draft-ietf-uta-tls-bcp-06.txt   draft-ietf-uta-tls-bcp-07.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 26, 2015 TUM Expires: May 15, 2015 TUM
P. Saint-Andre P. Saint-Andre
&yet &yet
October 23, 2014 November 11, 2014
Recommendations for Secure Use of TLS and DTLS Recommendations for Secure Use of TLS and DTLS
draft-ietf-uta-tls-bcp-06 draft-ietf-uta-tls-bcp-07
Abstract Abstract
Transport Layer Security (TLS) and Datagram Transport Layer Security Transport Layer Security (TLS) and Datagram Transport Layer Security
(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
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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 26, 2015. This Internet-Draft will expire on May 15, 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
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Security Services . . . . . . . . . . . . . . . . . . . . 4 3. General Recommendations . . . . . . . . . . . . . . . . . . . 4
2.2. Unauthenticated TLS . . . . . . . . . . . . . . . . . . . 5 3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 4
4. General Recommendations . . . . . . . . . . . . . . . . . . . 6 3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 5
4.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 6 3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 5
4.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 6 3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 7 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 7 3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 7
4.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 8 3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7
4.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8 3.6. Server Name Indication . . . . . . . . . . . . . . . . . 8
4.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 9 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8
4.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 9 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8
4.6. Server Name Indication . . . . . . . . . . . . . . . . . 10 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 9
5. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 10 4.2.1. Implementation Details . . . . . . . . . . . . . . . 10
5.1. General Guidelines . . . . . . . . . . . . . . . . . . . 10 4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 10
5.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 11 4.4. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 11
5.3. Cipher Suite Negotiation Details . . . . . . . . . . . . 12 4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 12
5.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 12 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 12
5.5. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 13 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 12
5.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 14 5.2. Unauthenticated TLS and Opportunistic Encryption . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 14 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 14
7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 15 7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 15
7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 16 7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 16
7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 16 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 17
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 17 9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 18 9.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 21 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 21
A.1. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 21 A.1. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 21
A.2. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 21 A.2. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 21
A.3. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 21 A.3. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 21
A.4. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 21 A.4. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 22
A.5. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 21 A.5. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 22
A.6. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 22 A.6. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 22
A.7. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 22 A.7. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 22
A.8. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 22 A.8. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 23
A.9. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 22 A.9. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 23
A.10. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 23 A.10. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 A.11. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction 1. Introduction
Transport Layer Security (TLS) [RFC5246] and Datagram Transport Transport Layer Security (TLS) [RFC5246] and Datagram Transport
Security Layer (DTLS) [RFC6347] are widely used to protect data Security Layer (DTLS) [RFC6347] are widely used to protect data
exchanged over application protocols such as HTTP, SMTP, IMAP, POP, exchanged over application protocols such as HTTP, SMTP, IMAP, POP,
SIP, and XMPP. Over the last few years, several serious attacks on SIP, and XMPP. Over the last few years, several serious attacks on
TLS have emerged, including attacks on its most commonly used cipher TLS have emerged, including attacks on its most commonly used cipher
suites and modes of operation. For instance, both the AES-CBC suites and modes of operation. For instance, both the AES-CBC
[RFC3602] and RC4 [I-D.ietf-tls-prohibiting-rc4] encryption [RFC3602] and RC4 [I-D.ietf-tls-prohibiting-rc4] encryption
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fact, this document calls for the deployment of algorithms that are fact, this document calls for the deployment of algorithms that are
widely implemented but not yet widely deployed. Concerning 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 authentication (be it one-way only or deployers who wish to add authentication (be it one-way only or
mutual), confidentiality, and data integrity protection to their mutual), confidentiality, and data integrity protection to their
communications. communications.
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 it is explicitly called out that a recommendation applies to
DTLS. TLS 1.3, when it is standardized and deployed in the field, TLS alone or to DTLS alone, each recommendation applies to both TLS
should resolve the current vulnerabilities while providing and DTLS.
significantly better functionality. It will very likely obsolete
this document.
These are minimum recommendations for the use of TLS for the It is expected that the TLS 1.3 specification will resolve many of
specified audience. Individual specifications can have stricter the vulnerabilities listed in this document. A system that deploys
TLS 1.3 will have fewer vulnerabilities than TLS 1.2 or below. This
document is likely to be updated after TLS 1.3 gets noticeable
deployment.
These are minimum recommendations for the use of TLS in the vast
majority of implementation and deployment scenarios, with the
exception of unauthenticated TLS (see Section 5). Other
specifications that reference this document can 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 (e.g., for use with a particular their particular circumstances (e.g., for use with a particular
application protocol). When that is the case, implementers are application protocol); when that is the case, implementers are
advised to adhere to those stricter requirements. advised to 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. Terminology
The deployment recommendations of this document address the operators
of application layer services that are most commonly used on the
Internet, including, but not limited to:
o Operators of WWW servers that wish to protect HTTP with TLS.
o Operators of email servers who wish to protect the application-
layer protocols with TLS (e.g., IMAP, POP3 or SMTP).
o Operators of instant-messaging services who wish to protect their
application-layer protocols with TLS (e.g., XMPP or IRC).
2.1. Security Services
This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following:
o Confidentiality: all (payload) communication is encrypted with the
goal that no party should be able to decrypt it except the
intended receiver.
o Data integrity: any changes made to the communication in transit
are detectable by the receiver.
o Authentication: an end-point of the TLS communication is
authenticated as the intended entity to communicate with. TLS
enables authentication of one or both end-points in the
communication. Some TLS usage scenarios do not require
authentication. They are not in the scope of this document. We
discuss them under Section 2.2.
If deployers deviate from the recommendations given in this document,
they MUST verify that they do not need one of the foregoing security
services.
This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that
enforce secrecy of the data-in-transit.
This document also assumes that data integrity protection is always
one of the goals of a deployment. In cases where 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 for
instance [DegabrieleP07] in the context of IPsec).
The intended audience covers those services that are most commonly
used on the Internet. Typically, all communication between TLS
clients and TLS servers requires all three of the above security
services. This is particularly true where TLS clients are user
agents like Web browsers or email software.
This document does not address the rarer deployment scenarios where
one of the above three properties is not desired, such as the use
case described under Section 2.2 below. Another 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.
2.2. Unauthenticated TLS
Several important applications use TLS to protect data between a TLS
client and a TLS server, but do so without the TLS client verifying
the server's certificate. The reader is referred to
[I-D.ietf-dane-smtp-with-dane] for an example and an explanation of
why this less secure practice will likely remain common in the
context of SMTP (especially for MTA-to-MTA communications). The
practice is also encountered in similar contexts such as server-to-
server XMPP traffic.
In some of these scenarios the use of TLS is optional, i.e. the
client decides dynamically ("opportunistically") whether to use TLS
with a particular server or to connect in the clear. (Opportunistic
encryption is described at length in Section 2 of
[I-D.farrelll-mpls-opportunistic-encrypt].) In other scenarios, the
use of TLS is required but certificates are not always checked (e.g.,
this is often the case on the XMPP network, where multi-tenant
hosting environments make it difficult for operators to obtain proper
certificates for all of the domains they service).
It can be argued that the recommendations provided in this document
ought to apply equally to unauthenticated TLS as well as
authenticated TLS. That would keep TLS implementations and
deployments in sync, which is a desirable property given that servers
can be used simultaneously for unauthenticated TLS and for
authenticated TLS (indeed, often a server will not know whether a
client might attempt authenticated or unauthenticated TLS). On the
other hand, it has been argued that some of the recommendations in
this document might be too strict for unauthenticated scenarios and
that any security is better than no security at all (i.e., sending
traffic in the clear), even if it means deploying outdated protocol
versions and ciphers in unauthenticated scenarios. The sense of the
UTA Working Group was to complete work on this document about
authenticated TLS and to initiate work on a separate document about
unauthenticated TLS.
In summary: this document does not apply to unauthenticated TLS use
cases.
3. Terminology
A number of security-related terms in this document are used in the A number of security-related terms in this document are used in the
sense defined in [RFC4949]. sense defined in [RFC4949].
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 3. 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 3.1. Protocol Versions
4.1.1. SSL/TLS Protocol Versions 3.1.1. 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, the TLS and to start using modern, more secure versions; therefore, the
following are the recommendations concerning TLS/SSL protocol following are the recommendations concerning TLS/SSL protocol
versions: versions:
o Implementations MUST NOT negotiate SSL version 2. o Implementations MUST NOT negotiate SSL version 2.
Rationale: Today, SSLv2 is considered 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. SSLv3 does not support TLS extensions, some strong cipher suites. SSLv3 does not support TLS extensions, some
of which (e.g. renegotiation_info) are security-critical. In of which (e.g., renegotiation_info) are security-critical. In
addition, with the emergence of the POODLE attack [POODLE], SSLv3 addition, with the emergence of the POODLE attack [POODLE], SSLv3
is now widely recognized as fundamentally insecure. is now widely recognized as fundamentally insecure.
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). In fact, the cipher suites TLS 1.2 (published in 2008). In fact, the cipher suites
recommended by this document (Section 5.2 below) are only recommended by this document (Section 4.2 below) are only
available in TLS 1.2. 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.
4.1.2. DTLS Protocol Versions 3.1.2. DTLS Protocol Versions
DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS
1.1 was published. The following are the recommendations with 1.1 was published. The following are the recommendations with
respect to DTLS: respect to DTLS:
o Implementations MAY negotiate DTLS version 1.0 [RFC4347]. o Implementations MAY negotiate DTLS version 1.0 [RFC4347].
Version 1.0 of DTLS correlates to version 1.1 of TLS (see above). Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
o Implementations MUST support, and prefer to negotiate, DTLS o Implementations MUST support, and prefer to negotiate, DTLS
version 1.2 [RFC6347]. version 1.2 [RFC6347].
Version 1.2 of DTLS correlates to Version 1.2 of TLS 1.2 (see Version 1.2 of DTLS correlates to Version 1.2 of TLS 1.2 (see
above). (There is no Version 1.1 of DTLS.) above). (There is no Version 1.1 of DTLS.)
Note: DTLS and TLS are nearly identical. The most notable exception 3.1.3. Fallback to Lower Versions
is that RC4, which is a stream-based bulk encryption algorithm,
cannot be supported by DTLS.
4.1.3. Fallback to Lower Versions
Clients that "fallback" to lower versions of the protocol after the Clients that "fallback" to lower versions of the protocol after the
server rejects higher versions of the protocol MUST NOT fallback to server rejects higher versions of the protocol MUST NOT fallback to
SSLv3. SSLv3.
Rationale: Some client implementations revert to lower versions of Rationale: Some client implementations revert to lower versions of
TLS or even to SSLv3 if the server rejected higher versions of the 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) 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 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.
4.2. Strict TLS 3.2. Strict TLS
To prevent SSL Stripping: To prevent SSL Stripping:
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 o In many application protocols, clients can be configured to use
TLS even if the server has not advertised that TLS is mandatory or TLS no matter whether the server offers TLS during a protocol
even supported (e.g., this is often the case in messaging exchange or advertises support for TLS (e.g., through a flag
protocols such as IMAP and XMPP). Application clients SHOULD use indicating that TLS is required). Application clients SHOULD use
TLS by default, and disable this default only through explicit TLS by default, and disable this default only through explicit
configuration by the user. configuration 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.
Rationale: Combining unprotected and TLS-protected communication Rationale: Combining unprotected and TLS-protected communication
opens the way to SSL Stripping and similar attacks, since an initial opens the way to SSL Stripping and similar attacks, since an initial
part of the communication is not integrity protected and therefore part of the communication is not integrity protected and therefore
can be manipulated by an attacker whose goal is to keep the can be manipulated by an attacker whose goal is to keep the
communication in the clear. communication in the clear.
4.3. Compression 3.3. Compression
Implementations and deployments SHOULD disable TLS-level compression Implementations and deployments SHOULD disable TLS-level compression
([RFC5246], Sec. 6.2.2). ([RFC5246], Section 6.2.2).
Rationale: TLS compression has been subject to security attacks, such Rationale: TLS compression has been subject to security attacks, such
as the CRIME attack. 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 Section 2.5 of [I-D.ietf-uta-tls-attacks] for
further details. further details.
4.4. TLS Session Resumption 3.4. 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
not to negate the benefits of forward secrecy (see Section 7.3 for as not to negate the benefits of forward secrecy (see Section 7.3
details on forward secrecy). for 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.
Rationale: session resumption is another kind of TLS handshake, and Rationale: session resumption is another kind of TLS handshake, and
therefore must be as secure as the initial handshake. This document therefore must be as secure as the initial handshake. This document
(Section 5) recommends the use of cipher suites that provide forward (Section 4) recommends the use of cipher suites that provide forward
secrecy, i.e. that prevent an attacker who gains momentary access to secrecy, i.e. that prevent an attacker who gains momentary access to
the TLS endpoint (either client or server) and its secrets from the TLS endpoint (either client or server) and its secrets from
reading either past or future communication. The tickets must be reading either past or future communication. The tickets must be
managed so as not to negate this security property. managed so as not to negate this security property.
4.5. TLS Renegotiation 3.5. 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.
4.6. Server Name Indication 3.6. 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 Rationale: SNI supports deployment of multiple TLS-protected virtual
servers on a single address, and therefore enables fine-grained servers on a single address, and therefore enables fine-grained
security for these virtual servers, by allowing each one to have its security for these virtual servers, by allowing each one to have its
own certificate. own certificate.
5. Recommendations: Cipher Suites 4. 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, some available cipher selection of cipher suites. Unfortunately, some available cipher
suites are insecure, some do not provide the targeted security suites are insecure, some do not provide the targeted security
services, and some no longer provide enough security. Incorrectly services, and some no longer provide enough security. Incorrectly
configuring a server leads to no or reduced security. This section configuring a server leads to no or reduced security. This section
includes recommendations on the selection and negotiation of cipher includes recommendations on the selection and negotiation of cipher
suites. suites.
5.1. General Guidelines 4.1. General Guidelines
Cryptographic algorithms weaken over time as cryptanalysis improves. Cryptographic algorithms weaken over time as cryptanalysis improves.
In other words, as time progresses, algorithms that were once In other words, as time progresses, algorithms that were once
considered strong but are now weak, need to be phased out over time considered strong but are now weak, need to be phased out over time
and replaced with more secure cipher suites to ensure that desired and replaced with more secure cipher suites to ensure that desired
security properties still hold. SSL/TLS has been in existence for security properties still hold. SSL/TLS has been in existence for
almost 20 years at this point and this section provides some much almost 20 years at this point and this section provides some much
needed recommendations concerning cipher suite selection: needed recommendations concerning cipher suite selection:
o Implementations MUST NOT negotiate the cipher suites with NULL o Implementations MUST NOT negotiate the cipher suites with NULL
skipping to change at page 11, line 5 skipping to change at page 9, line 5
with access to the connection can view the plaintext of contents with access to the connection can view the plaintext of contents
being exchanged by the client and server. 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]. We weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. We
note that this guideline does not apply to DTLS, which note that this guideline does not apply to DTLS, which
specifically forbids the use of RC4. specifically forbids the use of RC4.
o Implementations MUST NOT negotiate cipher suites offering only so-
called "export-level" encryption (including algorithms with 40
bits or 56 bits of security).
Rationale: These cipher suites are deliberately "dumbed down" and
are very easy to break.
o Implementations MUST NOT negotiate cipher suites offering less o Implementations MUST NOT negotiate cipher suites offering less
than 112 bits of security, including the so-called "export-level" than 112 bits of security, including the so-called "export-level"
encryption (which provide 40 or 56 bits of security). encryption (which provide 40 or 56 bits of security).
Rationale: Based on [RFC3766], at least 112 bits of security is Rationale: Based on [RFC3766], at least 112 bits of security is
needed. 40-bit and 56-bit security are considered insecure today. needed. 40-bit and 56-bit security are considered insecure today.
TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers. TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers.
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. algorithms offering less than 128 bits of security.
Rationale: Cipher suites that offer between 112-bits and 128-bits Rationale: Cipher suites that offer between 112-bits and 128-bits
of security are not considered weak at this time, however it is of security are not considered weak at this time, however it is
expected that their useful lifespan is short enough to justify expected that their useful lifespan is short enough to justify
supporting stronger cipher suites at this time. 128-bit ciphers supporting stronger cipher suites at this time. 128-bit ciphers
are expected to remain secure for at least several years, and are expected to remain secure for at least several years, and
256-bit ciphers "until the next fundamental technology 256-bit ciphers "until the next fundamental technology
breakthrough". Note that some legacy cipher suites (e.g. 168-bit breakthrough". Note that some legacy cipher suites (e.g., 168-bit
3DES) have an effective key length which is smaller than their 3DES) have an effective key length which is smaller than their
nominal key length (112 bits in the case of 3DES). Such cipher nominal key length (112 bits in the case of 3DES). Such cipher
suites should be evaluated according to their effective key suites should be evaluated according to their effective key
length. 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. See Section 7.3 for a detailed
discussion.
5.2. Recommended Cipher Suites 4.2. Recommended Cipher Suites
Given the foregoing considerations, implementation and deployment of Given the foregoing considerations, implementation and deployment of
the following 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
These cipher suites are supported only in TLS 1.2 since they are These cipher suites are supported only in TLS 1.2 since they are
authenticated encryption (AEAD) algorithms [RFC5116]. authenticated encryption (AEAD) algorithms [RFC5116].
Typically, in order to prefer these suites, the order of suites needs Typically, in order to prefer these suites, the order of suites needs
to be explicitly configured in server software. to be explicitly configured in server software.
[RFC4492] allows clients and servers to negotiate ECDH parameters 4.2.1. Implementation Details
(curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves" extension [RFC4492]. For interoperability, clients
and servers SHOULD support the NIST P-256 (secp256r1) curve
[RFC4492]. In addition, clients SHOULD send an ec_point_formats
extension with a single element, "uncompressed".
5.3. Cipher Suite Negotiation Details
Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
first proposal to any server, unless they have prior knowledge that first proposal to any server, unless they have prior knowledge that
the server cannot respond to a TLS 1.2 client_hello message. the server cannot respond to a TLS 1.2 client_hello message.
Servers SHOULD prefer this cipher suite whenever it is proposed, even Servers SHOULD prefer this cipher suite whenever it is proposed, even
if it is not the first proposal. if it is not the first proposal.
Clients are of course free to offer stronger cipher suites, e.g. Clients are of course free to offer stronger cipher suites, e.g.,
using AES-256; when they do, the server SHOULD prefer the stronger using AES-256; when they do, the server SHOULD prefer the stronger
cipher suite unless there are compelling reasons (e.g., seriously cipher suite unless there are compelling reasons (e.g., seriously
degraded performance) to choose otherwise. degraded performance) to choose otherwise.
Note that other profiles of TLS 1.2 exist that use different cipher
suites. For example, [RFC6460] defines a profile that uses the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
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 Section 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 Note that some profiles of TLS 1.2 use different cipher suites. For
example, [RFC6460] defines a profile that uses the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
[RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves" extension [RFC4492]. For interoperability, clients
and servers SHOULD support the NIST P-256 (secp256r1) curve
[RFC4492]. In addition, clients SHOULD send an ec_point_formats
extension with a single element, "uncompressed".
4.3. Public Key Length
When using the cipher suites recommended in this document, two public When using the cipher suites recommended in this document, two public
keys are normally used in the TLS handshake: one for the Diffie- keys are normally used in the TLS handshake: one for the Diffie-
Hellman key agreement and one for server authentication. Where a Hellman key agreement and one for server authentication. Where a
client certificate is used, a third one is added. client certificate is used, a third public key 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), DH 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: For various reasons, in practice DH keys are typically
be roughly equivalent to 80-bit symmetric keys, it is better to use generated in lengths that are powers of two (e.g., 2^10 = 1024 bits,
longer keys for the "DHE" family of cipher suites. Key lengths of at 2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits
least 2048 bits are estimated to be roughly equivalent to 112-bit would be roughly equivalent to only an 80-bit symmetric key
symmetric keys and might be sufficient for at least the next [RFC3766], it is better to use keys longer than that for the "DHE"
10 years. See Section 5.5 for additional information on the use of family of cipher suites. A DH key of 1926 bits would be roughly
modular Diffie-Hellman in TLS. equivalent to a 100-bit symmetric key [RFC3766] and a DH key of 2048
bits might be sufficient for at least the next 10 years. See
Section 4.4 for additional information on the use of modular Diffie-
Hellman in TLS.
Servers SHOULD authenticate using 2048-bit certificates. In As noted in [RFC3766], correcting for the emergence of a TWIRL
machine would imply that 1024-bit DH keys yield about 65 bits of
equivalent strength and that a 2048-bit DH key would yield about 92
bits of equivalent strength.
Servers SHOULD authenticate using at least 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 4.4. 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 4.2. 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. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, 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 perceptions of IPR concerns (which, for
are two related issues hindering effective use of modular Diffie- the most part, have now been resolved [RFC6090]). On the other hand,
Hellman cipher suites in TLS: there are two related issues hindering effective use of modular
Diffie-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 4.5. Truncated HMAC
Implementations MUST NOT use the Truncated HMAC extension, defined in Implementations MUST NOT use the Truncated HMAC extension, defined in
Sec. 7 of [RFC6066]. Section 7 of [RFC6066].
Rationale: the extension does not apply to the AEAD cipher suites Rationale: the extension does not apply to the AEAD cipher suites
recommended above. However it does apply to most other TLS cipher recommended above. However it does apply to most other TLS cipher
suites. Its use has been shown to be insecure in [PatersonRS11]. suites. Its use has been shown to be insecure in [PatersonRS11].
5. Applicability Statement
The deployment recommendations of this document address the operators
of application layer services that are most commonly used on the
Internet, including, but not limited to:
o Operators of web servers that wish to protect HTTP with TLS.
o Operators of email servers who wish to protect the application-
layer protocols with TLS (e.g., IMAP, POP3 or SMTP).
o Operators of instant-messaging services who wish to protect their
application-layer protocols with TLS (e.g., XMPP or IRC).
5.1. Security Services
This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following:
o Confidentiality: all application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.
o Data integrity: any changes made to the communication in transit
are detectable by the receiver.
o Authentication: an end-point of the TLS communication is
authenticated as the intended entity to communicate with.
With regard to authentication, TLS enables authentication of one or
both end-points in the communication. Although some TLS usage
scenarios do not require authentication, those scenarios are not in
scope for this document (a rationale for this decision is provided
under Section 5.2).
If deployers deviate from the recommendations given in this document,
they MUST verify that they do not need one of the foregoing security
services.
This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that
enforce secrecy of the data-in-transit.
This document also assumes that data integrity protection is always
one of the goals of a deployment. In cases where 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 for
instance [DegabrieleP07] in the context of IPsec).
The intended audience covers those services that are most commonly
used on the Internet. Typically, all communication between TLS
clients and TLS servers requires all three of the above security
services. This is particularly true where TLS clients are user
agents like Web browsers or email software.
This document does not address the rarer deployment scenarios where
one of the above three properties is not desired, such as the use
case described under Section 5.2 below. Another 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.
5.2. Unauthenticated TLS and Opportunistic Encryption
Several important applications use TLS to protect data between a TLS
client and a TLS server, but do so without the TLS client necessarily
verifying the server's certificate. This practice is often called
"unauthenticated TLS". The reader is referred to
[I-D.ietf-dane-smtp-with-dane] for an example and an explanation of
why this less secure practice will likely remain common in the
context of SMTP (especially for MTA-to-MTA communications). The
practice is also encountered in similar contexts such as server-to-
server traffic on the XMPP network (where multi-tenant hosting
environments make it difficult for operators to obtain proper
certificates for all of the domains they service).
Furthermore, in some scenarios the use of TLS itself is optional,
i.e. the client decides dynamically ("opportunistically") whether to
use TLS with a particular server or to connect in the clear. This
practice, often called "opportunistic encryption", and is described
at length in Section 2 of [I-D.farrelll-mpls-opportunistic-encrypt].
It can be argued that the recommendations provided in this document
ought to apply equally to unauthenticated TLS as well as
authenticated TLS. That would keep TLS implementations and
deployments in sync, which is a desirable property given that servers
can be used simultaneously for unauthenticated TLS and for
authenticated TLS (indeed, a server cannot know whether a client
might attempt authenticated or unauthenticated TLS). On the other
hand, it has been argued that some of the recommendations in this
document might be too strict for unauthenticated scenarios and that
any security is better than no security at all (i.e., sending traffic
in the clear), even if it means deploying outdated protocol versions
and ciphers in unauthenticated scenarios. The sense of the UTA
Working Group was to complete work on this document about
authenticated TLS and to initiate work on a separate document about
unauthenticated TLS.
In summary: this document does not apply to unauthenticated TLS use
cases.
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
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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 if frequently do not validate host names and must therefore determine 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 Section 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
implement a policy very different from HTTPS. implement a policy very different from HTTPS.
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-with-dane]). [I-D.ietf-dane-srv] and [I-D.ietf-dane-smtp-with-dane]).
Host name validation typically applies only to the leaf "end entity"
certificate. Naturally, in order to ensure proper authentication in
the context of the PKI, application clients need to verify the entire
certification path in accordance with [RFC5280] (see also [RFC6125]).
7.2. AES-GCM 7.2. AES-GCM
Section 5.2 above recommends the use of the AES-GCM authenticated Section 4.2 above recommends the use of the AES-GCM authenticated
encryption algorithm. Please refer to [RFC5246], Sec. 11 for general encryption algorithm. Please refer to [RFC5246], Section 11 for
security considerations when using TLS 1.2, and to [RFC5288], Sec. 6 general security considerations when using TLS 1.2, and to [RFC5288],
for security considerations that apply specifically to AES-GCM when Section 6 for security considerations that apply specifically to AES-
used with TLS. GCM when 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
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o A long-term key used on a device as a default key [Heninger2012]. o A long-term key used on a device as a default key [Heninger2012].
o A key generated by a Trusted Third Party like a CA, and later o A key generated by a Trusted Third Party like a CA, and later
retrieved from it either by extortion or compromise retrieved from it either by extortion or compromise
[Soghoian2011]. [Soghoian2011].
o A cryptographic break-through, or the use of asymmetric keys with o A cryptographic break-through, or the use of asymmetric keys with
insufficient length [Kleinjung2010]. insufficient length [Kleinjung2010].
PFS ensures in such cases that the session keys cannot be determined Forward secrecy ensures in such cases that the session keys cannot be
even by an attacker who obtains the long-term keys some time after determined even by an attacker who obtains the long-term keys some
the conversation. It also protects against an attacker who is in time after the conversation. It also protects against an attacker
possession of the long-term keys, but remains passive during the who is in possession of the long-term keys, but remains passive
conversation. during the conversation.
PFS is generally achieved by using the Diffie-Hellman scheme to Forward secrecy is generally achieved by using the Diffie-Hellman
derive session keys. The Diffie-Hellman scheme has both parties scheme to derive session keys. The Diffie-Hellman scheme has both
maintain private secrets and send parameters over the network as parties maintain private secrets and send parameters over the network
modular powers over certain cyclic groups. The properties of the so- as modular powers over certain cyclic groups. The properties of the
called Discrete Logarithm Problem (DLP) allow to derive the session so-called Discrete Logarithm Problem (DLP) allow to derive the
keys without an eavesdropper being able to do so. There is currently session keys without an eavesdropper being able to do so. There is
no known attack against DLP if sufficiently large parameters are currently no known attack against DLP if sufficiently large
chosen. A variant of the Diffie-Hellman scheme uses Elliptic Curves parameters are chosen. A variant of the Diffie-Hellman scheme uses
instead of the originally proposed modular arithmetics. Elliptic Curves 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 forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. We
strict use of PFS-only ciphers. thus advocate strict use of forward-secrecy-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 for group membership, in order to avoid some
tests are not standardized in TLS at the time of writing. See known attacks. These tests are not standardized in TLS at the
[RFC6989] for recipient tests required of IKEv2 implementations time of writing. See [RFC6989] for recipient tests required of
that reuse DH exponents. IKEv2 implementations that reuse DH exponents.
7.5. Certificate Revocation 7.5. Certificate Revocation
Unfortunately, no mechanism exists at this time that we can recommend Unfortunately, no mechanism exists at this time that we can recommend
as a complete and efficient solution for the problem of checking the as a complete and efficient solution for the problem of checking the
revocation status of common public key certificates (a.k.a. PKIX revocation status of common public key certificates (a.k.a. PKIX
certificates, [RFC5280]). The current state of the art is as certificates, [RFC5280]). The current state of the art is as
follows: follows:
o Certificate Revocation Lists (CRLs) are not scalable and therefore o Certificate Revocation Lists (CRLs) are not scalable and therefore
skipping to change at page 17, line 16 skipping to change at page 17, line 41
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.
With regard to PKIX certificates, servers SHOULD support OCSP and With regard to PKIX certificates, servers SHOULD support OCSP and
OCSP stapling, including the OCSP stapling extension defined in OCSP stapling, including the OCSP stapling extension defined in
[RFC6961], as a best practice given the current state of the art and [RFC6961], as a best practice given the current state of the art and
as a foundation for a possible future solution. as a foundation for a possible future solution.
The foregoing considerations do not apply to DANE certificates The foregoing considerations do not apply to scenarios where the
[RFC6698], since they do not require a revocation mechanism. DANE-TLSA resource record [RFC6698] is used to signal to a client
which certificate a server considers valid and good to use for TLS
connections.
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, Paul Hoffman, Simon Josefsson, Watson Ladd, Orit Levin,
Bodo Moeller, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom Ilari Liusvaara, Johannes Merkle, Bodo Moeller, Yoav Nir, Kenny
Ritter, Rich Salz, Sean Turner, and Aaron Zauner for their feedback Paterson, Patrick Pelletier, Tom Ritter, Rich Salz, Sean Turner, and
and suggested improvements. Thanks to Brian Smith whose "browser Aaron Zauner for their feedback and suggested improvements. Thanks
cipher suites" page is a great resource. Finally, thanks to all to Brian Smith, whose "browser cipher suites" page is a great
others who commented on the TLS, UTA and other lists and are not resource. Finally, thanks to all others who commented on the TLS,
mentioned here by name. UTA, and other discussion lists but who 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 20, line 23 skipping to change at page 20, line 46
Encryption", RFC 5116, January 2008. Encryption", RFC 5116, January 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. (CRL) Profile", RFC 5280, May 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011. Extension Definitions", RFC 6066, January 2011.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure [RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101, Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011. August 2011.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport [RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
Layer Security (TLS)", RFC 6460, January 2012. Layer Security (TLS)", RFC 6460, January 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012. Protocol: TLSA", RFC 6698, August 2012.
skipping to change at page 21, line 15 skipping to change at page 21, line 39
[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-06 A.1. draft-ietf-uta-tls-bcp-07
o WGLC feedback.
A.2. draft-ietf-uta-tls-bcp-06
o Undo unauthenticated TLS, following another long thread on the o Undo unauthenticated TLS, following another long thread on the
list. list.
A.2. draft-ietf-uta-tls-bcp-05 A.3. draft-ietf-uta-tls-bcp-05
o Lots of comments by Sean Turner. o Lots of comments by Sean Turner.
o Unauthenticated TLS, following a long thread on the list. o Unauthenticated TLS, following a long thread on the list.
A.3. draft-ietf-uta-tls-bcp-04 A.4. 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.4. draft-ietf-uta-tls-bcp-03 A.5. 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.5. draft-ietf-uta-tls-bcp-02 A.6. 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.6. draft-ietf-tls-bcp-01 A.7. 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.7. draft-ietf-tls-bcp-00 A.8. draft-ietf-tls-bcp-00
o Initial WG version, with only updated references. o Initial WG version, with only updated references.
A.8. draft-sheffer-tls-bcp-02 A.9. 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.9. draft-sheffer-tls-bcp-01 A.10. 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 forward secrecy.
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.10. draft-sheffer-tls-bcp-00 A.11. 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
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