Network Working Group D. Simon Request for Comments: 5216 B. Aboba Obsoletes: 2716 R. Hurst Category: Standards Track Microsoft Corporation March 2008
The EAP-TLS Authentication Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Extensible Authentication Protocol (EAP), defined in RFC 3748, provides support for multiple authentication methods. Transport Layer 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.
This document obsoletes RFC 2716. A summary of the changes between this document and RFC 2716 is available in Appendix A.
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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. EAP has been defined for use with a variety of lower layers, including the Point-to-Point Protocol (PPP) [RFC1661], Layer 2 tunneling protocols such as the Point-to-Point Tunneling Protocol (PPTP) [RFC2637] or Layer 2 Tunneling Protocol (L2TP) [RFC2661], IEEE 802 wired networks [IEEE-802.1X], and wireless technologies such as IEEE 802.11 [IEEE- 802.11] and IEEE 802.16 [IEEE-802.16e].
While the EAP methods defined in [RFC3748] did not support mutual authentication, the use of EAP with wireless technologies such as [IEEE-802.11] has resulted in development of a new set of
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requirements. As described in "Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs" [RFC4017], it is desirable for EAP methods used for wireless LAN authentication to support mutual authentication and key derivation. Other link layers can also make use of EAP to enable mutual authentication and key derivation.
This document defines EAP-Transport Layer Security (EAP-TLS), which includes support for certificate-based mutual authentication and key derivation, utilizing the protected ciphersuite negotiation, mutual authentication and key management capabilities of the TLS protocol, described in "The Transport Layer Security (TLS) Protocol Version 1.1" [RFC4346]. While this document obsoletes RFC 2716 [RFC2716], it remains backward compatible with it. A summary of the changes between this document and RFC 2716 is available in Appendix A.
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].
This document frequently uses the following terms:
authenticator The entity initiating EAP authentication.
peer The entity that responds to the authenticator. In [IEEE-802.1X], this entity 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].
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.
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Master Session Key (MSK) Keying material that is derived between the EAP peer and server and exported by the EAP method.
Extended Master Session Key (EMSK) Additional keying material derived between the EAP peer and server that is exported by the EAP method.
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 user-Id.
From this point forward, while nominally the EAP conversation occurs between the EAP authenticator and the peer, the authenticator MAY act as a pass-through device, with the EAP packets received from the peer being encapsulated for transmission to a backend authentication 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 peer's TLS version number, a sessionId, a random number, and a set of ciphersuites supported by the peer. The version offered by the peer 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
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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 peer'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 peer, indicating a resumption of the previously established session with that sessionId. The server will also choose a ciphersuite from those offered by the peer. If the session matches the peer's, then the ciphersuite MUST match the one negotiated during the handshake protocol execution that established the session.
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 Digital Signature Standard (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 peer to authenticate itself via public key. While the EAP server SHOULD require peer authentication, this is not mandatory, since there are circumstances in which peer authentication will not be needed (e.g., emergency services, as described in [UNAUTH]), or where the peer will authenticate via some other means.
If the peer supports EAP-TLS and is configured to use it, it MUST respond to the EAP-Request with an EAP-Response packet of EAP- Type=EAP-TLS. If the preceding server_hello message sent by the EAP server in the preceding EAP-Request packet did not indicate the resumption of a previous session, the data field of this packet MUST encapsulate one or more TLS records containing a TLS client_key_exchange, change_cipher_spec, and finished messages. If the EAP server sent a certificate_request message in the preceding EAP-Request packet, then unless the peer is configured for privacy (see Section 2.1.4) the peer MUST send, in addition, certificate and certificate_verify 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
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this packet, the EAP server will verify the peer's certificate and digital signature, if requested.
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.
In the case where the EAP-TLS mutual authentication is successful, the conversation will appear as follows:
The purpose of the sessionId within the TLS protocol is to allow for improved efficiency in the case where a peer 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 also can be used to provide "fast reconnect" functionality as defined in Section 7.2.1 of [RFC3748].
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 to choose a new session.
In the case where the EAP server and authenticator reside on the same device, the peer will only be able to continue sessions when connecting to the same authenticator. Should the authenticators be set up in a rotary or round-robin, then it may not be possible for the peer to know in advance the authenticator to which it will be connecting, 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 authenticators will utilize the same backend authentication server.
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.
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In the case where a previously established session is being resumed, and both sides authenticate successfully, the conversation will appear as follows:
If the peer's authentication is unsuccessful, the EAP server SHOULD 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 immediately terminating the conversation so as to allow the peer to inform the user or log 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 per-peer limit
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on the number of restarts, so as to protect against denial-of-service attacks.
If the peer 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 handshake messages. The latter contains the EAP server's authentication response to the peer. The peer will then verify the finished message in order to authenticate the EAP server.
If EAP server authentication is unsuccessful, the peer SHOULD delete the session from its cache, preventing reuse of the sessionId. 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.
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In the case where the server authenticates to the peer successfully, but the peer fails to authenticate to the server, the conversation will appear as follows:
EAP-TLS peer and server implementations MAY support privacy. Disclosure of the username is avoided by utilizing a privacy Network Access Identifier (NAI) [RFC4282] in the EAP-Response/Identity, and transmitting the peer certificate within a TLS session providing confidentiality.
In order to avoid disclosing the peer username, an EAP-TLS peer configured for privacy MUST negotiate a TLS ciphersuite supporting confidentiality and MUST provide a client certificate list containing no entries in response to the initial certificate_request from the EAP-TLS server.
An EAP-TLS server supporting privacy MUST NOT treat a certificate list containing no entries as a terminal condition; instead, it MUST bring up the TLS session and then send a hello_request. The handshake then proceeds normally; the peer sends a client_hello and the server replies with a server_hello, certificate, server_key_exchange, certificate_request, server_hello_done, etc.
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For the calculation of exported keying material (see Section 2.3), the master_secret derived within the second handshake is used.
An EAP-TLS peer supporting privacy MUST provide a certificate list containing at least one entry in response to the subsequent certificate_request sent by the server. If the EAP-TLS server supporting privacy does not receive a client certificate in response to the subsequent certificate_request, then it MUST abort the session.
EAP-TLS privacy support is designed to allow EAP-TLS peers that do not support privacy to interoperate with EAP-TLS servers supporting privacy. EAP-TLS servers supporting privacy MUST request a client certificate, and MUST be able to accept a client certificate offered by the EAP-TLS peer, in order to preserve interoperability with EAP- TLS peers that do not support privacy.
However, an EAP-TLS peer configured for privacy typically will not be able to successfully authenticate with an EAP-TLS server that does not support privacy, since such a server will typically treat the refusal to provide a client certificate as a terminal error. As a result, unless authentication failure is considered preferable to disclosure of the username, EAP-TLS peers SHOULD only be configured for privacy on networks known to support it.
This is most easily achieved with EAP lower layers that support network advertisement, so that the network and appropriate privacy configuration can be determined. In order to determine the privacy configuration on link layers (such as IEEE 802 wired networks) that do not support network advertisement, it may be desirable to utilize information provided in the server certificate (such as the subject and subjectAltName fields) or within identity selection hints [RFC4284] to determine the appropriate configuration.
In the case where the peer and server support privacy and mutual authentication, the conversation will appear as follows:
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 16 MB. The group of EAP-TLS messages sent in a single round may thus be larger than the MTU size or the maximum Remote Authentication Dail-In User Service (RADIUS) packet size of 4096 octets. As a result, an EAP-TLS implementation MUST provide its own support for fragmentation and reassembly. However, in order to ensure interoperability with existing implementations, TLS handshake messages SHOULD NOT be fragmented into multiple TLS records if they fit within a single TLS record.
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 single certificate is rarely longer than a few thousand octets, and no other field is likely to be anywhere near as long, a reasonable choice of maximum acceptable message length might be 64 KB.
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 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.
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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 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.
In the case where the EAP-TLS mutual authentication is successful, and fragmentation is required, the conversation will appear as follows:
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 mechanism for determining both the peer identity (Peer-Id in [KEYFRAME]) and server identity (Server-Id in [KEYFRAME]). For details, see Section 5.2.
Since the identity presented in the EAP-Response/Identity need not be related to the identity presented in the peer certificate, EAP-TLS implementations SHOULD NOT require that they be identical. However, if they are not identical, the identity presented in the EAP- Response/Identity is unauthenticated information, and SHOULD NOT be used for access control or accounting purposes.
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TLS-PRF-X = TLS pseudo-random function defined in [RFC4346], computed to X octets.
In EAP-TLS, the MSK, EMSK, and Initialization Vector (IV) are derived from the TLS master secret via a one-way function. This ensures that the TLS master secret cannot be derived from the MSK, EMSK, or IV unless the one-way function (TLS PRF) is broken. Since the MSK and EMSK are derived from the TLS master secret, if the TLS master secret is compromised then the MSK and EMSK are also compromised.
The MSK is divided into two halves, corresponding to the "Peer to Authenticator Encryption Key" (Enc-RECV-Key, 32 octets) and "Authenticator to Peer Encryption Key" (Enc-SEND-Key, 32 octets).
The IV is a 64-octet quantity that is a known value; octets 0-31 are known as the "Peer to Authenticator IV" or RECV-IV, and octets 32-63 are known as the "Authenticator to Peer IV", or SEND-IV.
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| | pre_master_secret | server| | | client Random| V | Random | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +---->| master_secret |<----+ | | (TMS) | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | V V V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | key_block | | label == "key expansion" | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | | client | server | client | server | client | server | MAC | MAC | write | write | IV | IV | | | | | | V V V V V V
Enc-RECV-Key = MSK(0,31) = Peer to Authenticator Encryption Key (MS-MPPE-Recv-Key in [RFC2548]). Also known as the PMK in [IEEE-802.11]. Enc-SEND-Key = MSK(32,63) = Authenticator to Peer Encryption Key (MS-MPPE-Send-Key in [RFC2548]) RECV-IV = IV(0,31) = Peer to Authenticator Initialization Vector SEND-IV = IV(32,63) = Authenticator to Peer Initialization Vector Session-Id = 0x0D || client.random || server.random
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Where:
Key_Material(W,Z) = Octets W through Z inclusive of the key material. IV(W,Z) = Octets W through Z inclusive of the IV. MSK(W,Z) = Octets W through Z inclusive of the MSK. EMSK(W,Z) = Octets W through Z inclusive of the EMSK. TLS-PRF-X = TLS PRF function computed to X octets. client.random = Nonce generated by the TLS client. server.random = Nonce generated by the TLS server.
| | pre_master_secret | server| | | client Random| V | Random | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +---->| master_secret |<----+ | | | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | V V V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | MSK, EMSK | | label == "client EAP encryption" | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | MSK(0,31) | MSK(32,63) | EMSK(0,63) | | | | | | V V V
Figure 2 - EAP-TLS Key Hierarchy
The use of these keys is specific to the lower layer, as described in Section 2.1 of [KEYFRAME].
EAP-TLS implementations need not necessarily support all TLS ciphersuites listed in [RFC4346]. Not all TLS ciphersuites are supported by available TLS tool kits, and licenses may be required in some cases.
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To ensure interoperability, EAP-TLS peers and servers MUST support the TLS [RFC4346] mandatory-to-implement ciphersuite:
TLS_RSA_WITH_3DES_EDE_CBC_SHA
EAP-TLS peers and servers SHOULD also support and be able to negotiate the following TLS ciphersuites:
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 MUST NOT be used to negotiate the ciphersuites used to secure data.
TLS also supports compression as well as ciphersuite negotiation. However, during the EAP-TLS conversation the EAP peer and server MUST NOT request or negotiate compression.
RFC 5216 EAP-TLS Authentication Protocol March 2008
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
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 MUST be ignored on reception.
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 acknowledgment. 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.
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TLS data
The TLS data consists of the encapsulated TLS packet in TLS record format.
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 Data fields. Octets outside the range of the Length field should be treated as Data Link Layer padding and MUST be ignored on reception.
Type
13 -- EAP-TLS
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Flags
0 1 2 3 4 5 6 7 8 +-+-+-+-+-+-+-+-+ |L M R R R R R R| +-+-+-+-+-+-+-+-+
L = Length included M = More fragments 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. 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.
[1] A formal proof of the security of EAP-TLS when used with [IEEE-802.11] is provided in [He]. This proof relies on the assumption that the private key pairs used by the EAP peer and server are not shared with other parties or applications. For example, a backend authentication server supporting EAP-TLS SHOULD NOT utilize the same certificate with https.
[2] Privacy is an optional feature described in Section 2.1.4.
[3] Section 5 of BCP 86 [RFC3766] offers advice on the required RSA or Diffie-Hellman (DH) module and Digital Signature Algorithm (DSA) subgroup size in bits, for a given level of attack resistance in bits. For example, a 2048-bit RSA key is recommended to provide 128-bit equivalent key strength. The National Institute of Standards and Technology (NIST) also offers advice on appropriate key sizes in [SP800-57].
[4] EAP-TLS inherits the secure ciphersuite negotiation features of TLS, including key derivation function negotiation when utilized with TLS v1.2 [RFC4346bis].
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The EAP-TLS peer name (Peer-Id) represents the identity to be used for access control and accounting purposes. The Server-Id represents the identity of the EAP server. Together the Peer-Id and Server-Id name the entities involved in deriving the MSK/EMSK.
In EAP-TLS, the Peer-Id and Server-Id are determined from the subject or subjectAltName fields in the peer and server certificates. For details, see Section 4.1.2.6 of [RFC3280]. Where the subjectAltName field is present in the peer or server certificate, the Peer-Id or Server-Id MUST be set to the contents of the subjectAltName. If subject naming information is present only in the subjectAltName extension of a peer or server certificate, then the subject field MUST be an empty sequence and the subjectAltName extension MUST be critical.
Where the peer identity represents a host, a subjectAltName of type dnsName SHOULD be present in the peer certificate. Where the peer identity represents a user and not a resource, a subjectAltName of type rfc822Name SHOULD be used, conforming to the grammar for the Network Access Identifier (NAI) defined in Section 2.1 of [RFC4282]. If a dnsName or rfc822Name are not available, other field types (for example, a subjectAltName of type ipAddress or uniformResourceIdentifier) MAY be used.
A server identity will typically represent a host, not a user or a resource. As a result, a subjectAltName of type dnsName SHOULD be present in the server certificate. If a dnsName is not available other field types (for example, a subjectAltName of type ipAddress or uniformResourceIdentifier) MAY be used.
Conforming implementations generating new certificates with Network Access Identifiers (NAIs) MUST use the rfc822Name in the subject alternative name field to describe such identities. The use of the subject name field to contain an emailAddress Relative Distinguished Name (RDN) is deprecated, and MUST NOT be used. The subject name field MAY contain other RDNs for representing the subject's identity.
Where it is non-empty, the subject name field MUST contain an X.500 distinguished name (DN). If subject naming information is present only in the subject name field of a peer certificate and the peer identity represents a host or device, the subject name field SHOULD contain a CommonName (CN) RDN or serialNumber RDN. If subject naming information is present only in the subject name field of a server certificate, then the subject name field SHOULD contain a CN RDN or serialNumber RDN.
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It is possible for more than one subjectAltName field to be present in a peer or server certificate in addition to an empty or non-empty subject distinguished name. EAP-TLS implementations supporting export of the Peer-Id and Server-Id SHOULD export all the subjectAltName fields within Peer-Ids or Server-Ids, and SHOULD also export a non-empty subject distinguished name field within the Peer- Ids or Server-Ids. All of the exported Peer-Ids and Server-Ids are considered valid.
EAP-TLS implementations supporting export of the Peer-Id and Server- Id SHOULD export Peer-Ids and Server-Ids in the same order in which they appear within the certificate. Such canonical ordering would aid in comparison operations and would enable using those identifiers for key derivation if that is deemed useful. However, the ordering of fields within the certificate SHOULD NOT be used for access control.
Since the EAP-TLS server is typically connected to the Internet, it SHOULD support validating the peer certificate using RFC 3280 [RFC3280] compliant path validation, including the ability to retrieve intermediate certificates that may be necessary to validate the peer certificate. For details, see Section 4.2.2.1 of [RFC3280].
Where the EAP-TLS server is unable to retrieve intermediate certificates, either it will need to be pre-configured with the necessary intermediate certificates to complete path validation or it will rely on the EAP-TLS peer to provide this information as part of the TLS handshake (see Section 7.4.6 of [RFC4346]).
In contrast to the EAP-TLS server, the EAP-TLS peer may not have Internet connectivity. Therefore, the EAP-TLS server SHOULD provide its entire certificate chain minus the root to facilitate certificate validation by the peer. The EAP-TLS peer SHOULD support validating the server certificate using RFC 3280 [RFC3280] compliant path validation.
Once a TLS session is established, EAP-TLS peer and server implementations MUST validate that the identities represented in the certificate are appropriate and authorized for use with EAP-TLS. The authorization process makes use of the contents of the certificates as well as other contextual information. While authorization requirements will vary from deployment to deployment, it is RECOMMENDED that implementations be able to authorize based on the EAP-TLS Peer-Id and Server-Id determined as described in Section 5.2.
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In the case of the EAP-TLS peer, this involves ensuring that the certificate presented by the EAP-TLS server was intended to be used as a server certificate. Implementations SHOULD use the Extended Key Usage (see Section 4.2.1.13 of [RFC3280]) extension and ensure that at least one of the following is true:
1) The certificate issuer included no Extended Key Usage identifiers in the certificate. 2) The issuer included the anyExtendedKeyUsage identifier in the certificate (see Section 4.2.1.13 of [RFC3280]). 3) The issuer included the id-kp-serverAuth identifier in the certificate (see Section 4.2.1.13 [RFC3280]).
When performing this comparison, implementations MUST follow the validation rules specified in Section 3.1 of [RFC2818]. In the case of the server, this involves ensuring the certificate presented by the EAP-TLS peer was intended to be used as a client certificate. Implementations SHOULD use the Extended Key Usage (see Section 4.2.1.13 [RFC3280]) extension and ensure that at least one of the following is true:
1) The certificate issuer included no Extended Key Usage identifiers in the certificate. 2) The issuer included the anyExtendedKeyUsage identifier in the certificate (see Section 4.2.1.13 of [RFC3280]). 3) The issuer included the id-kp-clientAuth identifier in the certificate (see Section 4.2.1.13 of [RFC3280]).
Certificates are long-lived assertions of identity. Therefore, it is important for EAP-TLS implementations to be capable of checking whether these assertions have been revoked.
EAP-TLS peer and server implementations MUST support the use of Certificate Revocation Lists (CRLs); for details, see Section 3.3 of [RFC3280]. EAP-TLS peer and server implementations SHOULD also support the Online Certificate Status Protocol (OCSP), described in "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP" [RFC2560]. OCSP messages are typically much smaller than CRLs, which can shorten connection times especially in bandwidth-constrained environments. While EAP-TLS servers are typically connected to the Internet during the EAP conversation, an EAP-TLS peer may not have Internet connectivity until authentication completes.
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In the case where the peer is initiating a voluntary Layer 2 tunnel using PPTP [RFC2637] or L2TP [RFC2661], the peer will typically already have a PPP interface and Internet connectivity established at the time of tunnel initiation.
However, in the case where the EAP-TLS peer is attempting to obtain network access, it will not have network connectivity and is therefore not capable of checking for certificate revocation until after authentication completes and network connectivity is available. For this reason, EAP-TLS peers and servers SHOULD implement Certificate Status Request messages, as described in "Transport Layer Security (TLS) Extensions", Section 3.6 of [RFC4366]. To enable revocation checking in situations where servers do not support Certificate Status Request messages and network connectivity is not available prior to authentication completion, peer implementations MUST also support checking for certificate revocation after authentication completes and network connectivity is available, and they SHOULD utilize this capability by default.
The integrity protection of EAP-TLS packets does not extend to the EAP header fields (Code, Identifier, Length) or the Type or Flags fields. As a result, these fields can be modified by an attacker.
In most cases, modification of the Code or Identifier fields will only result in a denial-of-service attack. However, an attacker can add additional data to an EAP-TLS packet so as to cause it to be longer than implied by the Length field. EAP peers, authenticators, or servers that do not check for this could be vulnerable to a buffer overrun.
It is also possible for an attacker to modify the Type or Flags fields. By modifying the Type field, an attacker could cause one TLS-based EAP method to be negotiated instead of another. For example, the EAP-TLS Type field (13) could be changed to indicate another TLS-based EAP method. Unless the alternative TLS-based EAP method utilizes a different key derivation formula, it is possible that an EAP method conversation altered by a man-in-the-middle could run all the way to completion without detection. Unless the ciphersuite selection policies are identical for all TLS-based EAP methods utilizing the same key derivation formula, it may be possible for an attacker to mount a successful downgrade attack, causing the peer to utilize an inferior ciphersuite or TLS-based EAP method.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3268] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)", RFC 3268, June 2002.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The Network Access Identifier", RFC 4282, December 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366, April 2006.
[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.
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[IEEE-802.11] 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 Std. 802.11-2007, 2007.
[IEEE-802.16e] Institute of Electrical and Electronics Engineers, "IEEE Standard for Local and Metropolitan Area Networks: Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems: Amendment for Physical and Medium Access Control Layers for Combined Fixed and Mobile Operations in Licensed Bands", IEEE 802.16e, August 2005.
[He] He, C., Sundararajan, M., Datta, A., Derek, A. and J. Mitchell, "A Modular Correctness Proof of IEEE 802.11i and TLS", CCS '05, November 7-11, 2005, Alexandria, Virginia, USA
[KEYFRAME] Aboba, B., Simon, D. and P. Eronen, "Extensible Authentication Protocol (EAP) Key Management Framework", Work in Progress, November 2007.
[RFC1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W., and G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637, July 1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, August 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication Protocol", RFC 2716, October 1999.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs", RFC 4017, March 2005.
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[RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity Selection Hints for the Extensible Authentication Protocol (EAP)", RFC 4284, January 2006.
[SP800-57] National Institute of Standards and Technology, "Recommendation for Key Management", Special Publication 800-57, May 2006.
[RFC4346bis] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.2", Work in Progress, February 2008.
[UNAUTH] Schulzrinne. H., McCann, S., Bajko, G. and H. Tschofenig, "Extensions to the Emergency Services Architecture for dealing with Unauthenticated and Unauthorized Devices", Work in Progress, November 2007.
Acknowledgments
Thanks to Terence Spies, Mudit Goel, Anthony Leibovitz, and Narendra Gidwani of Microsoft, Glen Zorn of NetCube, Joe Salowey of Cisco, and Pasi Eronen of Nokia for useful discussions of this problem space.
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This appendix lists the major changes between [RFC2716] and this document. Minor changes, including style, grammar, spelling, and editorial changes, are not mentioned here.
o As EAP is now in use with a variety of lower layers, not just PPP for which it was first designed, mention of PPP is restricted to situations relating to PPP-specific behavior and reference is made to other lower layers such as IEEE 802.11, IEEE 802.16, etc.
o The document now cites TLS v1.1 as a normative reference (Sections 1 and 6.1).
o The terminology section has been updated to reflect definitions from [RFC3748] (Section 1.2), and the EAP Key Management Framework [KEYFRAME] (Section 1.2).
o Use for peer unauthenticated access is clarified (Section 2.1.1).
o Privacy is supported as an optional feature (Section 2.1.4).
o It is no longer recommended that the identity presented in the EAP-Response/Identity be compared to the identity provided in the peer certificate (Section 2.2).
o The EAP-TLS key hierarchy is defined, using terminology from [RFC3748]. This includes formulas for the computation of TEKs as well as the MSK, EMSK, IV, and Session-Id (Section 2.3).
o Mandatory and recommended TLS ciphersuites are provided. The use of TLS ciphersuite negotiation for determining the lower layer ciphersuite is prohibited (Section 2.4).
o The Start bit is not set within an EAP-Response packet (Section 3.2).
o A section on security claims has been added and advice on key strength is provided (Section 5.1).
o The Peer-Id and Server-Id are defined (Section 5.2), and requirements for certificate validation (Section 5.3) and revocation (Section 5.4) are provided.
o Packet modification attacks are described (Section 5.5).
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o The examples have been updated to reflect typical messages sent in the described scenarios. For example, where mutual authentication is performed, the EAP-TLS server is shown to request a client certificate and the peer is shown to provide a certificate_verify message. A privacy example is provided, and two faulty examples of session resume failure were removed.
Authors' Addresses
Dan Simon Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399
RFC 5216 EAP-TLS Authentication Protocol March 2008
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