Individual Contribution Pat R. Calhoun Internet-Draft Sun Microsystems, Inc. Category: Standards Track Stephen Farrell Baltimore Technologies William Bulley Merit Network, Inc. May 2001 Diameter End-2-End Security Extension Status of this Memo 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. Distribution of this memo is unlimited. Copyright (C) The Internet Society 2001. All Rights Reserved. Abstract The Diameter base protocol leverages either IPsec or SSL for integrity and confidentiality between two Diameter nodes. The base protocol also defines a Diameter proxy server, that forwards requests to other servers when it detects that a given request cannot be satisfied locally. Calhoun, Bulley, Farrell expires October 2001 [Page 1] Internet-Draft May 2001 The ROAMOPS Working Group has defined a requirement that allows for the Diameter servers communicating through the proxy to be able to provide for end-to-end AVP integrity and confidentiality, making it difficult for the proxy to be able to modify, and/or be able to view sensitive information, within the message. The Mobile-IP and NASREQ Working Groups have stated that strong authentication is a requirement for AAA data, such as accounting records, for the purposes of non-repudiation. This Diameter extension specifies how strong AVP authentication, integrity and encryption can be done using a mixture of symmetric and asymmetric transforms, by encapsulating Cryptographic Message Syntax (CMS) data into Diameter AVPs. The CMS data can also be used to carry X.509 certificates. Table of Contents 1.0 Introduction 1.1 Requirements language 1.2 Advertising extension support 2.0 AVP Format 3.0 Key Management 3.1 Usage Scenario 3.2 Certificate Requirements 3.3 Algorithms 3.4 Reuse of CMS Content Encryption Keys 4.0 Command-Codes Values 4.1 E2E-SA-Setup-Request (ESSR) Command 4.2 E2E-SA-Setup-Answer (ESSA) Command 5.0 End-to-End Security Association Message Flow 6.0 End-to-End Security AVPs 6.1 CMS-Signed-Data AVP 6.2 CMS-Encrypted-Data AVP 6.3 CMS-Cert AVP 6.4 Local-CA-Info AVP 6.4.1 CA-Name AVP 6.4.2 Key-Hash AVP 6.5 OCSP-Nonce AVP 6.6 AAA-Server-Certs AVP 6.7 OCSP-Responses AVP 6.8 CA-Chain AVP 6.9 Expected-Signed-AVP AVP 6.10 Expected-Encrypted-AVP AVP 6.11 AVP-Code AVP 7.0 Result-Code AVP Values 7.1 Transient Failures 7.2 Permanent Failures Calhoun, Bulley, Farrell expires October 2001 [Page 2] Internet-Draft May 2001 8.0 IANA Considerations 8.1 Command Codes 8.2 AVP Codes 8.3 Result-Code AVP Values 8.4 Extension Identifier 9.0 Security Considerations 10.0 References 11.0 Acknowledgements 12.0 Authors' Addresses 13.0 Full Copyright Statement 14.0 Expiration Date Calhoun, Bulley, Farrell expires October 2001 [Page 3] Internet-Draft May 2001 1.0 Introduction The Diameter base protocol [1] leverages either IPsec or SSL for integrity and confidentiality between two Diameter nodes. The base protocol also defines a Diameter proxy server, that forwards requests to other servers when it detects that a given request cannot be satisfied locally. The ROAMOPS Working Group has defined a requirement in [10] that allows for the Diameter servers communicating through the proxy to be able to provide for end-to-end AVP integrity and confidentiality, making it difficult for the proxy to be able to modify and see sensitive information within the message. The Mobile-IP and NASREQ Working Groups have stated in [6, 7, 8] that non-repudiation is a requirement for AAA data, such as accounting records. When a chain of proxies use hop-by-hop security (e.g. TLS, IPSec), a proxy may modify information in a Diameter message. It is virtually impossible for the rest of the nodes in the proxy chain to know that the message was modified in mid-stream. Figure 1 shows an example of such a network, where DIA3 modifies the contents of "foo" in both the request and the response. (Request) (Request) (Request) [AVP(foo)=x] [AVP(foo)=x] [AVP(foo)=y] +------+ -----> +------+ -----> +------+ -----> +------+ | | | | | | | | | NASB +----------+ DIA2 +----------+ DIA3 +----------+ DIA1 | | | | | | | | | +------+ <----- +------+ <----- +------+ <----- +------+ (Answer) (Answer) (Answer) [AVP(foo)=b] [AVP(foo)=b] [AVP(foo)=a] Figure 1: Proxy Chain This document describes how strong authentication and encryption can be provided in the Diameter protocol, by encapsulating CMS objects [3] in AVPs. The CMS object can also be used to carry X.509 certificates and revocation lists. In the example provided in Figure 1, the originator of the request and response adds a digital signature that covers a set of AVPs within the message. The protected AVPs should not be changed by an intermediate proxy server (DIA2, DIA3), since the signature validation performed by the end server would fail. The Diameter base protocol also allows a Diameter broker to provide redirect services, as shown in Figure 2. The Diameter broker MAY return information to a requesting server that would allow the Calhoun, Bulley, Farrell expires October 2001 [Page 4] Internet-Draft May 2001 servers to interact directly, bypassing the broker. This optimized approach reduces the complexity associated with end-to-end security. +------------------+ | Diameter | | Broker | +------------------+ ^ Request | Response + | Result Code = Local | Redirect Home ISP v ISP +----------+ +----------+ | abc.net | | xyz.net | | Diameter |<------------>| Diameter | | Server | | Server | +----------+ Direct +----------+ Communication Figure 2: Diameter Broker Returning Redirect Indication When redirect services are used, a network layer security protocol, such as IP Security, MAY be used to secure the traffic between the two Diameter servers. However, security at the application level may still be necessary in this network configuration, specifically the ability to authenticate a select set of AVPs. Brokers that operate in a redirect mode typically require that both Diameter servers sign the same set of AVPs, marked with the 'P' bit, in accounting records. The accounting record, signed by both parties is then forwarded to the broker via the local Diameter server. This provides the broker with some assurances that both networks agreed on the accounting data, which it MAY use for settlement purposes. If the underlying security protocol provides confidentiality, strong encryption MAY not be necessary in the redirect case. Given that asymmetric transform operations are expensive, Diameter servers may wish to use them only when dealing with inter-domain servers, as shown in Figure 3. This configuration is normally desirable since Diameter entities within a given administrative domain may inherently trust each other. Further, it is desirable to move this functionality to the edges, since NASes do not necessarily have the CPU power to perform expensive cryptographic operations. Calhoun, Bulley, Farrell expires October 2001 [Page 5] Internet-Draft May 2001 +------------------------+ | Foreign Network | |+-----+ +--------+ | +--------+ +--------+ || | |Diameter| | |Diameter| |Diameter| || NAS +------+ +--------+ +--------+ Home | || | | Proxy | | | Broker | | Server | |+-----+ +--------+ | +--------+ +--------+ | | +------------------------+ <------------> <--------------------------> Figure 3: Mixed Diameter Security Models 1.1 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 [5]. 1.2 Advertising extension support Diameter nodes conforming to this specification MAY advertise support by including the value of two (2) in either the Device-Reboot-Ind Command's [2] Auth-Extension-Id or Acct-Extension-Id AVPs. 2.0 AVP Format The Diameter base protocol [1] details the AVP header, which includes the 'P' bit, but does not specify how the 'P' bit is used. The 'P' bit, known as the protected AVP bit, is used to indicate whether the AVP is protected by a digital signature. When set, the AVP is protected and the contents cannot be changed by a Diameter proxy server without detection. Calhoun, Bulley, Farrell expires October 2001 [Page 6] Internet-Draft May 2001 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|R|V|R|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor ID (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+-+-+-+-+ Figure 4: Diameter AVP Header All Diameter specifications MUST specify whether the 'P' bit can be set or not, as is done in section 4.5 of [1] and section 6 below. AVPs that are designed to be changed at each hop (such as the Proxy- Info AVP) MUST NOT allow the 'P' bit to be set. 3.0 Key Management For e2e origin authentication, CMS itself already provides sufficient key management without the need for additional specification. Basically, the originating Diameter node signs and includes whatever certificates are necessary for validation of the digital signature. However, for encryption of AVPs more work is needed. In order to be able to encrypt AVPs for a recipient, the originating Diameter node must have a copy of the recipient's public key. There are many well- known key retrieval schemes (e.g. using LDAP [16]), however, in order to simplify Diameter implementations a specific Diameter key distribution mechanism is defined here. Another issue that must be addressed is how a Diameter node is to "know" that certain AVPs are required to use the end-to-end security extension. This is communicated during the End-to-End Security Association message exchange, listed in section 4.0. Finally, this section addresses the certificate profile to be used for this Diameter extension, which is a simplified profile of [4]. 3.1 Usage Scenario When a Diameter node is about to send a message which MAY use end- to-end security, it must determine whether to use the end-to-end security service or not. We assume the Diameter node knows the user's NAI, which determines the user's realm. Calhoun, Bulley, Farrell expires October 2001 [Page 7] Internet-Draft May 2001 In the present discussion we assume that the Diameter node has not cached any information. Where information can be cached this is noted. We use Diameter E2E-SA-Setup-Request (ESSR) and E2E-SA-Setup-Answer (ESSA) messages to establish whether end-to-end security is required and if so, for which AVPs and which public key(s) to use. The originating node sends the ESSR message to a server in the destination realm. The ESSR message contains: - the realm part of the user's NAI - the list of direct trust CA's that the originating Diameter node has configured into it for certificate validation. A "direct trust" CA is one that the node is willing to use as the "top" of a certificate chain, sometimes confusingly known as a "root CA." - a list of AVPs that expected to be protected (and how) for this realm - (optionally) a flag indicating that the originating Diameter node wishes to receive certificate status information (using OCSP messages) in which case a nonce to be used by the destination Diameter node in OCSP requests MAY also be supplied. See [9] for details of the certificate status protocol and messages. The destination node returns the ESSA message which contains: - TTL for this SA (seconds) - a chain of CA certificates (possibly empty) - public key certificates for the AAA servers in the realm, all of which MUST validate up to one of the CA's contained in the ESSR message, via the chain of CA certificates above; - a list of AVPs that expected to be protected (and how) for this realm - (optionally, if nonce received and OCSP supported) a list of OCSP responses for the certificates in question, each of which uses the nonce from the ESSR message [Issue: If one OCSP responder is used, do we need to append to the nonce for each request?] The originating Diameter node now has to check the response. Any failure results in error messages and auditing and not sending the Diameter message. Checks: Calhoun, Bulley, Farrell expires October 2001 [Page 8] Internet-Draft May 2001 - the certificate chain selected is cryptographically correct, passes the (relevant parts of the) rfc2459 path validation algorithm and terminates at a CA mentioned in the ESSR message - the realm part of the user's NAI must occur as a subjectAltName (with the rfc822Address option) in the AAA server's certificate. This rfc822Address MUST be of the form "Diameter- @" where is the NAI's domain component and can be anything (e.g. "Diameter-1@baltimore.com", "Diameter-west@sun.com") chosen by the AAA server administrator. - the ESSA message MUST be digitally signed and the signature MUST be validated and the signer's certificate chain MUST terminate as a CA mentioned in the ESSR message If certificate status (revocation) is an issue for the Diameter node, then the ESSR message MAY contain a nonce value. The idea is that the sender of the ESSA makes OCSP requests on behalf of the Diameter node and returns the OCSP responses to the Diameter node as part of the ESSA message. The use of the nonce value ensures that the sender of the ESSA cannot return cached or otherwise fake OCSP responses to the Diameter node. The nonce value is to be (the beginning of) the nonce in the OCSP response. [Issue The reason for "beginning" above is that an OCSP responder might produce an error if presented with the same nonce more than once.] The ESSR MAY be signed. If the originating node has a private key and protection expectations for AVPs are specified, then the ESSR SHOULD be signed. The ESSA MUST be signed to prevent an intermediate node from modifying the protection expectations for AVPs. The ESSA MUST be signed a Diameter node from the destination realm (based on the signer's name). If e2e confidentiality or digial signature is required, then the originating node prepares the CMS related AVPs as required. 3.2 Certificate Requirements Certificates used for the purposes of Diameter MUST conform to the PKIX profile [4], and MUST also include a Diameter node's NAI, which is typically added in the Host-Name AVP [1], as one of the values of the subjectAltName extension of the Certificate. The NAI is to be Calhoun, Bulley, Farrell expires October 2001 [Page 9] Internet-Draft May 2001 encoded as an rfc822Name within the subjectAltName. For Diameter nodes (capable of acting as recipients for e2e confidentiality), the NAI MUST be of the form "Diameter- @". Other Diameter nodes MAY use this naming scheme. Note that this naming constraint is for PKI purposes only, and in no way restricts a Diameter's host name. These names are used for two purposes: 1. Where a Diameter node is verifying a signature it needs to be able to compare the identity of the signer against the identity in the Host-Name AVP. 2. Where a Diameter node is encrypting AVPs, it needs to be able to ensure that it uses a public key for the intended recipient. This requires comparing the identity in a Certificate against the NAI of the intended recipient (which is assumed to be known). In either case, the presence of the required NAI as an rfc822Name value in the subjectAltName extension of a verified public key certificate satisfies the matching requirement. Note that there MAY also be other values in the subjectAltName extension, (either using rfc822Name or other elements of the CHOICE), these can be safely ignored, but implementations MUST be able to handle their presence. Note also that the PKIX profile [4], section 4.1.2.6, specifies the rules for the relationship between the subjectAltName extension and the subject field of public key certificates. [Issue: Future versions of this draft may specify a restricted profile of [4] in order to simplify implementation.] 3.3 Algorithms For all uses of CMS in this specification the mandatory to implement algorithms are as follows: - Asymmetric: RSA - Hashing: SHA-1 - Signature: RSAwithSHA1 - Symmetric Encryption: 3DES At some point in future, AES will replace 3DES. Calhoun, Bulley, Farrell expires October 2001 [Page 10] Internet-Draft May 2001 3.4 Reuse of CMS Content Encryption Keys Once a CMS-Encrypted-Data AVP has been exchanged between two Diameter nodes, then they share a symmetric cryptographic key (the content encryption key) which can be used to encrypt further Diameter AVPs between the nodes by using the scheme specified in [15]. Normally, the nodes first take part in an ESSR/ESSA exchange in order to distribute the required asymmetric keys. [15] leaves open some issues, namely how to handle loss of a shared secret (say following a node re-boot) and for how long to continue to use a shared secret (the maximum number of decryptions required). Where a Diameter node receives a CMS-Encrypted-Data AVP, but doesn't have the required shared secret, that node should return the DIAMETER_KEY_UNKNOWN error message. The Diameter nodes may then use the ESSR/ESSA exchange, or cached asymmetric keys to rebuild their security association. In [15], the default value for the maximum number of decryptions allowed (CEKMaxDecrypts) when re-using a content encryption key is one. In general this default SHOULD be used, but if a Diameter node "knows" that more than one CMS-Encrypted-Data AVP will be exchanged between the nodes (based on the Expected-Encrypted-AVP settings exchanged during the ESSR/ESSA exchange) then the CEKMaxDecrypts setting MAY be set higher. Diameter nodes MUST be able to support a maxDecrypts setting of 1000. [Issue: Are there reasonable expectations for the highest MUST support for maxDecrypts?] 4.0 Command-Codes Values This section defines new Command-Code [1] values that MUST be supported by all Diameter implementations that conform to this specification. The following Command Codes are currently defined in this document: Command-Name Abbrev. Code Reference -------------------------------------------------------- E2E-SA-Setup-Request ESSR 304 4.1 E2E-SA-Setup-Answer ESSA 304 4.2 4.1 E2E-SA-Setup-Request (ESSR) Command The E2E-SA-Setup-Request command, indicated by the Command-Code field Calhoun, Bulley, Farrell expires October 2001 [Page 11] Internet-Draft May 2001 set to 304 and the 'R' bit set in the message flags field, is sent by a Diameter node to establish a Diameter End-to-End Security Association. ::= < Diameter-Header: 304, R > { Origin-FQDN } { Origin-Realm } { Destination-Realm } + { Local-CA-info } { Auth-Extension-Id } [ Destination-FQDN ] * [ Expected-Signed-AVP ] * [ Expected-Encrypted-AVP ] * [ OCSP-Nonce ] 0*1[ CMS-Signed-Data ] * [ Proxy-Info ] * [ Route-Record ] 4.2 E2E-SA-Setup-Answer (ESSA) Command The E2E-SA-Setup-Answer command, indicated by the Command-Code field set to 304 and the 'A' bit set in the message flags field, is sent by a Diameter node in response to an ESSR message. ::= < Diameter-Header: 304, A > { Origin-FQDN } { Origin-Realm } 0*1{ CA-Chain } + { AAA-Server-Certs } * { OCSP-Responses } { Destination-FQDN } { Auth-Extension-Id } * [ Expected-Signed-AVP ] * [ Expected-Encrypted-AVP ] [ CMS-Signed-Data ] * [ Proxy-Info ] * [ Route-Record ] 5.0 End-to-End Security Association Message Flow This section contains an example of a NAS in domain xyz.com, communicating with its local proxy, which in turn communicates with a Diameter server in ABC.COM's network. In the following example, once the initial capabilities exchange is complete, the NAS receives a request for access from alice@abc.com, which causes an end-to-end Calhoun, Bulley, Farrell expires October 2001 [Page 12] Internet-Draft May 2001 security association message exchange, followed by an authentication request. +-------+ +-------+ +---------+ | NAS | | Proxy | | abc.com | | | | Srv | |Home Srv | +-------+ +-------+ +---------+ | | | |DRI extensions 1, 2 | | (a) |------------------->| | |DRI extension 1, -1 | | (b) |<-------------------| | | . |DRI extensions 1, 2 | (c) | |<-------------------| | |DRI extension 1, -1 | (d) | |------------------->| ->| User alice@abc.com | | (e) | Requests Access | | | | | | ESSR | | | Dest-Realm=abc.com | | | CMS-Cert | | (f) |--------------------+------------------->| | | ESSA | | | Origin-FQDN=foo | | | CMS-Cert | (g) |<-------------------+--------------------| | Auth-Request + | | | CMS-Signed-Data | | | Dest-FQDN=foo | | (h) |--------------------+------------------->| | | Auth-Answer + | | | CMS-Encrypted-Data | (i) |<-------------------+--------------------| Figure 5: Example of an End-to-End Security Association Setup (a) NAS sends a DRI message to its proxy server indicating that it supports extensions 1 (NASREQ) and 2 (E2E Security). (b) The proxy server sends a DRI message to the NAS indicating that it supports extension 1 (NASREQ), as well as the wildcard extension. (c) ABC.COM's Home Server sends a DRI message to a proxy server indicating that it supports extensions 1 (NASREQ) and 2 (E2E Security). (d) The proxy server sends a DRI message to ABC.COM's Home Server Calhoun, Bulley, Farrell expires October 2001 [Page 13] Internet-Draft May 2001 indicating that it supports extension 1 (NASREQ), as well as the wildcard extension. (e) The NAS receives a request for access from a user (alice@abc.com). (f) The NAS issues an ESSR message, with the Destination-Realm AVP set to abc.com, and its certificate in the CMS-Cert AVP. The ESSR includes the set of AVPs that the NAS expects to be encrypted, in the event that the home server returns messages that contain these AVPs. (g) ABC.COM's Home Server processes the ESSR message, and replies with the ESSA message. The ESSA also includes the set of AVPs that the home server is expecting to be authenticated, as well as its certificate in the CMS-Cert AVP. (h) The NAS issues an authentication request with the Destination-FQDN AVP set to the value of the Origin-FQDN AVP in the ESSA. The message includes the CMS-Signed-AVP, which authenticates the AVPs that were requested by the Home Server in the ESSA. (i) The Home Server successfully authenticates the user, and returns a reply, which includes the CMS-Encrypted-Data AVP, whose contents include the AVPs that were specified by the NAS in the ESSR. 6.0 End-to-End Security AVPs This section contains AVPs that are used to establish a Diameter End-to-End Security Association. Calhoun, Bulley, Farrell expires October 2001 [Page 14] Internet-Draft May 2001 +---------------------+ | AVP Flag rules | |----+-----+----+-----|----+ AVP Section | | |SHLD| MUST|MAY | Attribute Name Code Defined Value Type |MUST| MAY | NOT| NOT|Encr| -----------------------------------------|----+-----+----+-----|----| AAA-Server-Certs 351 6.6 OctetString| M | P | | V | N | AVP-Code 352 6.11 Unsigned32 | M | P | | V | N | CA-Chain 353 6.8 OctetString| M | P | | V | N | CA-Name 349 6.4.1 OctetString| M | P | | V | N | CMS-Cert 354 6.3 OctetString| M | | | P,V | N | CMS-Encrypted- 355 6.2 OctetString| M | P | | V | N | Data | | | | | | CMS-Signed-Data 310 6.1 OctetString| M | | | P,V | N | Key-Hash 350 6.4.2 OctetString| M | P | | V | N | Expected-Signed- 356 6.9 Grouped |M,P | | | V | N | AVP | | | | | | Expected- 357 6.10 Grouped |M,P | | | V | N | Encrypted-AVP | | | | | | Local-CA-Info 348 6.4 Grouped | M | P | | V | N | OCSP-Nonce 358 6.5 OctetString| M | P | | V | N | OCSP-Responses 359 6.7 OctetString| M | P | | V | N | 6.1 CMS-Signed-Data AVP The CMS-Signed-Data AVP (AVP Code 310) is of type OctetString and contains the Basic Encoding Rules (BER) encoding of a CMS object [3] of type ContentInfo. The profile of CMS algorithm and structure usage is as specified in the S/MIME v3 message specification [11]. This means that where a set of AVPs is protected using CMS, the set MUST first be encoded according to MIME encoding rules specified below. This method of encapsulating AVPs allows existing S/MIME toolkits to be used without changes in order to produce strongly protected Diameter messages. To package a set of AVPs as a MIME type, the AVPs are first concatenated in the order in which they occur in the Diameter message. The entire AVP MUST be input to the signing process, from the first byte of the AVP code to the last byte of the AVP data, including all other fields, length, reserved/flags, and optional vendor IDs, and padding. The AVP MUST be input to the signing process in network byte order. The signature is calculated over the catenation of all the 'P' bit AVPs, but the AVPs themselves are not carried within the CMS-Signed- Data AVP. Instead, the digest value within the SignedData structure contains the digest over these AVPs. Multiple Diameter entities MAY Calhoun, Bulley, Farrell expires October 2001 [Page 15] Internet-Draft May 2001 add their signatures to an existing CMS-Signed-Data AVP using the countersignature attribute, defined in section 11.4 of [3]. The countersignature attribute requires that the signatures occur sequentially, meaning that each node's signature covers the existing signatures in the CMS object. A countersignature MUST cover all AVPs in the message that have the 'P' bit enabled (i.e. the same AVPs as the first signature). Receiving Diameter nodes MAY, but need not, be able to support the use of the countersignature attribute. If a receiver detects that the contents of the CMS-Signed-Data AVP are invalid, it SHOULD return the new Result-Code AVP value defined in section 7.0. When AVPs are to be both encrypted and signed, the CMS-Encrypted-Data AVP MUST be created first. The resulting CMS object MUST then be MIME encoded producing an application/pkcs7-mime MIME type which is then used as the content of the EnvelopedData. This means that signing is "outside" encryption. The eContent field of the EncapsulatedContentInfo structure MUST be absent since the authentication covers data outside of the object. The signature is computed over all AVPs prior to the AVP that have the 'P' bit enabled. The order of the AVPs MUST be preserved and the computation begins with the first AVP immediately following the Diameter header. If the signature cannot be verified correctly, a response with the Result-Code AVP set to DIAMETER_INVALID_AUTH [1] MUST be returned. No more than one CMS-Signed-Data AVP MUST be present in any given Diameter message. 6.2 CMS-Encrypted-Data AVP The CMS-Encrypted-Data AVP (AVP Code 355) is of type OctetString and contains the Basic Encoding Rules (BER) encoding of a CMS object [3] of type ContentInfo. This means that where a set of AVPs is protected using CMS, the set MUST first be catenated a sequence of encoded AVPs. The entire AVP MUST be input to the encryption process, from the first byte of the AVP code to the last byte of the AVP data, including all other fields, length, reserved/flags, and optional vendor IDs, and padding. The AVP MUST be input to the encryption process in network byte order, and the encryptor is free to order AVPs whatever way it chooses. This value is then encrypted and used as the value of the EncryptedContent field within CMSEnvelopedData. Calhoun, Bulley, Farrell expires October 2001 [Page 16] Internet-Draft May 2001 If a receiver detects that the contents of the CMS-Data AVP is invalid, it SHOULD return the new Result-Code AVP value defined in section 7.0. When AVPs are to be both encrypted and authenticated, the CMS- Encrypted-Data AVP MUST be created first. Where AVPs are encapsulated within a CMS-Encrypted-Data AVP, the eContentType of the EncapsulatedContentInfo MUST be id-data [11]. CMS-Encrypted-Data MAY contain more than one CMS object, that is, implementations are REQUIRED to be able to add a new CMS-Encrypted- Data AVP value and are also REQUIRED to be able to decrypt all CMS- Encrypted-Data AVP values which are encrypted for them. When a conforming implementation receives a Diameter message which contains encrypted AVPs within a CMS EnvelopedData, then the recipient MUST check to see if it is on the list of recipients specified in the RecipientInfos of the EnvelopedData. If not, the recipient MAY choose to process the message or indicate an error. If the recipient is in the RecipientInfos and an error occurs during decryption, then the recipient MUST indicate an error. Diameter nodes SHOULD implement content encryption key re-use (see section 3.4 above). Zero or more CMS-Encrypted-Data AVP MAY be present in any Diameter message. 6.3 CMS-Cert AVP The CMS-Cert AVP (AVP Code 354) is of type OctetString and contains a "certs-only" CMS structure which is a degenerate form of CMS structure containing only PKI related information (see section 3.6 of [11] for details of the CMS certs-only structure). The CMS-Cert AVP contains one or more public key certificates (Certificate) and MAY optionally contains attribute certificates (AttributeCertificate) as allowed by CMS. Other legacy formats supported by CMS MUST NOT be used. Support for use of the Certificate structure is REQUIRED, while implementations MAY support use of the AttributeCertificate structure as defined in the PKIX attribute certificate profile [12]. The latter allows Diameter implementations to include a certificate from a trusted party that they are authorized to emit the AVPs contained in the message. Calhoun, Bulley, Farrell expires October 2001 [Page 17] Internet-Draft May 2001 This use of the CMS-Cert AVP can be used to "push" public key and attribute certificates and CRLs using Diameter, which MAY be useful in environments where repositories (e.g. LDAP servers) are either not used or not available (e.g. due to crossing a domain boundary). Conforming implementations MUST be able to emit a certs-only CMS structure which contains relevant PKI related information and MUST be able to process a CMS-Cert AVP which contains a certs-only CMS structure. Of course, the recipient of such a certs-only CMS structure SHOULD NOT use the PKI related information without first verifying it, e.g. by checking that issuer's are trusted, signatures verify etc. A CRL [4] MAY also be provided in the crls field of the SignedData, which MAY be used to assist in determining whether a certificate has been revoked. Optionally, the Diameter node MAY check the status of certificates using another mechanism, such as Online Certificate Status Protocol (OCSP) [9]. 6.4 Local-CA-Info AVP The Local-CA-Info AVP (AVP Code 348) is of type Grouped. The Grouped Data field has the following ABNF grammar: Local-CA-Info = CA-Name Key-Hash CA-Name = ; See Section 6.4.1 Key-Hash = ; See Section 6.4.2 The Local-CA-Info AVP Data field contains the Certificate Authority's name in the CA-Name AVP, as well as a hash of the key in the Key-Hash AVP. +---------------------------------------------------------------+ | AVP Header (AVP Code = 348) | +---------------------------------------------------------------+ | CA-Name AVP | +---------------------------------------------------------------+ | Key-Hash AVP | +---------------------------------------------------------------+ 6.4.1 CA-Name AVP The CA-Name AVP (AVP Code 349) is of type OctetString, encoded in the UTF-8 [24] format. The AVP contains the DN (in LDAP string syntax) of the Certificate Authority, e.g. "CN=CA;O=Baltimore Technologies;C=IE". Calhoun, Bulley, Farrell expires October 2001 [Page 18] Internet-Draft May 2001 6.4.2 Key-Hash AVP The Key-Hash AVP (AVP Code 350) is of type OctetString, and contains a SHA-1 hash of the key. The hash MUST be calculated over the representation of the CA public key which would be present in an X.509 public key certificate, specifically, the input for the hash algorithm MUST be the DER encoding of a SubjectPublicKeyInfo representation of the key. Note: This includes the AlgorithmIdentifier as well as the BIT STRING. The rules given in [4] for encoding keys MUST be followed. Since this AVP is used for indexing and not for security (since Diameter nodes SHOULD validate certificates), there is no need to support more than one hash algorithm here. 6.5 OCSP-Nonce AVP The OCSP-Nonce AVP (AVP Code 358) is of type OctetString, and contains a random value (RECOMMENDED 128 bits) generated by the Diameter node. [Issue: Currently the nonce value also indicates the wish to receive OCSP responses. It might be better to make this a grouped AVP with a flag and optional nonce.] 6.6 AAA-Server-Certs AVP The AAA-Server-Certs AVP (AVP Code 351) is of type OctetString and contains the certificates of the AAA Servers in the home domain. Note: this AVP contains no CA certificates, just those for AAA servers. 6.7 OCSP-Responses AVP The OCSP-Responses AVP (AVP Code 359) is of type OctetString, and contains an OCSP response message from an OCSP responder. This AVP MUST only be included in an ESSA message for which the corresponding ESSR message contained an OCSP nonce. 6.8 CA-Chain AVP The CA-Chain AVP (AVP Code 353) is of type OctetString, and contains a certificate chain, from one of the nominated locally trusted CAs Calhoun, Bulley, Farrell expires October 2001 [Page 19] Internet-Draft May 2001 down to the (one and only) CA which has issued the end entity certificates in the AAA-Server-Certs AVP. To produce this AVP in an ESSA message, one (and only one) of the Local-CA-info values from the corresponding ESSR message is selected (call this the "top" CA for the purposes of this description). This AVP then contains a certificate path (in order) from the "top" CA down to the (one and only) CA which has issued all the end entity certificates in the AAA-Servers-AVP. The (typically self-signed), certificate of the "top" CA MAY be included, or MAY be omitted. [Issue: Whether the "top" CA cert should be included or not can be determined later.] 6.9 Expected-Signed-AVP AVP The Expected-Signed-AVP AVP (AVP Code 356) is of type Grouped. The Grouped Data field has the following ABNF grammar: Expected-Signed-AVP = AVP-Code Vendor-Id AVP-Code = ; See Section 6.11 Vendor-Id = ; See [1] A Diameter node adds the Expected-Signed-AVP AVP to inform the ESSR/A peer that any message received which contains the AVP specified in this AVP MUST be authenticated via the CMS-Signed-Data AVP. The Vendor-Id MAY contain a non-zero value if the AVP specified in the AVP-Code AVP is vendor-specific. +---------------------------------------------------------------+ | AVP Header (AVP Code = 356) | +---------------------------------------------------------------+ | Vendor-Id AVP | +---------------------------------------------------------------+ | AVP-Code AVP | +---------------------------------------------------------------+ 6.10 Expected-Encrypted-AVP AVP The Expected-Encrypted-AVP AVP (AVP Code 357) is of type Grouped. The Grouped Data field has the following ABNF grammar: Expected-Encrypted-AVP = AVP-Code Vendor-Id AVP-Code = ; See Section 6.11 Vendor-Id = ; See [1] Calhoun, Bulley, Farrell expires October 2001 [Page 20] Internet-Draft May 2001 A Diameter node adds the Expected-Encrypted-AVP AVP to inform the ESSR/A peer that any message received which contains the AVP specified in this AVP MUST be encrypted via the CMS-Encrypted-Data AVP. The Vendor-Id MAY contain a non-zero value if the AVP specified in the AVP-Code AVP is vendor-specific. +---------------------------------------------------------------+ | AVP Header (AVP Code = 357) | +---------------------------------------------------------------+ | Vendor-Id AVP | +---------------------------------------------------------------+ | AVP-Code AVP | +---------------------------------------------------------------+ 6.11 AVP-Code AVP The AVP-Code AVP (AVP Code 352) is of type Unsigned32, and contains the AVP Code of the AVP that is to be authenticated or encrypted. 7.0 Result-Code AVP Values This section defines new Result-Code [1] values that MUST be supported by all Diameter implementations that conform to this specification. 7.1 Transient Failures Errors that fall within the transient failures category are used to inform a peer that the request could not be satisfied at the time it was received, but MAY be able to satisfy the request in the future. DIAMETER_KEY_UNKNOWN 4007 This error code is returned when a CMS-Signed-Data or CMS- Encrypted-Data AVP is received that was generated using a key that is not locally recognized. This error could be caused if one of the endpoints of an end-to-end security association lost a previously agreed upon key, perhaps as a result of a reboot. 7.2 Permanent Failures Errors that fall within the permanent failures category are used to inform the peer that the request failed, and should not be attempted again. Calhoun, Bulley, Farrell expires October 2001 [Page 21] Internet-Draft May 2001 DIAMETER_INVALID_CMS_DATA 5018 This error code is returned when a CMS-Data AVP is received with an invalid ContentInfo object. 8.0 IANA Considerations This section contains the namespaces that have either been created in this specification, or the values assigned to existing namespaces managed by IANA. 8.1 Command Codes This specification assigns the value 304 from the Command Code namespace defined in [1], and extended in [13] and [14]. See section 4.0 for the assignment of the namespace in this specification. 8.2 AVP Codes This specification assigns the values 348-359 from the AVP Code namespace defined in [1], and extended in [13] and [14]. See section 6.0 for the assignment of the namespace in this specification. 8.3 Result-Code AVP Values This specification assigns the values 4007, 5018 from the Result-Code AVP (AVP Code 268) value namespace defined in [1], and extended in [14]. See section 7.0 for the assignment of the namespace in this specification. 8.4 Extension Identifier This specification assigns the value two (2) to the Extension Identifier namespace defined in [1]. See section 1.2 for more information. 9.0 Security Considerations This document describes how end-to-end security can be achieved in the Diameter protocol by allowing S/MIME Cryptographic Message Syntax [3] objects to be carried as a Diameter AVP. Section 6.3 states that a certificate received in a CMS-Cert AVP Calhoun, Bulley, Farrell expires October 2001 [Page 22] Internet-Draft May 2001 SHOULD NOT be used prior to cert verification. In most cases, the verification will be according to the rules specified in [4], however, some communities have indicated that they wish to be allowed to specify alternative certificate verification mechanisms, hence the "SHOULD NOT" rather than the more typical "MUST NOT". The authors do however strongly RECOMMEND that the verification procedures specified in [4] are always applied, regardless of whatever other verification mechanisms are in use. 10.0 References [1] P. Calhoun, A. Rubens, H. Akhtar, E. Guttman, "Diameter Base Protocol", draft-ietf-aaa-Diameter-04.txt, IETF work in pro- gress, May 2001. [2] Kaufman, Perlman, Speciner, "Network Security: Private Communi- cations in a Public World", Prentice Hall, March 1995, ISBN 0- 13-061466-1. [3] R. Housley, "Cryptographic Message Syntax", RFC 2630, June 1999. [4] Housley, Ford, Polk, Solo, "Internet X.509 Public Key Infras- tructure Certificate and CRL Profile", RFC 2459, January 1999. [5] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [6] M. Beadles, D. Mitton, "Criteria for Evaluating Network Access Server Protocols", draft-ietf-nasreq-criteria-05.txt, IETF work in progress, June 2000. [7] T. Hiller et al., "Cdma2000 Wireless Data Requirements for AAA", draft-hiller-cdma2000-AAA-02.txt, IETF work in progress, Sep- tember 2000. [8] S. Glass, S. Jacobs, C. Perkins, "Mobile IP Authentication, Authorization, and Accounting Requirements". RFC 2977. October 2000. [9] Myers, Ankney, Malpani, Galperin, Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol (OCSP)", RFC 2560, June 1999. [10] Aboba, Zorn, "Criteria for Evaluating Roaming Protocols", RFC 2477, January 1999. Calhoun, Bulley, Farrell expires October 2001 [Page 23] Internet-Draft May 2001 [11] B. Ramsdell, "S/MIME v2 Message Specification", RFC2633, June 1999. [12] S. Farrell, R. Housley, "An Internet Attribute Certificate Pro- file for Authorization", draft-ietf-pkix-ac509prof-06.txt, IETF work in progress, January 2001. [13] P. Calhoun, W. Bulley, G. Zorn, "Diameter NASREQ Extension", draft-ietf-aaa-Diameter-nasreq-04.txt, IETF work in progress, May 2001. [14] P. Calhoun, C. Perkins, "Diameter Mobile IP Extensions", draft- ietf-aaa-Diameter-mobileip-04.txt, IETF work in progress, May 2001. [15] Farrell, Turner, "Reuse of CMS Content Encryption Keys", draft- ietf-smime-rcek-02.txt, IETF work in progress, May 2001. [16] Boyen, Howes, Richard, "Internet X.509 Public Key Infrastructure Operational Protocols - LDAPv2", RFC 2559, April 1999. 11.0 Acknowledgements The authors would also like to acknowledge the following people for their contribution in the development of the Diameter protocol: Bernard Aboba, Jari Arkko, William Bulley, Daniel C. Fox, Lol Grant, Ignacio Goyret, Nancy Greene, Peter Heitman, Paul Krumviede, Fergal Ladley, Ryan Moats, Victor Muslin, Kenneth Peirce, Sumit Vakil, John R. Vollbrecht, Jeff Weisberg and Glen Zorn 12.0 Authors' Addresses Questions about this memo can be directed to: Pat R. Calhoun Network and Security Research Center, Sun Labs Sun Microsystems, Inc. 15 Network Circle Menlo Park, California, 94025 USA Phone: +1 650-786-7733 Fax: +1 650-786-6445 E-mail: pcalhoun@eng.sun.com Calhoun, Bulley, Farrell expires October 2001 [Page 24] Internet-Draft May 2001 Stephen Farrell Baltimore Technologies 39 Parkgate Street, Dublin 8, IRELAND Phone: +353-1-881-6000 Fax: +353-1-881-7000 E-Mail: stephen.farrell@baltimore.ie William Bulley Merit Network, Inc. Building One, Suite 2000 4251 Plymouth Road Ann Arbor, Michigan, 48105-2785 USA Phone: +1 734-764-9993 Fax: +1 734-647-5185 E-mail: web@merit.edu 13.0 Full Copyright Statement Copyright (C) The Internet Society (2001). 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 implementation may be prepared, copied, published and distributed, in whole or in part, without restric- tion 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 pro- cess 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 con- tained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRAN- TIES, 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, Farrell expires October 2001 [Page 25] Internet-Draft May 2001 14.0 Expiration Date This memo is filed as and expires in October 2001. Calhoun, Bulley, Farrell expires October 2001 [Page 26]