draft-ietf-uta-tls-bcp-04.txt   draft-ietf-uta-tls-bcp-05.txt 
UTA Y. Sheffer UTA Y. Sheffer
Internet-Draft Porticor Internet-Draft Porticor
Intended status: Best Current Practice R. Holz Intended status: Best Current Practice R. Holz
Expires: April 3, 2015 TUM Expires: April 17, 2015 TUM
P. Saint-Andre P. Saint-Andre
&yet &yet
September 30, 2014 October 14, 2014
Recommendations for Secure Use of TLS and DTLS Recommendations for Secure Use of TLS and DTLS
draft-ietf-uta-tls-bcp-04 draft-ietf-uta-tls-bcp-05
Abstract Abstract
Transport Layer Security (TLS) and Datagram Transport Security Layer Transport Layer Security (TLS) and Datagram Transport Security Layer
(DTLS) are widely used to protect data exchanged over application (DTLS) are widely used to protect data exchanged over application
protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the
last few years, several serious attacks on TLS have emerged, last few years, several serious attacks on TLS have emerged,
including attacks on its most commonly used cipher suites and modes including attacks on its most commonly used cipher suites and modes
of operation. This document provides recommendations for improving of operation. This document provides recommendations for improving
the security of deployed services that use TLS and DTLS. The the security of deployed services that use TLS and DTLS. The
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 3, 2015. This Internet-Draft will expire on April 17, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Intended Audience and Applicability Statement . . . . . . . . 4 2. Intended Audience and Applicability Statement . . . . . . . . 4
2.1. Security Services . . . . . . . . . . . . . . . . . . . . 4 2.1. Security Services . . . . . . . . . . . . . . . . . . . . 4
2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Unauthenticated TLS . . . . . . . . . . . . . . . . . . . 5
3. Conventions used in this document . . . . . . . . . . . . . . 5 3. Conventions used in this document . . . . . . . . . . . . . . 5
4. General Recommendations . . . . . . . . . . . . . . . . . . . 5 4. General Recommendations . . . . . . . . . . . . . . . . . . . 6
4.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 5 4.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 6
4.2. Applicability to DTLS . . . . . . . . . . . . . . . . . . 6 4.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 6
4.3. Fallback to SSL . . . . . . . . . . . . . . . . . . . . . 6 4.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 7
4.4. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6 4.1.3. Fallback to Earlier Versions . . . . . . . . . . . . 7
4.5. Compression . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7
4.6. TLS Session Resumption . . . . . . . . . . . . . . . . . 7 4.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8
4.7. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7 4.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8
4.8. Server Name Indication . . . . . . . . . . . . . . . . . 8 4.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 9
5. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8 4.6. Server Name Indication . . . . . . . . . . . . . . . . . 9
5.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8 5. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 9
5.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 9 5.1. General Guidelines . . . . . . . . . . . . . . . . . . . 10
5.3. Cipher Suite Negotiation Details . . . . . . . . . . . . 10 5.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 11
5.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 10 5.3. Cipher Suite Negotiation Details . . . . . . . . . . . . 11
5.5. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 11 5.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 12
5.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 11 5.5. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 5.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 14
7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 12 7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.4. Diffie Hellman Exponent Reuse . . . . . . . . . . . . . . 13 7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 14
7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 14 7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 15 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 15 9.1. Normative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 18 9.2. Informative References . . . . . . . . . . . . . . . . . 17
A.1. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 18 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 20
A.2. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 18 A.1. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 20
A.3. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 18 A.2. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 20
A.4. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 18 A.3. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 20
A.5. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 19 A.4. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 20
A.6. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 19 A.5. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 21
A.7. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 19 A.6. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 21
A.8. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 20 A.7. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 A.8. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 21
A.9. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
Transport Layer Security (TLS) and Datagram Transport Security Layer Transport Layer Security (TLS) and Datagram Transport Security Layer
(DTLS) are widely used to protect data exchanged over application (DTLS) are widely used to protect data exchanged over application
protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the
last few years, several serious attacks on TLS have emerged, last few years, several serious attacks on TLS have emerged,
including attacks on its most commonly used cipher suites and modes including attacks on its most commonly used cipher suites and modes
of operation. For instance, both AES-CBC and RC4, which together of operation. For instance, both the AES-CBC and RC4 encryption
comprise most current usage, have been attacked in the context of algorithms, which together comprise most current usage, have been
TLS. A companion document [I-D.ietf-uta-tls-attacks] provides attacked in the context of TLS. A companion document
detailed information about these attacks. [I-D.ietf-uta-tls-attacks] provides detailed information about these
attacks.
Because of these attacks, those who implement and deploy TLS and DTLS Because of these attacks, those who implement and deploy TLS and DTLS
need updated guidance on how TLS can be used securely. Note that need updated guidance on how TLS can be used securely. Note that
this document provides guidance for deployed services, as well as this document provides guidance for deployed services as well as
software implementations, assuming the implementer expects his or her software implementations, assuming the implementer expects his or her
code to be deployed in environments defined in the following section. code to be deployed in environments defined in the following section.
In fact, this document calls for the deployment of algorithms that In fact, this document calls for the deployment of algorithms that
are widely implemented but not yet widely deployed. Concerning are widely implemented but not yet widely deployed. Concerning
deployment, this document targets a wide audience, namely all deployment, this document targets a wide audience, namely all
deployers who wish to add confidentiality and data integrity deployers who wish to add confidentiality and data integrity
protection to their communications. In many (but not all) cases protection to their communications. In many (but not all) cases
authentication is also desired. This document does not address the authentication is also desired. This document does not address the
rare deployment scenarios where no confidentiality is desired. rare deployment scenarios where no confidentiality is desired.
The recommendations herein take into consideration the security of The recommendations herein take into consideration the security of
various mechanisms, their technical maturity and interoperability, various mechanisms, their technical maturity and interoperability,
and their prevalence in implementations at the time of writing. and their prevalence in implementations at the time of writing.
Unless noted otherwise, these recommendations apply to both TLS and Unless noted otherwise, these recommendations apply to both TLS and
DTLS. TLS 1.3, when it is standardized and deployed in the field, DTLS. TLS 1.3, when it is standardized and deployed in the field,
should resolve the current vulnerabilities while providing should resolve the current vulnerabilities while providing
significantly better functionality, and will very likely obsolete significantly better functionality and will very likely obsolete this
this document. document.
These are minimum recommendations for the use of TLS for the These are minimum recommendations for the use of TLS for the
specified audience. Individual specifications may have stricter specified audience. Individual specifications may have stricter
requirements related to one or more aspects of the protocol, based on requirements related to one or more aspects of the protocol, based on
their particular circumstances. When that is the case, implementers their particular circumstances. When that is the case, implementers
MUST adhere to those stricter requirements. MUST adhere to those stricter requirements.
Community knowledge about the strength of various algorithms and Community knowledge about the strength of various algorithms and
feasible attacks can change quickly, and experience shows that a feasible attacks can change quickly, and experience shows that a
security BCP is a point-in-time statement. Readers are advised to security BCP is a point-in-time statement. Readers are advised to
seek out any errata or updates that apply to this document. seek out any errata or updates that apply to this document.
2. Intended Audience and Applicability Statement 2. Intended Audience and Applicability Statement
In the following, we specify which audience this document addresses The deployment recommendations address the operators of application
concerning deployment. This document applies only to environments layer services that are most commonly used on the Internet,
where confidentiality is required. It recommends algorithms and including, but not limited to:
configuration options that make secrecy of the data-in-transit
mandatory. While this includes the majority of the TLS use cases,
there are some notable exceptions.
This document assumes that data integrity protection is always one of o Operators of WWW servers that wish to protect HTTP with TLS.
the goals of a deployment. In cases when integrity is not required,
it does not make sense to employ TLS in the first place. There are o Operators of email servers who wish to protect the application-
attacks against confidentiality-only protection that utilize the lack layer protocols with TLS (e.g., IMAP, POP3 or SMTP).
of integrity to also break confidentiality (see e.g. [DegabrieleP07]
in the context of IPsec). Thus, even when using opportunistic o Operators of instant-messaging services who wish to protect their
encryption, it is essential to provide cryptographic data integrity application-layer protocols with TLS (e.g. XMPP or IRC).
protection
2.1. Security Services 2.1. Security Services
This document provides recommendations for an audience that wishes to This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following: secure their communication with TLS to achieve the following:
o Confidentiality: all (payload) communication is encrypted with the o Confidentiality: all (payload) communication is encrypted with the
goal that no party should be able to decrypt it except the goal that no party should be able to decrypt it except the
intended receiver. intended receiver.
o Data integrity: any changes made to the communication are o Data integrity: any changes made to the communication in transit
detectable by the receiver. are detectable by the receiver.
o Optionally, authentication: this means that an end-point of the o Authentication: this means that an end-point of the TLS
TLS communication is authenticated as the intended entity to communication is authenticated as the intended entity to
communicate with. TLS allows to authenticate one or both end- communicate with. TLS allows to authenticate one or both end-
points in the communication. points in the communication. Some TLS usage scenarios do not
require authentication, and are further discussed in Section 2.2.
Deployers MUST verify that they do not need one of the above security Deployers MUST verify that they do not need one of the above security
services if they deviate from the recommendations given in this services if they deviate from the recommendations given in this
document. document.
2.2. Examples This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that
enforce secrecy of the data-in-transit. While this includes the
majority of the TLS use cases, there are some notable exceptions.
This document assumes that data integrity protection is always one of
the goals of a deployment. In cases when integrity is not required,
it does not make sense to employ TLS in the first place. There are
attacks against confidentiality-only protection that utilize the lack
of integrity to also break confidentiality (see e.g. [DegabrieleP07]
in the context of IPsec).
The intended audience covers those services that are most commonly The intended audience covers those services that are most commonly
used on the Internet. Typically, all communication between clients used on the Internet. Typically, all communication between clients
and servers requires all three of the above security services. and servers requires all three of the above security services. This
is particularly true where clients are user agents like Web browsers
or email software.
o Operators of WWW servers (HTTPS). This document does not address the rare deployment scenarios where
one of the above three properties is not desired, with the exception
of the use case described in Section 2.2 below. An example of an
audience not needing confidentiality is the following: a monitored
network where the authorities in charge of the respective traffic
domain require full access to unencrypted (plaintext) traffic, and
where users collaborate and send their traffic in the clear.
o Operators of email servers who wish to protect the application- 2.2. Unauthenticated TLS
layer protocols with TLS (e.g., IMAP, POP3, or SMTP between client
and server).
o Operators of instant-messaging services who wish to protect their Several important applications use TLS to protect data between a
application-layer protocols with TLS (e.g. XMPP or IRC between client and a server, but do so without the client verifying the
client and server). server's certificate. The reader is referred to
[I-D.dukhovni-smtp-opportunistic-tls] for additional details and an
explanation why this insecure practice is still common and likely to
remain so for a while.
An example of an audience not needing confidentiality is the In many of these scenarios the actual use of TLS is optional, i.e.
following: a monitored network where the authorities in charge of the client decides dynamically ("opportunistically") whether to use
that traffic domain require full access to unencrypted (plaintext) TLS with a particular server or to connect in the clear.
traffic, and where users collaborate and send their traffic in the Opportunistic encryption is described at length in Sec. 2 of
clear. [I-D.farrelll-mpls-opportunistic-encrypt].
Despite the threat model differing from "standard" authenticated
usage of TLS, the recommendations in this document are applicable to
unauthenticated uses of TLS, with the obvious exception of peer
authentication.
3. Conventions used in this document 3. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
4. General Recommendations 4. General Recommendations
This section provides general recommendations on the secure use of This section provides general recommendations on the secure use of
TLS. Recommendations related to cipher suites are discussed in the TLS. Recommendations related to cipher suites are discussed in the
following section. following section.
4.1. Protocol Versions 4.1. Protocol Versions
4.1.1. SSL/TLS Protocol Versions
It is important both to stop using old, less secure versions of SSL/ It is important both to stop using old, less secure versions of SSL/
TLS and to start using modern, more secure versions. Therefore: TLS and to start using modern, more secure versions; therefore, the
following are the recommendations concerning TLS/SSL protocol
versions:
o Implementations MUST NOT negotiate SSL version 2. o Implementations MUST NOT negotiate SSL version 2.
Rationale: SSLv2 is considered today as insecure [RFC6176]. Rationale: Today, SSLv2 is considered insecure [RFC6176].
o Implementations MUST NOT negotiate SSL version 3. o Implementations MUST NOT negotiate SSL version 3.
Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
plugged some significant security holes, but did not support plugged some significant security holes, but did not support
strong cipher suites. In addition, SSLv3 does not support TLS strong cipher suites. In addition, SSLv3 does not support TLS
extensions, some of which are considered security-critical today. extensions, some of which (e.g. renegotiation_info) are security-
critical.
o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246]. o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246].
Rationale: TLS 1.0 (published in 1999) does not support many Rationale: TLS 1.0 (published in 1999) does not support many
modern, strong cipher suites. modern, strong cipher suites.
o Implementations MAY negotiate TLS version 1.1 [RFC4346]. o Implementations MAY negotiate TLS version 1.1 [RFC4346].
Rationale: TLS 1.1 (published in 2006) is a security improvement Rationale: TLS 1.1 (published in 2006) is a security improvement
over TLS 1.0, but still does not support certain stronger cipher over TLS 1.0, but still does not support certain stronger cipher
suites. suites.
o Implementations MUST support, and prefer to negotiate, TLS version o Implementations MUST support, and prefer to negotiate, TLS version
1.2 [RFC5246]. 1.2 [RFC5246].
Rationale: Several stronger cipher suites are available only with Rationale: Several stronger cipher suites are available only with
TLS 1.2 (published in 2008). TLS 1.2 (published in 2008). In fact, the cipher suites
recommended by this document (Section 5.2 below) are only
available in TLS 1.2.
This BCP applies to TLS 1.2. It is not safe for readers to assume This BCP applies to TLS 1.2. It is not safe for readers to assume
that the recommendations in this BCP apply to any future version of that the recommendations in this BCP apply to any future version of
TLS. TLS.
4.2. Applicability to DTLS 4.1.2. DTLS Protocol Versions
DTLS [RFC4347] [RFC6347] is an adaptation of TLS for UDP datagrams. DTLS is an adaptation of TLS for UDP datagrams.
With respect to the recommendations in the current document, DTLS 1.0 The following are the recommendations with respect to DTLS:
is equivalent to TLS 1.1. The only exception is RC4 which is
disallowed in DTLS. DTLS 1.2 is equivalent to TLS 1.2.
4.3. Fallback to SSL o Implementations MAY negotiate DTLS version 1.0 [RFC4347].
Some client implementations revert to lower versions of TLS or even o Implementations MUST negotiate DTLS version 1.2 [RFC6347].
to SSLv3 if the server rejected higher versions of the protocol.
This fall back can be forced by a man in the middle (MITM) attacker.
By default, such clients MUST NOT fall back to SSLv3.
Rationale: TLS 1.0 and SSLv3 are significantly less secure than TLS Rationale: DTLS is an adaptation of TLS for UDP that was introduced
when TLS 1.1 was published. Version 1.0 correlates to TLS 1.1 and
Version 1.2 correlates to TLS 1.2. There is no Version 1.1.
Note: DTLS and TLS are nearly identical. The most notable exception
is that RC4, which is a stream-based bulk encryption algorithm,
cannot be supported by DTLS.
4.1.3. Fallback to Earlier Versions
Clients that "fallback" to lower versions of the protocol after the
server rejects higher versions of the protocol MUST NOT fallback to
SSLv3.
Rationale: Some client implementations revert to lower versions of
TLS or even to SSLv3 if the server rejected higher versions of the
protocol. This fallback can be forced by a man in the middle (MITM)
attacker. TLS 1.0 and SSLv3 are significantly less secure than TLS
1.2, the version recommended by this document. While TLS 1.0-only 1.2, the version recommended by this document. While TLS 1.0-only
servers are still quite common, IP scans show that SSLv3-only servers servers are still quite common, IP scans show that SSLv3-only servers
amount to only about 3% of the current Web server population. amount to only about 3% of the current Web server population.
4.4. Strict TLS 4.2. Strict TLS
Combining unprotected and TLS-protected communication opens the way To prevent SSL Stripping:
to SSL Stripping and similar attacks. Therefore:
o In cases where an application protocol allows implementations or o In cases where an application protocol allows implementations or
deployments a choice between strict TLS configuration and dynamic deployments a choice between strict TLS configuration and dynamic
upgrade from unencrypted to TLS-protected traffic (such as upgrade from unencrypted to TLS-protected traffic (such as
STARTTLS), clients and servers SHOULD prefer strict TLS STARTTLS), clients and servers SHOULD prefer strict TLS
configuration. configuration.
o In many application protocols, clients can be configured to use
TLS even if the server has not advertised that TLS is mandatory or
even supported (e.g., this is often the case in messaging
protocols such as IMAP and XMPP). Application clients SHOULD use
TLS by default, and disable this default only through explicit
configration by the user.
o HTTP client and server implementations MUST support the HTTP o HTTP client and server implementations MUST support the HTTP
Strict Transport Security (HSTS) header [RFC6797], in order to Strict Transport Security (HSTS) header [RFC6797], in order to
allow Web servers to advertise that they are willing to accept allow Web servers to advertise that they are willing to accept
TLS-only clients. TLS-only clients.
o When applicable, Web servers SHOULD use HSTS to indicate that they o When applicable, Web servers SHOULD use HSTS to indicate that they
are willing to accept TLS-only clients. are willing to accept TLS-only clients.
4.5. Compression Rationale: Combining unprotected and TLS-protected communication
opens the way to SSL Stripping and similar attacks, since an initial
part of the communication is not integrity protected and therefore
can be manipulated by an attacker whose goal is to keep the
communication in the clear.
4.3. Compression
Implementations and deployments SHOULD disable TLS-level compression Implementations and deployments SHOULD disable TLS-level compression
([RFC5246], Sec. 6.2.2), because it has been subject to security ([RFC5246], Sec. 6.2.2).
attacks.
Rationale: TLS compression has been subject to security attacks, such
as the CRIME attack.
Implementers should note that compression at higher protocol levels Implementers should note that compression at higher protocol levels
can allow an active attacker to extract cleartext information from can allow an active attacker to extract cleartext information from
the connection. The BREACH attack is one such case. These issues the connection. The BREACH attack is one such case. These issues
can only be mitigated outside of TLS and are thus out of scope of the can only be mitigated outside of TLS and are thus out of scope of the
current document. See Sec. 2.5 of [I-D.ietf-uta-tls-attacks] for current document. See Sec. 2.5 of [I-D.ietf-uta-tls-attacks] for
further details. further details.
4.6. TLS Session Resumption 4.4. TLS Session Resumption
If TLS session resumption is used, care ought to be taken to do so If TLS session resumption is used, care ought to be taken to do so
safely. In particular, when using session tickets [RFC5077], the safely. In particular, when using session tickets [RFC5077], the
resumption information MUST be authenticated and encrypted to prevent resumption information MUST be authenticated and encrypted to prevent
modification or eavesdropping by an attacker. Further modification or eavesdropping by an attacker. Further
recommendations apply to session tickets: recommendations apply to session tickets:
o A strong cipher suite MUST be used when encrypting the ticket (as o A strong cipher suite MUST be used when encrypting the ticket (as
least as strong as the main TLS cipher suite). least as strong as the main TLS cipher suite).
o Ticket keys MUST be changed regularly, e.g. once every week, so as o Ticket keys MUST be changed regularly, e.g. once every week, so as
not to negate the benefits of forward secrecy (see Section 7.3 for not to negate the benefits of forward secrecy (see Section 7.3 for
details on forward secrecy). details on forward secrecy).
o Session ticket validity SHOULD be limited to a reasonable duration o Session ticket validity SHOULD be limited to a reasonable duration
(e.g. 1 day), for similar reasons. (e.g. 1 day), for similar reasons.
4.7. TLS Renegotiation Rationale: session resumption is another kind of TLS handshake, and
therefore must be as secure as the initial handshake. This document
(Section 5) recommends the use of cipher suites that provide forward
secrecy, i.e. that prevent an attacker who gains momentary access to
the TLS endpoint (either client or server) and its secrets from
reading either past or future communication. The tickets must be
managed so as not to negate this security property.
4.5. TLS Renegotiation
Where handshake renegotiation is implemented, both clients and Where handshake renegotiation is implemented, both clients and
servers MUST implement the renegotiation_info extension, as defined servers MUST implement the renegotiation_info extension, as defined
in [RFC5746]. in [RFC5746].
To counter the Triple Handshake attack, we adopt the recommendation To counter the Triple Handshake attack, we adopt the recommendation
from [triple-handshake]: TLS clients SHOULD ensure that all from [triple-handshake]: TLS clients SHOULD ensure that all
certificates received over a connection are valid for the current certificates received over a connection are valid for the current
server endpoint, and abort the handshake if they are not. In some server endpoint, and abort the handshake if they are not. In some
usages, it may be simplest to refuse any change of certificates usages, it may be simplest to refuse any change of certificates
during renegotiation. during renegotiation.
4.8. Server Name Indication 4.6. Server Name Indication
TLS implementations MUST support the Server Name Indication (SNI) TLS implementations MUST support the Server Name Indication (SNI)
extension for those higher level protocols which would benefit from extension for those higher level protocols which would benefit from
it, including HTTPS. However, unlike implementation, the use of SNI it, including HTTPS. However, unlike implementation, the use of SNI
in particular circumstances is a matter of local policy. in particular circumstances is a matter of local policy.
Rationale: SNI supports deployment of multiple TLS-protected virtual
servers on a single address, and therefore enables fine grain
security for these virtual servers, by allowing each one to have its
own certificate.
5. Recommendations: Cipher Suites 5. Recommendations: Cipher Suites
TLS and its implementations provide considerable flexibility in the TLS and its implementations provide considerable flexibility in the
selection of cipher suites. Unfortunately many available cipher selection of cipher suites. Unfortunately many available cipher
suites are insecure, and so misconfiguration can easily result in suites are insecure, and so misconfiguration can easily result in
reduced security. This section includes recommendations on the reduced security. This section includes recommendations on the
selection and negotiation of cipher suites. selection and negotiation of cipher suites.
5.1. General Guidelines 5.1. General Guidelines
It is important both to stop using old, insecure cipher suites and to Cryptographic algorithms weaken over time as cryptanalysis improves.
start using modern, more secure cipher suites. Therefore: In other words, as time progresses, algorithms that were once
considered strong but are now weak, need to be phased out over time
and replaced with more secure cipher suites to ensure that desired
security properties still hold. SSL/TLS has been in existence for
almost 20 years at this point and this section provides some much
needed recommendations concerning cipher suite selection:
o Implementations MUST NOT negotiate the NULL cipher suites. o Implementations MUST NOT negotiate the cipher suites with NULL
encryption.
Rationale: The NULL cipher suites offer no encryption whatsoever Rationale: The NULL cipher suites do not encrypt traffic and so
and thus are completely insecure. provide no confidentiality services. Any entity in the network
with access to the connection can view the plaintext of contents
being exchanged by the client and server.
o Implementations MUST NOT negotiate RC4 cipher suites o Implementations MUST NOT negotiate RC4 cipher suites.
Rationale: The RC4 stream cipher has a variety of cryptographic Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. We
note that this guideline does not apply to DTLS, which
specifically forbids the use of RC4.
o Implementations MUST NOT negotiate cipher suites offering only so- o Implementations MUST NOT negotiate cipher suites offering only so-
called "export-level" encryption (including algorithms with 40 called "export-level" encryption (including algorithms with 40
bits or 56 bits of security). bits or 56 bits of security).
Rationale: These cipher suites are deliberately "dumbed down" and Rationale: These cipher suites are deliberately "dumbed down" and
are very easy to break. are very easy to break.
o Applications MUST NOT negotiate cipher suites of less than 112 o Applications MUST NOT negotiate cipher suites of less than 112
bits of security. bits of security.
o Implementations SHOULD NOT negotiate cipher suites that use o Implementations SHOULD NOT negotiate cipher suites that use
algorithms offering less than 128 bits of security. Note that algorithms offering less than 128 bits of security.
some legacy cipher suites (e.g. 168-bit 3DES) have an effective
key length which is smaller than their nominal key length (112
bits in the case of 3DES). Such cipher suites should be evaluated
according to their effective key length.
Rationale: Although these cipher suites are not actively subject Rationale: Cipher suites that offer between 112-bits and 128-bits
to breakage, their useful lifespan is short enough that stronger of security are not considered weak at this time, however it is
cipher suites are desirable. 128-bit ciphers are expected to expected that their useful lifespan is short enough to justify
remain secure for at least several years, and 256-bit ciphers supporting stronger cipher suites at this time. 128-bit ciphers
"until the next fundamental technology breakthrough". are expected to remain secure for at least several years, and
256-bit ciphers "until the next fundamental technology
breakthrough". Note that some legacy cipher suites (e.g. 168-bit
3DES) have an effective key length which is smaller than their
nominal key length (112 bits in the case of 3DES). Such cipher
suites should be evaluated according to their effective key
length.
o Implementations MUST support, and SHOULD prefer to negotiate, o Implementations MUST support, and SHOULD prefer to negotiate,
cipher suites offering forward secrecy, such as those in the cipher suites offering forward secrecy, such as those in the
Ephemeral Diffie-Hellman and Elliptic Curve Ephemeral Diffie Ephemeral Diffie-Hellman and Elliptic Curve Ephemeral Diffie-
Hellman ("DHE" and "ECDHE") families. Hellman ("DHE" and "ECDHE") families.
Rationale: Forward secrecy (sometimes called "perfect forward Rationale: Forward secrecy (sometimes called "perfect forward
secrecy") prevents the recovery of information that was encrypted secrecy") prevents the recovery of information that was encrypted
with older session keys, thus limiting the amount of time during with older session keys, thus limiting the amount of time during
which attacks can be successful. which attacks can be successful.
5.2. Recommended Cipher Suites 5.2. Recommended Cipher Suites
Given the foregoing considerations, implementation of the following Given the foregoing considerations, implementation and deployment of
cipher suites is RECOMMENDED: the following cipher suites is RECOMMENDED:
o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
We suggest that TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 be preferred in
general. See [RFC5289] for additional implementation details.
It is noted that those cipher suites are supported only in TLS 1.2 It is noted that those cipher suites are supported only in TLS 1.2
since they are authenticated encryption (AEAD) algorithms [RFC5116]. since they are authenticated encryption (AEAD) algorithms [RFC5116].
[RFC4492] allows clients and servers to negotiate ECDH parameters [RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). Both clients and servers SHOULD include the "Supported (curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves" extension [RFC4492]. For interoperability, clients Elliptic Curves" extension [RFC4492]. For interoperability, clients
and servers SHOULD support the NIST P-256 (secp256r1) curve and servers SHOULD support the NIST P-256 (secp256r1) curve
[RFC4492]. In addition, clients SHOULD send an ec_point_formats [RFC4492]. In addition, clients SHOULD send an ec_point_formats
extension with a single element, "uncompressed". extension with a single element, "uncompressed".
skipping to change at page 10, line 31 skipping to change at page 12, line 19
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites. TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
This document is not an application profile standard, in the sense of This document is not an application profile standard, in the sense of
Sec. 9 of [RFC5246]. As a result, clients and servers are still Sec. 9 of [RFC5246]. As a result, clients and servers are still
REQUIRED to support the mandatory TLS cipher suite, REQUIRED to support the mandatory TLS cipher suite,
TLS_RSA_WITH_AES_128_CBC_SHA. TLS_RSA_WITH_AES_128_CBC_SHA.
5.4. Public Key Length 5.4. Public Key Length
When using the cipher suites recommended in this document, two public
keys are normally used in the TLS handshake: one for the Diffie-
Hellman key agreement and one for server authentication. Where a
client certificate is used, a third one is added.
With a key exchange based on modular Diffie-Hellman ("DHE" cipher With a key exchange based on modular Diffie-Hellman ("DHE" cipher
suites), key lengths of at least 2048 bits are RECOMMENDED. suites), DH key lengths of at least 2048 bits are RECOMMENDED.
Rationale: because Diffie-Hellman keys of 1024 bits are estimated to Rationale: because Diffie-Hellman keys of 1024 bits are estimated to
be roughly equivalent to 80-bit symmetric keys, it is better to use be roughly equivalent to 80-bit symmetric keys, it is better to use
longer keys for the "DHE" family of cipher suites. Unfortunately, longer keys for the "DHE" family of cipher suites. Key lengths of at
some existing software cannot handle (or cannot easily handle) key least 2048 bits are estimated to be roughly equivalent to 112-bit
lengths greater than 1024 bits. The most common workaround for these symmetric keys and might be sufficient for at least the next
systems is to prefer the "ECDHE" family of cipher suites instead of 10 years. See Section 5.5 for additional information on the use of
the "DHE" family. For modular groups, key lengths of at least 2048 modular Diffie-Hellman in TLS.
bits are estimated to be roughly equivalent to 112-bit symmetric keys
and might be sufficient for at least the next 10 years.
Servers SHOULD authenticate using 2048-bit certificates. In Servers SHOULD authenticate using 2048-bit certificates. In
addition, the use of SHA-256 fingerprints is RECOMMENDED (see addition, the use of SHA-256 fingerprints is RECOMMENDED (see
[CAB-Baseline] for more details). Clients SHOULD indicate to servers [CAB-Baseline] for more details). Clients SHOULD indicate to servers
that they request SHA-256, by using the "Signature Algorithms" that they request SHA-256, by using the "Signature Algorithms"
extension defined in TLS 1.2. extension defined in TLS 1.2.
5.5. Modular vs. Elliptic Curve DH Cipher Suites 5.5. Modular vs. Elliptic Curve DH Cipher Suites
Not all TLS implementations support both modular and EC Diffie- Not all TLS implementations support both modular and EC Diffie-
skipping to change at page 11, line 14 skipping to change at page 13, line 4
5.5. Modular vs. Elliptic Curve DH Cipher Suites 5.5. Modular vs. Elliptic Curve DH Cipher Suites
Not all TLS implementations support both modular and EC Diffie- Not all TLS implementations support both modular and EC Diffie-
Hellman groups, as required by Section 5.2. Some implementations are Hellman groups, as required by Section 5.2. Some implementations are
severely limited in the length of DH values. When such severely limited in the length of DH values. When such
implementations need to be accommodated, we recommend using (in implementations need to be accommodated, we recommend using (in
priority order): priority order):
1. Elliptic Curve DHE with negotiated parameters [RFC5289] 1. Elliptic Curve DHE with negotiated parameters [RFC5289]
2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit 2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
Diffie-Hellman parameters Diffie-Hellman parameters
3. The same cipher suite, with 1024-bit parameters. 3. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, with 1024-bit parameters.
Rationale: Elliptic Curve Cryptography is not universally deployed Rationale: Elliptic Curve Cryptography is not universally deployed
for several reasons, including its complexity compared to modular for several reasons, including its complexity compared to modular
arithmetic and longstanding IPR concerns. On the other hand, there arithmetic and longstanding IPR concerns. On the other hand, there
are two related issues hindering effective use of modular Diffie- are two related issues hindering effective use of modular Diffie-
Hellman cipher suites in TLS: Hellman cipher suites in TLS:
o There are no protocol mechanisms to negotiate the DH groups or o There are no protocol mechanisms to negotiate the DH groups or
parameter lengths supported by client and server. parameter lengths supported by client and server.
o There are widely deployed client implementations that reject o There are widely deployed client implementations that reject
received DH parameters if they are longer than 1024 bits. received DH parameters if they are longer than 1024 bits.
We note that with DHE and ECDHE cipher suites, the TLS master key We note that with DHE and ECDHE cipher suites, the TLS master key
only depends on the Diffie Hellman parameters and not on the strength only depends on the Diffie-Hellman parameters and not on the strength
of the RSA certificate; moreover, 1024 bit modular DH parameters are of the RSA certificate; moreover, 1024 bit modular DH parameters are
generally considered insufficient at this time. generally considered insufficient at this time.
With modular ephemeral DH, deployers SHOULD carefully evaluate With modular ephemeral DH, deployers SHOULD carefully evaluate
interoperability vs. security considerations when configuring their interoperability vs. security considerations when configuring their
TLS endpoints. TLS endpoints.
5.6. Truncated HMAC 5.6. Truncated HMAC
The truncated HMAC extension, defined in Sec. 7 of [RFC6066] does not Implementations MUST NOT use the Truncated HMAC extension, defined in
apply to the AEAD cipher suites recommended above. However it does Sec. 7 of [RFC6066].
apply to most other TLS cipher suites. Its use has been shown to be
insecure in [PatersonRS11], and implementations MUST NOT use it. Rationale: the extension does not apply to the AEAD cipher suites
recommended above. However it does apply to most other TLS cipher
suites. Its use has been shown to be insecure in [PatersonRS11].
6. IANA Considerations 6. IANA Considerations
This document requests no actions of IANA. [Note to RFC Editor: This document requests no actions of IANA. [Note to RFC Editor:
please remove this whole section before publication.] please remove this whole section before publication.]
7. Security Considerations 7. Security Considerations
This entire document discusses the security practices directly This entire document discusses the security practices directly
affecting applications using the TLS protocol. This section contains affecting applications using the TLS protocol. This section contains
broader security considerations related to technologies used in broader security considerations related to technologies used in
conjunction with or by TLS. conjunction with or by TLS.
7.1. Host Name Validation 7.1. Host Name Validation
Application authors should take note that TLS implementations Application authors should take note that TLS implementations
frequently do not validate host names, and must therefore determine frequently do not validate host names and must therefore determine if
if the TLS implementation they are using does, and if not write their the TLS implementation they are using does and, if not, write their
own validation code or consider changing the TLS implementation. own validation code or consider changing the TLS implementation.
It is noted that the requirements regarding host name validation (and It is noted that the requirements regarding host name validation (and
in general, binding between the TLS layer and the protocol that runs in general, binding between the TLS layer and the protocol that runs
above it) vary between different protocols. For HTTPS, these above it) vary between different protocols. For HTTPS, these
requirements are defined by Sec. 3 of [RFC2818]. requirements are defined by Sec. 3 of [RFC2818].
Readers are referred to [RFC6125] for further details regarding Readers are referred to [RFC6125] for further details regarding
generic host name validation in the TLS context. In addition, the generic host name validation in the TLS context. In addition, the
RFC contains a long list of example protocols, some of which RFC contains a long list of example protocols, some of which
skipping to change at page 12, line 38 skipping to change at page 14, line 31
If the host name is discovered indirectly and in an insecure manner If the host name is discovered indirectly and in an insecure manner
(e.g., by an insecure DNS query for an MX or SRV record), it SHOULD (e.g., by an insecure DNS query for an MX or SRV record), it SHOULD
NOT be used as a reference identifier [RFC6125] even when it matches NOT be used as a reference identifier [RFC6125] even when it matches
the presented certificate. This proviso does not apply if the host the presented certificate. This proviso does not apply if the host
name is discovered securely (for further discussion, see for example name is discovered securely (for further discussion, see for example
[I-D.ietf-dane-srv] and [I-D.ietf-dane-smtp]). [I-D.ietf-dane-srv] and [I-D.ietf-dane-smtp]).
7.2. AES-GCM 7.2. AES-GCM
Sec. Section 5.2 above recommends the use of the AES-GCM Section 5.2 above recommends the use of the AES-GCM authenticated
authenticated encryption algorithm. Please refer to [RFC5246], Sec. encryption algorithm. Please refer to [RFC5246], Sec. 11 for general
11 for general security considerations when using TLS 1.2, and to security considerations when using TLS 1.2, and to [RFC5288], Sec. 6
[RFC5288], Sec. 6 for security considerations that apply specifically for security considerations that apply specifically to AES-GCM when
to AES-GCM when used with TLS. used with TLS.
7.3. Forward Secrecy 7.3. Forward Secrecy
Forward secrecy (also often called Perfect Forward Secrecy or "PFS", Forward secrecy (also often called Perfect Forward Secrecy or "PFS"
and defined in [RFC4949]) is a defense against an attacker who and defined in [RFC4949]) is a defense against an attacker who
records encrypted conversations where the session keys are only records encrypted conversations where the session keys are only
encrypted with the communicating parties' long-term keys. Should the encrypted with the communicating parties' long-term keys. Should the
attacker be able to obtain these long-term keys at some point later attacker be able to obtain these long-term keys at some point later
in time, he will be able to decrypt the session keys and thus the in time, he will be able to decrypt the session keys and thus the
entire conversation. In the context of TLS and DTLS, such compromise entire conversation. In the context of TLS and DTLS, such compromise
of long-term keys is not entirely implausible. It can happen, for of long-term keys is not entirely implausible. It can happen, for
example, due to: example, due to:
o A client or server being attacked by some other attack vector, and o A client or server being attacked by some other attack vector, and
skipping to change at page 13, line 42 skipping to change at page 15, line 37
called Discrete Logarithm Problem (DLP) allow to derive the session called Discrete Logarithm Problem (DLP) allow to derive the session
keys without an eavesdropper being able to do so. There is currently keys without an eavesdropper being able to do so. There is currently
no known attack against DLP if sufficiently large parameters are no known attack against DLP if sufficiently large parameters are
chosen. A variant of the Diffie-Hellman scheme uses Elliptic Curves chosen. A variant of the Diffie-Hellman scheme uses Elliptic Curves
instead of the originally proposed modular arithmetics. instead of the originally proposed modular arithmetics.
Unfortunately, many TLS/DTLS cipher suites were defined that do not Unfortunately, many TLS/DTLS cipher suites were defined that do not
feature PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate feature PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate
strict use of PFS-only ciphers. strict use of PFS-only ciphers.
7.4. Diffie Hellman Exponent Reuse 7.4. Diffie-Hellman Exponent Reuse
For performance reasons, many TLS implementations reuse Diffie- For performance reasons, many TLS implementations reuse Diffie-
Hellman and Elliptic Curve Diffie-Hellman exponents across multiple Hellman and Elliptic Curve Diffie-Hellman exponents across multiple
connections. Such reuse can result in major security issues: connections. Such reuse can result in major security issues:
o If exponents are reused for a long time (e.g., more than a few o If exponents are reused for a long time (e.g., more than a few
hours), an attacker who gains access to the host can decrypt hours), an attacker who gains access to the host can decrypt
previous connections. In other words, exponent reuse negates the previous connections. In other words, exponent reuse negates the
effects of forward secrecy. effects of forward secrecy.
o TLS implementations that reuse exponents should test the DH public o TLS implementations that reuse exponents should test the DH public
key they receive, in order to avoid some known attacks. These key they receive, in order to avoid some known attacks. These
tests are not standardized in TLS at the time of writing. See tests are not standardized in TLS at the time of writing. See
[RFC6989] for recipient tests required of IKEv2 implementations [RFC6989] for recipient tests required of IKEv2 implementations
that reuse DH exponents. that reuse DH exponents.
7.5. Certificate Revocation 7.5. Certificate Revocation
Unfortunately there is currently no effective, Internet-scale Unfortunately there is currently no effective, Internet-scale
mechanism to affect certificate revocation: mechanism to effect certificate revocation:
o Certificate Revocation Lists (CRLs) are non-scalable and therefore o Certificate Revocation Lists (CRLs) are non-scalable and therefore
rarely used. rarely used.
o The On-Line Certification Status Protocol (OCSP) presents both o The On-Line Certification Status Protocol (OCSP) presents both
scaling and privacy issues when used for heavy traffic Web scaling and privacy issues when used for heavy traffic Web
servers. In addition, clients typically "soft-fail", meaning they servers. In addition, clients typically "soft-fail", meaning they
do not abort the TLS connection if the OCSP server does not do not abort the TLS connection if the OCSP server does not
respond. respond.
o OCSP stapling (Sec. 8 of [RFC6066]) resolves the operational o OCSP stapling (Sec. 8 of [RFC6066]) resolves the operational
issues with OCSP, but is still ineffective in the presence of a issues with OCSP, but is still ineffective in the presence of a
MITM attacker because they can simply ignore the client's request MITM attacker because the attacker can simply ignore the client's
for a stapled OCSP response. request for a stapled OCSP response.
o OCSP stapling as defined in [RFC6066] does not extend to o OCSP stapling as defined in [RFC6066] does not extend to
intermediate certificates used in a certificate chain. [RFC6961] intermediate certificates used in a certificate chain. [RFC6961]
addresses this shortcoming, but is a recent addition without much addresses this shortcoming, but is a recent addition without much
deployment. deployment.
o Proprietary mechanisms that embed revocation lists in the Web o Proprietary mechanisms that embed revocation lists in the Web
browser's configuration database cannot scale beyond a small browser's configuration database cannot scale beyond a small
number of the most heavily used Web servers. number of the most heavily used Web servers.
The current consensus appears to be that OCSP stapling, combined with The current consensus appears to be that OCSP stapling, combined with
a "must staple" mechanism similar to HSTS, would finally resolve this a "must staple" mechanism similar to HSTS, would finally resolve this
problem; in particular when used together with the extension defined problem; in particular when used together with the extension defined
in [RFC6961]. But such a mechanism has not been standardized yet. in [RFC6961]. But such a mechanism has not been standardized yet.
8. Acknowledgments 8. Acknowledgments
We would like to thank Uri Blumenthal, Viktor Dukhovni, Stephen We would like to thank Uri Blumenthal, Viktor Dukhovni, Stephen
Farrell, Simon Josefsson, Watson Ladd, Orit Levin, Johannes Merkle, Farrell, Simon Josefsson, Watson Ladd, Orit Levin, Johannes Merkle,
Bodo Moeller, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom Bodo Moeller, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom
Ritter, Rich Salz, Aaron Zauner for their review and improvements. Ritter, Rich Salz, Sean Turner, Aaron Zauner for their review and
Thanks to Brian Smith whose "browser cipher suites" page is a great improvements. Thanks to Brian Smith whose "browser cipher suites"
resource. Finally, thanks to all others who commented on the TLS, page is a great resource. Finally, thanks to all others who
UTA and other lists and are not mentioned here by name. commented on the TLS, UTA and other lists and are not mentioned here
by name.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
skipping to change at page 16, line 16 skipping to change at page 18, line 16
Degabriele, J. and K. Paterson, "Attacking the IPsec Degabriele, J. and K. Paterson, "Attacking the IPsec
standards in encryption-only configurations", 2007, standards in encryption-only configurations", 2007,
<http://dx.doi.org/10.1109/SP.2007.8>. <http://dx.doi.org/10.1109/SP.2007.8>.
[Heninger2012] [Heninger2012]
Heninger, N., Durumeric, Z., Wustrow, E., and J. Heninger, N., Durumeric, Z., Wustrow, E., and J.
Halderman, "Mining Your Ps and Qs: Detection of Widespread Halderman, "Mining Your Ps and Qs: Detection of Widespread
Weak Keys in Network Devices", Usenix Security Symposium Weak Keys in Network Devices", Usenix Security Symposium
2012, 2012. 2012, 2012.
[I-D.dukhovni-smtp-opportunistic-tls]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-dukhovni-smtp-
opportunistic-tls-01 (work in progress), July 2013.
[I-D.farrelll-mpls-opportunistic-encrypt]
Farrel, A. and S. Farrell, "Opportunistic Encryption in
MPLS Networks", draft-farrelll-mpls-opportunistic-
encrypt-02 (work in progress), February 2014.
[I-D.ietf-dane-smtp] [I-D.ietf-dane-smtp]
Finch, T., "Secure SMTP using DNS-Based Authentication of Finch, T., "Secure SMTP using DNS-Based Authentication of
Named Entities (DANE) TLSA records.", draft-ietf-dane- Named Entities (DANE) TLSA records.", draft-ietf-dane-
smtp-01 (work in progress), February 2013. smtp-01 (work in progress), February 2013.
[I-D.ietf-dane-srv] [I-D.ietf-dane-srv]
Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", draft-ietf-dane-srv-07 (work in with SRV Records", draft-ietf-dane-srv-07 (work in
progress), July 2014. progress), July 2014.
skipping to change at page 18, line 9 skipping to change at page 20, line 21
[triple-handshake] [triple-handshake]
Delignat-Lavaud, A., Bhargavan, K., and A. Pironti, Delignat-Lavaud, A., Bhargavan, K., and A. Pironti,
"Triple Handshakes Considered Harmful: Breaking and Fixing "Triple Handshakes Considered Harmful: Breaking and Fixing
Authentication over TLS", 2014, <https://secure- Authentication over TLS", 2014, <https://secure-
resumption.com/>. resumption.com/>.
Appendix A. Change Log Appendix A. Change Log
Note to RFC Editor: please remove this section before publication. Note to RFC Editor: please remove this section before publication.
A.1. draft-ietf-uta-tls-bcp-04 A.1. draft-ietf-uta-tls-bcp-05
o Lots of comments by Sean Turner.
o Unauthenticated TLS, following a long thread on the list.
A.2. draft-ietf-uta-tls-bcp-04
o Some cleanup, and input from TLS WG discussion on applicability. o Some cleanup, and input from TLS WG discussion on applicability.
A.2. draft-ietf-uta-tls-bcp-03 A.3. draft-ietf-uta-tls-bcp-03
o Disallow truncated HMAC. o Disallow truncated HMAC.
o Applicability to DTLS. o Applicability to DTLS.
o Some more text restructuring. o Some more text restructuring.
o Host name validation is sometimes irrelevant. o Host name validation is sometimes irrelevant.
o HSTS: MUST implement, SHOULD deploy. o HSTS: MUST implement, SHOULD deploy.
o Session identities are not protected, only tickets are. o Session identities are not protected, only tickets are.
o Clarified the target audience. o Clarified the target audience.
A.3. draft-ietf-uta-tls-bcp-02 A.4. draft-ietf-uta-tls-bcp-02
o Rearranged some sections for clarity and re-styled the text so o Rearranged some sections for clarity and re-styled the text so
that normative text is followed by rationale where possible. that normative text is followed by rationale where possible.
o Removed the recommendation to use Brainpool curves. o Removed the recommendation to use Brainpool curves.
o Triple Handshake mitigation. o Triple Handshake mitigation.
o MUST NOT negotiate algorithms lower than 112 bits of security. o MUST NOT negotiate algorithms lower than 112 bits of security.
o MUST implement SNI, but use per local policy. o MUST implement SNI, but use per local policy.
o Changed SHOULD NOT negotiate or fall back to SSLv3 to MUST NOT. o Changed SHOULD NOT negotiate or fall back to SSLv3 to MUST NOT.
o Added hostname validation. o Added hostname validation.
o Non-normative discussion of DH exponent reuse. o Non-normative discussion of DH exponent reuse.
A.4. draft-ietf-tls-bcp-01 A.5. draft-ietf-tls-bcp-01
o Clarified that specific TLS-using protocols may have stricter o Clarified that specific TLS-using protocols may have stricter
requirements. requirements.
o Changed TLS 1.0 from MAY to SHOULD NOT. o Changed TLS 1.0 from MAY to SHOULD NOT.
o Added discussion of "optional TLS" and HSTS. o Added discussion of "optional TLS" and HSTS.
o Recommended use of the Signature Algorithm and Renegotiation Info o Recommended use of the Signature Algorithm and Renegotiation Info
extensions. extensions.
o Use of a strong cipher for a resumption ticket: changed SHOULD to o Use of a strong cipher for a resumption ticket: changed SHOULD to
MUST. MUST.
o Added an informational discussion of certificate revocation, but o Added an informational discussion of certificate revocation, but
no recommendations. no recommendations.
A.5. draft-ietf-tls-bcp-00 A.6. draft-ietf-tls-bcp-00
o Initial WG version, with only updated references. o Initial WG version, with only updated references.
A.6. draft-sheffer-tls-bcp-02 A.7. draft-sheffer-tls-bcp-02
o Reorganized the content to focus on recommendations. o Reorganized the content to focus on recommendations.
o Moved description of attacks to a separate document (draft- o Moved description of attacks to a separate document (draft-
sheffer-uta-tls-attacks). sheffer-uta-tls-attacks).
o Strengthened recommendations regarding session resumption. o Strengthened recommendations regarding session resumption.
A.7. draft-sheffer-tls-bcp-01 A.8. draft-sheffer-tls-bcp-01
o Clarified our motivation in the introduction. o Clarified our motivation in the introduction.
o Added a section justifying the need for PFS. o Added a section justifying the need for PFS.
o Added recommendations for RSA and DH parameter lengths. Moved o Added recommendations for RSA and DH parameter lengths. Moved
from DHE to ECDHE, with a discussion on whether/when DHE is from DHE to ECDHE, with a discussion on whether/when DHE is
appropriate. appropriate.
o Recommendation to avoid fallback to SSLv3. o Recommendation to avoid fallback to SSLv3.
o Initial information about browser support - more still needed! o Initial information about browser support - more still needed!
o More clarity on compression. o More clarity on compression.
o Client can offer stronger cipher suites. o Client can offer stronger cipher suites.
o Discussion of the regular TLS mandatory cipher suite. o Discussion of the regular TLS mandatory cipher suite.
A.8. draft-sheffer-tls-bcp-00 A.9. draft-sheffer-tls-bcp-00
o Initial version. o Initial version.
Authors' Addresses Authors' Addresses
Yaron Sheffer Yaron Sheffer
Porticor Porticor
29 HaHarash St. 29 HaHarash St.
Hod HaSharon 4501303 Hod HaSharon 4501303
Israel Israel
Email: yaronf.ietf@gmail.com Email: yaronf.ietf@gmail.com
Ralph Holz Ralph Holz
Technische Universitaet Muenchen Technische Universitaet Muenchen
Boltzmannstr. 3 Boltzmannstr. 3
Garching 85748 Garching 85748
Germany Germany
Email: holz@net.in.tum.de Email: ralph.ietf@gmail.com
Peter Saint-Andre Peter Saint-Andre
&yet &yet
P.O. Box 787
Parker, CO 80134
USA
Email: peter@andyet.com Email: peter@andyet.com
URI: https://andyet.com/
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