from the RADIUS
   server, it sends an error response to the HTTP-style request it has
   received.  If the RADIUS client does not receive a response, it
   retransmits or fails over to another RADIUS server as described in
   [RFC2865].

2.1.5.  Obtaining Nonces

   The RADIUS client has two ways to obtain nonces: it has received one
   in a Digest-Nextnonce Attribute of a previously received Access-
   Accept packet, or it asks the RADIUS server for one.  To do the
   latter, it sends an Access-Request containing a Digest-Method and a
   Digest-URI Attribute, but without a Digest-Nonce Attribute.  It adds
   a Message-Authenticator (see [RFC3579]) Attribute to the Access-
   Request packet.  The RADIUS server chooses a nonce and responds with
   an Access-Challenge containing a Digest-Nonce Attribute.

   The RADIUS client constructs a (Proxy-)Authenticate header using the
   received Digest-Nonce and Digest-Realm attributes to fill the nonce
   and realm directives.  The RADIUS server can send Digest-Qop,
   Digest-Algorithm, Digest-Domain, and Digest-Opaque attributes in the
   Access-Challenge carrying the nonce.  If these attributes are
   present, the client MUST use them.

2.2.  RADIUS Server Behavior

   If the RADIUS server receives an Access-Request packet with a
   Digest-Method and a Digest-URI Attribute but without a Digest-Nonce
   Attribute, it chooses a nonce.  It puts the nonce into a Digest-Nonce
   Attribute and sends it in an Access-Challenge packet to the RADIUS
   client.  The RADIUS server MUST add Digest-Realm, Message-
   Authenticator (see [RFC3579]), SHOULD add Digest-Algorithm and one or



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   more Digest-Qop, and MAY add Digest-Domain or Digest-Opaque
   attributes to the Access-Challenge packet.

2.2.1.  General Attribute Checks

   If the RADIUS server receives an Access-Request packet containing a
   Digest-Response Attribute, it looks for the following attributes:

   Digest-Realm, Digest-Nonce, Digest-Method, Digest-URI, Digest-Qop,
   Digest-Algorithm, and Digest-Username.  Depending on the content of
   Digest-Algorithm and Digest-Qop, it looks for Digest-Entity-Body-
   Hash, Digest-CNonce, and Digest-AKA-Auts, too.  See [RFC2617] and
   [RFC3310] for details.  If the Digest-Algorithm Attribute is missing,
   'MD5' is assumed.  If the RADIUS server has issued a Digest-Opaque
   Attribute along with the nonce, the Access-Request MUST have a
   matching Digest-Opaque Attribute.

   If mandatory attributes are missing, it MUST respond with an Access-
   Reject packet.

   The RADIUS server removes '\' characters that escape quote and '\'
   characters from the text values it has received in the Digest-*
   attributes.

   If the mandatory attributes are present, the RADIUS server MUST check
   if the RADIUS client is authorized to serve users of the realm
   mentioned in the Digest-Realm Attribute.  If the RADIUS client is not
   authorized, the RADIUS server MUST send an Access-Reject.  The RADIUS
   server SHOULD log the event so as to notify the operator, and MAY
   take additional action such as sending an Access-Reject in response
   to all future requests from this client, until this behavior is reset
   by management action.

   The RADIUS server determines the age of the nonce in the Digest-Nonce
   by using an embedded timestamp or by looking it up in a local table.
   The RADIUS server MUST check the integrity of the nonce if it embeds
   the timestamp in the nonce.  Section 2.2.2 describes how the server
   handles old nonces.

2.2.2.  Authentication

   If the Access-Request message passes the checks described above, the
   RADIUS server calculates the digest response as described in
   [RFC2617].  To look up the password, the RADIUS server uses the
   RADIUS User-Name Attribute.  The RADIUS server MUST check if the user
   identified by the User-Name Attribute:

   o  is authorized to access the protection space and



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   o  is authorized to use the URI included in the SIP-AOR Attribute, if
      this attribute is present.

   If any of those checks fails, the RADIUS server MUST send an Access-
   Reject.

   Correlation between User-Name and SIP-AOR AVP values is required just
   to avoid any user from registering or misusing a SIP-AOR that has
   been allocated to a different user.

   All values required for the digest calculation are taken from the
   Digest attributes described in this document.  If the calculated
   digest response equals the value received in the Digest-Response
   Attribute, the authentication was successful.

   If the response values match, but the RADIUS server considers the
   nonce in the Digest-Nonce Attribute too old, it sends an Access-
   Challenge packet containing a new nonce and a Digest-Stale Attribute
   with a value of 'true' (without surrounding quotes).

   If the response values don't match, the RADIUS server responds with
   an Access-Reject.

2.2.3.  Constructing the Reply

   If the authentication was successful, the RADIUS server adds an
   attribute to the Access-Accept packet that can be used by the RADIUS
   client to construct an Authentication-Info header:

   o  If the Digest-Qop Attribute's value is 'auth' or unspecified, the
      RADIUS server SHOULD put a Digest-Response-Auth Attribute into the
      Access-Accept packet.

   o  If the Digest-Qop Attribute's value is 'auth-int' and at least one
      of the following conditions is true, the RADIUS server SHOULD put
      a Digest-HA1 Attribute into the Access-Accept packet:

      *  The Digest-Algorithm Attribute's value is 'MD5-sess' or
         'AKAv1-MD5-sess'.

      *  IPsec is configured to protect traffic between the RADIUS
         client and RADIUS server with IPsec (see Section 8).

   In all other cases, Digest-Response-Auth or Digest-HA1 MUST NOT be
   sent.

   RADIUS servers MAY construct a Digest-Nextnonce Attribute and add it
   to the Access-Accept packet.  This is useful to limit the lifetime of



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   a nonce and to save a round-trip in future requests (see nextnonce
   discussion in [RFC2617], Section 3.2.3).  The RADIUS server adds a
   Message-Authenticator Attribute (see [RFC3579]) and sends the
   Access-Accept packet to the RADIUS client.

   If the RADIUS server does not accept the nonce received in an
   Access-Request packet but authentication was successful, the RADIUS
   server MUST send an Access-Challenge packet containing a Digest-Stale
   Attribute set to 'true' (without surrounding quotes).  The RADIUS
   server MUST add Message-Authenticator (see [RFC3579]), Digest-Nonce,
   Digest-Realm, SHOULD add Digest-Algorithm and one or more Digest-
   Qops, and MAY add Digest-Domain or Digest-Opaque attributes to the
   Access-Challenge packet.

3.  New RADIUS Attributes

   If not stated otherwise, the attributes have the following format:

   0                   1                   2
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length       | Text ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Quote and backslash characters in Digest-* attributes representing
   HTTP-style directives with a quoted-string syntax are escaped.  The
   surrounding quotes are removed.  They are syntactical delimiters that
   are redundant in RADIUS.  For example, the directive

   realm="the \"example\" value"

   is represented as follows:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Digest-Realm  |       23      | the \"example\" value |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.  Digest-Response Attribute

   Description
         If this attribute is present in an Access-Request message, a
         RADIUS server implementing this specification MUST treat the
         Access-Request as a request for Digest Authentication.  When a
         RADIUS client receives a (Proxy-)Authorization header, it puts
         the request-digest value into a Digest-Response Attribute.
         This attribute (which enables the user to prove possession of
         the password) MUST only be used in Access-Request packets.




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   Type
         103 for Digest-Response.
   Length
         >= 3
   Text
         When using HTTP Digest, the text field is 32 octets long and
         contains a hexadecimal representation of a 16-octet digest
         value as it was calculated by the authenticated client.  Other
         digest algorithms MAY define different digest lengths.  The
         text field MUST be copied from request-digest of digest-
         response [RFC2617] without surrounding quotes.

3.2.  Digest-Realm Attribute

   Description
         This attribute describes a protection space component of the
         RADIUS server.  HTTP-style protocols differ in their definition
         of the protection space.  See [RFC2617], Section 1.2, for
         details.  It MUST only be used in Access-Request, Access-
         Challenge, and Accounting-Request packets.
   Type
         104 for Digest-Realm
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         realm directive (realm-value according to [RFC2617]) without
         surrounding quotes from the HTTP-style request it wants to
         authenticate.  In Access-Challenge packets, the RADIUS server
         puts the expected realm value into this attribute.

3.3.  Digest-Nonce Attribute

   Description
         This attribute holds a nonce to be used in the HTTP Digest
         calculation.  If the Access-Request had a Digest-Method and a
         Digest-URI but no Digest-Nonce Attribute, the RADIUS server
         MUST put a Digest-Nonce Attribute into its Access-Challenge
         packet.  This attribute MUST only be used in Access-Request and
         Access-Challenge packets.
   Type
         105 for Digest-Nonce
   Length
         >= 3







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   Text
         In Access-Requests, the RADIUS client takes the value of the
         nonce directive (nonce-value in [RFC2617]) without surrounding
         quotes from the HTTP-style request it wants to authenticate.
         In Access-Challenge packets, the attribute contains the nonce
         selected by the RADIUS server.

3.4.  Digest-Response-Auth Attribute

   Description
         This attribute enables the RADIUS server to prove possession of
         the password.  If the previously received Digest-Qop Attribute
         was 'auth-int' (without surrounding quotes), the RADIUS server
         MUST send a Digest-HA1 Attribute instead of a Digest-Response-
         Auth Attribute.  The Digest-Response-Auth Attribute MUST only
         be used in Access-Accept packets.  The RADIUS client puts the
         attribute value without surrounding quotes into the rspauth
         directive of the Authentication-Info header.
   Type
         106 for Digest-Response-Auth.
   Length
         >= 3
   Text
         The RADIUS server calculates a digest according to Section
         3.2.3 of [RFC2617] and copies the result into this attribute.
         Digest algorithms other than the one defined in [RFC2617] MAY
         define digest lengths other than 32.

3.5.  Digest-Nextnonce Attribute

   This attribute holds a nonce to be used in the HTTP Digest
   calculation.

   Description
         The RADIUS server MAY put a Digest-Nextnonce Attribute into an
         Access-Accept packet.  If this attribute is present, the RADIUS
         client MUST put the contents of this attribute into the
         nextnonce directive of an Authentication-Info header in its
         HTTP-style response.  This attribute MUST only be used in
         Access-Accept packets.
   Type
         107 for Digest-Nextnonce
   Length
         >= 3
   Text
         It is recommended that this text be base64 or hexadecimal data.





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RFC 5090         RADIUS Extension Digest Authentication    February 2008


3.6.  Digest-Method Attribute

   Description
         This attribute holds the method value to be used in the HTTP
         Digest calculation.  This attribute MUST only be used in
         Access-Request and Accounting-Request packets.
   Type
         108 for Digest-Method
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         request method from the HTTP-style request it wants to
         authenticate.

3.7.  Digest-URI Attribute

   Description
         This attribute is used to transport the contents of the
         digest-uri directive or the URI of the HTTP-style request.  It
         MUST only be used in Access-Request and Accounting-Request
         packets.
   Type
         109 for Digest-URI
   Length
         >= 3
   Text
         If the HTTP-style request has an Authorization header, the
         RADIUS client puts the value of the uri directive found in the
         HTTP-style request Authorization header (known as "digest-uri-
         value" in Section 3.2.2 of [RFC2617]) without surrounding
         quotes into this attribute.  If there is no Authorization
         header, the RADIUS client takes the value of the request URI
         from the HTTP-style request it wants to authenticate.

3.8.  Digest-Qop Attribute

   Description
         This attribute holds the Quality of Protection parameter that
         influences the HTTP Digest calculation.  This attribute MUST
         only be used in Access-Request, Access-Challenge, and
         Accounting-Request packets.  A RADIUS client SHOULD insert one
         of the Digest-Qop attributes it has received in a previous
         Access-Challenge packet.  RADIUS servers SHOULD insert at least
         one Digest-Qop Attribute in an Access-Challenge packet.
         Digest-Qop is optional in order to preserve backward
         compatibility with a minimal implementation of [RFC2069].




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   Type
         110 for Digest-Qop
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         qop directive (qop-value as described in [RFC2617]) from the
         HTTP-style request it wants to authenticate.  In Access-
         Challenge packets, the RADIUS server puts a desired qop-value
         into this attribute.  If the RADIUS server supports more than
         one "quality of protection" value, it puts each qop-value into
         a separate Digest-Qop Attribute.

3.9.  Digest-Algorithm Attribute

   Description
         This attribute holds the algorithm parameter that influences
         the HTTP Digest calculation.  It MUST only be used in Access-
         Request, Access-Challenge and Accounting-Request packets.  If
         this attribute is missing, MD5 is assumed.
   Type
         111 for Digest-Algorithm
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         algorithm directive (as described in [RFC2617], Section 3.2.1)
         from the HTTP-style request it wants to authenticate.  In
         Access-Challenge packets, the RADIUS server SHOULD put the
         desired algorithm into this attribute.

3.10.  Digest-Entity-Body-Hash Attribute

   Description
         When using the qop-value 'auth-int', a hash of the HTTP-style
         message body's contents is required for digest calculation.
         Instead of sending the complete body of the message, only its
         hash value is sent.  This hash value can be used directly in
         the digest calculation.

         The clarifications described in section 22.4 of [RFC3261] about
         the hash of empty entity bodies apply to the Digest-Entity-
         Body-Hash Attribute.  This attribute MUST only be sent in
         Access-Request packets.
   Type
         112 for Digest-Entity-Body-Hash
   Length
         >= 3



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   Text
         The attribute holds the hexadecimal representation of
         H(entity-body).  This hash is required by certain
         authentication mechanisms, such as HTTP Digest with quality of
         protection set to 'auth-int'.  RADIUS clients MUST use this
         attribute to transport the hash of the entity body when HTTP
         Digest is the authentication mechanism and the RADIUS server
         requires that the integrity of the entity body (e.g., qop
         parameter set to 'auth-int') be verified.  Extensions to this
         document may define support for authentication mechanisms other
         than HTTP Digest.

3.11.  Digest-CNonce Attribute

   Description
         This attribute holds the client nonce parameter that is used in
         the HTTP Digest calculation.  It MUST only be used in Access-
         Request packets.
   Type
         113 for Digest-CNonce
   Length
         >= 3
   Text
         This attribute includes the value of the cnonce-value [RFC2617]
         without surrounding quotes, taken from the HTTP-style request.

3.12.  Digest-Nonce-Count Attribute

   Description
         This attribute includes the nonce count parameter that is used
         to detect replay attacks.  The attribute MUST only be used in
         Access-Request packets.
   Type
         114 for Digest-Nonce-Count
   Length
         10
   Text
         In Access-Requests, the RADIUS client takes the value of the nc
         directive (nc-value according to [RFC2617]) without surrounding
         quotes from the HTTP-style request it wants to authenticate.

3.13.  Digest-Username Attribute

   Description
         This attribute holds the user name used in the HTTP Digest
         calculation.  The RADIUS server MUST use this attribute only
         for the purposes of calculating the digest.  In order to
         determine the appropriate user credentials, the RADIUS server



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         MUST use the User-Name (1) Attribute, and MUST NOT use the
         Digest-Username Attribute.  This attribute MUST only be used in
         Access-Request and Accounting-Request packets.
   Type
         115 for Digest-Username
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         username directive (username-value according to [RFC2617])
         without surrounding quotes from the HTTP-style request it wants
         to authenticate.

3.14.  Digest-Opaque Attribute

   Description
         This attribute holds the opaque parameter that is passed to the
         HTTP-style client.  The HTTP-style client will pass this value
         back to the server (i.e., the RADIUS client) without
         modification.  This attribute MUST only be used in Access-
         Request and Access-Challenge packets.
   Type
         116 for Digest-Opaque
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         opaque directive (opaque-value according to [RFC2617]) without
         surrounding quotes from the HTTP-style request it wants to
         authenticate and puts it into this attribute.  In Access-
         Challenge packets, the RADIUS server MAY include this
         attribute.

3.15.  Digest-Auth-Param Attribute

   Description
         This attribute is a placeholder for future extensions and
         corresponds to the auth-param parameter defined in Section
         3.2.1 of [RFC2617].  The Digest-Auth-Param is the mechanism
         whereby the RADIUS client and RADIUS server can exchange auth-
         param extension parameters contained within Digest headers that
         are not understood by the RADIUS client and for which there are
         no corresponding stand-alone attributes.

         Unlike the previously listed Digest-* attributes, the Digest-
         Auth-Param contains not only the value but also the parameter
         name, since the parameter name is unknown to the RADIUS client.
         If the Digest header contains several unknown parameters, then



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         the RADIUS implementation MUST repeat this attribute, and each
         instance MUST contain one different unknown Digest
         parameter/value combination.  This attribute MUST ONLY be used
         in Access-Request, Access-Challenge, Access-Accept, and
         Accounting-Request packets.
   Type
         117 for Digest-Auth-Param
   Length
         >= 3
   Text
         The text consists of the whole parameter, including its name,
         the equal sign ('='), and quotes.

3.16.  Digest-AKA-Auts Attribute

   Description
         This attribute holds the auts parameter that is used in the
         Digest AKA [RFC3310] calculation.  It is only used if the
         algorithm of the digest-response denotes a version of AKA
         Digest [RFC3310].  This attribute MUST only be used in Access-
         Request packets.
   Type
         118 for Digest-AKA-Auts
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         auts directive (auts-param according to Section 3.4 of
         [RFC3310]) without surrounding quotes from the HTTP-style
         request it wants to authenticate.

3.17.  Digest-Domain Attribute

   Description
         When a RADIUS client has asked for a nonce, the RADIUS server
         MAY send one or more Digest-Domain attributes in its Access-
         Challenge packet.  The RADIUS client puts them into the quoted,
         space-separated list of URIs of the domain directive of a WWW-
         Authenticate header.  Together with Digest-Realm, the URIs in
         the list define the protection space (see [RFC2617], Section
         3.2.1) for some HTTP-style protocols.  This attribute MUST only
         be used in Access-Challenge and Accounting-Request packets.
   Type
         119 for Digest-Domain
   Length
         3





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   Text
         This attribute consists of a single URI that defines a
         protection space component.

3.18.  Digest-Stale Attribute

   Description
         This attribute is sent by a RADIUS server in order to notify
         the RADIUS client whether it has accepted a nonce.  If the
         nonce presented by the RADIUS client was stale, the value is
         'true' and is 'false' otherwise.  The RADIUS client puts the
         content of this attribute into a stale directive of the WWW-
         Authenticate header in the HTTP-style response to the request
         it wants to authenticate.  The attribute MUST only be used in
         Access-Challenge packets.
   Type
         120 for Digest-Stale
   Length
         3
   Text
         The attribute has either the value 'true' or 'false' (both
         values without surrounding quotes).

3.19.  Digest-HA1 Attribute

   Description
         This attribute is used to allow the generation of an
         Authentication-Info header, even if the HTTP-style response's
         body is required for the calculation of the rspauth value.  It
         SHOULD be used in Access-Accept packets if the required quality
         of protection (qop) is 'auth-int'.

         This attribute MUST NOT be sent if the qop parameter was not
         specified or has a value of 'auth' (in this case, use Digest-
         Response-Auth instead).

         The Digest-HA1 Attribute MUST only be sent by the RADIUS server
         or processed by the RADIUS client if at least one of the
         following conditions is true:

         +  The Digest-Algorithm Attribute's value is 'MD5-sess' or
            'AKAv1-MD5-sess'.

         +  IPsec is configured to protect traffic between the RADIUS
            client and RADIUS server with IPsec (see Section 8).

         This attribute MUST only be used in Access-Accept packets.




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   Type
         121 for Digest-HA1
   Length
         >= 3
   Text
         This attribute contains the hexadecimal representation of H(A1)
         as described in [RFC2617], Sections 3.1.3, 3.2.1, and 3.2.2.2.

3.20.  SIP-AOR Attribute

   Description
         This attribute is used for the authorization of SIP messages.
         The SIP-AOR Attribute identifies the URI, the use of which must
         be authenticated and authorized.  The RADIUS server uses this
         attribute to authorize the processing of the SIP request.  The
         SIP-AOR can be derived from, for example, the To header field
         in a SIP REGISTER request (user under registration), or the
         From header field in other SIP requests.  However, the exact
         mapping of this attribute to SIP can change due to new
         developments in the protocol.  This attribute MUST only be used
         when the RADIUS client wants to authorize SIP users and MUST
         only be used in Access-Request packets.
   Type
         122 for SIP-AOR
   Length
         >= 3
   Text
         The syntax of this attribute corresponds either to a SIP URI
         (with the format defined in [RFC3261] or a tel URI (with the
         format defined in [RFC3966]).

         The SIP-AOR Attribute holds the complete URI, including
         parameters and other parts.  It is up to the RADIUS server as
         to which components of the URI are regarded in the
         authorization decision.

4.  Diameter Compatibility

   This document defines support for Digest Authentication in RADIUS.  A
   companion document "Diameter Session Initiation Protocol (SIP)
   Application" [RFC4740] defines support for Digest Authentication in
   Diameter, and addresses compatibility issues between RADIUS and
   Diameter.

5.  Table of Attributes

   The following table provides a guide to which attributes may be found
   in which kinds of packets, and in what quantity.



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 Access- Access- Access- Access-    Acct-
 Request Accept  Reject  Challenge  Req   #  Attribute
  0-1      0      0      0          0-1   1  User-Name
  0-1      0      0      1          0    24  State [4]
  1        1      1      1          0-1  80  Message-Authenticator
  0-1      0      0      0          0   103  Digest-Response
  0-1      0      0      1          0-1 104  Digest-Realm
  0-1      0      0      1          0   105  Digest-Nonce
  0        0-1    0      0          0   106  Digest-Response-Auth [1][2]
  0        0-1    0      0          0   107  Digest-Nextnonce
  1        0      0      0          0-1 108  Digest-Method
  0-1      0      0      0          0-1 109  Digest-URI
  0-1      0      0      0+         0-1 110  Digest-Qop
  0-1      0      0      0-1        0-1 111  Digest-Algorithm [3]
  0-1      0      0      0          0   112  Digest-Entity-Body-Hash
  0-1      0      0      0          0   113  Digest-CNonce
  0-1      0      0      0          0   114  Digest-Nonce-Count
  0-1      0      0      0          0-1 115  Digest-Username
  0-1      0      0      0-1        0   116  Digest-Opaque
  0+       0+     0      0+         0+  117  Digest-Auth-Param
  0-1      0      0      0          0   118  Digest-AKA-Auts
  0        0      0      0+         0+  119  Digest-Domain
  0        0      0      0-1        0   120  Digest-Stale
  0        0-1    0      0          0   121  Digest-HA1 [1][2]
  0-1      0      0      0          0   122  SIP-AOR

   The following table defines the meaning of the above table entries.

      0     This attribute MUST NOT be present in the packet.
      0+    Zero or more instances of this attribute MAY be
            present in the packet.
      0-1   Zero or one instance of this attribute MAY be
            present in the packet.

   [Note 1] Digest-HA1 MUST be used instead of Digest-Response-Auth if
            Digest-Qop is 'auth-int'.

   [Note 2] Digest-Response-Auth MUST be used instead of Digest-HA1 if
            Digest-Qop is 'auth'.

   [Note 3] If Digest-Algorithm is missing, 'MD5' is assumed.

   [Note 4] An Access-Challenge MUST contain a State attribute, which is
            copied to the subsequent Access-Request.  A server receiving
            an Access-Request that contains a State attribute MUST
            respond with either an Access-Accept or an Access-Reject;
            the server MUST NOT respond with an Access-Challenge.




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6.  Examples

   This is an example selected from the traffic between a softphone (A),
   a Proxy Server (B), and an example.com RADIUS server (C).  The
   communication between the Proxy Server and a SIP Public Switched
   Telephone Network (PSTN) gateway is omitted for brevity.  The SIP
   messages are not shown completely.

   The password of user '12345678' is 'secret'.  The shared secret
   between the RADIUS client and server is 'secret'.  To ease testing,
   only the last byte of the RADIUS authenticator changes between
   requests.  In a real implementation, this would be a serious flaw.

   A->B

      INVITE sip:97226491335@example.com SIP/2.0
      From: <sip:12345678@example.com>
      To: <sip:97226491335@example.com>

   B->A

      SIP/2.0 100 Trying

   B->C

      Code = Access-Request (1)
      Packet identifier = 0x7c (124)
      Length = 97
      Authenticator = F5E55840E324AA49D216D9DBD069807C
      NAS-IP-Address = 192.0.2.38
      NAS-Port = 5
      User-Name = 12345678
      Digest-Method = INVITE
      Digest-URI = sip:97226491335@example.com
      Message-Authenticator = 7600D5B0BDC33987A60D5C6167B28B3B

   C->B

      Code = Access-challenge (11)
      Packet identifier = 0x7c (124)
      Length = 72
      Authenticator = EBE20199C26EFEAD69BF8AB0E786CA4D
      Digest-Nonce = 3bada1a0
      Digest-Realm = example.com
      Digest-Qop = auth
      Digest-Algorithm = MD5
      Message-Authenticator = 5DA18ED3BBC9513DCBDE0A37F51B7DE3




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   B->A

      SIP/2.0 407 Proxy Authentication Required
      Proxy-Authenticate: Digest realm="example.com"
           ,nonce="3bada1a0",qop=auth,algorithm=MD5
      Content-Length: 0

   A->B

      ACK sip:97226491335@example.com SIP/2.0

   A->B

      INVITE sip:97226491335@example.com SIP/2.0
      Proxy-Authorization: Digest nonce="3bada1a0"
           ,realm="example.com"
           ,response="756933f735fcd93f90a4bbdd5467f263"
           ,uri="sip:97226491335@example.com",username="12345678"
           ,qop=auth,algorithm=MD5
           ,cnonce="56593a80,nc="00000001"

      From: <sip:12345678@example.com>
      To: <sip:97226491335@example.com>

   B->C

      Code = Access-Request (1)
      Packet identifier = 0x7d (125)
      Length = 221
      Authenticator = F5E55840E324AA49D216D9DBD069807D
      NAS-IP-Address = 192.0.2.38
      NAS-Port = 5
      User-Name = 12345678
      Digest-Method = INVITE
      Digest-URI = sip:97226491335@example.com
      Digest-Realm = example.com
      Digest-Qop = auth
      Digest-Algorithm = MD5
      Digest-CNonce = 56593a80
      Digest-Nonce = 3bada1a0
      Digest-Nonce-Count = 00000001
      Digest-Response = 756933f735fcd93f90a4bbdd5467f263
      Digest-Username = 12345678
      SIP-AOR = sip:12345678@example.com
      Message-Authenticator = B6C7F7F8D11EF261A26933D234561A60






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   C->B

      Code = Access-Accept (2)
      Packet identifier = 0x7d (125)
      Length = 72
      Authenticator = FFDD74D6470D21CB6FC4D6056BE245D2
      Digest-Response-Auth = f847de948d12285f8f4199e366f1af21
      Message-Authenticator = 7B76E2F10A7067AF601938BF13B0A62E

   B->A

      SIP/2.0 180 Ringing

   B->A

      SIP/2.0 200 OK

   A->B

      ACK sip:97226491335@example.com SIP/2.0

   A second example shows the traffic between a web browser (A), a web
   server (B), and a RADIUS server (C).

   A->B

      GET /index.html HTTP/1.1

   B->C

      Code = Access-Request (1)
      Packet identifier = 0x7e (126)
      Length = 68
      Authenticator = F5E55840E324AA49D216D9DBD069807E
      NAS-IP-Address = 192.0.2.38
      NAS-Port = 5
      Digest-Method = GET
      Digest-URI = /index.html
      Message-Authenticator = 690BFC95E88DF3B185F15CD78E469992












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   C->B

      Code = Access-challenge (11)
      Packet identifier = 0x7e (126)
      Length = 72
      Authenticator = 2EE5EB01C02C773B6C6EC8515F565E8E
      Digest-Nonce = a3086ac8
      Digest-Realm = example.com
      Digest-Qop = auth
      Digest-Algorithm = MD5
      Message-Authenticator = 646DB2B0AF9E72FFF2CF7FEB33C4952A

   B->A

      HTTP/1.1 401 Authentication Required
      WWW-Authenticate: Digest realm="example.com",
          nonce="a3086ac8",qop=auth,algorithm=MD5
      Content-Length: 0

   A->B

      GET /index.html HTTP/1.1
      Authorization: Digest = algorithm=MD5,qop=auth,nonce="a3086ac8"
           ,nc="00000001",cnonce="56593a80"
           ,realm="example.com"
           ,response="a4fac45c27a30f4f244c54a2e99fa117"
           ,uri="/index.html",username="12345678"

   B->C

      Code = Access-Request (1)
      Packet identifier = 0x7f (127)
      Length = 176
      Authenticator = F5E55840E324AA49D216D9DBD069807F
      NAS-IP-Address = 192.0.2.38
      NAS-Port = 5
      User-Name = 12345678
      Digest-Method = GET
      Digest-URI = /index.html
      Digest-Realm = example.com
      Digest-Qop = auth
      Digest-Algorithm = MD5
      Digest-CNonce = 56593a80
      Digest-Nonce = a3086ac8
      Digest-Nonce-Count = 00000001
      Digest-Response = a4fac45c27a30f4f244c54a2e99fa117
      Digest-Username = 12345678
      Message-Authenticator = 237D85C1478C70C67EEAF22A9C456821



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RFC 5090         RADIUS Extension Digest Authentication    February 2008


   C->B

      Code = Access-Accept (2)
      Packet identifier = 0x7f (127)
      Length = 72
      Authenticator = 6364FA6ED66012847C05A0895607C694
      Digest-Response-Auth = 08c4e942d1d0a191de8b3aa98cd35147
      Message-Authenticator = 43795A3166492AD2A890AD57D5F97D56

   B->A

      HTTP/1.1 200 OK
      ...

      <html>
      ...

7.  IANA Considerations

   The following values from the RADIUS Attribute Types number space
   were assigned in [RFC4590].  This document requests that the values
   in the table below be entered within the existing registry.

   Attribute               #
   ---------------        ----
   Digest-Response         103
   Digest-Realm            104
   Digest-Nonce            105
   Digest-Response-Auth    106
   Digest-Nextnonce        107
   Digest-Method           108
   Digest-URI              109
   Digest-Qop              110
   Digest-Algorithm        111
   Digest-Entity-Body-Hash 112
   Digest-CNonce           113
   Digest-Nonce-Count      114
   Digest-Username         115
   Digest-Opaque           116
   Digest-Auth-Param       117
   Digest-AKA-Auts         118
   Digest-Domain           119
   Digest-Stale            120
   Digest-HA1              121
   SIP-AOR                 122






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RFC 5090         RADIUS Extension Digest Authentication    February 2008


8.  Security Considerations

   The RADIUS extensions described in this document enable RADIUS to
   transport the data that is required to perform a digest calculation.
   As a result, RADIUS inherits the vulnerabilities of HTTP Digest (see
   [RFC2617], Section 4) in addition to RADIUS security vulnerabilities
   described in [RFC2865], Section 8, and [RFC3579], Section 4.

   An attacker compromising a RADIUS client or proxy can carry out man-
   in-the-middle attacks even if the paths between A, B and B, C (Figure
   2) have been secured with TLS or IPsec.

   The RADIUS server MUST check the Digest-Realm Attribute it has
   received from a client.  If the RADIUS client is not authorized to
   serve HTTP-style clients of that realm, it might be compromised.

8.1.  Denial of Service

   RADIUS clients implementing the extension described in this document
   may authenticate HTTP-style requests received over the Internet.  As
   compared with the use of RADIUS to authenticate link-layer network
   access, attackers may find it easier to cover their tracks in such a
   scenario.

   An attacker can attempt a denial-of-service attack on one or more
   RADIUS servers by sending a large number of HTTP-style requests.  To
   make simple denial-of-service attacks more difficult, the RADIUS
   server MUST check whether it has generated the nonce received from an
   HTTP-style client.  This SHOULD be done statelessly.  For example, a
   nonce could consist of a cryptographically random part and some kind
   of signature provided by the RADIUS client, as described in
   [RFC2617], Section 3.2.1.

8.2.  Confidentiality and Data Integrity

   The attributes described in this document are sent in cleartext.
   RADIUS servers SHOULD include Digest-Qop and Digest-Algorithm
   attributes in Access-Challenge messages.  A man in the middle can
   modify or remove those attributes in a bidding down attack, causing
   the RADIUS client to use a weaker authentication scheme than
   intended.

   The Message-Authenticator Attribute, described in [RFC3579], Section
   3.2 MUST be included in Access-Request, Access-Challenge, Access-
   Reject, and Access-Accept messages that contain attributes described
   in this specification.





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   The Digest-HA1 Attribute contains no random components if the
   algorithm is 'MD5' or 'AKAv1-MD5'.  This makes offline dictionary
   attacks easier and enables replay attacks.

   Some parameter combinations require the protection of RADIUS packets
   against eavesdropping and tampering.  Implementations SHOULD try to
   determine automatically whether IPsec is configured to protect
   traffic between the RADIUS client and the RADIUS server.  If this is
   not possible, the implementation checks a configuration parameter
   telling it whether IPsec will protect RADIUS traffic.  The default
   value of this configuration parameter tells the implementation that
   RADIUS packets will not be protected.

   HTTP-style clients can use TLS with server-side certificates together
   with HTTP-Digest Authentication.  Instead of TLS, IPsec can be used,
   too.  TLS or IPsec secure the connection while Digest Authentication
   authenticates the user.  The RADIUS transaction can be regarded as
   one leg on the path between the HTTP-style client and the HTTP-style
   server.  To prevent RADIUS from representing the weak link, a RADIUS
   client receiving an HTTP-style request via TLS or IPsec could use an
   equally secure connection to the RADIUS server.  There are several
   ways to achieve this, for example:

   o  The RADIUS client may reject HTTP-style requests received over TLS
      or IPsec.

   o  The RADIUS client may require that traffic be sent and received
      over IPsec.

   RADIUS over IPsec, if used, MUST conform to the requirements
   described in [RFC3579], Section 4.2.

9.  References

9.1.  Normative References

   [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.

   [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
             "Remote Authentication Dial In User Service (RADIUS)", RFC
             2865, June 2000.




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   [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
             A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
             "SIP: Session Initiation Protocol", RFC 3261, June 2002.

   [RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
             Dial In User Service) Support For Extensible Authentication
             Protocol (EAP)", RFC 3579, September 2003.

   [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
             3966, December 2004.

9.2.  Informative References

   [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
             Protocol (CHAP)", RFC 1994, August 1996.

   [RFC2069] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
             Luotonen, A., Sink, E., and L. Stewart, "An Extension to
             HTTP : Digest Access Authentication", RFC 2069, January
             1997.

   [RFC3310] Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer
             Protocol (HTTP) Digest Authentication Using Authentication
             and Key Agreement (AKA)", RFC 3310, September 2002.

   [RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
             Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
             Extensions (S/MIME) Version 3.1 Message Specification", RFC
             3851, July 2004.

   [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4590] Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
             and W. Beck, "RADIUS Extension for Digest Authentication",
             RFC 4590, July 2006.

   [RFC4740] Garcia-Martin, M., Ed., Belinchon, M., Pallares-Lopez, M.,
             Canales-Valenzuela, C., and K. Tammi, "Diameter Session
             Initiation Protocol (SIP) Application", RFC 4740, November
             2006.








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Appendix A - Changes from RFC 4590

   This Appendix lists the major changes between [RFC4590] and this
   document.  Minor changes, including style, grammar, spelling, and
   editorial changes are not mentioned here.

   o  The Table of Attributes (Section 5) now indicates that the
      Digest-Method Attribute is required within an Access-Request.
      Also, an entry has been added for the State attribute.  The table
      also includes entries for Accounting-Request messages.  As noted
      in the examples, the User-Name Attribute is not necessary when
      requesting a nonce.

   o  Two errors in attribute assignment have been corrected within the
      IANA Considerations (Section 7).  Digest-Response-Auth is assigned
      attribute 106, and Digest-Nextnonce is assigned attribute 107.

   o Several errors in the examples section have been corrected.

Acknowledgments

   The authors would like to thank Mike McCauley for his help in working
   through the details of the examples.

   We would like to acknowledge Kevin McDermott (Cisco Systems) for
   providing comments and experimental implementation.

   Many thanks to all reviewers, especially to Miguel Garcia, Jari
   Arkko, Avi Lior, and Jun Wang.






















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Authors' Addresses

   Baruch Sterman
   Kayote Networks
   P.O. Box 1373
   Efrat  90435
   Israel

   EMail: baruch@kayote.com

   Daniel Sadolevsky
   SecureOL, Inc.
   Jerusalem Technology Park
   P.O. Box 16120
   Jerusalem  91160
   Israel

   EMail: dscreat@dscreat.com

   David Schwartz
   Kayote Networks
   P.O. Box 1373
   Efrat  90435
   Israel

   EMail: david@kayote.com

   David Williams
   Cisco Systems
   7025 Kit Creek Road
   P.O. Box 14987
   Research Triangle Park  NC 27709
   USA

   EMail: dwilli@cisco.com

   Wolfgang Beck
   Deutsche Telekom AG
   Deutsche Telekom Allee 7
   Darmstadt  64295
   Germany

   EMail: beckw@t-systems.com








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