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rfc:rfc5090

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.

Sterman, et al. Standards Track [Page 1] RFC 5090 RADIUS Extension Digest Authentication February 2008

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

Sterman, et al. Standards Track [Page 2] RFC 5090 RADIUS Extension Digest Authentication February 2008

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.

Sterman, et al. Standards Track [Page 3] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 4] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 5] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 6] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 7] RFC 5090 RADIUS Extension Digest Authentication February 2008

       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.

Sterman, et al. Standards Track [Page 8] RFC 5090 RADIUS Extension Digest Authentication February 2008

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

Sterman, et al. Standards Track [Page 9] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 10] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 11] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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.

Sterman, et al. Standards Track [Page 12] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 13] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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.

Sterman, et al. Standards Track [Page 14] 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].

Sterman, et al. Standards Track [Page 15] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 16] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 17] RFC 5090 RADIUS Extension Digest Authentication February 2008

       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

Sterman, et al. Standards Track [Page 18] RFC 5090 RADIUS Extension Digest Authentication February 2008

       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

Sterman, et al. Standards Track [Page 19] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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.

Sterman, et al. Standards Track [Page 20] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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.

Sterman, et al. Standards Track [Page 21] RFC 5090 RADIUS Extension Digest Authentication February 2008

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.

Sterman, et al. Standards Track [Page 22] RFC 5090 RADIUS Extension Digest Authentication February 2008

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

Sterman, et al. Standards Track [Page 23] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 24] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 25] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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

Sterman, et al. Standards Track [Page 26] 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

Sterman, et al. Standards Track [Page 27] 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.

Sterman, et al. Standards Track [Page 28] RFC 5090 RADIUS Extension Digest Authentication February 2008

 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.

Sterman, et al. Standards Track [Page 29] RFC 5090 RADIUS Extension Digest Authentication February 2008

 [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.

Sterman, et al. Standards Track [Page 30] RFC 5090 RADIUS Extension Digest Authentication February 2008

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.

Sterman, et al. Standards Track [Page 31] RFC 5090 RADIUS Extension Digest Authentication February 2008

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

Sterman, et al. Standards Track [Page 32] RFC 5090 RADIUS Extension Digest Authentication February 2008

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Sterman, et al. Standards Track [Page 33]

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