Network Working Group Dan Simon INTERNET-DRAFT Bernard Aboba Category: Proposed Standard Microsoft <draft-simon-emu-rfc2716bis-00.txt> 13 February 2006 The EAP TLS Authentication Protocol 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. This Internet-Draft will expire on August 20, 2006. Copyright Notice Copyright (C) The Internet Society 2006. Abstract The Extensible Authentication Protocol (EAP), defined in RFC 3748, provides support for multiple authentication methods. Transport Level Security (TLS) provides for mutual authentication, integrity- protected ciphersuite negotiation and key exchange between two endpoints. This document defines EAP-TLS, which includes support for certificate-based mutual authentication and key derivation. Simon & Aboba Proposed Standard [Page 1]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 Table of Contents 1. Introduction.............................................. 3 1.1 Requirements Language ........................... 3 1.2 Terminology ..................................... 3 2. Protocol Overview ........................................ 4 2.1 Overview of the EAP-TLS Conversation ............ 4 2.2 Retry Behavior .................................. 7 2.3 Fragmentation ................................... 8 2.4 Identity Verification ........................... 9 2.5 Key Derivation .................................. 10 2.6 Ciphersuite and Compression Negotiation ......... 11 3. Detailed Description of the EAP-TLS Protocol ............. 11 3.1 EAP TLS Packet Format ........................... 11 3.2 EAP TLS Request Packet .......................... 12 3.3 EAP TLS Response Packet ......................... 14 4. IANA Considerations ...................................... 15 5. Security Considerations .................................. 15 5.1 Certificate Revocation .......................... 15 5.2 Separation of EAP Authenticator and Server ...... 16 5.3 PPP Encryption and Other Security Mechanisms .... 16 6. References ............................................... 17 6.1 Normative references ............................ 17 6.2 Informative references .......................... 17 Acknowledgments .............................................. 18 Authors' Addresses ........................................... 18 Appendix A - Examples ........................................ 19 Intellectual Property Statement .............................. 25 Disclaimer of Validity ....................................... 25 Copyright Statement .......................................... 25 Simon & Aboba Proposed Standard [Page 2]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 1. Introduction The Extensible Authentication Protocol (EAP), described in [RFC3748], provides a standard mechanism for support of multiple authentication methods. Through the use of EAP, support for a number of authentication schemes may be added, including smart cards, Kerberos, Public Key, One Time Passwords, and others. While the EAP methods defined in [RFC3748] did not support mutual authentication, the use of EAP with wireless technologies such as [IEEE-802.11i] has resulted in development of a new set of requirements [RFC4017]. As described in [RFC4017] it is desirable for EAP methods used for wireless LAN authentication to support mutual authentication and key derivation. Since PPP encryption protocols such as [RFC2419] and [RFC2420] assume existence of a session key, it is useful to have a mechanism for session key establishment. Since design of secure key management protocols is non-trivial, it is desirable to avoid creating new mechanisms for this. The EAP protocol described in this document allows a EAP peer to take advantage of the protected ciphersuite negotiation, mutual authentication and key management capabilities of the TLS protocol, described in [RFC2246]. 1.1. Requirements In this document, several words are used to signify the requirements of the specification. 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 "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119]. 1.2. Terminology This document frequently uses the following terms: authenticator The end of the link initiating EAP authentication. The term authenticator is used in [IEEE-802.1X], and has the same meaning in this document. peer The end of the link that responds to the authenticator. In [IEEE-802.1X], this end is known as the Supplicant. backend authentication server A backend authentication server is an entity that provides an authentication service to an authenticator. When used, this server typically executes EAP methods for the authenticator. This terminology is also used in [IEEE-802.1X]. Simon & Aboba Proposed Standard [Page 3]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 EAP server The entity that terminates the EAP authentication method with the peer. In the case where no backend authentication server is used, the EAP server is part of the authenticator. In the case where the authenticator operates in pass-through mode, the EAP server is located on the backend authentication server. Master Session Key (MSK) Keying material that is derived between the EAP peer and server and exported by the EAP method. The MSK is at least 64 octets in length. Extended Master Session Key (EMSK) Additional keying material derived between the EAP client and server that is exported by the EAP method. The EMSK is at least 64 octets in length. 2. Protocol Overview 2.1. Overview of the EAP-TLS Conversation As described in [RFC3748], the EAP-TLS conversation will typically begin with the authenticator and the peer negotiating EAP. The authenticator will then typically send an EAP-Request/Identity packet to the peer, and the peer will respond with an EAP-Response/Identity packet to the authenticator, containing the peer's userId. From this point forward, while nominally the EAP conversation occurs between the EAP authenticator and the peer, the authenticator MAY act as a passthrough device, with the EAP packets received from the peer being encapsulated for transmission to a backend security server. In the discussion that follows, we will use the term "EAP server" to denote the ultimate endpoint conversing with the peer. Once having received the peer's Identity, the EAP server MUST respond with an EAP-TLS/Start packet, which is an EAP-Request packet with EAP-Type=EAP-TLS, the Start (S) bit set, and no data. The EAP-TLS conversation will then begin, with the peer sending an EAP-Response packet with EAP-Type=EAP-TLS. The data field of that packet will encapsulate one or more TLS records in TLS record layer format, containing a TLS client_hello handshake message. The current cipher spec for the TLS records will be TLS_NULL_WITH_NULL_NULL and null compression. This current cipher spec remains the same until the change_cipher_spec message signals that subsequent records will have the negotiated attributes for the remainder of the handshake. The client_hello message contains the client's TLS version number, a sessionId, a random number, and a set of ciphersuites supported by Simon & Aboba Proposed Standard [Page 4]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 the client. The version offered by the client MUST correspond to TLS v1.0 or later. The EAP server will then respond with an EAP-Request packet with EAP- Type=EAP-TLS. The data field of this packet will encapsulate one or more TLS records. These will contain a TLS server_hello handshake message, possibly followed by TLS certificate, server_key_exchange, certificate_request, server_hello_done and/or finished handshake messages, and/or a TLS change_cipher_spec message. The server_hello handshake message contains a TLS version number, another random number, a sessionId, and a ciphersuite. The version offered by the server MUST correspond to TLS v1.0 or later. If the client's sessionId is null or unrecognized by the server, the server MUST choose the sessionId to establish a new session; otherwise, the sessionId will match that offered by the client, indicating a resumption of the previously established session with that sessionID. The server will also choose a ciphersuite from those offered by the client; if the session matches the client's, then the ciphersuite MUST match the one negotiated during the handshake protocol execution that established the session. The purpose of the sessionId within the TLS protocol is to allow for improved efficiency in the case where a client repeatedly attempts to authenticate to an EAP server within a short period of time. While this model was developed for use with HTTP authentication, it may also have application to PPP authentication (e.g. multilink). As a result, it is left up to the peer whether to attempt to continue a previous session, thus shortening the TLS conversation. Typically the peer's decision will be made based on the time elapsed since the previous authentication attempt to that EAP server. Based on the sessionId chosen by the peer, and the time elapsed since the previous authentication, the EAP server will decide whether to allow the continuation, or whether to choose a new session. In the case where the EAP server and authenticator reside on the same device, then client will only be able to continue sessions when connecting to the same NAS or tunnel server. Should these devices be set up in a rotary or round-robin then it may not be possible for the peer to know in advance the authenticator it will be connecting to, and therefore which sessionId to attempt to reuse. As a result, it is likely that the continuation attempt will fail. In the case where the EAP authentication is remoted then continuation is much more likely to be successful, since multiple NAS devices and tunnel servers will remote their EAP authentications to the same backend authentication server. Simon & Aboba Proposed Standard [Page 5]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 If the EAP server is resuming a previously established session, then it MUST include only a TLS change_cipher_spec message and a TLS finished handshake message after the server_hello message. The finished message contains the EAP server's authentication response to the peer. If the EAP server is not resuming a previously established session, then it MUST include a TLS server_certificate handshake message, and a server_hello_done handshake message MUST be the last handshake message encapsulated in this EAP-Request packet. The certificate message contains a public key certificate chain for either a key exchange public key (such as an RSA or Diffie-Hellman key exchange public key) or a signature public key (such as an RSA or DSS signature public key). In the latter case, a TLS server_key_exchange handshake message MUST also be included to allow the key exchange to take place. The certificate_request message is included when the server desires the client to authenticate itself via public key. While the EAP server SHOULD require client authentication, this is not a requirement, since it may be possible that the server will require that the peer authenticate via some other means. The peer MUST respond to the EAP-Request with an EAP-Response packet of EAP-Type=EAP-TLS. The data field of this packet will encapsulate one or more TLS records containing a TLS change_cipher_spec message and finished handshake message, and possibly certificate, certificate_verify and/or client_key_exchange handshake messages. If the preceding server_hello message sent by the EAP server in the preceding EAP-Request packet indicated the resumption of a previous session, then the peer MUST send only the change_cipher_spec and finished handshake messages. The finished message contains the peer's authentication response to the EAP server. If the preceding server_hello message sent by the EAP server in the preceeding EAP-Request packet did not indicate the resumption of a previous session, then the peer MUST send, in addition to the change_cipher_spec and finished messages, a client_key_exchange message, which completes the exchange of a shared master secret between the peer and the EAP server. If the EAP server sent a certificate_request message in the preceding EAP-Request packet, then the peer MUST send, in addition, certificate and certificate_verify handshake messages. The former contains a certificate for the peer's signature public key, while the latter contains the peer's signed authentication response to the EAP server. After receiving this packet, the EAP server will verify the peer's certificate and digital signature, if requested. If the peer's authentication is unsuccessful, the EAP server SHOULD Simon & Aboba Proposed Standard [Page 6]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a TLS record containing the appropriate TLS alert message. The EAP server SHOULD send a TLS alert message rather immediately terminating the conversation so as to allow the peer to inform the user of the cause of the failure and possibly allow for a restart of the conversation. To ensure that the peer receives the TLS alert message, the EAP server MUST wait for the peer to reply with an EAP-Response packet. The EAP-Response packet sent by the peer MAY encapsulate a TLS client_hello handshake message, in which case the EAP server MAY allow the EAP-TLS conversation to be restarted, or it MAY contain an EAP-Response packet with EAP-Type=EAP-TLS and no data, in which case the EAP-Server MUST send an EAP-Failure packet, and terminate the conversation. It is up to the EAP server whether to allow restarts, and if so, how many times the conversation can be restarted. An EAP Server implementing restart capability SHOULD impose a limit on the number of restarts, so as to protect against denial of service attacks. If the peers authenticates successfully, the EAP server MUST respond with an EAP-Request packet with EAP-Type=EAP-TLS, which includes, in the case of a new TLS session, one or more TLS records containing TLS change_cipher_spec and finished handshke messages. The latter contains the EAP server's authentication response to the peer. The peer will then verify the hash in order to authenticate the EAP server. If the EAP server authenticates unsuccessfully, the peer MAY send an EAP-Response packet of EAP-Type=EAP-TLS containing a TLS Alert message identifying the reason for the failed authentication. The peer MAY send a TLS alert message rather than immediately terminating the conversation so as to allow the EAP server to log the cause of the error for examination by the system administrator. To ensure that the EAP Server receives the TLS alert message, the peer MUST wait for the EAP-Server to reply before terminating the conversation. The EAP Server MUST reply with an EAP-Failure packet since server authentication failure is a terminal condition. If the EAP server authenticates successfully, the peer MUST send an EAP-Response packet of EAP-Type=EAP-TLS, and no data. The EAP-Server then MUST respond with an EAP-Success message. 2.2. Retry Behavior As with other EAP protocols, the EAP server is responsible for retry behavior. This means that if the EAP server does not receive a reply from the peer, it MUST resend the EAP-Request for which it has not Simon & Aboba Proposed Standard [Page 7]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 yet received an EAP-Response. However, the peer MUST NOT resend EAP- Response packets without first being prompted by the EAP server. For example, if the initial EAP-TLS start packet sent by the EAP server were to be lost, then the peer would not receive this packet, and would not respond to it. As a result, the EAP-TLS start packet would be resent by the EAP server. Once the peer received the EAP-TLS start packet, it would send an EAP-Response encapsulating the client_hello message. If the EAP-Response were to be lost, then the EAP server would resend the initial EAP-TLS start, and the peer would resend the EAP-Response. As a result, it is possible that a peer will receive duplicate EAP- Request messages, and may send duplicate EAP-Responses. Both the peer and the EAP-Server should be engineered to handle this possibility. 2.3. Fragmentation A single TLS record may be up to 16384 octets in length, but a TLS message may span multiple TLS records, and a TLS certificate message may in principle be as long as 16MB. The group of EAP-TLS messages sent in a single round may thus be larger than the PPP MTU size, the maximum RADIUS packet size of 4096 octets, or even the Multilink Maximum Received Reconstructed Unit (MRRU). As described in [RFC1990], the multilink MRRU is negotiated via the Multilink MRRU LCP option, which includes an MRRU length field of two octets, and thus can support MRRUs as large as 64 KB. However, note that in order to protect against reassembly lockup and denial of service attacks, it may be desirable for an implementation to set a maximum size for one such group of TLS messages. Since a typical certificate chain is rarely longer than a few thousand octets, and no other field is likely to be anwhere near as long, a reasonable choice of maximum acceptable message length might be 64 KB. If this value is chosen, then fragmentation can be handled via the multilink PPP fragmentation mechanisms described in [RFC1990]. While this is desirable, there may be cases in which multilink or the MRRU LCP option cannot be negotiated. As a result, an EAP-TLS implementation MUST provide its own support for fragmentation and reassembly. Since EAP is a simple ACK-NAK protocol, fragmentation support can be added in a simple manner. In EAP, fragments that are lost or damaged in transit will be retransmitted, and since sequencing information is provided by the Identifier field in EAP, there is no need for a Simon & Aboba Proposed Standard [Page 8]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 fragment offset field as is provided in IPv4. EAP-TLS fragmentation support is provided through addition of a flags octet within the EAP-Response and EAP-Request packets, as well as a TLS Message Length field of four octets. Flags include the Length included (L), More fragments (M), and EAP-TLS Start (S) bits. The L flag is set to indicate the presence of the four octet TLS Message Length field, and MUST be set for the first fragment of a fragmented TLS message or set of messages. The M flag is set on all but the last fragment. The S flag is set only within the EAP-TLS start message sent from the EAP server to the peer. The TLS Message Length field is four octets, and provides the total length of the TLS message or set of messages that is being fragmented; this simplifies buffer allocation. When an EAP-TLS peer receives an EAP-Request packet with the M bit set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS and no data. This serves as a fragment ACK. The EAP server MUST wait until it receives the EAP-Response before sending another fragment. In order to prevent errors in processing of fragments, the EAP server MUST increment the Identifier field for each fragment contained within an EAP-Request, and the peer MUST include this Identifier value in the fragment ACK contained within the EAP-Response. Retransmitted fragments will contain the same Identifier value. Similarly, when the EAP server receives an EAP-Response with the M bit set, it MUST respond with an EAP-Request with EAP-Type=EAP-TLS and no data. This serves as a fragment ACK. The EAP peer MUST wait until it receives the EAP-Request before sending another fragment. In order to prevent errors in the processing of fragments, the EAP server MUST use increment the Identifier value for each fragment ACK contained within an EAP-Request, and the peer MUST include this Identifier value in the subsequent fragment contained within an EAP- Response. 2.4. Identity Verification As noted in [RFC3748] Section 5.1: It is RECOMMENDED that the Identity Response be used primarily for routing purposes and selecting which EAP method to use. EAP Methods SHOULD include a method-specific mechanism for obtaining the identity, so that they do not have to rely on the Identity Response. As part of the TLS negotiation, the server presents a certificate to the peer, and if mutual authentication is requested, the peer presents a certificate to the server. EAP-TLS therefore provides a Simon & Aboba Proposed Standard [Page 9]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 mechanism for determining both the peer and server identities. The peer identity (Peer-ID in [KEYFRAME]) is either directly determined from the altSubjectName in the peer certificate or by a mapping of the altSubjectName to the Peer-ID using a directory service. The server identity (Server-ID in [KEYFRAME]) is either directly determined from the altSubjectName in the server certificate or by a mapping of the altSubjectName to the Server-ID using a directory service. The peer MUST verify the validity of the EAP server certificate, and SHOULD also examine the EAP server name presented in the certificate, in order to determine whether the EAP server can be trusted. Please note that in the case where the EAP authentication is remoted that the EAP server will not reside on the same machine as the authenticator, and therefore the name in the EAP server's certificate cannot be expected to match that of the intended destination. In this case, a more appropriate test might be whether the EAP server's certificate is signed by a CA controlling the intended destination and whether the EAP server exists within a target sub-domain. 2.5. Key Derivation Since the normal TLS keys are used in the handshake, and therefore should not be used in a different context, new encryption keys must be derived from the TLS master secret for use with PPP encryption. For both peer and EAP server, the derivation proceeds as follows: given the master secret negotiated by the TLS handshake, the pseudorandom function (PRF) defined in the specification for the version of TLS in use, and the value random defined as the concatenation of the handshake message fields client_hello.random and server_hello.random (in that order), the value PRF(master secret, "client EAP encryption", random) is computed up to 128 bytes, and the value PRF("", "client EAP encryption", random) is computed up to 64 bytes (where "" is an empty string). The peer encryption key (the one used for encrypting data from peer to EAP server) is obtained by truncating to the correct length the first 32 bytes of the first PRF of these two output strings. TheEAP server encryption key (the one used for encrypting data from EAP server to peer), if different from the client encryption key, is obtained by truncating to the correct length the second 32 bytes of this same PRF output string. The client authentication key (the one used for computing MACs for messages from peer to EAP server), if used, is obtained by truncating to the correct length the third 32 bytes of this same PRF output string. The EAP server authentication key (the one used for computing MACs for messages from EAP server to peer), if used, and if different from the peer authentication key, is obtained by truncating to the correct length the fourth 32 bytes of this same PRF output string. The peer initialization vector (IV), used for messages from Simon & Aboba Proposed Standard [Page 10]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 peer to EAP server if a block cipher has been specified, is obtained by truncating to the cipher's block size the first 32 bytes of the second PRF output string mentioned above. Finally, the server initialization vector (IV), used for messages from peer to EAP server if a block cipher has been specified, is obtained by truncating to the cipher's block size the second 32 bytes of this second PRF output. The use of these encryption and authentication keys is specific to the PPP encryption mechanism used, such as those defined in [RFC2419] and [RFC2420]. Additional keys or other non-secret values (such as IVs) can be obtained as needed for future PPP encryption methods by extending the outputs of the PRF beyond 128 bytes and 64 bytes, respectively. 2.6. Ciphersuite and Compression Negotiation Since TLS supports ciphersuite negotiation, peers completing the TLS negotiation will also have selected a ciphersuite, which includes encryption and hashing methods. Since the ciphersuite negotiated within EAP-TLS applies only to the EAP conversation, TLS ciphersuite negotiation SHOULD NOT be used to negotiate the ciphersuites used to secure data. TLS also supports compression as well as ciphersuite negotiation. Since compression negotiated within EAP-TLS applies only to the EAP conversation, TLS compression negotiation MUST NOT be used to negotiate compression mechanisms to be applied to data. 3. Detailed description of the EAP-TLS protocol 3.1. EAP TLS Packet Format A summary of the EAP TLS Request/Response packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Data... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 - Request 2 - Response Simon & Aboba Proposed Standard [Page 11]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 Identifier The identifier field is one octet and aids in matching responses with requests. Length The Length field is two octets and indicates the length of the EAP packet including the Code, Identifier, Length, Type, and Data fields. Octets outside the range of the Length field should be treated as Data Link Layer padding and should be ignored on reception. Type 13 - EAP TLS Data The format of the Data field is determined by the Code field. 3.2. EAP TLS Request Packet A summary of the EAP TLS Request packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Flags | TLS Message Length +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TLS Message Length | TLS Data... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 Identifier The Identifier field is one octet and aids in matching responses with requests. The Identifier field MUST be changed on each Request packet. Length Simon & Aboba Proposed Standard [Page 12]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 The Length field is two octets and indicates the length of the EAP packet including the Code, Identifier, Length, Type, and TLS Response fields. Type 13 - EAP TLS Flags 0 1 2 3 4 5 6 7 8 +-+-+-+-+-+-+-+-+ |L M S R R R R R| +-+-+-+-+-+-+-+-+ L = Length included M = More fragments S = EAP-TLS start R = Reserved The L bit (length included) is set to indicate the presence of the four octet TLS Message Length field, and MUST be set for the first fragment of a fragmented TLS message or set of messages. The M bit (more fragments) is set on all but the last fragment. The S bit (EAP-TLS start) is set in an EAP-TLS Start message. This differentiates the EAP-TLS Start message from a fragment acknowledgement. Implementations of this specification MUST set the reserved bits to zero, and MUST ignore them on reception. TLS Message Length The TLS Message Length field is four octets, and is present only if the L bit is set. This field provides the total length of the TLS message or set of messages that is being fragmented. TLS data The TLS data consists of the encapsulated TLS packet in TLS record format. Simon & Aboba Proposed Standard [Page 13]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 3.3. EAP TLS Response Packet A summary of the EAP TLS Response packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Flags | TLS Message Length +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TLS Message Length | TLS Data... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 2 Identifier The Identifier field is one octet and MUST match the Identifier field from the corresponding request. Length The Length field is two octets and indicates the length of the EAP packet including the Code, Identifier, Length, Type, and TLS data fields. Type 13 - EAP TLS Flags 0 1 2 3 4 5 6 7 8 +-+-+-+-+-+-+-+-+ |L M S R R R R R| +-+-+-+-+-+-+-+-+ L = Length included M = More fragments S = EAP-TLS start R = Reserved The L bit (length included) is set to indicate the presence of the four octet TLS Message Length field, and MUST be set for the first Simon & Aboba Proposed Standard [Page 14]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 fragment of a fragmented TLS message or set of messages. The M bit (more fragments) is set on all but the last fragment. The S bit (EAP-TLS start) is set in an EAP-TLS Start message. This differentiates the EAP-TLS Start message from a fragment acknowledgement. Implementations of this specification MUST set the reserved bits to zero, and MUST ignore them on reception. TLS Message Length The TLS Message Length field is four octets, and is present only if the L bit is set. This field provides the total length of the TLS message or set of messages that is being fragmented. TLS data The TLS data consists of the encapsulated TLS packet in TLS record format. 4. IANA Considerations This specification does not request the creation of any new parameter registries, nor does it require any other IANA assignments. 5. Security Considerations 5.1. Certificate revocation Since the EAP server is on the Internet during the EAP conversation, the server is capable of following a certificate chain or verifying whether the peer's certificate has been revoked. In contrast, the peer may or may not have Internet connectivity, and thus while it can validate the EAP server's certificate based on a pre-configured set of CAs, it may not be able to follow a certificate chain or verify whether the EAP server's certificate has been revoked. In the case where the peer is initiating a voluntary Layer 2 tunnel using PPTP or L2TP, the peer will typically already have a PPP interface and Internet connectivity established at the time of tunnel initiation. As a result, during the EAP conversation it is capable of checking for certificate revocation. However, in the case where the peer is initiating an intial PPP conversation, it will not have Internet connectivity and is therefore not capable of checking for certificate revocation until after NCP negotiation completes and the peer has access to the Internet. In this case, the peer SHOULD check for certificate revocation after connecting to the Internet. Simon & Aboba Proposed Standard [Page 15]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 5.2. Separation of the EAP Authenticator and Server As a result of the EAP-TLS conversation, the EAP peer and server endpoints will mutually authenticate and derive the MSK and EMSK. Subsequently the EAP peer and authenticator may negotiate a ciphersuite for protection of data and derive a session key for subsequent use. Since the peer and EAP client reside on the same machine, it is necessary for the EAP client module to pass the required keying material to the lower layer, as described in [KEYFRAME]. The situation may be more complex on the EAP authenticator, which may or may not reside on the same machine as the EAP server. In the case where the EAP server and authenticator reside on different machines, there are several implications for security. Firstly, the mutual authentication defined in EAP-TLS will occur between the EAP peer and server, not between the peer and the authenticator. This means that as a result of the EAP-TLS conversation, it is not possible for the EAP peer to validate the identity of the NAS or tunnel server that it is speaking to. The second issue is that the EAP keying material derived between the peer and EAP server will need to be transported to the authenticator. The implications of this are described in more detail in [KEYFRAME]; however the specification of this transport mechanism is outside the scope of this document. 5.3. PPP Encryption and Other Security Mechanisms EAP-TLS has been deployed for use with a variety of lower layers, including PPP, Layer 2 tunneling protocols such as PPTP and L2TP, IEEE 802 wired networks [IEEE-802.1X] and wireless technologies such as IEEE 802.11 [IEEE-802.11i] and IEEE 802.16 [IEEE-802.16e]. In compulsory layer 2 tunneling, a PPP peer makes a connection to a NAS or router which tunnels the PPP packets to a tunnel server. Since with compulsory tunneling a PPP peer cannot tell whether its packets are being tunneled, let alone whether the network device is securing the tunnel, if security is required then the client must make its own arrangements. In the case where all endpoints cannot be relied upon to implement IPSEC, TLS, or another suitable security protocol, PPP encryption provides a convenient means to ensure the privacy of packets transiting between the client and the tunnel server. Simon & Aboba Proposed Standard [Page 16]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2246] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246, November 1998. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H. Lefkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. 6.2. Informative References [DES] National Bureau of Standards, "DES Modes of Operation", FIPS PUB 81 (December 1980). [FIPS] National Bureau of Standards, "Data Encryption Standard", FIPS PUB 46 (January 1977). [IEEE-802.11] Institute of Electrical and Electronics Engineers, "Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", IEEE Standard 802.11-2003, 2003. [IEEE-802.1X] Institute of Electrical and Electronics Engineers, "Local and Metropolitan Area Networks: Port-Based Network Access Control", IEEE Standard 802.1X-2004, December 2004. [KEYFRAME] Aboba, B., Simon, D., Arkko, J., Eronen, P. and H. Levkowetz, "Extensible Authentication Protocol (EAP) Key Management Framework", Internet-drafts (work in progress), draft-ietf- eap-keying-10.txt, February 2006. [RFC1321] Rivest, R. and S. Dusse, "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. Simon & Aboba Proposed Standard [Page 17]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 [RFC1570] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, January 1994. [RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1962] D. Rand, "The PPP Compression Control Protocol", RFC 1962, Novell, June 1996. [RFC1968] Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968, June 1996. [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996. [RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998. [RFC2420] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)", RFC 2420, September 1998. [RFC4017] Stanley, D., Walker, J. and B. Aboba, "Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs", RFC 4017, March 2005. Acknowledgments Thanks to Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft for useful discussions of this problem space. Author Addresses Dan Simon Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 706 6711 EMail: dansimon@microsoft.com Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 706 6605 EMail: bernarda@microsoft.com Simon & Aboba Proposed Standard [Page 18]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 Appendix A - Examples In the case where the EAP-TLS mutual authentication is successful, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> <- EAP-Request/ EAP-Type=EAP-TLS (TLS Start) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello)-> <- EAP-Request/ EAP-Type=EAP-TLS (TLS server_hello, TLS certificate, [TLS server_key_exchange,] [TLS certificate_request,] TLS server_hello_done) EAP-Response/ EAP-Type=EAP-TLS (TLS certificate, TLS client_key_exchange, [TLS certificate_verify,] TLS change_cipher_spec, TLS finished) -> <- EAP-Request/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) EAP-Response/ EAP-Type=EAP-TLS -> <- EAP-Success Simon & Aboba Proposed Standard [Page 19]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 In the case where the EAP-TLS mutual authentication is successful, and fragmentation is required, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> <- EAP-Request/ EAP-Type=EAP-TLS (TLS Start, S bit set) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello)-> <- EAP-Request/ EAP-Type=EAP-TLS (TLS server_hello, TLS certificate, [TLS server_key_exchange,] [TLS certificate_request,] TLS server_hello_done) (Fragment 1: L, M bits set) EAP-Response/ EAP-Type=EAP-TLS -> <- EAP-Request/ EAP-Type=EAP-TLS (Fragment 2: M bit set) EAP-Response/ EAP-Type=EAP-TLS -> <- EAP-Request/ EAP-Type=EAP-TLS (Fragment 3) EAP-Response/ EAP-Type=EAP-TLS (TLS certificate, TLS client_key_exchange, [TLS certificate_verify,] TLS change_cipher_spec, TLS inished)(Fragment 1: L, M bits set)-> <- EAP-Request/ EAP-Type=EAP-TLS EAP-Response/ EAP-Type=EAP-TLS (Fragment 2)-> <- EAP-Request/ Simon & Aboba Proposed Standard [Page 20]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) EAP-Response/ EAP-Type=EAP-TLS -> <- EAP-Success In the case where the server authenticates to the client successfully, but the client fails to authenticate to the server, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> <- EAP-Request/ EAP-Type=EAP-TLS (TLS Start) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello)-> <- EAP-Request/ EAP-Type=EAP-TLS (TLS server_hello, TLS certificate, [TLS server_key_exchange,] TLS certificate_request, TLS server_hello_done) EAP-Response/ EAP-Type=EAP-TLS (TLS certificate, TLS client_key_exchange, TLS certificate_verify, TLS change_cipher_spec, TLS finished) -> <- EAP-Request/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) EAP-Response/ EAP-Type=EAP-TLS -> <- EAP-Request EAP-Type=EAP-TLS (TLS Alert message) EAP-Response/ EAP-Type=EAP-TLS -> Simon & Aboba Proposed Standard [Page 21]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 <- EAP-Failure (User Disconnected) In the case where server authentication is unsuccessful, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> <- EAP-Request/ EAP-Type=EAP-TLS (TLS Start) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello)-> <- EAP-Request/ EAP-Type=EAP-TLS (TLS server_hello, TLS certificate, [TLS server_key_exchange,] [TLS certificate_request,] TLS server_hello_done) EAP-Response/ EAP-Type=EAP-TLS (TLS certificate, TLS client_key_exchange, [TLS certificate_verify,] TLS change_cipher_spec, TLS finished) -> <- EAP-Request/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) EAP-Response/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) <- EAP-Request/ EAP-Type=EAP-TLS EAP-Response/ EAP-Type=EAP-TLS (TLS Alert message) -> <- EAP-Failure (User Disconnected) Simon & Aboba Proposed Standard [Page 22]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 In the case where a previously established session is being resumed, and both sides authenticate successfully, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> <- EAP-Request/ EAP-Request/ EAP-Type=EAP-TLS (TLS Start) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello)-> <- EAP-Request/ EAP-Type=EAP-TLS (TLS server_hello, TLS change_cipher_spec TLS finished) EAP-Response/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) -> <- EAP-Success In the case where a previously established session is being resumed, and the server authenticates to the client successfully but the client fails to authenticate to the server, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> <- EAP-Request/ EAP-Request/ EAP-Type=EAP-TLS (TLS Start) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello) -> <- EAP-Request/ EAP-Type=EAP-TLS Simon & Aboba Proposed Standard [Page 23]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 (TLS server_hello, TLS change_cipher_spec, TLS finished) EA-Response/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) -> <- EAP-Request EAP-Type=EAP-TLS (TLS Alert message) EAP-Response EAP-Type=EAP-TLS -> <- EAP-Failure (User Disconnected) In the case where a previously established session is being resumed, and the server authentication is unsuccessful, the conversation will appear as follows: Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> <- EAP-Request/ EAP-Request/ EAP-Type=EAP-TLS (TLS Start) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello)-> <- EAP-Request/ EAP-Type=EAP-TLS (TLS server_hello, TLS change_cipher_spec, TLS finished) EAP-Response/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) <- EAP-Request/ EAP-Type=EAP-TLS EAP-Response/ EAP-Type=EAP-TLS (TLS Alert message) -> <- EAP-Failure (User Disconnected) Simon & Aboba Proposed Standard [Page 24]
INTERNET-DRAFT EAP TLS Authentication Protocol 13 February 2006 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. 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. Copyright Statement Copyright (C) The Internet Society (2006). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Simon & Aboba Proposed Standard [Page 25]
