Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Network Working Group N. Cam-Winget Internet Draft D. McGrew Category: Informational J. Salowey Expires: July 11, 2007 H. Zhou Cisco Systems January 11, 2007 Dynamic Provisioning using EAP-FAST draft-cam-winget-eap-fast-provisioning-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. Copyright Notice Copyright (C) The Internet Society (2007). All Rights Reserved. Cam-Winget, et al. Expires July 11, 2007 [Page 1]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Abstract EAP-FAST is an flexible EAP method that enables secure communication between a client and a server by using the Transport Layer Security (TLS) to establish a mutually authenticated tunnel. EAP-FAST also enables the provisioning credentials or other information thru this protected tunnel. This document describes the use of EAP-FAST for dynamic provisioning. Table of Contents 1. Introduction...................................................3 1.1. Specification Requirements.............................3 1.2. Terminology............................................3 2. EAP-FAST Provisioning Modes....................................4 3. Dynamic Provisioning using EAP-FAST Conversation...............5 3.1 Network Access after EAP-FAST Provisioning.................7 3.2 Authenticating Using EAP-MSCHAPv2..........................8 3.3 Use of other Inner EAP Methods for EAP-FAST Provisioning...9 3.4 Key Derivations Used in the EAP-FAST Provisioning Exchange10 3.5 Peer-Id, Server-Id and Session-Id.........................11 3.6 Provisioning or Refreshment of a PAC......................11 4. Information Provisioned in EAP-FAST...........................12 4.1 Protected Access Credential...............................12 4.2 Provisioning PACs through PAC TLV.........................13 4.2.1 Formats for PAC TLV Attributes ......................14 4.2.2 PAC-Key .............................................15 4.2.3 PAC-Opaque ..........................................16 4.2.4 PAC-Info ............................................17 4.2.5 PAC-Acknowledgement TLV..............................19 4.2.6 PAC-Type TLV.........................................20 4.3 Server Trusted Root Certificate...........................20 4.3.1 Server-Trusted-Root TLV .............................21 4.3.2 PKCS #7 TLV .........................................22 5. Security Considerations.......................................24 5.1 User Identity Protection and Validation...................24 5.2 Mitigation of Dictionary Attacks..........................24 5.3 Mitigation of Man-in-the-middle (MitM) attacks in server- unauthenticated provisioning mode.............................26 5.4 Mitigation of Man-in-the-middle (MitM) attacks in server- authenticated provisioning mode...............................27 Cam-Winget, et al. Expires July 11, 2007 [Page 2]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 5.5 Generation of Diffie-Hellman Groups.......................27 5.6 PAC Storage Considerations................................28 5.7 Security Claims...........................................29 6. IANA Considerations...........................................31 7. References....................................................31 7.1 Normative.................................................31 7.2 Informative...............................................32 8. Acknowledgments...............................................32 9. Author's Addresses............................................32 10. Appendix: Examples...........................................33 10.1 Example 1: Successful Tunnel PAC Provisioning............33 10.2 Example 2: Failed Provisioning...........................35 10.3 Example 3: Provisioning a Authentication Server's Trusted Root Certificate..............................................37 11. Intellectual Property Statement..............................39 12. Disclaimer of Validity.......................................39 13. Copyright Statement..........................................39 14. Expiration Date..............................................40 1. Introduction [EAP-FAST] is an EAP method that can be used to mutually authenticate peer and server. However, to mutually authenticate with EAP-FAST, credentials such as a preshared key, trusted anchor or a Protected Access Credential (PAC) MUST be provisioned to the peer before it can establish a secure communication channel with the server. In some cases, the provisioning of such information present deployment hurdles. Through the use of the protected tunnel, EAP-FAST can also be used to enable the means for dynamic in-band provisioning to address such deployment obstacles. 1.1. Specification Requirements 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]. 1.2. Terminology Much of the terminology in this document comes from [RFC3748]. Additional terms are defined below: Cam-Winget, et al. Expires July 11, 2007 [Page 3]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Man in the Middle (MitM) An adversary that can successfully inject itself between a peer and EAP server. The MitM succeeds by impersonating itself as a valid peer, authenticator or authentication server. Provisioning Providing peer with a trust anchor, shared secret or other appropriate information based on which a security association can be established. Protected Access Credential (PAC) Credentials distributed to a peer for future optimized network authentication. The PAC consists of at most three components: a shared secret, an opaque element and optionally other information. The shared secret part contains the pre-shared key between the peer and authentication server. The opaque part is provided to the peer and is presented to the authentication server when the peer wishes to obtain access to network resources. Finally, a PAC may optionally include other information that may be useful to the peer. Tunnel PAC A set of credentials stored by the peer and consumed by both the peer and the server to establish a TLS tunnel. 2. EAP-FAST Provisioning Modes EAP-FAST supports two modes for provisioning: 1) Server-Authenticated Mode: Provisioning inside a server- authenticated TLS tunnel. 2) Server-Unauthenticated Mode: Provisioning inside a server- unauthenticated TLS tunnel. In the Server-Authenticated Provisioning mode, the peer has successfully authenticated the EAP server as part of EAP-FAST Phase 1 (i.e. TLS tunnel establishment). Additional exchanges MAY be needed inside the tunnel for the EAP Server to authenticate the peer before any information can be provisioned. Cam-Winget, et al. Expires July 11, 2007 [Page 4]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 In the Server-Unauthenticated Provisioning mode, an unauthenticated TLS tunnel is established in the EAP-FAST Phase 1 where the server is not authenticated. This provisioning mode is defined to enable bootstrapping or initial configuration of peers where the peer lacks strong credentials (if any) to mutually authenticate with the server and configuration of such credentials through out-of-band mechanisms are prohibitive. In the Server-Unauthenticated Provisioning mode, the peer and server do not achieve mutual authentication during EAP-FAST Phase 1. It is expected that the peer negotiates TLS_DH_anon based cipher suites to signal that it can not provide proof of authenticity. While other cipher suites such as those requiring the use of server certificates may be used, the peer may lack the necessary trust anchors to validate the certificate and authenticate the server. Since the server is not authenticated in the Server-Unauthenticated Provisioning mode, it is possible that the TLS tunnel may be terminated by an attacker. It is strongly recommended that an inner EAP method be used to provide some authenticity assurances and MitM detection and warning outlined in Section 5 MUST be applied. The EAP-FAST Phase 2 conversation is unchanged in either Provisioning mode. However, if the server is not authenticated in Phase 1 the peer MUST negotiate an EAP method supporting mutual authentication and key derivation that is compatible with its initial or bootstrapping credentials (such as a password-based EAP method). The peer then uses the Crypto-Binding TLV to validate that the same server terminates both the TLS tunnel and the inner EAP method, thereby verifying that the exchange was not subject to a man-in-the- middle attack. Assuming that the Crypto-Binding TLV exchange is successful, the server will subsequently provide the information such as a shared key or the trusted root(s) of server certificate using a PAC TLV or a Server-Trusted-Root TLV respectively. Once the EAP-FAST Provisioning conversation completes, the peer is expected to use the provisioned credentials in subsequent EAP-FAST authentications. 3. Dynamic Provisioning using EAP-FAST Conversation Cam-Winget, et al. Expires July 11, 2007 [Page 5]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 The provisioning EAP-FAST exchange uses same sequence as the EAP-FAST Authentication Phase 1 to establish a protected TLS tunnel. Once a tunnel is secured between the two parties, the client and server can then negotiate and execute an EAP authentication method by which both parties can achieve mutual authentication. Provisioning in EAP-FAST is negotiated by the client in the first communication exchange when EAP-FAST is requested from the server. If the client does not have a Protected Access Credential (PAC) or requires provisioning of other information (such as the server's Trusted Root certificate), it can request to initiate a provisioning EAP-FAST exchange and dynamically obtain a PAC or other information from the server. The EAP-FAST provisioning conversation will typically occur between the peer and an authentication server; more specifically, the server that can provision the peer with the requested information. The conversation between a peer and authentication server commences as a normal EAP-FAST exchange as defined in [EAP-FAST]. On receipt of the EAP-FAST Start message, the peer determines it must be provisioned with a new PAC or server's trusted root certificate. This version of the EAP-FAST provisioning mode implementation MUST support the following TLS ciphersuites: TLS_RSA_WITH_RC4_128_SHA TLS_RSA_WITH_AES_128_CBC_SHA TLS_DH_anon_WITH_AES_128_CBC_SHA [RFC 3268] TLS_DHE_RSA_WITH_AES_128_CBC_SHA [RFC 3268] Other TLS ciphersuites MAY be supported. To provide best security practices, it is highly recommended that the peer obtain the server's public key or trust anchor to enable server-side authentication. However, as the provisioning of the public key or trust anchor must also be secured to ensure the public key is to be trusted, some deployments may be willing to trade off the security risks for ease of deployment and thus use TLS_DH_anon_WITH_AES_128_CBC_SHA that is allowed for EAP-FAST provisioning mode only. With a successful EAP-FAST Phase 1 tunnel established, subsequent messages exchanged between peer and authentication server are protected using the negotiated TLS cipher suites as defined by both Cam-Winget, et al. Expires July 11, 2007 [Page 6]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 [RFC 2246], [RFC4346] and [RFC 3268] to provide message confidentiality and integrity respectively. With a protected tunnel, the peer must authenticate itself to the server before the server can provision it with information such as a PAC. Following a successful authentication exchange and successful Intermediate Result TLV and Crypto-Binding TLV exchange, the server can then provision the peer with a unique PAC. The provisioning is invoked through the a PAC-TLV exchange that is executed following the successful authentication exchange including the Intermediate Result TLV and Crypto-Binding TLV exchange, with the server distributing the information such as a PAC in a corresponding PAC TLV to the peer and the peer confirming its receipt in a final PAC TLV Acknowledgement message. 3.1 Network Access after EAP-FAST Provisioning Depending on server policy, network access can be granted or denied based on the EAP-FAST Provisioning mode, the credential(s) or other information that have been provisioned, and the inner EAP methods used. For example, in the Server-Authenticated Provisioning Mode, access can be granted after the EAP server has authenticated the peer and provisioned the peer with a Tunnel PAC (e.g. a PAC used to mutually authenticate and establish the EAP-FAST tunnel). Additionally, peer policy may also be used to disconnect the current provisioning connection and initiate a new EAP-FAST exchange for authentication utilizing the newly provisioned information and ensure the inner methods are conducted with the trusted server. The peer policy may be required as the peer determines whether it can authenticate the EAP Server. In the case where a peer lacks the trust anchors to validate the server's certificate, the peer SHOULD negotiate using one of the TLS_DH_anon based cipher suites to signal the EAP server that it lacks the trust anchors to authenticate the server. At the end of the Server-Unauthenticated Provisioning Mode, network access SHOULD NOT be granted. EAP server SHOULD conclude with an EAP Failure to acknowledge that this conversation was intended for provisioning only and thus no network access is authorized. Upon Cam-Winget, et al. Expires July 11, 2007 [Page 7]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 completion of the exchange, the EAP Server SHALL NOT grant network access or distribute any session keys to the NAS as this phase is not intended to provide network access. Even though provisioning mode completes with a successful inner termination (e.g. successful Result TLV), server policy defines whether the peer gains network access or not. Thus, it is feasible for the server, while providing a successful Result TLV may conclude with an EAP Failure. The EAP-FAST server, when denying network access after EAP-FAST Provisioning, may choose to instead, immediately invoke another EAP- FAST Start and thus initiate the EAP-FAST Phase 1 conversation. This server based implementation policy may be chosen to avoid applications such as wireless devices from being disrupted (e.g. in 802.11 devices, an EAP Failure may trigger a full 802.11 disassociation) and allow them to smoothly transition to the subsequent EAP-FAST authentications to enable network access. As an alternative, both the peer and server can initiate TLS renegotiation, where the newly provisioned credentials can be used to establish a server authenticated or mutually authenticated TLS tunnel for authentication. Upon completion of the TLS negotiation and subsequent authentication, normal network access policy on EAP-FAST authentication can be applied. Similarly, if Server-Authenticated Provisioning Mode is used and the server policy is to disallow network access, the EAP Server SHALL NOT grant network access or distribute any session keys to the NAS as this phase is not intended to provide network access. Even though provisioning mode completes with a successful inner termination (e.g. successful Result TLV), the EAP-FAST Server-Authenticated Provisioning Mode MUST conclude with an EAP Failure to acknowledge that this conversation was intended for provisioning only and thus no network access is authorized. The EAP-FAST server may choose to instead, immediately invoke another EAP authentication transaction. 3.2 Authenticating Using EAP-MSCHAPv2 This version of the EAP-FAST provisioning mode implementation MUST support EAP-MSCHAPv2 as the inner authentication method for enabling Server-Unauthenticated Provisioning Mode. While other authentication methods are allowed and exist to achieve mutual authentication, when using an anonymous or unauthenticated TLS tunnel, MSCHAPv2 was chosen for several reasons: Cam-Winget, et al. Expires July 11, 2007 [Page 8]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 * Provide the ability of slowing an active attack by obscuring the password through some hash * Especially in the Server-Unauthenticated EAP-FAST Provisioning conversation, MSCHAPv2 provides the ability to detect, based on the challenge responses, whether there is a possible attack. * It is understood that a large deployed base is already able to support MSCHAPv2 * Allow support for password change during the EAP-FAST Provisioning mode. The MSCHAPv2 exchange forces the server to provide a valid ServerChallengeResponse which must be a function of the server challenge, client challenge and password as part of its response. This reduces the window of vulnerability in the EAP-FAST for in-band provisioning mode to force the man-in-the-middle, acting as the server, to successfully break the password within the client's challenge response time limit. EAP-FAST for provisioning MUST support EAP-MSCHAPv2 as the inner method when using an anonymous DH key agreement. However, with support of signed DH key agreement, the provisioning protocol of EAP- FAST may support other methods such as EAP-GTC to enable peers (using other password databases such as LDAP and OTP) to be provisioned in- band as well. However, the replacement may only be achieved when used with cipher suites that enable server side authentication (for example, using TLS_DHE_RSA_WITH_AES_128_CBC_SHA) to ensure no loss in security. When using an anonymous DH key agreement and MSCHAPv2, a binding between the tunnel and the MSCHAPv2 exchanges is formed by using keying material generated during the EAP-FAST tunnel establishment as the MSCHAPv2 challenges. A detailed description of the challenge generation is described in Section 3.4. 3.3 Use of other Inner EAP Methods for EAP-FAST Provisioning Once a protected tunnel is established, the peer must authenticate itself to the server before the server can provision the peer. When using TLS_DH_anon_WITH_AES_128_CBC_SHA cipher suite in the EAP-FAST Cam-Winget, et al. Expires July 11, 2007 [Page 9]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Phase 1 conversation, an EAP method providing both mutual authentication and keying material MUST be employed. With the use of additional TLS cipher suites, especially when server authenticity is verified as part of the TLS tunnel establishment, other inner EAP methods with weaker protection than EAP-MSCHAPv2 can be used safely inside tunnel. Hence, in addition to EAP-MSCHAPV2 as the inner method, EAP-GTC MAY be used in Server-Authenticated Provisioning Mode. This will enable peers using other user databases such as LDAP and OTP to be provisioned in-band as well. However, the replacement may only be achieved when used with the TLS cipher suites that ensure server authentication, such as TLS_DHE_RSA_WITH_AES_128_CBC_SHA, to ensure no loss in security. This version of the EAP-FAST provisioning mode implementation MUST support both EAP-GTC and EAP-MS-CHAPv2 within the tunnel in Server- Authenticated Provisioning Mode. It should be noted that Server-Authenticated Provisioning Mode provides significant security advantages over Server-Unauthenticated Provisioning even when EAP-MSCHAPv2 is being used as inner method. It protects the EAP-MSCHAPv2 exchanges from potential MitM attacks by verifying server's authenticity before exchanging MSCHAPv2. Thus Server-Authenticated Provisioning Mode is the preferred provisioning mode. The EAP-FAST peer MUST use the Server-Authenticated Provisioning Mode whenever a certificate or (server's) public key is available to authenticate the server, in order to ensure best security practices. 3.4 Key Derivations Used in the EAP-FAST Provisioning Exchange When generating keys in the EAP-FAST Provisioning conversation, the DH computation is used as the pre_master_secret and is converted into the master_secret as specified by [RFC 2246]. The TLS tunnel key is calculated similar to the TLS key calculation with an extra 72 octets generated. Portions of the extra 72 octets are used for the EAP-FAST provisioning exchange session key seed and as the random challenges in the MSCHAPv2 exchange. Cam-Winget, et al. Expires July 11, 2007 [Page 10]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 To generate the key material, compute key_block = PRF(master_secret, "key expansion", server_random + client_random); until enough output has been generated. Then the key_block is partitioned as follows: client_write_MAC_secret[hash_size] server_write_MAC_secret[hash_size] client_write_key[Key_material_length] server_write_key[key_material_length] client_write_IV[IV_size] server_write_IV[IV_size] session_key_seed[seed_size= 40] MSCHAPv2 ServerChallenge[16] MSCHAPv2 ClientChallenge[16] The extra key material, session_key_seed is used for the Crypto- Binding while the ServerChallenge and ClientChallenge correspond to the authentication server's MSCHAPv2 challenge and the peer's MSCHAPv2 challenge respectively. The ServerChallenge and ClientChallenge are only used for the MSCHAPv2 exchange when DH anonymous key agreement is used in the EAP-FAST tunnel establishment. 3.5 Peer-Id, Server-Id and Session-Id The provisioning modes of EAP-FAST does not change the general EAP- FAST protocol and thus how the Peer-Id, Server-Id and Session-Id are determined is based on the [EAP-FAST] techniques. [EAP-FAST] Section 3.4 describes how the Peer-Id and Server-Id are determined; Section 3.5 describes how the Session-Id is generated. 3.6 Provisioning or Refreshment of a PAC The server may provision or refresh information by use of the Protected Access Credential (PAC) after a successful user Cam-Winget, et al. Expires July 11, 2007 [Page 11]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 authentication. A PAC TLV is defined to facilitate the distribution and refreshing of information and is defined in Section 4.2. A fresh PAC may be distributed after a successful Intermediate Result TLV and Crypto-Binding TLV exchange, if the server detects that the PAC is expiring soon. A successful EAP-FAST inner method authentication, including a successful Crypto-Binding exchange must ensue before an EAP-FAST server can distribute a fresh PAC. A PAC TLV should not be accepted if it is not TLS tunnel-encapsulated. N.B. In-band PAC refreshing is enforced by server policy. The server, based on the PAC-Opaque information, may determine not to refresh a peer's PAC through the PAC TLV mechanism even if the PAC- Key has expired. 4. Information Provisioned in EAP-FAST In addition to the Tunnel PAC (the one used to establish the EAP-FAST Phase 1 TLS tunnel), other types of credentials and information can also be provisioned in EAP-FAST. They may include trusted root certificates for the server certificates, application specific PACs, and user identities to name a few. Typically, provisioning is invoked after both peer and server validate their authenticities and after a successful Crypto-Binding TLV exchange. However, depending on the information being provisioned, mutual authentication MAY not be needed. At minimum, at least one entity (peer or server) must prove authenticity before credentials are provisioned to ensure that information is not freely provisioned to or by adversaries. For example, the EAP server MAY not need to authenticate the peer to provision the peer with trusted root certificates. However, the peer MUST authenticate the server before it can accept a trusted server root certificate. 4.1 Protected Access Credential A Protected Access Credential (PAC) is a security credential provided by the Authentication Server (AS) that holds application specific information. The server distributes all PAC information through the use of a PAC TLV. Each type of PAC information is typed through a PAC Type and PAC TLV Attribute defined in this section. Cam-Winget, et al. Expires July 11, 2007 [Page 12]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 For instance, a Tunnel PAC holds a shared secret mutually and uniquely shared between the peer and AS and is used to secure an EAP- FAST (TLS) tunnel. EAP-FAST uses the Tunnel PAC to facilitate the storage of secure information between a peer and a server on the peer and minimize the per user state management on the AS. The Tunnel PAC is distributed by the server to the peer and used to establish a secure EAP-FAST TLS tunnel and convey the server policy of what must and can occur in the tunnel. The server policy can include EAP methods, TLV exchanges and identities allowed in the tunnel. It is up to the server policy to include what's necessary in a PAC to enforce the policy in subsequent authentications that use the PAC. For example, user identity, I-ID, can be included as the part of the server policy. This I-ID information limits the inner EAP methods to be carried only on the specified user identity. Other types of information can also be included, such as which EAP method(s) and which cipher suite is allowed. If the server policy is not included in a PAC, then there is no validation or limitation on the inner EAP methods or user identities inside the tunnel established by the use of that PAC. To request provisioning of a Tunnel PAC, a peer MUST send a PAC TLV with a PAC-Type PAC TLV with its TLVs field and '1' (Tunnel PAC Type). The request may be issued after the peer has determined that it has successfully authenticated the EAP Server and the tunnel and inner EAP methods were between the same peer and EAP Server by validating the Crypto-Binding TLV. This would differentiate the Tunnel PAC request from other types of PAC provisioning requests. If anonymous DH is negotiated and the peer does not send any PAC-TLV to request provisioning, then Tunnel PAC is provisioned automatically by the server. PAC-Acknowledge TLV MUST be used for peer to acknowledge the receipt of the Tunnel PAC. Please see Section 10.1 for an example of packet exchanges to provision a Tunnel PAC. 4.2 Provisioning PACs through PAC TLV The PAC TLV is defined to enable the provisioning of PAC information. Cam-Winget, et al. Expires July 11, 2007 [Page 13]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Additionally, the PAC-Type in PAC TLV MAY be used by the peer to request provisioning for specific types of information. Conversely, the PAC TLV is used by the server to provision the requested information to a peer. The PAC TLV provides support for Protected Access Credential (PAC) defined within [EAP-FAST]. A consistent PAC format will allow it to be used by multiple applications beyond EAP-FAST. A general PAC TLV format is defined as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|R| TLV Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PAC Attributes... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ M 0 - Non-mandatory TLV 1 - Mandatory TLV R Reserved, set to zero (0) TLV Type 11 Length The length of the PAC Attributes field in octets. PAC Attributes A list of PAC attributes in the TLV format. 4.2.1 Formats for PAC TLV Attributes A common encapsulating format is used to convey the different fields that comprise a PAC attribute. The common encapsulation is defined as follows: Cam-Winget, et al. Expires July 11, 2007 [Page 14]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type The type field is two octets, denoting the attribute type. Allocated Types include: 1 - PAC-Key 2 - PAC-Opaque 3 - PAC-Lifetime 4 - A-ID 5 - I-ID 6 - Reserved 7 - A-ID-Info 8 - PAC-Acknowledgement 9 - PAC-Info 10 - PAC-Type Length The Length filed is two octets, which contains the length of the Value field in octets. Value The value of the PAC Attribute. 4.2.2 PAC-Key The PAC-Key is distributed as an attribute of type PAC-Key (Type=1). The key is a randomly generated octet string. The key is represented as an octet string whose length is determined by the length field. The generator of this key is the issuer of the credential, identified by the A-ID. 0 1 2 3 Cam-Winget, et al. Expires July 11, 2007 [Page 15]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Key ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 1 - PAC-Key Length The Length filed is two octets. For this version of EAP-FAST, PAC-Key is 32 octets. Key The Key field contains the key used to establish the TLS tunnel. 4.2.3 PAC-Opaque The PAC-Opaque contains data that is opaque to the recipient, the peer is not the intended consumer of PAC-Opaque and thus should not attempt to interpret it. A peer that has been issued a PAC-Opaque by a server MUST store that data, and present it back to the server as is, in the ClientHello SessionTicket extension field [RFC4507]. If a client has opaque data issued to it by multiple servers, then it MUST store the data issued by each server separately according to A-ID. This requirement allows the client to maintain and use each opaque data as an independent PAC pairing, with a PAC-Key mapping to a PAC- Opaque identified by the A-ID. As there is a one to one correspondence between PAC-Key and PAC-Opaque, the peer must determine the PAC-Key and corresponding PAC-Opaque based on the A-ID provided in the EAP-FAST/Start message and the A-ID provided in the PAC-Info when it was provisioned with a PAC-Opaque. As the PAC-Opaque is server specific, its contents and definition are specific to the issuer of the PAC, e.g. the PAC server. The PAC-Opaque field is embedded as part of the PAC TLV when the Cam-Winget, et al. Expires July 11, 2007 [Page 16]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 server has determined that the PAC must be provisioned or refreshed. The PAC-Opaque field format is summarized as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 2 - PAC-Opaque Length The Length filed is two octets, which contains the length of the value field in octets. Value The Value field contains the actual data for PAC-Opaque. The PAC-Opaque field is also passed from the peer to the server during the EAP-FAST Authentication Phase 1 conversation to enable the server to validate and recreate the PAC-Key. When it is passed from the peer, it is encapsulated as defined above in the ClientHello SessionTicket Extension [RFC4507]. 4.2.4 PAC-Info PAC-Info is comprised of a set of PAC attributes. At minimum, the A-ID, A-ID-Info, and PAC-Type attribute is required to convey the issuing identity to the peer. Other optional fields may be included in the PAC to provide more information to the peer. It is a container attribute for various types of information each of which is encoded in conformance to the PAC TLV attribute field format as defined in Section 4.2. 0 1 2 3 Cam-Winget, et al. Expires July 11, 2007 [Page 17]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attributes... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 9 - PAC-Info Length The Length filed is two octets, which contains the length of the Attributes field in octets. Attributes The Attributes field contains a list of PAC Attributes. Each mandatory and optional field type is defined as follows: PAC-LIFETIME (type 3) This is a 4 octet quantity representing the expiration time of the credential in UNIX UTC time. This is a mandatory field contained in the PAC-Opaque field to enable the server to validate the PAC. This field may also be optionally provided to the peer as part of PAC-Info. A- ID (type 4) Authority identifier is the name of the authority that issued the PAC. The A-ID is intended to be unique across all issuing servers to avoid namespace collisions. Server implementations should use measures to ensure the A-ID used is globally unique to avoid name collisions. The A-ID is used by the peer to determine which PAC to employ. Similarly, the server uses the A-ID to both authenticate the PAC-Opaque and determine which master key was used to issue the PAC. This field is mandatory and included in both the PAC-Opaque and PAC-Info. I-ID (type 5) Initiator identifier (I-ID) is the peer identity associated with the credential. The server employs the I-ID in the EAP- FAST Phase 2 conversation to validate that the same peer identity used to execute EAP-FAST Phase 1 is also used in at Cam-Winget, et al. Expires July 11, 2007 [Page 18]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 minimum one inner EAP method in EAP-FAST Phase 2. This field is a mandatory field in PAC-Opaque and may optionally be included in the PAC-Info. If the AS is enforcing the I-ID validation on inner EAP method, then I-ID is mandatory in PAC-Info, to enable the client to also enforce a unique PAC for each unique user. If I-ID is missing from the PAC-Info, it is assumed that the Tunnel PAC can be used for multiple users and client will not enforce the unique Tunnel PAC per user policy. A-ID-Info (type 7) Authority Identifier Information is a mandatory TLV intended to provide a user-friendly name for the A-ID. It may contain the enterprise name and server name in a more human-readable format. This TLV serves as an aid to the peer to better inform the end-user about the A-ID. This field is a mandatory field in the PAC-Info. PAC-Type (type 10) PAC-Type is a mandatory TLV intended to provide the type of PAC. This field is a mandatory field in the PAC-Info. For legacy implementations, if PAC-Type is not present, then it defaults to a Tunnel PAC (Type 1). 4.2.5 PAC-Acknowledgement TLV The PAC-Acknowledgement TLV is used to acknowledge the receipt of the Tunnel PAC by the peer. Peer sends this TLV in response to the PAC TLV to indicate the result of the retrieving and storing of the new Tunnel PAC. This TLV is only used when Tunnel PAC is provisioned. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Result | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 8 - PAC-Acknowledgement Cam-Winget, et al. Expires July 11, 2007 [Page 19]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Length The length of this field is two octets and value must be 2. Result The resulting value must be one of the following: 1 - Success 2 - Failure 4.2.6 PAC-Type TLV The PAC-Type TLV is a TLV intended to specify the PAC type. Its format is described below. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PAC Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 10 - PAC-Type Length The length of this field is two octets and value must be 2. PAC Type This two octet field defined the type of PAC being requested or provisioned. Its value must be one of the following: 1 - Tunnel PAC 4.3 Server Trusted Root Certificate It is desirable to provision the peer with the server's trusted root certificates (or CA certificates), which can later be used for enabling PKI based cipher suites. Server-Trusted-Root TLV is introduced to facilitate the request for and delivery of server Cam-Winget, et al. Expires July 11, 2007 [Page 20]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 trusted root certificates. Within the EAP-FAST Phase 2 conversation, a peer MAY request for a server's trusted root certificate using a Server-Trusted-Root TLV, and the EAP server MAY respond with a Server-Trusted-Root TLV containing the trusted root certificate in the PCKS#7 TLV to the peer. The Server-Trusted-Root TLV can be exchanged in regular EAP-FAST Authentication mode or Provisioning mode. After the peer has determined that it has successfully authenticated the EAP server and determined that the tunnel and inner EAP methods were between the same peer and EAP Server by validating the Crypto- Binding TLV, it MAY send one or more Server-Trusted-Root TLVs (marked as optional) to request for the certificate trust anchors of the server certificate from the EAP server. The EAP server will send the trusted root(s) of server certificate after its internal policy has been satisfied; or it may ignore the request or request additional authentications based on its policy. The peer may receive a trusted root of server certificate, but is not required to use it. Please see Section 10.3 for an example of a server provisioning a server trusted root certificate. 4.3.1 Server-Trusted-Root TLV The Server-Trusted-Root TLV allows the peer to send a request to the EAP server for a trusted root in PKCS#7 [RFC2315]format. The Server-Trusted-Root TLV is always marked as optional, and cannot be responded to with a NAK TLV. The Server-Trusted-Root TLV can only be sent as an inner TLV (inside the protection of the tunnel). The peer MUST NOT request, or accept the trusted root sent inside the Server-Trusted-Root-TLV by the EAP server until it has completed authentication of the EAP server, and validated the Crypto-Binding TLV. The peer may receive a trusted root, but is not required to use the trusted root sent from the EAP server. If the EAP server sets credential-format to PKCS#7-Server- Certificate-Root, then the Server-Trusted-Root TLV MUST contain the root of the certificate chain of the certificate issued to the EAP server packaged in a PKCS#7 TLV. If the Server certificate is a Cam-Winget, et al. Expires July 11, 2007 [Page 21]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 self-signed certificate, then the root is the self-signed certificate. If the Server-Trusted-Root TLV credential format does not contain one of the known values, then the EAP-server MUST ignore the TLV. The Server-Trusted-Root TLV is defined as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|R| TLV Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Credential-Format | TLVs... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- M 0 - Optional TLV R Reserved, set to zero (0) TLV Type 18 Length >=2 Credential-Format The Credential-Format field is two octets. Values include: 1 - PKCS#7-Server-Certificate-Root. TLVs This field is of indefinite length. It contains TLVs associated with the certificate request and response. 4.3.2 PKCS #7 TLV Cam-Winget, et al. Expires July 11, 2007 [Page 22]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 The PKCS#7 TLV is sent by the EAP server to the peer inside the Server-Trusted-Root TLV. It contains the PKCS #7 [RFC2315] wrapped X.509 certificate. This field contains a certificate or certificate chain in PKCS#7 format requested by the peer as defined in [RFC2315]. The PKCS#7 TLV is always marked as optional, which cannot be responded to with a NAK TLV. EAP-FAST server implementations that claim to support dynamic provisioning MUST support this TLV. EAP- FAST peer implementations MAY not support this TLV. If the PKCS#7 TLV contains a certificate or certificate chain that is not acceptable to the peer, then peer MUST ignore the TLV. The PKCS#7 TLV is defined as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|R| TLV Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PKCS #7 Data... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- M 0 - Optional TLV R Reserved, set to zero (0) TLV Type 20 (for PKCS #7 TLV) Length The length of the PKCS #7 Data field PKCS #7 Data This field contains the PKCS #7 wrapped X.509 certificate or certificate chain in the PKCS #7 format. Cam-Winget, et al. Expires July 11, 2007 [Page 23]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 5. Security Considerations The Dynamic Provisioning EAP-FAST protocol shares the same security considerations outlined in [EAP-FAST]. Additionally, it also has its unique security considerations described below: 5.1 User Identity Protection and Validation EAP-FAST for provisioning employs the TLS key agreement (as defined in the TLS protocol) to establish a protected tunnel; the initial EAP-Identity request/response may be omitted as it must be transmitted in the clear and thus subject to snooping and forgery. Alternately, an anonymous identity may be used in the EAP-Identity response to prevent disclosure of the peer's true identity. As the provisioning EAP-FAST exchange is used for provisioning a PAC to a specific identity, e.g. I-ID, it is expected that the server will assign the I-ID based on the identity provided in the protected inner EAP authentication method. Thus, the protected identity may not be identical to the cleartext identity presented in the initial EAP identity exchange messages. In order to shield the user identity from snooping, the cleartext Identity may only provide enough information to enable routing of the authentication request to the correct realm. For example, the peer may initially claim the identity of "anonymous@example.com" in order to route the authentication request to the example.com EAP server. Subsequently, once the EAP- FAST session has been negotiated, in the inner authentication method, the peer may claim the identity of "user1@example.com". Thus, the EAP-FAST protocol for provisioning can provide protection for the user's identity, though not necessarily the destination realm, unless the provisioning EAP-FAST conversation terminates at the local authentication server. 5.2 Mitigation of Dictionary Attacks When EAP-FAST is invoked for provisioning, the peer specifies the means for securing the communications for the provisioning. As such, it can invoke the TLS key agreement in one of two ways: anonymously or server-authenticated. With a server-authenticated TLS key agreement, the server must provide its certificate and be validated Cam-Winget, et al. Expires July 11, 2007 [Page 24]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 by the peer, whereas in an anonymous TLS key agreement, there is no ability to authenticate the server. In a server authenticated TLS key agreement, the protected communications is assured that the AS is authentic as the peer must have been pre-provisioned with the AS's certificate or public (RSA) key prior to the negotiation. In this instance, the AS provides proof of identity through an identity and (certificate) credential, preventing an adversary from posing as an AS to mount a dictionary attack. An EAP-FAST compliant implementation must assure that provisioning of the AS public key, certificate or root certificate to the peer must be achieved through a secure mechanism. Only through a secure mechanism can server-authenticated DH key agreement provide resistance to dictionary attacks. While this option affords best security practices, it presents deployment issues as, especially for wireless clients where there is little means to provide secure configuration, peers must be configured with a means to validate the server's credential (e.g. public key). In an anonymous DH key agreement, an adversary may attempt to impersonate a client and enable EAP-FAST for provisioning. However, it must successfully authenticate inside the DH tunnel to succeed and gain a PAC credential from a server. Thus, peer impersonation is mitigated through the enabling of peer authentication inside a protected tunnel. However, an adversary may impersonate as a valid AS and obtains the MSCHAPv2 exchanges in order to gain peer's identity and credentials. While the adversary must successfully gain contact with a peer that is willing to negotiate EAP-FAST for provisioning and provide a valid A-ID that a client accepts, this occurrence is feasible and enables an adversary to mount a dictionary attack. For this reason, an EAP-FAST compliant implementation must support an MSCHAPv2 or stronger EAP method for peer authentication when an anonymous DH key agreement is used for the tunnel establishment. With MSCHAPv2, a peer may detect it is under attack when the AS that has provided an acceptable Authority ID (A-ID) fails to provide a successful MSCHAPv2 server challenge response. By employing the ServerChallenge and ClientChallenge derived during tunnel establishment; detection of a MitM is feasible during the MSCHAPv2 exchange. The peer MAY choose to use a more secure out-of-band mechanism for PAC provisioning that affords better security than the anonymous DH Cam-Winget, et al. Expires July 11, 2007 [Page 25]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 key agreement. Similarly, the peer MAY find a means of pre- provisioning the server's public key securely to invoke the server- authenticated DH key agreement. The anonymous DH key agreement is presented as a viable option as there may be deployments that can physically confine devices during the provisioning or are willing to accept the risk of an active dictionary attack. Further, it is the only option that enables zero out-of-band provisioning and facilitates simpler deployments requiring little to no peer configuration. 5.3 Mitigation of Man-in-the-middle (MitM) attacks in server- unauthenticated provisioning mode EAP-FAST invocation of provisioning addresses MitM attacks in server- unauthenticated provisioning mode in the following way: * Generating MSCHAPv2 server and client challenges as a function of the DH key agreement: in enforcing the dependence of the MSCHAP challenges on the DH exchange, a MitM is prevented from successfully establishing a secure tunnel with both the peer and legitimate server and succeed in obtaining the PAC credential. * Cryptographic binding of EAP-FAST Phase 1 and the Phase 2 authentication method: by cryptographically binding key material generated in all phases, both peer and AS are assured that they were the sole participants of all transpired phases. The binding of the MSCHAPv2 random challenge derivations to the DH key agreement protocol enables early detection of a MitM attack. This is required to guard from adversaries who may otherwise reflect the inner EAP authentication messages between the true peer and AS and enforces that the adversary successfully respond with a valid challenge response. The cryptographic binding is another reassurance that indeed the true peer and AS were the two parties ensuing both the tunnel establishment and inner EAP authentication conversations. While it would be sufficient to only support the cryptographic binding to mitigate the MitM; the extra precaution of binding the MSCHAP challenge to the DH key agreement affords the client earlier detection of a MitM and further guards the peer from having to respond to the success or failure of the adversary's attempt to Cam-Winget, et al. Expires July 11, 2007 [Page 26]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 respond with a challenge response (e.g. indication of whether the adversary succeeded in breaking the peer's identity and password). A failure in either step, results in no PAC provisioning. EAP-FAST invocation of provisioning using an unauthenticated tunnel can invoke certain procedures to guard implementations for potential MitM attacks. Detectors can be devised to warn the user when the peer encounters error conditions that warrant the likelihood of a MitM. For example, when the MSCHAPv2 server challenge response is never received or fails, the peer implementation can impose policy decisions to warn the user and respond to the likelihood that the failure was due to a MitM attack. Similarly, to guard against attacks in the EAP-FAST Authentication that may force a peer to invoke in-band provisioning, guards and detectors can and should be implemented as part of the EAP-FAST Authentication protocols. 5.4 Mitigation of Man-in-the-middle (MitM) attacks in server- authenticated provisioning mode EAP-FAST provisioning in server-authenticated mode addresses MitM attacks by enforcing the server to present a valid certificate as part of the TLS negotiation. To ensure the authenticity of the server and address MitM attacks, the peer MUST verify the validity of the EAP server certificate to guarantee it is not subject to a MitM attack. The cryptographic binding is another reassurance that indeed the true peer and AS were the two parties communicating in both the tunnel establishment and inner EAP authentication conversations. 5.5 Generation of Diffie-Hellman Groups The security of the DH key exchange is based on the difficulty of solving the Discrete Logarithm Problem (DLP). As algorithms and adversaries become more efficient in their abilities to precompute values for a given fixed group, it becomes more important for a server to generate new groups as a means to allay this threat. The server could, for instance, constantly compute new groups in the background. Such an example is cited in [SECSH-DH]. Cam-Winget, et al. Expires July 11, 2007 [Page 27]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Thus, the server can maintain a list of safe primes and corresponding generators to choose from. A prime p is safe, if: p = 2q + 1 and q is prime New primes may be generated in the background. Initial implementations of the EAP-FAST provisioning exchange limit the generator to be 2 as it both improves the multiplication efficiency and still covers half of the space of possible residues. Furthermore, as the server defines the group used for the DH exchange, it may restrict the prime size to be 1024 bits. Additionally, since the EAP-FAST provisioning exchange employs DH per [RFC 3268] to generate AES keys, the DH keys must provide enough entropy to ensure that a strong 128bit results from the DH key agreement. EAP-FAST employs the 2048 bit DH groups defined in [RFC 3526]. 5.6 PAC Storage Considerations The main premise behind EAP-FAST is to protect the authentication stream over the media link. However, physical security is still an issue. Some care should be taken to protect the PAC on both the peer and server. The peer must store securely both the PAC-Key and PAC- Opaque, while the server must secure storage of its security association context used to consume the PAC-Opaque. Additionally, if manual provisioning is employed, the transportation mechanism used to distribute the PAC must also be secured. Most of the attacks described here would require some level of effort to execute; conceivably greater than their value. The main focus therefore, should be to ensure that proper protections are used on both the client and server. There are a number of potential attacks which can be considered against secure key storage such as: * weak passphrases On the peer side, keys are usually protected by a passphrase. On some environments, this passphrase may be associated with the user's password. In either case, if an attacker can obtain the Cam-Winget, et al. Expires July 11, 2007 [Page 28]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 encrypted key for a range of users, he may be able to successfully attack a weak passphrase. The tools are already in place today to allow an attacker to easily attack all email users in an enterprise environment. Most viruses or worms of this sort attract attention to themselves by their action, but that need not be the case. A simple, genuine appearing email could surreptitiously access keys from known locations and email them directly to the attacker, attracting little notice. * key finding attacks Key finding attacks are usually mentioned in reference to web servers, where the private SSL key may be stored securely, but at some point it must be decrypted and stored in system memory. An attacker with access to system memory can actually find the key by identifying their mathematical properties. To date, this attack appears to be purely theoretical and primarily acts to argue strongly for secure access controls on the server itself to prevent such unauthorized code from executing. * key duplication , key substitution, key modification Once keys are accessible to an attacker on either the client or server, they fall under three forms of attack: key duplication, key substitution and key modification. The first option would be the most common, allowing the attacker to masquerade as the user in question. The second option could have some use if an attacker could implement it on the server. Alternatively, an attacker could use one of the latter two attacks on either the peer or server to force a PAC re-key, and take advantage of the MitM/dictionary attack weakness of the EAP-FAST provisioning protocol. Another consideration is the use of secure mechanisms afforded by the particular device. For instance, some laptops enable secure key storage through a special chip. It would be worthwhile for implementations to explore the use of such a mechanism. 5.7 Security Claims This section provides needed security claim requirement for EAP [RFC3748]. Auth. mechanism: Certificate based, shared secret based and various tunneled authentication mechanisms. Ciphersuite negotiation: Yes Cam-Winget, et al. Expires July 11, 2007 [Page 29]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 Mutual authentication: Yes Integrity protection: Yes, Any method executed within the EAP- FAST tunnel is integrity protected. The cleartext EAP headers outside the tunnel are not integrity protected. Replay protection: Yes Confidentiality: Yes Key derivation: Yes Key strength: [1] Dictionary attack prot.: Yes Fast reconnect: Yes Cryptographic binding: Yes Session independence: Yes Fragmentation: Yes Key Hierarchy: Yes Channel binding: No, but TLVs could be defined for this. Notes 1. BCP 86 [RFC3766] offers advice on appropriate key sizes. The National Institute for Standards and Technology (NIST) also offers advice on appropriate key sizes in [SP800-57]. [RFC3766] Section 5 advises use of the following required RSA or DH module and DSA subgroup size in bits, for a given level of attack resistance in bits. Based on the table below, a 2048-bit RSA key is required to provide 128-bit equivalent key strength: Attack Resistance RSA or DH Modulus DSA subgroup (bits) size (bits) size (bits) ----------------- ----------------- ------------ 70 947 129 80 1228 148 90 1553 167 100 1926 186 150 4575 284 200 8719 383 250 14596 482 Cam-Winget, et al. Expires July 11, 2007 [Page 30]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 6. IANA Considerations This section explains the criteria to be used by the IANA for assignment of Type value in PAC TLV attribute, PAC Type value in PAC- Type TLV, Credential-Format value in Server-Trusted-Root TLV. The "Specification Required" policy is used here with the meaning defined in BCP 26 [RFC2434]. 7. References 7.1 Normative [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC4346] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1", RFC 4346, April 2006. [EAP] Blunk, L., et. al., "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [RFC3268] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)", RFC 3268, June 2002. [RFC2119] Bradner, S., "Key words for use in RFCs to indicate Requirement Levels", RFC 2119, March 1997. [RFC3546] Blake-Wilson, S., et al., "Transport Layer Security (TLS) Extensions", RFC 3546, June 2003. [EAP-FAST] Cam-Winget, N., et al., "EAP Flexible Authentication via Secure Tunneling (EAP-FAST) ", draft-cam-winget-eap-fast- 07 (work in progress), January 2007. [RFC4507] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 4507, May 2006. Cam-Winget, et al. Expires July 11, 2007 [Page 31]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 [MSCHAPv2] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC 2759, January 2000. [RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, March 1998. 7.2 Informative [RFC2434] Narten, T., and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631, January 1999. [RFC3526] Kivinen, T., "More Modular Exponential (MODP) Diffie- Hellman groups for Internet Key Exchange (IKE)", RFC 3526, May 2003 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [MITM] Puthenkulam, J., "The Compound Authentication Binding Problem", draft-puthenkulam-eap-binding-04 (expired), October 2003. 8. Acknowledgments The EAP-FAST design and protocol specification is based on the ideas and contributions from Pad Jakkahalli, Mark Krischer, Doug Smith, Ilan Frenkel and Jeremy Steiglitz. 9. Author's Addresses Nancy Cam-Winget Cisco Systems Cam-Winget, et al. Expires July 11, 2007 [Page 32]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 190 W Tasman Drive San Jose, CA 95134 US Phone: +1 408 853 0532 E-mail: ncamwing@cisco.com David McGrew Cisco Systems San Jose, CA 95134 US E-mail: mcgrew@cisco.com Joseph Salowey Cisco Systems 2901 3rd Ave Seattle, WA 98121 US Phone: +1 206 256 3380 E-mail: jsalowey@cisco.com Hao Zhou Cisco Systems 4125 Highlander Parkway Richfield, OH 44286 US Phone : +1 330 523 2132 E-mail: hzhou@cisco.com 10. Appendix: Examples 10.1 Example 1: Successful Tunnel PAC Provisioning The following exchanges show anonymous DH with a successful EAP- MSCHAPv2 exchange within Phase 2 to provision a Tunnel PAC, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID1) -> <- EAP-Request/ EAP-Type=EAP-FAST, V=1 Cam-Winget, et al. Expires July 11, 2007 [Page 33]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 (EAP-FAST Start, S bit set, A-ID) EAP-Response/ EAP-Type=EAP-FAST, V=1 (TLS client_hello without PAC-Opaque extension)-> <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (TLS server_hello, TLS Server Key Exchange TLS Server Hello Done) EAP-Response/ EAP-Type=EAP-FAST, V=1 -> (TLS Client Key Exchange TLS change_cipher_spec, TLS finished) <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (TLS change_cipher_spec TLS finished) EAP-Response/ EAP-Type=EAP-FAST, V=1 -> (Acknowledgement) TLS channel established (messages sent within the TLS channel) <- EAP Payload TLV, EAP-Request/ EAP Identity Request EAP Payload TLV, EAP-Response/ EAP Identity Response -> <- EAP Payload TLV, EAP-Request, EAP-MSCHAPV2, Challenge EAP Payload TLV, EAP-Response, EAP-MSCHAPV2, Response) -> <- EAP Payload TLV, EAP-Request, EAP-MSCHAPV2, Success) Cam-Winget, et al. Expires July 11, 2007 [Page 34]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 EAP Payload TLV, EAP-Response, EAP-MSCHAPV2, Success) -> <- Intermediate Result TLV (Success) Crypto-Binding-TLV=(Version=1, EAP-FAST Version=1, Nonce, CompoundMAC) Intermediate Result TLV (Success) Crypto-Binding-TLV=(Version=1, EAP-FAST Version=1, Nonce, CompoundMAC) <- Result TLV (Success) PAC TLV Result TLV (Success) PAC Acknowledgment -> TLS channel torn down (messages sent in cleartext) <- EAP-Failure 10.2 Example 2: Failed Provisioning The following exchanges show a failed EAP-MSCHAPV2 exchange within Phase 2, where the peer failed to authenticate the Server. The conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID1) -> <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (EAP-FAST Start, S bit set, A-ID) EAP-Response/ EAP-Type=EAP-FAST, V=1 (TLS client_hello without Ticket extension)-> Cam-Winget, et al. Expires July 11, 2007 [Page 35]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (TLS Server Key Exchange TLS Server Hello Done) EAP-Response/ EAP-Type=EAP-FAST, V=1 -> (TLS Client Key Exchange TLS change_cipher_spec, TLS finished) <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (TLS change_cipher_spec TLS finished) EAP-Response/ EAP-Type=EAP-FAST, V=1 -> (Acknowledgement) TLS channel established (messages sent within the TLS channel) <- EAP Payload TLV EAP-Request/EAP Identity Request EAP Payload TLV EAP-Response/ EAP Identity Response -> <- EAP Payload TLV, EAP-Request, EAP-MSCHAPV2, Challenge EAP Payload TLV, EAP-Response, EAP-MSCHAPV2, Response -> <- EAP Payload TLV, EAP-Request, EAP-MSCHAPV2, Success) // peer failed to verify server MSCHAPv2 response EAP Payload TLV, EAP-Response, EAP-MSCHAPV2, Failure) -> <- Result TLV (Failure) Result TLV (Failure) -> Cam-Winget, et al. Expires July 11, 2007 [Page 36]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 TLS channel torn down (messages sent in cleartext) <- EAP-Failure 10.3 Example 3: Provisioning a Authentication Server's Trusted Root Certificate The following exchanges show a successful provisioning of a server trusted root certificate using anonymous DH and EAP-MSCHAPV2 exchange within Phase 2, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID1) -> <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (EAP-FAST Start, S bit set, A-ID) EAP-Response/ EAP-Type=EAP-FAST, V=1 (TLS client_hello without Ticket extension)-> <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (TLS server_hello, (TLS Server Key Exchange TLS Server Hello Done) EAP-Response/ EAP-Type=EAP-FAST, V=1 -> (TLS Client Key Exchange TLS change_cipher_spec, TLS finished) <- EAP-Request/ EAP-Type=EAP-FAST, V=1 (TLS change_cipher_spec Cam-Winget, et al. Expires July 11, 2007 [Page 37]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 TLS finished) EAP-Payload-TLV[ EAP-Request/Identity]) // TLS channel established (messages sent within the TLS channel) // First EAP Payload TLV is piggybacked to the TLS Finished as Application Data and protected by the TLS tunnel EAP-Payload TLV/ [EAP Identity Response] -> <- EAP Payload TLV, EAP-Request, [EAP-MSCHAPV2, Challenge] EAP Payload TLV, EAP-Response, [EAP-MSCHAPV2, Response] -> <- EAP Payload TLV, EAP-Request, [EAP-MSCHAPV2, Success Request] EAP Payload TLV, EAP-Response, [EAP-MSCHAPV2, Success Response] -> <- Crypto-Binding TLV (Version=1, EAP-FAST Version=1, Nonce, CompoundMAC), Crypto-Binding TLV (Version=1 EAP-FAST Version=1, Nonce, CompoundMAC) Server-Trusted-Root TLV [Type = PKCS#7 ] -> <- Result TLV (Success) Server-Trusted-Root TLV [PKCS#7 TLV] Result TLV (Success) -> // TLS channel torn down (messages sent in cleartext) <- EAP-Failure Cam-Winget, et al. Expires July 11, 2007 [Page 38]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 11. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. 12. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 13. Copyright Statement Copyright (C) The Internet Society (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Cam-Winget, et al. Expires July 11, 2007 [Page 39]
Internet-Draft Dynamic Provisioning using EAP-FAST January 2007 14. Expiration Date This memo is filed as <draft-cam-winget-eap-fast-provisioning- 03.txt>, and expires July 11, 2007. Cam-Winget, et al. Expires July 11, 2007 [Page 40]
