| Network Working Group | J. Mattsson |
| Internet-Draft | M. Sethi |
| Updates: 5216 (if approved) | Ericsson |
| Intended status: Standards Track | August 3, 2019 |
| Expires: February 4, 2020 |
Using EAP-TLS with TLS 1.3
draft-ietf-emu-eap-tls13-06
This document specifies the use of EAP-TLS with TLS 1.3 while remaining backwards compatible with existing implementations of EAP-TLS. TLS 1.3 provides significantly improved security, privacy, and reduced latency when compared to earlier versions of TLS. EAP-TLS with TLS 1.3 further improves security and privacy by mandating use of privacy and revocation checking. This document updates RFC 5216.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on February 4, 2020.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
The Extensible Authentication Protocol (EAP), defined in [RFC3748], provides a standard mechanism for support of multiple authentication methods. EAP-Transport Layer Security (EAP-TLS) [RFC5216] specifies an EAP authentication method with certificate-based mutual authentication and key derivation utilizing the TLS handshake protocol for cryptographic algorithms and protocol version negotiation, mutual authentication, and establishment of shared secret keying material. EAP-TLS is widely supported for authentication in IEEE 802.11 [IEEE-802.11] networks (Wi-Fi) using IEEE 802.1X [IEEE-802.1X] and it's the default mechanism for certificate based authentication in 3GPP 5G [TS.33.501] and MulteFire [MulteFire] networks. EAP-TLS [RFC5216] references TLS 1.0 [RFC2246] and TLS 1.1 [RFC4346], but works perfectly also with TLS 1.2 [RFC5246]. TLS 1.0 and 1.1 are formally deprecated and prohibited to negotiate and use [I-D.ietf-tls-oldversions-deprecate].
Weaknesses found in TLS 1.2, as well as new requirements for security, privacy, and reduced latency has led to the specification of TLS 1.3 [RFC8446], which obsoletes TLS 1.2 [RFC5246]. TLS 1.3 is in large parts a complete remodeling of the TLS handshake protocol including a different message flow, different handshake messages, different key schedule, different cipher suites, different resumption, and different privacy protection. This means that significant parts of the normative text in the previous EAP-TLS specification [RFC5216] are not applicable to EAP-TLS with TLS 1.3 (or higher). Therefore, aspects such as resumption, privacy handling, and key derivation need to be appropriately addressed for EAP-TLS with TLS 1.3 (or higher).
This document defines how to use EAP-TLS with TLS 1.3 (or higher) and does not change how EAP-TLS is used with older versions of TLS. While this document updates EAP-TLS [RFC5216], it remains backwards compatible with it and existing implementations of EAP-TLS. This document only describes differences compared to [RFC5216].
In addition to the improved security and privacy offered by TLS 1.3, there are other significant benefits of using EAP-TLS with TLS 1.3. Privacy is mandatory and achieved without any additional round-trips, revocation checking is mandatory and easy with OCSP stapling, and TLS 1.3 introduces more possibilities to reduce fragmentation when compared to earlier versions of TLS.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED","MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
Readers are expected to be familiar with the terms and concepts used in EAP-TLS [RFC5216] and TLS [RFC8446].
TLS 1.3 changes both the message flow and the handshake messages compared to earlier versions of TLS. Therefore, much of Section 2.1 of [RFC5216] does not apply for TLS 1.3 (or higher).
After receiving an EAP-Request packet with EAP-Type=EAP-TLS as described in [RFC5216] the conversation will continue with the TLS handshake protocol encapsulated in the data fields of EAP-Response and EAP-Request packets. When EAP-TLS is used with TLS version 1.3 or higher, the formatting and processing of the TLS handshake SHALL be done as specified in that version of TLS. This document only lists additional and different requirements, restrictions, and processing compared to [RFC8446] and [RFC5216].
The EAP server MUST authenticate with a certificate and SHOULD require the EAP peer to authenticate with a certificate. Certificates can be of any type supported by TLS including raw public keys. Pre-Shared Key (PSK) authentication SHALL NOT be used except for resumption. SessionID is deprecated in TLS 1.3 and the EAP server SHALL ignore the legacy_session_id field if TLS 1.3 is negotiated. TLS 1.3 introduced early application data which is not used in EAP-TLS. A server which receives an "early_data" extension MUST ignore the extension or respond with a HelloRetryRequest as described in Section 4.2.10 of [RFC8446]. Resumption is handled as described in Section 2.1.6. After the TLS handshake has completed and all Post-Handshake messages have been sent, the EAP server sends EAP-Success.
In the case where EAP-TLS with mutual authentication is successful, the conversation will appear as shown in Figure 1. The EAP server commits to not send any more handshake messages by sending a TLS record with the application data 0x00, see Section 2.5.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
TLS Finished,
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
<-------- EAP-Success
Figure 1: EAP-TLS mutual authentication
TLS 1.3 changes both the message flow and the handshake messages compared to earlier versions of TLS. Therefore, some normative text in Section 2.1.3 of [RFC5216] does not apply for TLS 1.3 or higher. The two paragraphs below replaces the corresponding paragraphs in Section 2.1.3 of [RFC5216] when EAP-TLS is used with TLS 1.3 or higher. The other paragraphs in Section 2.1.3 of [RFC5216] still apply with the exception that SessionID is deprecated.
Figures 2, 3, and 4 illustrate message flows in several cases where the EAP peer or EAP server sends a TLS fatal alert message. TLS warning alerts generally mean that the connection can continue normally and does not change the message flow. Note that the party receiving a TLS warning alert may choose to terminate the connection by sending a TLS fatal alert, which may add an extra round-trip, see [RFC8446].
In the case where the server rejects the ClientHello, the conversation will appear as shown in Figure 2.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Fatal Alert)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Failure
Figure 2: EAP-TLS server rejection of ClientHello
In the case where server authentication is unsuccessful, the conversation will appear as shown in Figure 3.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
TLS Finished,
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Fatal Alert)
-------->
<-------- EAP-Failure
Figure 3: EAP-TLS unsuccessful server authentication
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 shown in Figure 4.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
TLS Finished,
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Fatal Alert)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Failure
Figure 4: EAP-TLS unsuccessful client authentication
In the case where EAP-TLS is used without peer authentication (e.g., emergency services, as described in [RFC7406]) the conversation will appear as shown in Figure 5.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS Certificate,
TLS CertificateVerify,
TLS Finished,
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Finished) -------->
<-------- EAP-Success
Figure 5: EAP-TLS without peer authentication
TLS 1.3 [RFC8446] defines that TLS servers can send a HelloRetryRequest message in response to a ClientHello if the server finds an acceptable set of parameters but the initial ClientHello does not contain all the needed information to continue the handshake.
An EAP-TLS peer and server SHOULD support the use of HelloRetryRequest message. As noted in Section 4.1.4 of [RFC8446], the server MUST provide the supported_versions extensions and SHOULD contain the minimal set of extensions necessary for the client to generate a correct ClientHello pair. A HelloRetryRequest MUST NOT contain any extensions that were not first offered by the client in its ClientHello, with the exception of optionally the cookie extension.
The case of a successful EAP-TLS mutual authentication after the server has sent a HelloRetryRequest message is shown in Figure 6. Note the extra round-trip as a result of the HelloRetryRequest.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS HelloRetryRequest)
<--------
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS Finished,
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Finished) -------->
<-------- EAP-Success
Figure 6: EAP-TLS with Hello Retry Request
When using EAP-TLS with TLS 1.3, the EAP server MUST indicate support of resumption in the initial authentication. To indicate support of resumption, the EAP server sends a NewSessionTicket message (containing a PSK and other parameters) after it has received the Finished message. The NewSessionTicket message MUST NOT include an "early_data" extension.
In the case where EAP-TLS with mutual authentication and ticket establishment is successful, the conversation will appear as shown in Figure 7.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
<-------- TLS Finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS NewSessionTicket,
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Success
Figure 7: EAP-TLS ticket establishment
TLS 1.3 replaces the session resumption mechanisms in earlier versions of TLS with a new PSK exchange. When EAP-TLS is used with TLS version 1.3 or higher, EAP-TLS SHALL use a resumption mechanism compatible with that version of TLS.
For TLS 1.3, resumption is described in Section 2.2 of [RFC8446]. If the client has received a NewSessionTicket message from the server, the client can use the PSK identity received in the ticket to negotiate the use of the associated PSK. If the server accepts it, then the security context of the new connection is tied to the original connection and the key derived from the initial handshake is used to bootstrap the cryptographic state instead of a full handshake. It is left up to the EAP peer whether to use resumption, but it is RECOMMENDED that the EAP server accept resumption as long as the ticket is valid. However, the server MAY choose to require a full authentication.
A subsequent authentication using resumption, where both sides authenticate successfully is shown in Figure 8.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS Finished,
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS
(TLS Finished) -------->
<-------- EAP-Success
Figure 8: EAP-TLS resumption
As specified in Section 2.2 of [RFC8446], the EAP peer SHOULD supply a "key_share" extension when offering resumption, which allows the EAP server to decline resumption and continue the handshake as a full handshake. The message flow in case of mutual authentication is given by Figure 1. If the EAP peer did not supply a "key_share" extension when offering resumption, the EAP server needs to reject the ClientHello and the EAP peer needs to restart a full handshake. The message flow in this case is given by Figure 2 followed by Figure 1.
Also during resumption, the server can respond with a Hello Retry Request (see Section 2.1.4) and issue a new ticket (see Section 2.1.5)
TLS 1.3 significantly improves privacy when compared to earlier versions of TLS by forbidding cipher suites without confidentiality and encrypting large parts of the TLS handshake including the certificate messages.
EAP-TLS peer and server implementations supporting TLS 1.3 or higher MUST support anonymous NAIs (Network Access Identifiers) (Section 2.4 in [RFC7542]) and a client supporting TLS 1.3 MUST NOT send its username in cleartext in the Identity Response. It is RECOMMENDED to use anonymous NAIs, but other privacy-friendly identities (e.g. encrypted usernames) MAY be used.
As the certificate messages in TLS 1.3 are encrypted, there is no need to send an empty certificate_list and perform a second handshake for privacy (as needed by EAP-TLS with earlier versions of TLS). When EAP-TLS is used with TLS version 1.3 or higher the EAP-TLS peer and EAP-TLS server SHALL follow the processing specified by the used version of TLS. For TLS 1.3 this means that the EAP-TLS peer only sends an empty certificate_list if it does not have an appropriate certificate to send, and the EAP-TLS server MAY treat an empty certificate_list as a terminal condition.
EAP-TLS with TLS 1.3 is always used with privacy. This does not add any extra round-trips and the message flow with privacy is just the normal message flow as shown in Figure 1.
Including ContentType and ProtocolVersion a single TLS record may be up to 16387 octets in length. 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. Implementations MUST NOT set the L bit in unfragmented messages, but MUST accept unfragmented messages with and without the L bit set.
Some EAP implementations and access networks may limit the number of EAP packet exchanges that can be handled. To avoid fragmentation, it is RECOMMENDED to keep the sizes of client, server, and trust anchor certificates small and the length of the certificate chains short. In addition, it is RECOMMENDED to use mechanisms that reduce the sizes of Certificate messages.
While Elliptic Curve Cryptography (ECC) was optional for earlier version of TLS, TLS 1.3 mandates support of ECC (see Section 9 of [RFC8446]). To avoid fragmentation, the use of ECC in certificates, signature algorithms, and groups are RECOMMENDED when using EAP-TLS with TLS 1.3 or higher. At a 128-bit security level, this reduces public key sizes from 384 bytes (RSA and DHE) to 32-64 bytes (ECDHE) and signatures from 384 bytes (RSA) to 64 bytes (ECDSA and EdDSA). An EAP-TLS deployment MAY further reduce the certificate sizes by limiting the number of Subject Alternative Names.
Endpoints SHOULD reduce the sizes of Certificate messages by omitting certificates that the other endpoint is known to possess. When using TLS 1.3, all certificates that specifies a trust anchor may be omitted (see Section 4.4.2 of [RFC8446]). When using TLS 1.2, only the self-signed certificate that specifies the root certificate authority may be omitted (see Section 7.4.2 of [RFC5246]). EAP-TLS peers and servers SHOULD support and use the Cached Information Extension as specified in [RFC7924]. EAP-TLS peers and servers MAY use other extensions for reducing the sizes of Certificate messages, e.g. certificate compression [I-D.ietf-tls-certificate-compression].
The identity provided in the EAP-Response/Identity is not authenticated by EAP-TLS. Unauthenticated information SHALL NOT be used for accounting purposes or to give authorization. The authenticator and the EAP server MAY examine the identity presented in EAP-Response/Identity for purposes such as routing and EAP method selection. They MAY reject conversations if the identity does not match their policy. Note that this also applies to resumption, see Sections 2.1.6, 5.6, and 5.7.
TLS 1.3 replaces the TLS pseudorandom function (PRF) used in earlier versions of TLS with HKDF and completely changes the Key Schedule. The key hierarchies shown in Section 2.3 of [RFC5216] are therefore not correct when EAP-TLS is used with TLS version 1.3 or higher. For TLS 1.3 the key schedule is described in Section 7.1 of [RFC8446].
When EAP-TLS is used with TLS version 1.3 or higher the Key_Material, IV, and Method-Id SHALL be derived from the exporter_master_secret using the TLS exporter interface [RFC5705] (for TLS 1.3 this is defined in Section 7.5 of [RFC8446]).
Type-Code = 0x0D
Key_Material = TLS-Exporter("EXPORTER_EAP_TLS_Key_Material",
Type-Code, 128)
IV = TLS-Exporter("EXPORTER_EAP_TLS_IV",
Type-Code, 64)
Method-Id = TLS-Exporter("EXPORTER_EAP_TLS_Method-Id",
Type-Code, 64)
Session-Id = Type-Code || Method-Id
All other parameters such as MSK and EMSK are derived in the same manner as with EAP-TLS [RFC5216], Section 2.3. The definitions are repeated below for simplicity:
MSK = Key_Material(0, 63) EMSK = Key_Material(64, 127) Enc-RECV-Key = MSK(0, 31) Enc-SEND-Key = MSK(32, 63) RECV-IV = IV(0, 31) SEND-IV = IV(32, 63)
The use of these keys is specific to the lower layer, as described [RFC5247].
Note that the key derivation MUST use the length values given above. While in TLS 1.2 and earlier it was possible to truncate the output by requesting less data from the TLS-Exporter function, this practice is not possible with TLS 1.3. If an implementation intends to use only a part of the output of the TLS-Exporter function, then it MUST ask for the full output and then only use the desired part. Failure to do so will result in incorrect values being calculated for the above keying material.
By using the TLS exporter, EAP-TLS can use any TLS 1.3 implementation without having to extract the Master Secret, ClientHello.random, and ServerHello.random in a non-standard way.
TLS 1.3 cipher suites are defined differently than in earlier versions of TLS (see Section B.4 of [RFC8446]), and the cipher suites discussed in Section 2.4 of [RFC5216] can therefore not be used when EAP-TLS is used with TLS version 1.3 or higher.
When EAP-TLS is used with TLS version 1.3 or higher, the EAP-TLS peers and servers MUST comply with the compliance requirements (mandatory-to-implement cipher suites, signature algorithms, key exchange algorithms, extensions, etc.) for the TLS version used. For TLS 1.3 the compliance requirements are defined in Section 9 of [RFC8446].
While EAP-TLS does not protect any application data, the negotiated cipher suites and algorithms MAY be used to secure data as done in other TLS-based EAP methods.
TLS 1.3 [RFC8446] introduces Post-Handshake messages. These Post-Handshake messages use the handshake content type and can be sent after the main handshake. One such Post-Handshake message is NewSessionTicket. The NewSessionTicket can be used for resumption. After sending TLS Finished, the EAP server may send any number of Post-Handshake messages in separate EAP-Requests. To decrease the uncertainty for the EAP peer, the following procedure MUST be followed:
When an EAP server has sent its last handshake message (Finished or a Post-Handshake), it commits to not sending any more handshake messages by sending a TLS record with application data 0x00 (i.e. a TLS record with TLSPlaintext.type = application_data, TLSPlaintext.length = 1, and TLSPlaintext.fragment = 0x00). EAP server implementations MUST set TLSPlaintext.fragment to 0x00, but EAP peer implementations MUST accept any application data as a commit from the EAP server to not send any more handshake messages. The TLS record with application data may be sent in the same EAP-Request as the last handshake record or in a separate EAP-Request. Sending the commit in a separate EAP-Request adds an additional round-trip, but may be necessary in TLS implementations that only implement a subset of TLS 1.3. In the case where the server sends the commit in a separate commit, the conversation will appear as shown in Figure 9. After sending the application data record, the EAP server may only send an EAP-Success, an EAP-Failure, or an EAP-Request with a TLS Alert Message.
EAP Peer EAP Server
EAP-Request/
<-------- Identity
EAP-Response/
Identity (Privacy-Friendly) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS ClientHello) -------->
EAP-Request/
EAP-Type=EAP-TLS
(TLS ServerHello,
TLS EncryptedExtensions,
TLS CertificateRequest,
TLS Certificate,
TLS CertificateVerify,
<-------- TLS Finished,
EAP-Response/
EAP-Type=EAP-TLS
(TLS Certificate,
TLS CertificateVerify,
TLS Finished) -------->
EAP-Request/
EAP-Type=EAP-TLS
<-------- TLS Application Data)
EAP-Response/
EAP-Type=EAP-TLS -------->
<-------- EAP-Success
Figure 9: Commit in separate EAP-Request
No updates to [RFC5216].
This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to the EAP-TLS 1.3 protocol in accordance with [RFC8126].
This memo requires IANA to add the following labels to the TLS Exporter Label Registry defined by [RFC5705]. These labels are used in derivation of Key_Material, IV and Method-Id as defined in Section 2.3:
Using EAP-TLS with TLS 1.3 does not change the security claims for EAP-TLS as given in Section 4.1 of [RFC5216]. However, it strengthens several of the claims as described in the following updates to the notes given in Section 4.1 of [RFC5216].
[1] Mutual authentication: By mandating revocation checking of certificates, the authentication in EAP-TLS with TLS 1.3 is stronger as authentication with revoked certificates will always fail.
[2] Confidentiality: The TLS 1.3 handshake offers much better confidentiality than earlier versions of TLS by mandating cipher suites with confidentiality and encrypting certificates and some of the extensions, see [RFC8446]. When using EAP-TLS with TLS 1.3, the use of privacy is mandatory and does not cause any additional round-trips.
[3] Key strength: TLS 1.3 forbids all algorithms with known weaknesses including 3DES, CBC mode, RC4, SHA-1, and MD5. TLS 1.3 only supports cryptographic algorithms offering at least 112-bit security, see [RFC8446].
[4] Cryptographic Negotiation: TLS 1.3 increases the number of cryptographic parameters that are negotiated in the handshake. When EAP-TLS is used with TLS 1.3, EAP-TLS inherits the cryptographic negotiation of AEAD algorithm, HKDF hash algorithm, key exchange groups, and signature algorithm, see Section 4.1.1 of [RFC8446].
No updates to [RFC5216].
No updates to [RFC5216].
While certificates often have a long validity period spanning several years, there are a number of reasons (e.g. key compromise, CA compromise, privilege withdrawn, etc.) why client, server, or sub-CA certificates have to be revoked before their expiry date. Revocation of the EAP server’s certificate is complicated by the fact that the EAP peer may not have Internet connectivity until authentication completes.
EAP-TLS peers and servers supporting TLS 1.3 MUST support Certificate Status Requests (OCSP stapling) as specified in [RFC6066] and Section 4.4.2.1 of [RFC8446]. When EAP-TLS is used with TLS 1.3, the peer and server MUST use Certificate Status Requests [RFC6066] for the server’s certificate chain and the EAP peer MUST treat a CertificateEntry (except the trust anchor) without a valid CertificateStatus extension as invalid and abort the handshake with an appropriate alert. When EAP-TLS is used with TLS 1.3, the server MUST check the revocation status of the certificates in the certificates in the client’s certificate chain.
The OCSP status handling in TLS 1.3 is different from earlier versions of TLS, see Section 4.4.2.1 of [RFC8446]. In TLS 1.3 the OCSP information is carried in the CertificateEntry containing the associated certificate instead of a separate CertificateStatus message as in [RFC4366]. This enables sending OCSP information for all certificates in the certificate chain.
No updates to [RFC5216].
EAP-TLS is typically encapsulated in other protocols, such as PPP [RFC1661], RADIUS [RFC2865], Diameter [RFC6733], or PANA [RFC5191]. The encapsulating protocols can also provide additional, non-EAP information to an EAP server. This information can include, but is not limited to, information about the authenticator, information about the EAP peer, or information about the protocol layers above or below EAP (MAC addresses, IP addresses, port numbers, WiFi SSID, etc.). Servers implementing EAP-TLS inside those protocols can make policy decisions and enforce authorization based on a combination of information from the EAP-TLS exchange and non-EAP information.
As noted in Section 2.2, the identity presented in EAP-Response/Identity is not authenticated by EAP-TLS and is therefore trivial for an attacker to forge, modify, or replay. Authorization and accounting MUST be based on authenticated information such as information in the certificate or the PSK identity and cached data provisioned for resumption as described in Section 5.7. Note that the requirements for Network Access Identifiers (NAIs) specified in Section 4 of [RFC7542] still apply and MUST be followed.
EAP-TLS servers MAY reject conversations based on non-EAP information provided by the encapsulating protocol, for example, if the MAC address of the authenticator does not match the expected policy.
There are a number of security issues related to resumption that are not described in [RFC5216]. The problems, guidelines, and requirements in this section therefore applies to all version of TLS.
When resumption occurs, it is based on cached information at the TLS layer. To perform resumption in a secure way, the EAP-TLS peer and EAP-TLS server need to be able to securely retrieve authorization information such as certificate chains, revocation status, etc. from the initial full handshake. We use the term "cached data" to describe such information. Authorization during resumption MUST be based on such cached data.
There are two ways to retrieve the cached information from the original full handshake. The first method is that the TLS server and client cache the information locally. The cached information is identified by an identifier. For TLS versions before 1.3, the identifier can be the session ID, for TLS 1.3, the identifier is the PSK identity. The second method for retrieving cached information is via [RFC5077] or [RFC8446], where the TLS server encapsulates the information into a ticket and sends it to the client. The client can subsequently do resumption using the obtained ticket. Note that the client still needs to cache the information locally. The following requirements apply to both methods.
If the EAP server or EAP client do not apply any authorization policies, they MAY allow resumption where no cached data is available. In all other cases, they MUST cache data during the initial full authentication to enable resumption. The cached data MUST be sufficient to make authorization decisions during resumption. If cached data cannot be retrieved in a secure way, resumption MUST NOT be done.
The above requirements also apply if the EAP server expects some system to perform accounting for the session. Since accounting must be tied to an authenticated identity, and resumption does not supply such an identity, accounting is impossible without access to cached data.
Information from the EAP-TLS exchange (e.g. the identity provided in EAP-Response/Identity) as well as non-EAP information (e.g. IP addresses) may change between the initial full handshake and resumption. This change creates a "Time-of-check time-of-use" (TOCTOU) security vulnerability. A malicious or compromised user could supply one set of data during the initial authentication, and a different set of data during resumption, potentially leading to them obtaining access that they should not have.
If any authorization, accounting, or policy decisions were made with information that have changed between the initial full handshake and resumption, and if change may lead to a different decision, such decisions MUST be reevaluated. It is RECOMMENDED that authorization, accounting, and policy decisions are reevaluated based on the information given in the resumption. EAP servers MAY reject resumption where the information supplied during resumption does not match the information supplied during the original authentication. Where a good decision is unclear, EAP servers SHOULD reject the resumption.
Any security policies for authorization MUST be followed also for resumption. The EAP-TLS client and server MAY need to recheck the authorization and revocation status of the other party. The certificates may have been revoked since the initial full handshake and the authorizations of the other party may have been reduced. If the cached revocation information is not sufficiently current, the EAP Peer or EAP Server needs to force a full TLS handshake.
[RFC6973] suggests that the privacy considerations of IETF protocols be documented.
TLS 1.3 offers much better privacy than earlier versions of TLS as discussed in Section 2.1.7. In this section, we only discuss the privacy properties of EAP-TLS with TLS 1.3. For privacy properties of TLS 1.3 itself, see [RFC8446].
EAP-TLS sends the standard TLS 1.3 handshake messages encapsulated in EAP packets. Additionally, the EAP peer sends an identity in the first EAP-Response. The other fields in the EAP-TLS Request and the EAP-TLS Response packets do not contain any cleartext privacy sensitive information.
Tracking of users by eavesdropping on identity responses or certificates is a well-known problem in many EAP methods. When EAP-TLS is used with TLS 1.3, all certificates are encrypted, and the username part of the identity response is always confidentiality protected (e.g. using Anonymous NAIs). However, as with other EAP methods, even when privacy-friendly identifiers or EAP tunneling is used, the domain name (i.e. the realm) in the NAI is still typically visible. How much privacy sensitive information the domain name leaks is highly dependent on how many other users are using the same domain name in the particular access network. If all EAP peers have the same domain, no additional information is leaked. If a domain name is used by a small subset of the EAP peers, it may aid an attacker in tracking or identifying the user.
If Anonymous NAIs are not used, the privacy-friendly identifiers need to be generated with care. The identities MUST be generated in a cryptographically secure way so that that it is computationally infeasible for an attacker to differentiate two identities belonging to the same user from two identities belonging to different users in the same realm. This can be achieved, for instance, by using random or pseudo-random usernames such as random byte strings or ciphertexts. Note that the privacy-friendly usernames also MUST NOT include substrings that can be used to relate the identity to a specific user. Similarly, privacy-friendly username SHOULD NOT be formed by a fixed mapping that stays the same across multiple different authentications.
An EAP peer with a policy allowing communication with EAP servers supporting only TLS 1.2 without privacy and with a static RSA key exchange is vulnerable to disclosure of the peer username. An active attacker can in this case make the EAP peer believe that an EAP server supporting TLS 1.3 only supports TLS 1.2 without privacy. The attacker can simply impersonate the EAP server and negotiate TLS 1.2 with static RSA key exchange and send an TLS alert message when the EAP peer tries to use privacy by sending an empty certificate message. Since the attacker (impersonating the EAP server) does not provide a proof-of-possession of the private key until the Finished message when a static RSA key exchange is used, an EAP peer may inadvertently disclose its identity (username) to an attacker. Therefore, it is RECOMMENDED for EAP peers to not use EAP-TLS with TLS 1.2 and static RSA based cipher suites without privacy.
As required by [RFC7258], work on IETF protocols needs to consider the effects of pervasive monitoring and mitigate them when possible.
Pervasive Monitoring is widespread surveillance of users. By encrypting more information and by mandating the use of privacy, TLS 1.3 offers much better protection against pervasive monitoring. In addition to the privacy attacks discussed above, surveillance on a large scale may enable tracking of a user over a wider geographical area and across different access networks. Using information from EAP-TLS together with information gathered from other protocols increases the risk of identifying individual users.
Over the years, there have been several serious attacks on earlier versions of Transport Layer Security (TLS), including attacks on its most commonly used ciphers and modes of operation. [RFC7457] summarizes the attacks that were known at the time of publishing and [RFC7525] provides recommendations for improving the security of deployed services that use TLS. However, many of the attacks are less serious for EAP-TLS as EAP-TLS only uses the TLS handshake and does not protect any application data. EAP-TLS implementations SHOULD mitigate known attacks and follow the recommendations in [RFC7525] and [I-D.ietf-tls-oldversions-deprecate]. The use of TLS 1.3 mitigates most of the known attacks.
All the following references in [RFC5216] are updated as specified below when EAP-TLS is used with TLS 1.3 or higher.
All references to [RFC2560] are updated with [RFC6960].
All references to [RFC3280] are updated with [RFC5280].
All references to [RFC4282] are updated with [RFC7542].
The authors want to thank Bernard Aboba, Jari Arkko, Alan DeKok, Ari Keraenen, Jouni Malinen, Oleg Pekar, Eric Rescorla, Jim Schaad, and Vesa Torvinen for comments and suggestions on the draft.
Alan DeKok, FreeRADIUS