INTERNET DRAFT Pat R. Calhoun Category: Standards Track Sun Microsystems, Inc. Title: draft-calhoun-diameter-proxy-02.txt William Bulley Date: August 1999 Merit Network, Inc. DIAMETER End-to-End Security/Referral Extensions Status of this Memo This document is an individual contribution for consideration by the AAA Working Group of the Internet Engineering Task Force. Comments should be submitted to the diameter@ipass.com mailing list. Distribution of this memo is unlimited. This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at: http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at: http://www.ietf.org/shadow.html. Abstract The DIAMETER base protocol [2] allows for secure communication between two DIAMETER nodes, and introduces the concept of proxying through the Proxy-State AVP. However, the base protocol only allows for hop-by-hop security, and the work done in the ROAMOPS WG [8] shows that support for end-to-end security through proxies. This document describes the extensions necessary to provide for secure Calhoun, Bulley expires January 2000 [Page 1]
INTERNET DRAFT August 1999 communication through DIAMETER proxies. Calhoun, Bulley expires January 2000 [Page 2]
INTERNET DRAFT August 1999 Table of Contents 1.0 Introduction 1.1 Copyright Statement 1.2 Requirements language 1.3 Changes in version -02 2.0 Extended AVP Format 3.0 Command Codes 3.1 Domain-Discovery-Request (DDR) 3.2 Domain-Discovery-Answer (DDA) 4.0 DIAMETER AVPs 4.1 Digital-Signature 4.2 X509-Certificate 4.3 X509-Certificate-URL 4.4 Next-Hop 5.0 Protocol Definition 5.1 Feature Advertisement/Discovery 5.2 DIAMETER Secure Proxying 5.3 Domain Discovery 5.4 Data Integrity 5.4.1 Using Digital Signatures 5.4.1 Using Mixed Data Integrity AVPs 5.5 AVP Data Encryption 5.5.1 AVP Encryption with Public Keys 5.6 Public Key Cryptography Support 5.6.1 X509-Certificate 5.6.2 X509-Certificate-URL 5.6.3 Static Public Key Configuration 6.0 IANA Considerations 7.0 References 8.0 Acknowledgements 9.0 Author's Address 10.0 Full Copyright Statement 1.0 Introduction Many services, including ROAMOPS and MobileIP, have a requirement for DIAMETER Server to proxy a request to another DIAMETER Server. The concept of proxying AAA requests was introduced by RADIUS and has been in use for many years. The DIAMETER base protocol [2] does provide the basic capability for proxying, but only defines hop-by-hop security, which has some known security flaws. Specifically a fraudulent proxy server can modify some portions of an AAA request in order to make the next hop improperly believe that some services were rendered. For example, a DIAMETER Proxy Server could modify an accounting request, such as the Calhoun, Bulley expires January 2000 [Page 3]
INTERNET DRAFT August 1999 number of bytes that a user transfered, and the end system would have no way of determining that this change occured. This specification contains the extensions necessary to DIAMETER to allow for end-to-end message integrity and privacy. The document also describes a method that DIAMETER can provide referal services to clients. The Extension number for this draft is two (2). This value is used in the Extension-Id AVP as defined in [2]. 1.1 Copyright Statement Copyright (C) The Internet Society 1999. All Rights Reserved. 1.2 Requirements language In this document, the key words "MAY", "MUST, "MUST NOT", "optional", "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as described in [13]. 1.3 Changes in version -02 The following changes were made in version 02 of the document: - New title - A good cleanup of the abstract and the introduction. - Fixed up text in section 4.2 that stated that all AVPs with the 'P' disabled were protected. This should have stated enabled. - Added Section 2 which describes the extended AVP Header Format. - Moved the Proxy State AVP to the base protocol. - Changed the description of the Digital-Signature AVP. - The Next-Hop AVP now requires a preceeding Digital-Signature AVP instead of a Host-IP-Address AVP. This change was necessary since the base protocol does not explicitely state that the Host-IP- Address AVP may appear multiple times in a single message. Such a change would be a big departure from the current RADIUS model where the Host-IP-Address contains the IP address of the originator of the message, not the address of intermediate hops. Calhoun, Bulley expires January 2000 [Page 4]
INTERNET DRAFT August 1999 - Fixed various references to sections that were incorrect. - Added clarification about the use of ICV and Digital Signatures within a single message. - Updated the AVP Header flags. - Re-wrote a good portion of section 5.1 ... - ... well, re-wrote a good portion of all everything in section 5. - Added a reference to RFC 2459 (x.509 certs) - Added an IANA Considerations section. 2.0 Extended AVP Format The DIAMETER Proxy specification introduces a new bit in the AVP flags field of the AVP Header. The following AVP header is used when proxy support is enabled. The attribute format is shown below and MUST be sent in network byte order. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Length | Cmd Flags |Reservd|P|E|T|V|H|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor ID (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+-+-+-+-+ Command Flags All Command Codes defined in this spec MUST set all bits in this field to zero (0). AVP Flags The AVP Flags field informs the DIAMETER host how each attribute Calhoun, Bulley expires January 2000 [Page 5]
INTERNET DRAFT August 1999 must be handled. The 'P' bit, known as the protected AVP bit, is used to indicate whether the AVP is protected by a Digital Signature AVP. When set, the AVP is protected and the contents cannot be changed by a DIAMETER proxy server. Note that unless noted, the 'P' bit can be set on any DIAMETER AVP. The Proxy-State AVP MUST not have the 'P' bit set since this AVP will be removed at each hop. Any other AVP that have similar properties (e.g. it will be removed or modified at each hop) MUST NOT have the 'P' bit enabled. When the 'E' bit is enabled it indicates that the AVP data is encrypted using end-to-end encryption. Note that the User-Name AVP [2] MUST NOT have the 'E' bit set since intermediate proxies require the domain information in order to determine the target proxy. 3.0 Command Codes This document defines the following DIAMETER Commands. All DIAMETER implementations supporting this extension MUST support all of the following commands: Command Name Command Code ----------------------------------- Domain-Discovery-Request 261 Domain-Discovery-Answer 262 3.1 Domain-Discovery-Request (DDR) Description The Domain-Discovery-Request message is used by a DIAMETER device wishing to get contact information about a domain's home authentication server as well as to receive password policy information. This message MUST contain the User-Name attribute in order to pass along the user's domain information. It is not necessary for an implementation to issue a DDR in order to make use of a proxy server. The message MUST include either the Host-Name AVP or Host-IP- Address AVP. The X509-Certificate or the X509-Certificate-URL [2] Calhoun, Bulley expires January 2000 [Page 6]
INTERNET DRAFT August 1999 MUST be present in this message in order to inform the home authentication server of the issuing host's certificate. At least one Extension-Id AVP MUST be present in the DDR in order to inform the peer about the locally supported extensions. Message Format <Domain-Discovery-Req> ::= <DIAMETER Header> <Domain-Discovery-Req Command AVP> <Host-IP-Address AVP> [<Host-Name AVP>] <Extension-Id AVPs> <User-Name AVP> [<X509-Certificate AVP>] [<X509-Certificate-URL AVP>] <Timestamp AVP> <Nonce AVP> {<Integrity-Check-Vector AVP> || <Digital-Signature AVP } AVP Format A summary of the Domain-Discovery-Request packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Length | Reserved |P|E|T|V|H|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Command Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AVP Code 256 DIAMETER Command AVP Length The length of this attribute MUST be 12. AVP Flags The 'M' bit MUST be set. The 'P' bits MAY be set if end to end message integrity is required. The 'V', 'H' and 'T' bits MUST Calhoun, Bulley expires January 2000 [Page 7]
INTERNET DRAFT August 1999 NOT be set. Command Type The Command Type field MUST be set to 261 (Domain-Discovery- Request). 3.2 Domain-Discovery-Answer (DDA) Description The Domain-Discovery-Answer message is sent in response to the Domain-Discovery-Request message by the domain's Home authentication server. The message MUST contain either the Host- Name AVP or Host-IP-Address AVP and either the X509-Certificate or the X509-Certificate-URL attribute and MAY contain at least one Framed-Password-Policy AVP. At least one Extension-Id AVP MUST be present in the DDA in order to inform the requestor about the locally supported extensions. The Domain-Discovery-Answer message MUST include the Result-Code AVP to indicate whether the request was successful or not. The following Error Codes are defined for this command: DIAMETER_ERROR_UNKNOWN_DOMAIN 1 This error code is used to indicate to the initiator of the request that the requested domain is unknown and cannot be resolved. DIAMETER_ERROR_BAD_CERT 2 This error code is used to indicate that the X509-Certificate or the X509-Certificate-URL in the Domain- Discovery-Request was invalid, or could not be verified. DIAMETER_ERROR_CANNOT_REPLY 3 This error code is returned when either an intermediate DIAMETER node or the home authentication server cannot reply to DIAMETER messages directly. This could be that the policy of an intermediate DIAMETER server does not permit direct contact and therefore requires proxying. It could also signify that the home authentication server does not have public key support. Message Format Calhoun, Bulley expires January 2000 [Page 8]
INTERNET DRAFT August 1999 <Domain-Discovery-Answer> ::= <DIAMETER Header> <Domain-Disc-Answer Command AVP> <Result-Code AVP> [<Error-Code AVP>] <Host-IP-Address AVP> [<Host-Name AVP>] <Extension-Id AVPs> <Framed-Password-Policy AVP> [<X509-Certificate AVP>] [<X509-Certificate-URL AVP>] <Timestamp AVP> <Nonce AVP> {<Integrity-Check-Vector AVP> || <Digital-Signature AVP } AVP Format A summary of the Domain-Discovery-Answer packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Length | Reserved |P|E|T|V|H|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Command Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AVP Code 256 DIAMETER Command AVP Length The length of this attribute MUST be 12. AVP Flags The 'M' bit MUST be set. The 'P' bits MAY be set if end to end message integrity is required. The 'V', 'H' and 'T' bits MUST NOT be set. Command Type The Command Type field MUST be set to 262 (Domain-Discovery- Answer). Calhoun, Bulley expires January 2000 [Page 9]
INTERNET DRAFT August 1999 4.0 DIAMETER AVPs This section will define the mandatory AVPs which MUST be supported by all DIAMETER implementations claiming support for this specification. The following AVPs are defined in this document: Attribute Name Attribute Code ----------------------------------- Digital-Signature 260 X509-Certificate 264 X509-Certificate-URL 265 Next-Hop 278 4.1 Digital-Signature Description The Digital-Signature AVP is used to provide for authentication, integrity and non-repudiation of DIAMETER AVPs. A DIAMETER entity adding AVPs to a message that must be protected by the Digital Signature MUST ensure that they apprear prior to this AVP. The only exception being the Integrity-Check-Vector AVP, which MUST appear after the Digital-Signature AVP, since it is stripped at each hop. Any other AVP that is stripped at each hop (e.g. Proxy- State AVP) also MUST NOT be protected by the Digital-Signature AVP. AVPs are marked as being protected by enabling their 'P' bit. A DIAMETER node adding a Digital-Signature to a message that already has such an AVP MUST sign all of the existing AVP that have the 'P' bit set in addition to the new protected AVPs added. It is imperative that Proxy servers NOT change the order of the AVPs with the 'P' bit set, otherwise the signature verification would fail. Proxy servers also MUST NOT remove, add, or change any AVP that has the 'P' bit set. The Digital Signature also includes the DIAMETER header. However, when computing the signature, it is necessary for the header's length field to be set to zero (0). This is necessary since the message size may change from one proxy server to another as AVPs are added and others are deleted. The Digital-Signature is generated in the method described in section 5.4.1. Calhoun, Bulley expires January 2000 [Page 10]
INTERNET DRAFT August 1999 All DIAMETER implementations supporting this extension MUST support this AVP. AVP Format A summary of the Digital-Signature AVP format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Length | Reserved |P|E|T|V|H|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transform ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+-+-+-+-+ AVP Code 260 Digital-Signature AVP Length The length of this attribute MUST be at least 17. AVP Flags The 'M' bit MUST be set. The 'P', 'V', 'H' and 'T' bits MUST NOT be set. Address The Address field contains the IP address of the DIAMETER host which generated the Digital-Signature. Transform ID The Transform ID field contains a value that identifies the transform that was used to compute the signature. The following values are defined in this document: RSA [9] 1 Calhoun, Bulley expires January 2000 [Page 11]
INTERNET DRAFT August 1999 Data The Data field contains the digital signature of the packet up to this AVP. 4.2 X509-Certificate Description The X509-Certificate [12] is used in order to send a DIAMETER peer the local system's X.509 certificate chain, which is used in order to validate the Digital-Signature attribute. Section 5.6 contains more information about the use of certificates. AVP Format A summary of the X509-Certificate AVP format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Length | Reserved |P|E|T|V|H|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+-+-+-+-+ AVP Code 264 X509-Certificate AVP Length The length of this attribute MUST be at least 9. AVP Flags The 'M' bit MUST be set. The 'P' bits MAY be set if end to end message integrity is required. The 'V', 'H' and 'T' bits MUST NOT be set. Data Calhoun, Bulley expires January 2000 [Page 12]
INTERNET DRAFT August 1999 The Data field contains the X.509 Certificate Chain. 4.3 X509-Certificate-URL Description The X509-Certificate-URL is used in order to send a DIAMETER peer a URL to the local system's X.509 certificate chain [12], which is used in order to validate the Digital-Signature attribute. Section 5.6 contains more information about the use of certificates. AVP Format A summary of the X509-Certificate-URL AVP format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Length | Reserved |P|E|T|V|H|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | String ... +-+-+-+-+-+-+-+-+ AVP Code 265 X509-Certificate-URL AVP Length The length of this attribute MUST be at least 9. AVP Flags The 'M' bit MUST be set. The 'P' bits MAY be set if end to end message integrity is required. The 'V', 'H' and 'T' bits MUST NOT be set. String The String field contains the X.509 Certificate Chain URL. Calhoun, Bulley expires January 2000 [Page 13]
INTERNET DRAFT August 1999 4.4 Next-Hop Description The Next-Hop AVP MUST preceed a Digital-Signature AVP and is used to validate that a packet traversed the proxy chain that was intended. A DIAMETER message with the Next-Hop Address being different than the address found in the preceeding Digital- Signature AVP is considered invalid. AVP Format A summary of the Next-Hop AVP format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Length | Reserved |P|E|T|V|H|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AVP Code 278 Next-Hop AVP Length The length of this attribute MUST be 12. AVP Flags The 'M' bit and 'P' bits MUST be set. The 'V', 'H' and 'T' bits MUST NOT be set. Address This field contains the IP Address of the next DIAMETER Server. 5.0 Protocol Definition This section will describe how the base protocol works (or is at least an attempt to). Calhoun, Bulley expires January 2000 [Page 14]
INTERNET DRAFT August 1999 5.1 Feature Advertisement/Discovery As defined in [2], the Reboot-Ind and Device-Feature-Query messages can be used to inform a peer about locally supported DIAMETER Extensions. In order to advertise support of this extension, the Extension-Id AVP must be transmitted with a value of two (2). 5.2 DIAMETER Secure Proxying The ROAMOPS specification [11] discusses how RADIUS servers can be arranged in a hierarchical manner, allowing message exchanges across domain boundaries. The specification also describes some security flaws encountered when RADIUS is used in a proxy environment. The DIAMETER extension described in this document introduces end-to-end security, which solves many of the problems encountered when RADIUS is used. (Request) (Request) +------+ -----> +------+ ------> +------+ | | | | | | | NASB +--------------------+ DIA2 +--------------------+ DIA1 | | | | | | | +------+ <----- +------+ <------ +------+ (Answer) (Answer) In this example NASB generates a Request that is forwarded to DIA2. The Request contains a Digital-Signature AVP which "protects" all preceeding AVPs bit the 'P' bit set (known as protected AVPs) within the request. All AVPs which may be modified, or removed by intermediate DIAMETRE Proxies MUST NOT have the 'P' bit set. Such AVPs include the Integrity-Check-Vector, Proxy-State, etc. Once DIA2 receives the request, it MAY validate the signature in the request to ensure that it was originated by NASB. Verification may not be necessary if the signature was added by a DIAMETER node one hop away since the Integrity-Check-Vector (or any whatever security mechanism used for hop-by-hop security) may be sufficient. The DIA2 Server SHOULD add the Proxy-State AVP [2], which contains opaque data that MUST be present in the response and is used to identify state information related to the request or response. If the Proxy-State AVP is already present in the request, it MUST be replaced with DIA2's Proxy-State AVP. This means that the Proxy-State AVP cannot have the 'P' bit set. The Server MAY also add other new AVPs to the request. All new AVPs that are protected by the new Digital-Signature AVP MUST have the 'P' bit set, and MUST preceed the Digital-Signature AVP. The message is then forwarded towards the DIA1 server. Calhoun, Bulley expires January 2000 [Page 15]
INTERNET DRAFT August 1999 The use of link level encryption, such as IPSec, cannot be used for end-to-end message integrity between NASB and DIA1, since all messages are processed by DIA2. What is needed is an application level security mechanism, which is what the Digital-Signature AVP provides. However, Digital-Signatures may not be necessary if the messages do not traverse proxies, unless non-repudiation is required. This mechanism now provides a method for DIA1 to be able to identify that NAS was the initiator of the request, and that no "critial" AVPs were modified mid-stream by intermediate proxies. Therefore, DIA2 cannot modify any protected AVPs (such as duration of a call, number of bytes transfered, etc). This mechanism also provides the application with the integrity, and non-repudiation, information it may need should it deem it necessary to log such information. This extension also provides for end-to-end AVP encryption, by using the peer's public key. However, given that asymetric encryption is very costly, it's use should be minimal. An attack has been identified in this proposal which allows a malicous man in the middle attack as shown in the following diagram. (Request) (Request) (Request) +------+ -----> +------+ -----> +------+ -----> +------+ | | | | | | | | | NASB +----------+ DIA2 +----------+ DIA3 +----------+ DIA1 | | | | | | | | | +------+ <----- +------+ <----- +------+ <----- +------+ (Answer) (Answer) (Answer) In this example, DIA3 traps packets generated from DIA2 towards DIA1, removes the AVPs added by DIA2 and inserts its own AVPs (possibly by trying to convince DIA1 to pay DIA3 for the services). This attack can be prevented by supporting a new Next-Hop AVP. In this case when NASB prepares a request it inserts a protected Next-Hop AVP which contains DIA2's identitity. DIA2 also adds a Next-Hop AVP with DIA1 as the next hop. This mechanism ensures that a man in the middle cannot alter the packet by overriding the previous hop's additions and signature. DIA1 could easily validate the packet's path with the use of the Next-Hop AVPs. 5.3 Domain Discovery The Domain Discovery message set is very useful in determining the Home authentication server, the password policies for the domain, as Calhoun, Bulley expires January 2000 [Page 16]
INTERNET DRAFT August 1999 a mechanism to retrieve a certificate (or a pointer to a certificate). Note that it is not necessary for a host to issue a Domain Discovery in order to make use of a proxy. A DIAMETER Request MAY be proxied by an intermediate server without the knowledge of the client, however the client will be unable to validate any Digital-Signatures if the home authentication server's certificate or public key is not known. The following example shows a case where DIA1 needs to communicate with DIA3. In this example it is necessary to use DIA2 as a proxy in order for both ISPs to communicate. Although this MAY be desireable in some business models, there are cases where it is beneficial to remove the proxy altogether and allow both DIA3 and DIA1 to communicate in a secure fashion. (DD-Request) (DD-Request) +------+ -----> +------+ ------> +------+ | | | | | | | DIA1 +--------------------+ DIA2 +--------------------+ DIA3 | | | | | | | +------+ <----- +------+ <------ +------+ (DD-Response) (DD-Response) The way the Domain Discovery works is that prior to sending out an authentication request DIA1 would issue a Domain Discovery message towards DIA2. This message is protected with the digital signature as well as the Next-Hop AVP. DIA2 would then forward the request to DIA3 including the Next-Hop and the digital signature AVP. When DIA3 receives the request, it MUST save the certificate (or the pointer to the certificate) and respond back including the local password policy, DIA3's certificate, its contact information (i.e. IP address) and protect the response with the digital signature. Note that in all cases the TimeStamp AVP is also present to ensure no replay packets are accepted. When DIA2 receives the packet, it must add the Next-Hop AVP as well as the digital signature AVP. When DIA1 receives the packet it then knows a direct route to communicate with DIA3 since the contact information is present in the response. The fact that both DIA1 and DIA3 can now communicate directly allows both peers to use IPSEC to protect the message exchange (it may be desirable to use the Digital- Signature AVP in instances where records of digitally signed packets must be kept). Calhoun, Bulley expires January 2000 [Page 17]
INTERNET DRAFT August 1999 (Request) +------+ -----> +------+ | | | | | DIA1 +--------------------+ DIA3 | | | | | +------+ <----- +------+ (Answer) In addition, the password policy is also present which can indicate whether DIA3 is willing to accept CHAP, PAP or EAP authentication. Note that the Domain-Discovery-Request/Answer MUST include at least one Extension-Id AVP [2]. 5.4 Data Integrity This section will describe how data integrity and non-repudiation is achieved using the Digital-Signature AVP. Note that the Timestamp and Nonce AVPs MUST be present in the message PRIOR to the Digital-Signature AVPs discussed in this section. The Timestamp AVP provides replay protection and the Nonce AVP provides randomness. 5.4.1 Using Digital Signatures In the case of a simple peer to peer relationship the use of IPSEC is sufficient for data integrity and confidentiality. However there are instances where a peer must communicate with another peer through the use of a proxy server. IPSEC does not provide a mechanism to protect traffic when two peers must use an intermediary node to communicate at the application layer therefore the Digital-Signature AVP MUST be used. The following diagram shows an example of a router initiating a DIAMETER message to DIA1. Once DIA1 has finished processing the message it adds its signature and forwards the message to the non- trusted DIA2 proxy server. If DIA2 needs to add or change any protected AVPs it SHOULD add its digital signature before forwarding the message to DIA3. Calhoun, Bulley expires January 2000 [Page 18]
INTERNET DRAFT August 1999 +------+ -----> +------+ -----> +------+ -----> +------+ | | | | | non- | | | |router+----------+ DIA1 +----------+trustd+----------+ DIA3 | | | | | | DIA2 | | | +------+ <----- +------+ <----- +------+ <----- +------+ Since intermediate DIAMETER proxies may add, or delete unprotected AVPs "en route" towards the final DIAMETER destination, it is necessary for the length in the header to be set to zero (0) prior to the signature computation. The length field must be restored once the computation is complete. The following is an example of a message that include end-to-end security: <DIAMETER Message> ::= <DIAMETER Header> <Command AVP> [<Additional AVPs>] <Next-Hop AVP> <Timestamp AVP> <Nonce AVP> <Digital-Signature AVP> The AVP Header's 'P' bit is used to identify which AVPs are considered protected when applying a digital signature to a DIAMETER message. Protected AVPs cannot be changed "en route" since they are protected by the Digital Signature AVP. All protected AVPs added by a DIAMETER entity MUST appear prior to the new Digital Signature AVP. The Next-Hop AVP indicates the intended recipient of the DIAMETER message. When a DIAMETER message is received with a Next-Hop AVP that does not correspond with the address information with the preceeding Digital-Signature AVP, the message MUST be considered invalid and MUST be rejected. The Next-Hop AVP MUST be protected. The Data field of the Digital-Signature AVP contains the RSA/MD5 signature algorithm as defined in [9]. 5.4.2 Using Mixed Data Integrity AVPs The previous sections described the Integrity-Check-Vector and the Digital-Signature AVP. Since the ICV offers hop-by-hop integrity and the digital signature offers end to end integrity, it is possible to use both AVPs within a single DIAMETER message. In fact, the use of the ICV and the Digital-Signature is recommended to provide both types of message integrity, which is necessary when messages are proxied. In the event that two peers use an out-of-band message Calhoun, Bulley expires January 2000 [Page 19]
INTERNET DRAFT August 1999 integrity mechanism (e.g. IPSec) for hop-by-hop message integrity, the ICV AVP is not necessary and should not be used. The following diagram provides an example where DIAMETER Server 1 (DIA1) communicates with DIA3 using Digital-Signatures through DIA2. In this example ICVs are used between DIA1 and DIA2 as well as between DIA2 and DIA3. <Public-Key> -----------------------------> <Shared-Secret> <Shared-Secret> +------+ -----> +------+ -----> +------+ | | | | | | | DIA1 +----------+ DIA2 +----------+ DIA3 | | | | | | | +------+ +------+ +------+ Using the previous diagram, the following message would be sent between DIA1 and DIA2: <DIAMETER Message> ::= <DIAMETER Header> <Command AVP> [<Additional AVPs>] <Timestamp AVP> <Nonce AVP> <Digital-Signature AVP> <Integrity-Check-Vector AVP (DIA1->DIA2)> The following message would be sent between DIA2 and DIA3: <DIAMETER Message> ::= <DIAMETER Header> <Command AVP> [<Additional AVPs>] <Timestamp AVP> <Nonce AVP> <Digital-Signature AVP> <Timestamp AVP> <Nonce AVP> <Integrity-Check-Vector AVP (DIA2->DIA3)> Note that in the above example messages the ICV AVP appear after the Digital-Signature AVP. This is necessary since DIA2 above removes the ICV AVP (DIA1->DIA2) and adds its own ICV AVP (DIA2->DIA3). The ICVs provide hop-by-hop security while the Digital-Signature provides integrity of the message between DIA1 and DIA3. Calhoun, Bulley expires January 2000 [Page 20]
INTERNET DRAFT August 1999 <Public-Key> ----------------------------> <Shared-Secret> <Shared-Secret> <Shared-Secret> +------+ -----> +------+ -----> +------+ -----> +------+ | | | | | | | | |router+----------+ DIA1 +----------+ DIA2 +----------+ DIA2 | | | | | | | | | +------+ <----- +------+ <----- +------+ <----- +------+ There are cases, such as in remote access, where the device initiating the DIAMETER request does not have the processing power to generate Digital-Signatures as required by the protocol. In such an arrangement, there normally exists a first hop DIAMETER Server (DIA1) which acts as a proxy to relay the request to the final authenticating DIAMETER server (DIA2). It is valid for the first hop server to remove the Integrity-Check-Vector AVP inserted by the router and replace it with a Digital-Signature AVP. 5.5 AVP Encryption with Public Keys AVP encryption using public keys is much more complex than the previously decribed method, yet it is desirable to use it in cases where the DIAMETER message will be processed by an untrusted intermediate node (proxy). Public Key encryption SHOULD be supported, however it is permissible for a low powered device initiating the DIAMETER message to use shared secret encryption with the first hop (proxy) DIAMETER server, which would decrypt and encrypt using the Public Key method. The PK-Encrypted-Data bit MUST only be set if the final DIAMETER host is aware of the sender's public key. This information can be relayed in three different methods as described in section 5.6. The AVP is encrypted in the method described in [9]. 5.6 Public Key Cryptography Support A DIAMETER peer's public key is required in order to validate a message which includes the the Digital-Signature AVP. There are three possibilities on retrieving public keys: 5.6.1 X509-Certificate A message which includes a Digital-Signature MAY include the X509-Certificate AVP. Given the size of a typical certificate, this Calhoun, Bulley expires January 2000 [Page 21]
INTERNET DRAFT August 1999 is very wasteful and in most cases DIAMETER peers would cache such information in order to minimize per packet processing overhead. It is however valid for a DIAMETER host to provide its X509-Certificate in certain cases, such as when issuing the Device- Reboot-Indication, or in the Domain-Discovery messages. It is envisioned that the peer would validate and cache the certificate at that time. 5.6.2 X509-Certificate-URL The X509-Certificate-URL is a method for a DIAMETER host sending a message that includes the Digital-Signature to provide a pointer to its certificate. Upon receiving such a message a DIAMETER host MAY choose to retrieve the certificate if it is not locally cached. Of course the process of retrieving and validating a certificate is lengthy and will require the initiator of the message to retransmit the request. However once cached the certificate can be used until it expires. 5.6.3 Static Public Key Configuration Given that using certificates requires a PKI infrastructure which is very costly, it is also possible to use this technology by locally configuring DIAMETER peers' public keys. Note that in a network involving many DIAMETER proxies this may not scale well. 6.0 IANA Considerations The numbers for the Command Code AVPs (section 3) is taken from the numbering space defined for Command Codes in [2]. The numbers for the various AVPs defined in section 4 were taken from the AVP numbering space defined in [2]. The numbering for the AVP and Command Codes MUST NOT conflict with values specified in [2] and other DIAMETER related Internet Drafts. This document also introduces two new bits to the AVP Header, which MUST NOT conflict with the base protocol [2] and any other DIAMETER extension. 7.0 References Calhoun, Bulley expires January 2000 [Page 22]
INTERNET DRAFT August 1999 [1] Rigney, et alia, "RADIUS", RFC-2138, April 1997 [2] Calhoun, Rubens, "DIAMETER Base Protocol", Internet-Draft, draft-calhoun-diameter-08.txt, August 1999. [3] Rivest, Dusse, "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [4] Kaufman, Perlman, Speciner, "Network Security: Private Communications in a Public World", Prentice Hall, March 1995, ISBN 0-13-061466-1. [5] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, January 1997. [6] Calhoun, Bulley, "DIAMETER User Authentication Extensions", draft-calhoun-diameter-authen-06.txt, August 1999. [7] Aboba, Beadles "The Network Access Identifier." RFC 2486. January 1999. [8] Aboba, Zorn, "Criteria for Evaluating Roaming Protocols", RFC 2477, January 1999. [9] Kaliski, "PKCS #1: RSA Encryption Version 1.5", Internet- Draft, draft-hoffman-pkcs-rsa-encrypt-03.txt, October 1997. [10] Calhoun, Zorn, Pan, "DIAMETER Framework", draft-calhoun-diameter-framework-02.txt, Work in Progress, December 1998. [11] Aboba, Vollbrecht, "Proxy Chaining and Policy Implementation in Roaming", RFC 2607, June 1999. [12] Housley, Ford, Polk, Solo, "Internet X.509 Public Key Infrastructure Certificate and CRL Profile", RFC 2459, January 1999. [13] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 8.0 Acknowledgements The Authors would like to acknowledge the following people for their contribution in the development of the DIAMETER protocol: Bernard Aboba, Jari Arkko, , Daniel C. Fox, Lol Grant, Nancy Greene, Peter Heitman, Ryan Moats, Victor Muslin, Kenneth Peirce, Allan Rubens, Sumit Vakil, John R. Vollbrecht, Jeff Weisberg and Glen Zorn 9.0 Author's Address Questions about this memo can be directed to: Pat R. Calhoun Network and Security Research Center, Sun Labs Sun Microsystems, Inc. 15 Network Circle Calhoun, Bulley expires January 2000 [Page 23]
INTERNET DRAFT August 1999 Menlo Park, California, 94025 USA Phone: 1-650-786-7733 Fax: 1-650-786-6445 E-mail: pcalhoun@eng.sun.com William Bulley Merit Network, Inc. 4251 Plymouth Road, Suite C Ann Arbor, Michigan, 48105-2785 USA Phone: 1-734-764-9993 Fax: 1-734-647-3185 E-mail: web@merit.edu 10.0 Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this docu- ment itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Inter- net organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permis- sions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WAR- RANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Calhoun, Bulley expires January 2000 [Page 24]