draft-ietf-uta-rfc7525bis-00.txt   draft-ietf-uta-rfc7525bis-01.txt 
UTA Working Group Y. Sheffer UTA Working Group Y. Sheffer
Internet-Draft Intuit Internet-Draft Intuit
Obsoletes: 7525 (if approved) R. Holz Obsoletes: 7525 (if approved) R. Holz
Intended status: Best Current Practice University of Twente Updates: 5288, 6066 (if approved) University of Twente
Expires: May 3, 2021 P. Saint-Andre Intended status: Best Current Practice P. Saint-Andre
Mozilla Expires: 8 January 2022 Mozilla
October 30, 2020 T. Fossati
arm
7 July 2021
Recommendations for Secure Use of Transport Layer Security (TLS) and Recommendations for Secure Use of Transport Layer Security (TLS) and
Datagram Transport Layer Security (DTLS) Datagram Transport Layer Security (DTLS)
draft-ietf-uta-rfc7525bis-00 draft-ietf-uta-rfc7525bis-01
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 their including attacks on its most commonly used cipher suites and their
modes of operation. This document provides recommendations for modes of operation. This document provides recommendations for
improving the security of deployed services that use TLS and DTLS. improving the security of deployed services that use TLS and DTLS.
skipping to change at page 1, line 46 skipping to change at page 1, line 48
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 May 3, 2021. This Internet-Draft will expire on 8 January 2022.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. General Recommendations . . . . . . . . . . . . . . . . . . . 4 3. General Recommendations . . . . . . . . . . . . . . . . . . . 5
3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 4 3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 5
3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 4 3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 5
3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 6 3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 6
3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 6 3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 7
3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8 3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8
3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 8 3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 9
3.6. Server Name Indication . . . . . . . . . . . . . . . . . 9 3.6. Post-Handshake Authentication . . . . . . . . . . . . . . 10
4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 9 3.7. Server Name Indication . . . . . . . . . . . . . . . . . 10
4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 9 3.8. Application-Layer Protocol Negotiation . . . . . . . . . 10
4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 11 3.9. Zero Round Trip Time (0-RTT) Data in TLS 1.3 . . . . . . 11
4.2.1. Implementation Details . . . . . . . . . . . . . . . 11 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 11
4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 12 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 12
4.4. Modular Exponential vs. Elliptic Curve DH Cipher Suites . 13 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 13
4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 14 4.2.1. Implementation Details . . . . . . . . . . . . . . . 14
5. Applicability Statement . . . . . . . . . . . . . . . . . . . 14 4.3. Cipher Suites for TLS 1.3 . . . . . . . . . . . . . . . . 14
5.1. Security Services . . . . . . . . . . . . . . . . . . . . 15 4.4. Limits on Key Usage . . . . . . . . . . . . . . . . . . . 15
5.2. Opportunistic Security . . . . . . . . . . . . . . . . . 16 4.5. Public Key Length . . . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 4.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 16
6.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 17 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 16
6.2. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 17 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 17
6.3. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 18 5.2. Opportunistic Security . . . . . . . . . . . . . . . . . 18
6.4. Certificate Revocation . . . . . . . . . . . . . . . . . 18 6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 6.1. Host Name Validation . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . 20 6.2.1. Nonce Reuse in TLS 1.2 . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . 22 6.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 20
6.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 21
Appendix A. Differences from RFC 7525 . . . . . . . . . . . . . 28 6.5. Certificate Revocation . . . . . . . . . . . . . . . . . 22
Appendix B. Document History . . . . . . . . . . . . . . . . . . 28 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
B.1. draft-ietf-uta-rfc7525bis-00 . . . . . . . . . . . . . . 28 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
B.2. draft-sheffer-uta-rfc7525bis-00 . . . . . . . . . . . . . 28 8.1. Normative References . . . . . . . . . . . . . . . . . . 23
B.3. draft-sheffer-uta-bcp195bis-00 . . . . . . . . . . . . . 28 8.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 Appendix A. Differences from RFC 7525 . . . . . . . . . . . . . 32
Appendix B. Document History . . . . . . . . . . . . . . . . . . 33
B.1. draft-ietf-uta-rfc7525bis-01 . . . . . . . . . . . . . . 33
B.2. draft-ietf-uta-rfc7525bis-00 . . . . . . . . . . . . . . 33
B.3. draft-sheffer-uta-rfc7525bis-00 . . . . . . . . . . . . . 34
B.4. draft-sheffer-uta-bcp195bis-00 . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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 years leading to 2015, several serious SIP, and XMPP. Over the years leading to 2015, several serious
attacks on TLS have emerged, including attacks on its most commonly attacks on TLS have emerged, including attacks on its most commonly
used cipher suites and their modes of operation. For instance, both used cipher suites and their modes of operation. For instance, both
the AES-CBC [RFC3602] and RC4 [RFC7465] encryption algorithms, which the AES-CBC [RFC3602] and RC4 [RFC7465] encryption algorithms, which
together have been the most widely deployed ciphers, have been together have been the most widely deployed ciphers, have been
attacked in the context of TLS. A companion document [RFC7457] attacked in the context of TLS. A companion document [RFC7457]
provides detailed information about these attacks and will help the provides detailed information about these attacks and will help the
reader understand the rationale behind the recommendations provided reader understand the rationale behind the recommendations provided
here. here.
The TLS community reacted to these attacks in two ways: The TLS community reacted to these attacks in two ways:
o Detailed guidance was published on the use of TLS 1.2 and earlier * Detailed guidance was published on the use of TLS 1.2 and earlier
protocol versions. This guidance is included in the original protocol versions. This guidance is included in the original
[RFC7525] and mostly retained in this revised version. [RFC7525] and mostly retained in this revised version.
o A new protocol version was released, TLS 1.3 [RFC8446], which
* A new protocol version was released, TLS 1.3 [RFC8446], which
largely mitigates or resolves these attacks. largely mitigates or resolves these attacks.
Those who implement and deploy TLS and DTLS, in particular versions Those who implement and deploy TLS and DTLS, in particular versions
1.2 or earlier of these protocols, need guidance on how TLS can be 1.2 or earlier of these protocols, need guidance on how TLS can be
used securely. This document provides guidance for deployed services used securely. This document provides guidance for deployed services
as well as for software implementations, assuming the implementer as well as for software implementations, assuming the implementer
expects his or her code to be deployed in environments defined in expects his or her code to be deployed in environments defined in
Section 5. Concerning deployment, this document targets a wide Section 5. Concerning deployment, this document targets a wide
audience - namely, all deployers who wish to add authentication (be audience - namely, all deployers who wish to add authentication (be
it one-way only or mutual), confidentiality, and data integrity it one-way only or mutual), confidentiality, and data integrity
protection to their communications. protection to their 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 it is explicitly called out that a recommendation applies to Unless it is explicitly called out that a recommendation applies to
TLS alone or to DTLS alone, each recommendation applies to both TLS TLS alone or to DTLS alone, each recommendation applies to both TLS
and DTLS. and DTLS.
This document attempts to minimize new guidance to TLS 1.2
implementations, and the overall approach is to encourage systems to
move to TLS 1.3. However this is not always practical. Newly
discovered attacks, as well as ecosystem changes, necessitated some
new requirements that apply to TLS 1.2 environments. Those are
summarized in Appendix A.
As noted, the TLS 1.3 specification resolves many of the As noted, the TLS 1.3 specification resolves many of the
vulnerabilities listed in this document. A system that deploys TLS vulnerabilities listed in this document. A system that deploys TLS
1.3 should have fewer vulnerabilities than TLS 1.2 or below. This 1.3 should have fewer vulnerabilities than TLS 1.2 or below. This
document is being republished with this in mind, and with an explicit document is being republished with this in mind, and with an explicit
goal to migrate most uses of TLS 1.2 into TLS 1.3. goal to migrate most uses of TLS 1.2 into TLS 1.3.
These are minimum recommendations for the use of TLS in the vast These are minimum recommendations for the use of TLS in the vast
majority of implementation and deployment scenarios, with the majority of implementation and deployment scenarios, with the
exception of unauthenticated TLS (see Section 5). Other exception of unauthenticated TLS (see Section 5). Other
specifications that reference this document can have stricter specifications that reference this document can have stricter
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3.1. Protocol Versions 3.1. Protocol Versions
3.1.1. 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. * 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.
* 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 strong plugged some significant security holes but did not support strong
cipher suites. SSLv3 does not support TLS extensions, some of cipher suites. SSLv3 does not support TLS extensions, some of
which (e.g., renegotiation_info [RFC5746]) are security-critical. which (e.g., renegotiation_info [RFC5746]) are security-critical.
In addition, with the emergence of the POODLE attack [POODLE], In addition, with the emergence of the POODLE attack [POODLE],
SSLv3 is now widely recognized as fundamentally insecure. See SSLv3 is now widely recognized as fundamentally insecure. See
[DEP-SSLv3] for further details. [DEP-SSLv3] for further details.
o Implementations MUST NOT negotiate TLS version 1.0 [RFC2246].
* Implementations MUST 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. In addition, TLS 1.0 lacks a per- modern, strong cipher suites. In addition, TLS 1.0 lacks a per-
record Initialization Vector (IV) for CBC-based cipher suites and record Initialization Vector (IV) for CBC-based cipher suites and
does not warn against common padding errors. does not warn against common padding errors. This and other
recommendations in this section are in line with [RFC8996].
NOTE: This recommendation has been changed from SHOULD NOT to MUST * Implementations MUST NOT negotiate TLS version 1.1 [RFC4346].
NOT on the assumption that [I-D.ietf-tls-oldversions-deprecate]
will be published as an RFC before this document.
o Implementations MUST NOT 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.
NOTE: This recommendation has been changed from SHOULD NOT to MUST NOTE: This recommendation has been changed from SHOULD NOT to MUST
NOT on the assumption that [I-D.ietf-tls-oldversions-deprecate] NOT on the assumption that [I-D.ietf-tls-oldversions-deprecate]
will be published as an RFC before this document. will be published as an RFC before this document.
o Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to
* Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to
negotiate TLS version 1.2 over earlier versions of TLS. negotiate TLS version 1.2 over earlier versions of TLS.
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 for TLS 1.2 (Section 4.2 below) are recommended by this document for TLS 1.2 (Section 4.2 below) are
only available in this version. only available in this version.
o Implementations SHOULD support TLS 1.3 [RFC8446] and if
* Implementations SHOULD support TLS 1.3 [RFC8446] and if
implemented, MUST prefer to negotiate TLS 1.3 over earlier implemented, MUST prefer to negotiate TLS 1.3 over earlier
versions of TLS. versions of TLS.
Rationale: TLS 1.3 is a major overhaul to the protocol and Rationale: TLS 1.3 is a major overhaul to the protocol and
resolves many of the security issues with TLS 1.2. We note that resolves many of the security issues with TLS 1.2. We note that
as long as TLS 1.2 is still allowed by a particular as long as TLS 1.2 is still allowed by a particular
implementation, even if it defaults to TLS 1.3, implementers MUST implementation, even if it defaults to TLS 1.3, implementers MUST
still follow all the recommendations in this document. still follow all the recommendations in this document.
o Implementations of "greenfield" protocols or deployments, where
* Implementations of "greenfield" protocols or deployments, where
there is no need to support legacy endpoints, SHOULD support TLS there is no need to support legacy endpoints, SHOULD support TLS
1.3, with no negotiation of earlier versions. Similarly, we 1.3, with no negotiation of earlier versions. Similarly, we
RECOMMEND that new protocol designs that embed the TLS mechanisms RECOMMEND that new protocol designs that embed the TLS mechanisms
(such as QUIC has done [I-D.ietf-quic-tls]) include TLS 1.3. (such as QUIC has done [RFC9001]) include TLS 1.3.
Rationale: secure deployment of TLS 1.3 is significantly easier Rationale: secure deployment of TLS 1.3 is significantly easier
and less error prone than the secure deployment of TLS 1.2. and less error prone than the secure deployment of TLS 1.2.
This BCP applies to TLS 1.2, 1.3 and to earlier versions. It is not This BCP applies to TLS 1.2, 1.3 and to earlier versions. It is not
safe for readers to assume that the recommendations in this BCP apply safe for readers to assume that the recommendations in this BCP apply
to any future version of TLS. to any future version of TLS.
3.1.2. 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 MUST NOT negotiate DTLS version 1.0 [RFC4347]. * Implementations MUST NOT 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).
NOTE: This recommendation has been changed from SHOULD NOT to MUST * Implementations MUST support and (unless a higher version is
NOT on the assumption that [I-D.ietf-tls-oldversions-deprecate]
will be published as an RFC before this document.
o Implementations MUST support and (unless a higher version is
available) MUST prefer to negotiate DTLS version 1.2 [RFC6347] available) MUST prefer to negotiate DTLS version 1.2 [RFC6347]
Version 1.2 of DTLS correlates to version 1.2 of TLS (see above). Version 1.2 of DTLS correlates to version 1.2 of TLS (see above).
(There is no version 1.1 of DTLS.) (There is no version 1.1 of DTLS.)
o Implementations SHOULD support and, if available, MUST prefer to * Implementations SHOULD support and, if available, MUST prefer to
negotiate DTLS version 1.3 as specified in [I-D.ietf-tls-dtls13]. negotiate DTLS version 1.3 as specified in [I-D.ietf-tls-dtls13].
Version 1.3 of DTLS correlates to version 1.3 of TLS (see above). Version 1.3 of DTLS correlates to version 1.3 of TLS (see above).
3.1.3. Fallback to Lower Versions 3.1.3. Fallback to Lower Versions
Clients that "fall back" to lower versions of the protocol after the Clients that "fall back" to lower versions of the protocol after the
server rejects higher versions of the protocol MUST NOT fall back to server rejects higher versions of the protocol MUST NOT fall back to
SSLv3 or earlier. Implementations of TLS/DTLS 1.2 or earlier MUST SSLv3 or earlier. Implementations of TLS/DTLS 1.2 or earlier MUST
implement the Fallback SCSV mechanism [RFC7507] to prevent such implement the Fallback SCSV mechanism [RFC7507] to prevent such
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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 but at least TLS 1.0 is still allowed by many web servers. As of 1.2 but at least TLS 1.0 is still allowed by many web servers. As of
this writing, the Fallback SCSV solution is widely deployed and this writing, the Fallback SCSV solution is widely deployed and
proven as a robust solution to this problem. proven as a robust solution to this problem.
3.2. Strict TLS 3.2. Strict TLS
The following recommendations are provided to help prevent SSL The following recommendations are provided to help prevent SSL
Stripping (an attack that is summarized in Section 2.1 of [RFC7457]): Stripping (an attack that is summarized in Section 2.1 of [RFC7457]):
o In cases where an application protocol allows implementations or * 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 Application protocols typically provide a way for the server to
* Application protocols typically provide a way for the server to
offer TLS during an initial protocol exchange, and sometimes also offer TLS during an initial protocol exchange, and sometimes also
provide a way for the server to advertise support for TLS (e.g., provide a way for the server to advertise support for TLS (e.g.,
through a flag indicating that TLS is required); unfortunately, through a flag indicating that TLS is required); unfortunately,
these indications are sent before the communication channel is these indications are sent before the communication channel is
encrypted. A client SHOULD attempt to negotiate TLS even if these encrypted. A client SHOULD attempt to negotiate TLS even if these
indications are not communicated by the server. indications are not communicated by the server.
o HTTP client and server implementations MUST support the HTTP
* 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 Web servers SHOULD use HSTS to indicate that they are willing to
* Web servers SHOULD use HSTS to indicate that they are willing to
accept TLS-only clients, unless they are deployed in such a way accept TLS-only clients, unless they are deployed in such a way
that using HSTS would in fact weaken overall security (e.g., it that using HSTS would in fact weaken overall security (e.g., it
can be problematic to use HSTS with self-signed certificates, as can be problematic to use HSTS with self-signed certificates, as
described in Section 11.3 of [RFC6797]). described in Section 11.3 of [RFC6797]).
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.
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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 outside the scope can only be mitigated outside of TLS and are thus outside the scope
of this document. See Section 2.6 of [RFC7457] for further details. of this document. See Section 2.6 of [RFC7457] for further details.
3.4. TLS Session Resumption 3.4. TLS Session Resumption
If TLS session resumption is used in the context of TLS 1.2, care Session resumption drastically reduces the number of TLS handshakes
ought to be taken to do so safely. In particular, when using session and thus is an essential performance feature for most deployments.
tickets [RFC5077], the resumption information MUST be authenticated
and encrypted to prevent modification or eavesdropping by an
attacker. Further recommendations apply to session tickets:
o A strong cipher suite MUST be used when encrypting the ticket (as Stateless session resumption with session tickets is a popular
strategy. For TLS 1.2, it is specified in [RFC5077]. For TLS 1.3,
an equivalent PSK-based mechanism is described in Section 4.6.1 of
[RFC8446]. When it is used, the resumption information MUST be
authenticated and encrypted to prevent modification or eavesdropping
by an attacker. Further recommendations apply to session tickets:
* 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 not to negate the benefits of forward secrecy (see Section 6.2 * Ticket keys MUST be changed regularly, e.g., once every week, so
as not to negate the benefits of forward secrecy (see Section 6.3
for details on forward secrecy). for details on forward secrecy).
o For similar reasons, session ticket validity SHOULD be limited to
* For similar reasons, session ticket validity SHOULD be limited to
a reasonable duration (e.g., half as long as ticket key validity). a reasonable duration (e.g., half as long as ticket key validity).
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 4) 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.
TLS 1.3 provides the powerful option of forward secrecy even within a
long-lived connection that is periodically resumed. Section 2.2 of
[RFC8446] recommends that clients SHOULD send a "key_share" when
initiating session resumption. In order to gain forward secrecy,
this document recommends that server implementations SHOULD respond
with a "key_share", to complete an ECDHE exchange on each session
resumption.
TLS session resumption introduces potential privacy issues where the
server is able to track the client, in some cases indefinitely. See
[Sy2018] for more details.
3.5. 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]. Note: this recommendation applies to TLS 1.2 only, in [RFC5746]. Note: this recommendation applies to TLS 1.2 only,
because renegotiation has been removed from TLS 1.3. because renegotiation has been removed from TLS 1.3.
The most secure option for countering the Triple Handshake attack is The most secure option for countering the Triple Handshake attack is
to refuse any change of certificates during renegotiation. In to refuse any change of certificates during renegotiation. In
addition, TLS clients SHOULD apply the same validation policy for all addition, TLS clients SHOULD apply the same validation policy for all
certificates received over a connection. The [triple-handshake] certificates received over a connection. The [triple-handshake]
document suggests several other possible countermeasures, such as document suggests several other possible countermeasures, such as
binding the master secret to the full handshake (see [SESSION-HASH]) binding the master secret to the full handshake (see [SESSION-HASH])
and binding the abbreviated session resumption handshake to the and binding the abbreviated session resumption handshake to the
original full handshake. Although the latter two techniques are original full handshake. Although the latter two techniques are
still under development and thus do not qualify as current practices, still under development and thus do not qualify as current practices,
those who implement and deploy TLS are advised to watch for further those who implement and deploy TLS are advised to watch for further
development of appropriate countermeasures. development of appropriate countermeasures.
3.6. Server Name Indication 3.6. Post-Handshake Authentication
Renegotiation in TLS 1.2 was replaced in TLS 1.3 by separate post-
handshake authentication and key update mechanisms. In the context
of protocols that multiplex requests over a single connection (such
as HTTP/2), post-handshake authentication has the same problems as
TLS 1.2 renegotiation. Multiplexed protocols SHOULD follow the
advice provided for HTTP/2 in [RFC8740].
3.7. Server Name Indication
TLS implementations MUST support the Server Name Indication (SNI) TLS implementations MUST support the Server Name Indication (SNI)
extension defined in Section 3 of [RFC6066] for those higher-level extension defined in Section 3 of [RFC6066] for those higher-level
protocols that would benefit from it, including HTTPS. However, the protocols that would benefit from it, including HTTPS. However, the
actual use of SNI in particular circumstances is a matter of local actual use of SNI in particular circumstances is a matter of local
policy. Implementers are strongly encouraged to support TLS policy. Implementers are strongly encouraged to support TLS
Encrypted Client Hello (formerly called Encrypted SNI) once Encrypted Client Hello (formerly called Encrypted SNI) once
[I-D.ietf-tls-esni] has been standardized. [I-D.ietf-tls-esni] has been standardized.
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. However, SNI also leaks the target domain for a own certificate. However, SNI also leaks the target domain for a
given connection; this information leak will be plugged by use of TLS given connection; this information leak will be plugged by use of TLS
Encrypted Client Hello. Encrypted Client Hello.
In order to prevent the attacks described in [ALPACA], a server that
does not recognize the presented server name SHOULD NOT continue the
handshake and instead fail with a fatal-level
"unrecognized_name(112)" alert. Note that this recommendation
updates Section 3 of [RFC6066]: "If the server understood the
ClientHello extension but does not recognize the server name, the
server SHOULD take one of two actions: either abort the handshake by
sending a fatal-level "unrecognized_name(112)" alert or continue the
handshake." It is also RECOMMENDED that clients abort the handshake
if the server acknowledges the SNI hostname with a different hostname
than the one sent by the client.
3.8. Application-Layer Protocol Negotiation
TLS implementations (both client- and server-side) MUST support the
Application-Layer Protocol Negotiation (ALPN) extension [RFC7301].
In order to prevent "cross-protocol" attacks resulting from failure
to ensure that a message intended for use in one protocol cannot be
mistaken for a message for use in another protocol, servers should
strictly enforce the behavior prescribed in Section 3.2 of [RFC7301]:
"In the event that the server supports no protocols that the client
advertises, then the server SHALL respond with a fatal
"no_application_protocol" alert." It is also RECOMMENDED that
clients abort the handshake if the server acknowledges the ALPN
extension, but does not select a protocol from the client list.
Failure to do so can result in attacks such those described in
[ALPACA].
3.9. Zero Round Trip Time (0-RTT) Data in TLS 1.3
The 0-RTT early data feature is new in TLS 1.3. It provides improved
latency when TLS connections are resumed, at the potential cost of
security. As a result, it requires special attention from
implementers on both the server and the client side. Typically this
extends to both the TLS library as well as protocol layers above it.
For use in HTTP-over-TLS, readers are referred to [RFC8470] for
guidance.
For QUIC-on-TLS, refer to Sec. 9.2 of [RFC9001].
For other protocols, generic guidance is given in Sec. 8 and
Appendix E.5 of [RFC8446]. Given the complexity, we RECOMMEND to
avoid this feature altogether unless an explicit specification exists
for the application protocol in question to clarify when 0-RTT is
appropriate and secure. This can take the form of an IETF RFC, a
non-IETF standard, or even documentation associated with a non-
standard protocol.
4. 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.
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Cryptographic algorithms weaken over time as cryptanalysis improves: Cryptographic algorithms weaken over time as cryptanalysis improves:
algorithms that were once considered strong become weak. Such algorithms that were once considered strong become weak. Such
algorithms need to be phased out over time and replaced with more algorithms need to be phased out over time and replaced with more
secure cipher suites. This helps to ensure that the desired security secure cipher suites. This helps to ensure that the desired security
properties still hold. SSL/TLS has been in existence for almost 20 properties still hold. SSL/TLS has been in existence for almost 20
years and many of the cipher suites that have been recommended in years and many of the cipher suites that have been recommended in
various versions of SSL/TLS are now considered weak or at least not various versions of SSL/TLS are now considered weak or at least not
as strong as desired. Therefore, this section modernizes the as strong as desired. Therefore, this section modernizes the
recommendations concerning cipher suite selection. recommendations concerning cipher suite selection.
o Implementations MUST NOT negotiate the cipher suites with NULL * Implementations MUST NOT negotiate the cipher suites with NULL
encryption. encryption.
Rationale: The NULL cipher suites do not encrypt traffic and so Rationale: The NULL cipher suites do not encrypt traffic and so
provide no confidentiality services. Any entity in the network provide no confidentiality services. Any entity in the network
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.
(Nevertheless, this document does not discourage software from
Nevertheless, this document does not discourage software from
implementing NULL cipher suites, since they can be useful for implementing NULL cipher suites, since they can be useful for
testing and debugging.) testing and debugging.
o Implementations MUST NOT negotiate RC4 cipher suites.
* 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 [RFC7465]. Note that DTLS weaknesses, as documented in [RFC7465]. Note that DTLS
specifically forbids the use of RC4 already. specifically forbids the use of RC4 already.
o Implementations MUST NOT negotiate cipher suites offering less
* Implementations MUST NOT negotiate cipher suites offering less
than 112 bits of security, including so-called "export-level" than 112 bits of security, including 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
* 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, because of so-called "meet-in-the- breakthrough. Note that, because of so-called "meet-in-the-
middle" attacks [Multiple-Encryption], some legacy cipher suites middle" attacks [Multiple-Encryption], some legacy cipher suites
(e.g., 168-bit 3DES) have an effective key length that is smaller (e.g., 168-bit 3DES) have an effective key length that is smaller
than their nominal key length (112 bits in the case of 3DES). than their nominal key length (112 bits in the case of 3DES).
Such cipher suites should be evaluated according to their Such cipher suites should be evaluated according to their
effective key length. effective key length.
o Implementations SHOULD NOT negotiate cipher suites based on RSA
* Implementations SHOULD NOT negotiate cipher suites based on RSA
key transport, a.k.a. "static RSA". key transport, a.k.a. "static RSA".
Rationale: These cipher suites, which have assigned values Rationale: These cipher suites, which have assigned values
starting with the string "TLS_RSA_WITH_*", have several drawbacks, starting with the string "TLS_RSA_WITH_*", have several drawbacks,
especially the fact that they do not support forward secrecy. especially the fact that they do not support forward secrecy.
o Implementations MUST support and prefer to negotiate cipher suites
* Implementations MUST support and prefer to negotiate cipher suites
offering forward secrecy, such as those in the Ephemeral Diffie- offering forward secrecy, such as those in the Ephemeral Diffie-
Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("DHE" and Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("DHE" and
"ECDHE") families. "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. See Section 6.2 for a detailed which attacks can be successful. See Section 6.3 for a detailed
discussion. discussion.
4.2. 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 * TLS_DHE_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_ECDHE_RSA_WITH_AES_256_GCM_SHA384
These cipher suites are supported only in TLS 1.2 because they are * TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
authenticated encryption (AEAD) algorithms [RFC5116].
* TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
* TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
These cipher suites are supported only in TLS 1.2 and not in earlier
protocol versions, because they are 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. (See [BETTERCRYPTO] to be explicitly configured in server software. (See [BETTERCRYPTO]
for helpful deployment guidelines, but note that its recommendations for helpful deployment guidelines, but note that its recommendations
differ from the current document in some details.) It would be ideal differ from the current document in some details.) It would be ideal
if server software implementations were to prefer these suites by if server software implementations were to prefer these suites by
default. default.
Some devices have hardware support for AES-CCM but not AES-GCM, so Some devices have hardware support for AES-CCM but not AES-GCM, so
they are unable to follow the foregoing recommendations regarding they are unable to follow the foregoing recommendations regarding
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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.
[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".
4.3. Public Key Length 4.3. Cipher Suites for TLS 1.3
This document does not specify any cipher suites for TLS 1.3.
Readers are referred to Sec. 9.1 of [RFC8446] for cipher suite
recommendations.
4.4. Limits on Key Usage
All ciphers have an upper limit on the amount of traffic that can be
securely encrypted with any given key. In the case of AEAD cipher
suites, the limit is typically determined by the cipher's integrity
guarantees. When the amount of traffic for a particular connection
has reached the limit, an implementation SHOULD perform a new
handshake (or in TLS 1.3, a Key Update) to rotate the session key.
For all AES-GCM cipher suites recommended for TLS 1.2 in this
document, the limit for one connection is 2^(24.5) full-size records
(about 24 million). This is the same number as for TLS 1.3 with the
equivalent cipher suites.
// TODO: refer to {{I-D.irtf-cfrg-aead-limits}} once it has added the
// derivation for TLS 1.2, which is different from TLS 1.3.
// Different derivation, same numbers.
For all TLS 1.3 cipher suites, readers are referred to Section 5.5 of
[RFC8446].
4.5. 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 public key is added. client certificate is used, a third public key is added.
With a key exchange based on modular exponential (MODP) Diffie- With a key exchange based on modular exponential (MODP) Diffie-
Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048 Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048
bits are RECOMMENDED. bits are REQUIRED.
Rationale: For various reasons, in practice, DH keys are typically Rationale: For various reasons, in practice, DH keys are typically
generated in lengths that are powers of two (e.g., 2^10 = 1024 bits, generated in lengths that are powers of two (e.g., 2^(10) = 1024
2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits bits, 2^(11) = 2048 bits, 2^(12) = 4096 bits). Because a DH key of
would be roughly equivalent to only an 80-bit symmetric key 1228 bits would be roughly equivalent to only an 80-bit symmetric key
[RFC3766], it is better to use keys longer than that for the "DHE" [RFC3766], it is better to use keys longer than that for the "DHE"
family of cipher suites. A DH key of 1926 bits would be roughly family of cipher suites. A DH key of 1926 bits would be roughly
equivalent to a 100-bit symmetric key [RFC3766] and a DH key of 2048 equivalent to a 100-bit symmetric key [RFC3766]. A DH key of 2048
bits might be sufficient for at least the next 10 years bits (equivalent to a 112-bit symmetric key) is the minimum allowed
[NIST.SP.800-56A]. See Section 4.4 for additional information on the by the latest revision of [NIST.SP.800-56A], as of this writing (see
use of MODP Diffie-Hellman in TLS. in particular Appendix D).
As noted in [RFC3766], correcting for the emergence of a TWIRL As noted in [RFC3766], correcting for the emergence of a TWIRL
machine would imply that 1024-bit DH keys yield about 65 bits of 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 equivalent strength and that a 2048-bit DH key would yield about 92
bits of equivalent strength. bits of equivalent strength. The Logjam attack [Logjam] further
demonstrates that 1024-bit Diffie Hellman parameters should be
avoided.
With regard to ECDH keys, the IANA "EC Named Curve Registry" (within With regard to ECDH keys, implementers are referred to the IANA
the "Transport Layer Security (TLS) Parameters" registry [IANA_TLS]) "Supported Groups Registry" (former "EC Named Curve Registry"),
contains 160-bit elliptic curves that are considered to be roughly within the "Transport Layer Security (TLS) Parameters" registry
equivalent to only an 80-bit symmetric key [ECRYPT-II]. Curves of [IANA_TLS], and in particular to the "recommended" groups. Curves of
less than 192 bits SHOULD NOT be used. less than 224 bits MUST NOT be used. This recommendation is in-line
with the latest revision of [NIST.SP.800-56A].
When using RSA, servers SHOULD authenticate using certificates with When using RSA, servers SHOULD authenticate using certificates with
at least a 2048-bit modulus for the public key. In addition, the use at least a 2048-bit modulus for the public key. In addition, the use
of the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for of the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for
more details). Clients SHOULD indicate to servers that they request more details). Clients SHOULD indicate to servers that they request
SHA-256, by using the "Signature Algorithms" extension defined in TLS SHA-256, by using the "Signature Algorithms" extension defined in TLS
1.2. 1.2.
4.4. Modular Exponential vs. Elliptic Curve DH Cipher Suites 4.6. Truncated HMAC
Not all TLS implementations support both modular exponential (MODP)
and elliptic curve (EC) Diffie-Hellman groups, as required by
Section 4.2. Some implementations are severely limited in the length
of DH values. When such implementations need to be accommodated, the
following are RECOMMENDED (in priority order):
1. Elliptic Curve DHE with appropriately negotiated parameters
(e.g., the curve to be used) and a Message Authentication Code
(MAC) algorithm stronger than HMAC-SHA1 [RFC5289]
2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
Diffie-Hellman parameters
3. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, with 1024-bit parameters
Rationale: Although Elliptic Curve Cryptography is widely deployed,
there are some communities where its adoption has been limited for
several reasons, including its complexity compared to modular
arithmetic and longstanding perceptions of IPR concerns (which, for
the most part, have now been resolved [RFC6090]). Note that ECDHE
cipher suites exist for both RSA and ECDSA certificates, so moving to
ECDHE cipher suites does not require moving away from RSA-based
certificates. On the other hand, there are two related issues
hindering effective use of MODP Diffie-Hellman cipher suites in TLS:
o There are no standardized, widely implemented protocol mechanisms
to negotiate the DH groups or parameter lengths supported by
client and server.
o Many servers choose DH parameters of 1024 bits or fewer.
o There are widely deployed client implementations that reject
received DH parameters if they are longer than 1024 bits. In
addition, several implementations do not perform appropriate
validation of group parameters and are vulnerable to attacks
referenced in Section 2.9 of [RFC7457].
Note that with DHE and ECDHE cipher suites, the TLS master key only
depends on the Diffie-Hellman parameters and not on the strength of
the RSA certificate; moreover, 1024 bit MODP DH parameters are
generally considered insufficient at this time.
With MODP ephemeral DH, deployers ought to carefully evaluate
interoperability vs. security considerations when configuring their
TLS endpoints.
4.5. Truncated HMAC
Implementations MUST NOT use the Truncated HMAC extension, defined in Implementations MUST NOT use the Truncated HMAC extension, defined in
Section 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 5. Applicability Statement
The recommendations of this document primarily apply to the The recommendations of this document primarily apply to the
implementation and deployment of application protocols that are most implementation and deployment of application protocols that are most
commonly used with TLS and DTLS on the Internet today. Examples commonly used with TLS and DTLS on the Internet today. Examples
include, but are not limited to: include, but are not limited to:
o Web software and services that wish to protect HTTP traffic with * Web software and services that wish to protect HTTP traffic with
TLS. TLS.
o Email software and services that wish to protect IMAP, POP3, or
* Email software and services that wish to protect IMAP, POP3, or
SMTP traffic with TLS. SMTP traffic with TLS.
o Instant-messaging software and services that wish to protect
* Instant-messaging software and services that wish to protect
Extensible Messaging and Presence Protocol (XMPP) or Internet Extensible Messaging and Presence Protocol (XMPP) or Internet
Relay Chat (IRC) traffic with TLS. Relay Chat (IRC) traffic with TLS.
o Realtime media software and services that wish to protect Secure
* Realtime media software and services that wish to protect Secure
Realtime Transport Protocol (SRTP) traffic with DTLS. Realtime Transport Protocol (SRTP) traffic with DTLS.
This document does not modify the implementation and deployment This document does not modify the implementation and deployment
recommendations (e.g., mandatory-to-implement cipher suites) recommendations (e.g., mandatory-to-implement cipher suites)
prescribed by existing application protocols that employ TLS or DTLS. prescribed by existing application protocols that employ TLS or DTLS.
If the community that uses such an application protocol wishes to If the community that uses such an application protocol wishes to
modernize its usage of TLS or DTLS to be consistent with the best modernize its usage of TLS or DTLS to be consistent with the best
practices recommended here, it needs to explicitly update the practices recommended here, it needs to explicitly update the
existing application protocol definition (one example is [TLS-XMPP], existing application protocol definition (one example is [TLS-XMPP],
which updates [RFC6120]). which updates [RFC6120]).
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best practices recommended here, unless they provide documentation of best practices recommended here, unless they provide documentation of
compelling reasons that would prevent such conformance (e.g., compelling reasons that would prevent such conformance (e.g.,
widespread deployment on constrained devices that lack support for widespread deployment on constrained devices that lack support for
the necessary algorithms). the necessary algorithms).
5.1. Security Services 5.1. 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 application-layer communication is encrypted * Confidentiality: all application-layer communication is encrypted
with the goal that no party should be able to decrypt it except with the goal that no party should be able to decrypt it except
the intended receiver. the intended receiver.
o Data integrity: any changes made to the communication in transit
* Data integrity: any changes made to the communication in transit
are detectable by the receiver. are detectable by the receiver.
o Authentication: an endpoint of the TLS communication is
* Authentication: an endpoint of the TLS communication is
authenticated as the intended entity to communicate with. authenticated as the intended entity to communicate with.
With regard to authentication, TLS enables authentication of one or With regard to authentication, TLS enables authentication of one or
both endpoints in the communication. In the context of opportunistic both endpoints in the communication. In the context of opportunistic
security [RFC7435], TLS is sometimes used without authentication. As security [RFC7435], TLS is sometimes used without authentication. As
discussed in Section 5.2, considerations for opportunistic security discussed in Section 5.2, considerations for opportunistic security
are not in scope for this document. are not in scope for this document.
If deployers deviate from the recommendations given in this document, If deployers deviate from the recommendations given in this document,
they need to be aware that they might lose access to one of the they need to be aware that they might lose access to one of the
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whether to use TLS with a particular server or to connect in the whether to use TLS with a particular server or to connect in the
clear. This practice, often called "opportunistic security", is clear. This practice, often called "opportunistic security", is
described at length in [RFC7435] and is often motivated by a desire described at length in [RFC7435] and is often motivated by a desire
for backward compatibility with legacy deployments. for backward compatibility with legacy deployments.
In these scenarios, some of the recommendations in this document In these scenarios, some of the recommendations in this document
might be too strict, since adhering to them could cause fallback to might be too strict, since adhering to them could cause fallback to
cleartext, a worse outcome than using TLS with an outdated protocol cleartext, a worse outcome than using TLS with an outdated protocol
version or cipher suite. version or cipher suite.
This document specifies best practices for TLS in general. A
separate document containing recommendations for the use of TLS with
opportunistic security is to be completed in the future.
6. Security Considerations 6. 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. ## Host Name Validation conjunction with or by TLS.
6.1. Host Name Validation
Application authors should take note that some TLS implementations do Application authors should take note that some TLS implementations do
not validate host names. If the TLS implementation they are using not validate host names. If the TLS implementation they are using
does not validate host names, authors might need to write their own does not validate host names, authors might need to write their own
validation code or consider using a different TLS implementation. validation code or consider using a different TLS implementation.
It is noted that the requirements regarding host name validation It is noted that the requirements regarding host name validation
(and, in general, binding between the TLS layer and the protocol that (and, in general, binding between the TLS layer and the protocol that
runs above it) vary between different protocols. For HTTPS, these runs above it) vary between different protocols. For HTTPS, these
requirements are defined by Section 3 of [RFC2818]. requirements are defined by Sections 4.3.3, 4.3.4 and 4.3.5 of
[I-D.ietf-httpbis-semantics].
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, that generic host name validation in the TLS context. In addition, that
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 [DANE-SRV] name is discovered securely (for further discussion, see [DANE-SRV]
and [DANE-SMTP]). and [DANE-SMTP]).
Host name validation typically applies only to the leaf "end entity" Host name validation typically applies only to the leaf "end entity"
certificate. Naturally, in order to ensure proper authentication in certificate. Naturally, in order to ensure proper authentication in
the context of the PKI, application clients need to verify the entire the context of the PKI, application clients need to verify the entire
certification path in accordance with [RFC5280] (see also [RFC6125]). certification path in accordance with [RFC5280] (see also [RFC6125]).
6.1. AES-GCM 6.2. AES-GCM
Section 4.2 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 Section 11 of [RFC5246] for encryption algorithm. Please refer to Section 11 of [RFC5246] for
general security considerations when using TLS 1.2, and to Section 6 general security considerations when using TLS 1.2, and to Section 6
of [RFC5288] for security considerations that apply specifically to of [RFC5288] for security considerations that apply specifically to
AES-GCM when used with TLS. AES-GCM when used with TLS.
6.2. Forward Secrecy 6.2.1. Nonce Reuse in TLS 1.2
The existence of deployed TLS stacks that mistakenly reuse the AES-
GCM nonce is documented in [Boeck2016], showing there is an actual
risk of AES-GCM getting implemented in an insecure way and thus
making TLS sessions that use an AES-GCM cipher suite vulnerable to
attacks such as [Joux2006]. (See [CVE] records: CVE-2016-0270, CVE-
2016-10213, CVE-2016-10212, CVE-2017-5933.)
While this problem has been fixed in TLS 1.3, which enforces a
deterministic method to generate nonces from record sequence numbers
and shared secrets for all of its AEAD cipher suites (including AES-
GCM), TLS 1.2 implementations could still choose their own
(potentially insecure) nonce generation methods.
It is therefore RECOMMENDED that TLS 1.2 implementations use the
64-bit sequence number to populate the "nonce_explicit" part of the
GCM nonce, as described in the first two paragraphs of Section 5.3 of
[RFC8446]. Note that this recommendation updates Section 3 of
[RFC5288]: "The nonce_explicit MAY be the 64-bit sequence number."
We note that at the time of writing there are no cipher suites
defined for nonce reuse resistant algorithms such as AES-GCM-SIV
[RFC8452].
6.3. Forward Secrecy
Forward secrecy (also called "perfect forward secrecy" or "PFS" and Forward secrecy (also called "perfect forward secrecy" or "PFS" and
defined in [RFC4949]) is a defense against an attacker who records defined in [RFC4949]) is a defense against an attacker who records
encrypted conversations where the session keys are only encrypted encrypted conversations where the session keys are only encrypted
with the communicating parties' long-term keys. with the communicating parties' long-term keys.
Should the attacker be able to obtain these long-term keys at some Should the attacker be able to obtain these long-term keys at some
point later in time, the session keys and thus the entire point later in time, the session keys and thus the entire
conversation could be decrypted. conversation could be decrypted.
In the context of TLS and DTLS, such compromise of long-term keys is In the context of TLS and DTLS, such compromise of long-term keys is
not entirely implausible. It can happen, for example, due to: not entirely implausible. It can happen, for example, due to:
o A client or server being attacked by some other attack vector, and * A client or server being attacked by some other attack vector, and
the private key retrieved. the private key retrieved.
o A long-term key retrieved from a device that has been sold or
* A long-term key retrieved from a device that has been sold or
otherwise decommissioned without prior wiping. otherwise decommissioned without prior wiping.
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 * A long-term key used on a device as a default key [Heninger2012].
* 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
* A cryptographic break-through, or the use of asymmetric keys with
insufficient length [Kleinjung2010]. insufficient length [Kleinjung2010].
o Social engineering attacks against system administrators.
o Collection of private keys from inadequately protected backups. * Social engineering attacks against system administrators.
* Collection of private keys from inadequately protected backups.
Forward secrecy ensures in such cases that it is not feasible for an Forward secrecy ensures in such cases that it is not feasible for an
attacker to determine the session keys even if the attacker has attacker to determine the session keys even if the attacker has
obtained the long-term keys some time after the conversation. It obtained the long-term keys some time after the conversation. It
also protects against an attacker who is in possession of the long- also protects against an attacker who is in possession of the long-
term keys but remains passive during the conversation. term keys but remains passive during the conversation.
Forward secrecy is generally achieved by using the Diffie-Hellman Forward secrecy is generally achieved by using the Diffie-Hellman
scheme to derive session keys. The Diffie-Hellman scheme has both scheme to derive session keys. The Diffie-Hellman scheme has both
parties maintain private secrets and send parameters over the network parties maintain private secrets and send parameters over the network
as modular powers over certain cyclic groups. The properties of the as modular powers over certain cyclic groups. The properties of the
so-called Discrete Logarithm Problem (DLP) allow the parties to so-called Discrete Logarithm Problem (DLP) allow the parties to
derive the session keys without an eavesdropper being able to do so. derive the session keys without an eavesdropper being able to do so.
There is currently no known attack against DLP if sufficiently large There is currently no known attack against DLP if sufficiently large
parameters are chosen. A variant of the Diffie-Hellman scheme uses parameters are chosen. A variant of the Diffie-Hellman scheme uses
Elliptic Curves instead of the originally proposed modular Elliptic Curves instead of the originally proposed modular
arithmetics. arithmetic.
Unfortunately, many TLS/DTLS cipher suites were defined that do not Unfortunately, many TLS/DTLS cipher suites were defined that do not
feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This
document therefore advocates strict use of forward-secrecy-only document therefore advocates strict use of forward-secrecy-only
ciphers. ciphers.
6.3. Diffie-Hellman Exponent Reuse 6.4. 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 too long (e.g., even more than a few * If exponents are reused for too long (e.g., even 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
* TLS implementations that reuse exponents should test the DH public
key they receive for group membership, in order to avoid some key they receive for group membership, in order to avoid some
known attacks. These tests are not standardized in TLS at the known attacks. These tests are not standardized in TLS at the
time of writing. See [RFC6989] for recipient tests required of time of writing. See [RFC6989] for recipient tests required of
IKEv2 implementations that reuse DH exponents. IKEv2 implementations that reuse DH exponents.
6.4. Certificate Revocation * Under certain conditions, the use of static DH keys, or of
ephemeral DH keys that are reused across multiple connections, can
lead to timing attacks (such as those described in [RACCOON]) on
the shared secrets used in Diffie-Hellman key exchange.
To address these concerns, TLS implementations SHOULD NOT use static
DH keys and SHOULD NOT reuse ephemeral DH keys across multiple
connections.
// TODO: revisit when draft-bartle-tls-deprecate-ffdhe becomes a TLS
// WG item, since it specifies MUST NOT rather than SHOULD NOT.
6.5. Certificate Revocation
The following considerations and recommendations represent the The following considerations and recommendations represent the
current state of the art regarding certificate revocation, even current state of the art regarding certificate revocation, even
though no complete and efficient solution exists for the problem of though no complete and efficient solution exists for the problem of
checking the revocation status of common public key certificates checking the revocation status of common public key certificates
[RFC5280]: [RFC5280]:
o Although Certificate Revocation Lists (CRLs) are the most widely * Although Certificate Revocation Lists (CRLs) are the most widely
supported mechanism for distributing revocation information, they supported mechanism for distributing revocation information, they
have known scaling challenges that limit their usefulness (despite have known scaling challenges that limit their usefulness (despite
workarounds such as partitioned CRLs and delta CRLs). workarounds such as partitioned CRLs and delta CRLs).
o Proprietary mechanisms that embed revocation lists in the Web
* 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.
o The On-Line Certification Status Protocol (OCSP) [RFC6960]
* The On-Line Certification Status Protocol (OCSP) [RFC6960]
presents both scaling and privacy issues. In addition, clients presents both scaling and privacy issues. In addition, clients
typically "soft-fail", meaning that they do not abort the TLS typically "soft-fail", meaning that they do not abort the TLS
connection if the OCSP server does not respond. (However, this connection if the OCSP server does not respond. (However, this
might be a workaround to avoid denial-of-service attacks if an might be a workaround to avoid denial-of-service attacks if an
OCSP responder is taken offline.) OCSP responder is taken offline.)
o The TLS Certificate Status Request extension (Section 8 of
* The TLS Certificate Status Request extension (Section 8 of
[RFC6066]), commonly called "OCSP stapling", resolves the [RFC6066]), commonly called "OCSP stapling", resolves the
operational issues with OCSP. However, it is still ineffective in operational issues with OCSP. However, it is still ineffective in
the presence of a MITM attacker because the attacker can simply the presence of a MITM attacker because the attacker can simply
ignore the client's request for a stapled OCSP response. ignore the client's request for a stapled OCSP response.
o OCSP stapling as defined in [RFC6066] does not extend to
* OCSP stapling as defined in [RFC6066] does not extend to
intermediate certificates used in a certificate chain. Although intermediate certificates used in a certificate chain. Although
the Multiple Certificate Status extension [RFC6961] addresses this the Multiple Certificate Status extension [RFC6961] addresses this
shortcoming, it is a recent addition without much deployment. shortcoming, it is a recent addition without much deployment.
o Both CRLs and OCSP depend on relatively reliable connectivity to
* Both CRLs and OCSP depend on relatively reliable connectivity to
the Internet, which might not be available to certain kinds of the Internet, which might not be available to certain kinds of
nodes (such as newly provisioned devices that need to establish a nodes (such as newly provisioned devices that need to establish a
secure connection in order to boot up for the first time). secure connection in order to boot up for the first time).
With regard to common public key certificates, servers SHOULD support With regard to common public key certificates, servers SHOULD support
the following as a best practice given the current state of the art the following as a best practice given the current state of the art
and as a foundation for a possible future solution: and as a foundation for a possible future solution:
1. OCSP [RFC6960] 1. OCSP [RFC6960]
2. Both the status_request extension defined in [RFC6066] and the 2. Both the status_request extension defined in [RFC6066] and the
skipping to change at page 19, line 29 skipping to change at page 23, line 7
secure connection in order to boot up for the first time). secure connection in order to boot up for the first time).
With regard to common public key certificates, servers SHOULD support With regard to common public key certificates, servers SHOULD support
the following as a best practice given the current state of the art the following as a best practice given the current state of the art
and as a foundation for a possible future solution: and as a foundation for a possible future solution:
1. OCSP [RFC6960] 1. OCSP [RFC6960]
2. Both the status_request extension defined in [RFC6066] and the 2. Both the status_request extension defined in [RFC6066] and the
status_request_v2 extension defined in [RFC6961] (This might status_request_v2 extension defined in [RFC6961] (This might
enable interoperability with the widest range of clients.) enable interoperability with the widest range of clients.)
3. The OCSP stapling extension defined in [RFC6961] 3. The OCSP stapling extension defined in [RFC6961]
The considerations in this section do not apply to scenarios where The considerations in this section do not apply to scenarios where
the DANE-TLSA resource record [RFC6698] is used to signal to a client the DANE-TLSA resource record [RFC6698] is used to signal to a client
which certificate a server considers valid and good to use for TLS which certificate a server considers valid and good to use for TLS
connections. connections.
7. Acknowledgments 7. Acknowledgments
The following acknowledgements are inherited from [RFC7525]. The following acknowledgments are inherited from [RFC7525].
Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen
Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson
Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller, Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller,
Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom
Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean
Turner, and Aaron Zauner for their feedback and suggested Turner, and Aaron Zauner for their feedback and suggested
improvements. Thanks also to Brian Smith, who has provided a great improvements. Thanks also to Brian Smith, who has provided a great
resource in his "Proposal to Change the Default TLS Ciphersuites resource in his "Proposal to Change the Default TLS Ciphersuites
Offered by Browsers" [Smith2013]. Finally, thanks to all others who Offered by Browsers" [Smith2013]. Finally, thanks to all others who
skipping to change at page 20, line 24 skipping to change at page 24, line 5
Universitaet Muenchen. Universitaet Muenchen.
The authors gratefully acknowledge the assistance of Leif Johansson The authors gratefully acknowledge the assistance of Leif Johansson
and Orit Levin as the working group chairs and Pete Resnick as the and Orit Levin as the working group chairs and Pete Resnick as the
sponsoring Area Director. sponsoring Area Director.
8. References 8. References
8.1. Normative References 8.1. Normative References
[I-D.ietf-httpbis-semantics]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-16, 27 May 2021,
<https://www.ietf.org/archive/id/draft-ietf-httpbis-
semantics-16.txt>.
[I-D.ietf-tls-dtls13] [I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-38 (work in progress), May 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
2020. dtls13-43, 30 April 2021,
<https://www.ietf.org/archive/id/draft-ietf-tls-
dtls13-43.txt>.
[I-D.ietf-tls-oldversions-deprecate] [I-D.ietf-tls-oldversions-deprecate]
Moriarty, K. and S. Farrell, "Deprecating TLSv1.0 and Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
TLSv1.1", draft-ietf-tls-oldversions-deprecate-08 (work in 1.1", Work in Progress, Internet-Draft, draft-ietf-tls-
progress), October 2020. oldversions-deprecate-12, 21 January 2021,
<https://www.ietf.org/archive/id/draft-ietf-tls-
oldversions-deprecate-12.txt>.
[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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86, Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, DOI 10.17487/RFC3766, April 2004, RFC 3766, DOI 10.17487/RFC3766, April 2004,
<https://www.rfc-editor.org/info/rfc3766>. <https://www.rfc-editor.org/info/rfc3766>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006, DOI 10.17487/RFC4492, May 2006,
<https://www.rfc-editor.org/info/rfc4492>. <https://www.rfc-editor.org/info/rfc4492>.
skipping to change at page 21, line 25 skipping to change at page 25, line 10
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>. <https://www.rfc-editor.org/info/rfc5246>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois [RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008, DOI 10.17487/RFC5288, August 2008,
<https://www.rfc-editor.org/info/rfc5288>. <https://www.rfc-editor.org/info/rfc5288>.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
DOI 10.17487/RFC5289, August 2008,
<https://www.rfc-editor.org/info/rfc5289>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication "Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>. <https://www.rfc-editor.org/info/rfc5746>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>. <https://www.rfc-editor.org/info/rfc6066>.
skipping to change at page 22, line 9 skipping to change at page 25, line 35
2011, <https://www.rfc-editor.org/info/rfc6125>. 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer [RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March
2011, <https://www.rfc-editor.org/info/rfc6176>. 2011, <https://www.rfc-editor.org/info/rfc6176>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>. January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, [RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015, DOI 10.17487/RFC7465, February 2015,
<https://www.rfc-editor.org/info/rfc7465>. <https://www.rfc-editor.org/info/rfc7465>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher [RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015, Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<https://www.rfc-editor.org/info/rfc7507>. <https://www.rfc-editor.org/info/rfc7507>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC8740] Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740,
DOI 10.17487/RFC8740, February 2020,
<https://www.rfc-editor.org/info/rfc8740>.
[RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.
8.2. Informative References 8.2. Informative References
[ALPACA] Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D.,
Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel,
"ALPACA: Application Layer Protocol Confusion - Analyzing
and Mitigating Cracks in TLS Authentication", 30th USENIX
Security Symposium (USENIX Security 21) , 2021,
<https://www.usenix.org/conference/usenixsecurity21/
presentation/brinkmann>.
[BETTERCRYPTO] [BETTERCRYPTO]
bettercrypto.org, "Applied Crypto Hardening", April 2015, bettercrypto.org, "Applied Crypto Hardening", April 2015,
<https://bettercrypto.org/static/applied-crypto- <https://bettercrypto.org/>.
hardening.pdf>.
[Boeck2016]
Böck, H., Zauner, A., Devlin, S., Somorovsky, J., and P.
Jovanovic, "Nonce-Disrespecting Adversaries: Practical
Forgery Attacks on GCM in TLS", May 2016,
<https://eprint.iacr.org/2016/475.pdf>.
[CAB-Baseline] [CAB-Baseline]
CA/Browser Forum, "Baseline Requirements for the Issuance CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates Version and Management of Publicly-Trusted Certificates Version
1.1.6", 2013, <https://www.cabforum.org/documents.html>. 1.1.6", 2013, <https://www.cabforum.org/documents.html>.
[CVE] MITRE, "Common Vulnerabilities and Exposures",
<https://cve.mitre.org>.
[DANE-SMTP] [DANE-SMTP]
Dukhovni, V. and W. Hardaker, "SMTP Security via Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672, (DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015, DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>. <https://www.rfc-editor.org/info/rfc7672>.
[DANE-SRV] [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", RFC 7673, DOI 10.17487/RFC7673, October with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October
2015, <https://www.rfc-editor.org/info/rfc7673>. 2015, <https://www.rfc-editor.org/info/rfc7673>.
[DegabrieleP07] [DegabrieleP07]
Degabriele, J. and K. Paterson, "Attacking the IPsec Degabriele, J. and K. Paterson, "Attacking the IPsec
Standards in Encryption-only Configurations", 2007 IEEE Standards in Encryption-only Configurations", 2007 IEEE
Symposium on Security and Privacy (SP '07), Symposium on Security and Privacy (SP '07),
DOI 10.1109/sp.2007.8, May 2007. DOI 10.1109/sp.2007.8, May 2007,
<https://doi.org/10.1109/sp.2007.8>.
[DEP-SSLv3] [DEP-SSLv3]
Barnes, R., Thomson, M., Pironti, A., and A. Langley, Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568, "Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015, DOI 10.17487/RFC7568, June 2015,
<https://www.rfc-editor.org/info/rfc7568>. <https://www.rfc-editor.org/info/rfc7568>.
[ECRYPT-II]
Smart, N., "ECRYPT II Yearly Report on Algorithms and
Keysizes (2011-2012)", 2012,
<http://www.ecrypt.eu.org/ecrypt2/>.
[Heninger2012] [Heninger2012]
Heninger, N., Durumeric, Z., Wustrow, E., and J. Heninger, N., Durumeric, Z., Wustrow, E., and J.A.
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 Weak Keys in Network Devices", Usenix Security
Symposium 2012, 2012. Symposium 2012, 2012.
[I-D.ietf-quic-tls]
Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
draft-ietf-quic-tls-32 (work in progress), October 2020.
[I-D.ietf-tls-esni] [I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", draft-ietf-tls-esni-08 (work in Encrypted Client Hello", Work in Progress, Internet-Draft,
progress), October 2020. draft-ietf-tls-esni-11, 14 June 2021,
<https://www.ietf.org/archive/id/draft-ietf-tls-esni-
11.txt>.
[IANA_TLS] [I-D.irtf-cfrg-aead-limits]
IANA, "Transport Layer Security (TLS) Parameters", Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-02, 22 February 2021,
<https://www.ietf.org/archive/id/draft-irtf-cfrg-aead-
limits-02.txt>.
[IANA_TLS] IANA, "Transport Layer Security (TLS) Parameters",
<http://www.iana.org/assignments/tls-parameters>. <http://www.iana.org/assignments/tls-parameters>.
[Joux2006] Joux, A., "Authentication Failures in NIST version of
GCM", 2006, <https://csrc.nist.gov/csrc/media/projects/
block-cipher-techniques/documents/bcm/comments/800-38-
series-drafts/gcm/joux_comments.pdf>.
[Kleinjung2010] [Kleinjung2010]
Kleinjung, T., Aoki, K., Franke, J., Lenstra, A., Thome, Kleinjung, T., Aoki, K., Franke, J., Lenstra, A., Thomé,
E., Bos, J., Gaudry, P., Kruppa, A., Montgomery, P., E., Bos, J., Gaudry, P., Kruppa, A., Montgomery, P.,
Osvik, D., te Riele, H., Timofeev, A., and P. Zimmermann, Osvik, D., te Riele, H., Timofeev, A., and P. Zimmermann,
"Factorization of a 768-Bit RSA Modulus", Advances in "Factorization of a 768-Bit RSA Modulus", Advances in
Cryptology - CRYPTO 2010 pp. 333-350, Cryptology - CRYPTO 2010 pp. 333-350,
DOI 10.1007/978-3-642-14623-7_18, 2010. DOI 10.1007/978-3-642-14623-7_18, 2010,
<https://doi.org/10.1007/978-3-642-14623-7_18>.
[Krawczyk2001] [Krawczyk2001]
Krawczyk, H., "The Order of Encryption and Authentication Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (Or: How Secure is SSL?)", for Protecting Communications (Or: How Secure is SSL?)",
CRYPTO 01, 2001, CRYPTO 01, 2001,
<https://www.iacr.org/archive/crypto2001/21390309.pdf>. <https://www.iacr.org/archive/crypto2001/21390309.pdf>.
[Logjam] Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
Green, M., Halderman, J., Heninger, N., Springall, D.,
Thomé, E., Valenta, L., VanderSloot, B., Wustrow, E.,
Zanella-Béguelin, S., and P. Zimmermann, "Imperfect
Forward Secrecy: How Diffie-Hellman Fails in Practice",
Proceedings of the 22nd ACM SIGSAC Conference on Computer
and Communications Security, DOI 10.1145/2810103.2813707,
October 2015, <https://doi.org/10.1145/2810103.2813707>.
[Multiple-Encryption] [Multiple-Encryption]
Merkle, R. and M. Hellman, "On the security of multiple Merkle, R. and M. Hellman, "On the security of multiple
encryption", Communications of the ACM Vol. 24, pp. encryption", Communications of the ACM Vol. 24, pp.
465-467, DOI 10.1145/358699.358718, July 1981. 465-467, DOI 10.1145/358699.358718, July 1981,
<https://doi.org/10.1145/358699.358718>.
[NIST.SP.800-56A] [NIST.SP.800-56A]
Barker, E., Chen, L., Roginsky, A., and M. Smid, Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
"Recommendation for Pair-Wise Key Establishment Schemes Davis, "Recommendation for pair-wise key-establishment
Using Discrete Logarithm Cryptography", NIST Special schemes using discrete logarithm cryptography", National
Publication 800-56A, 2013, Institute of Standards and Technology report,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/ DOI 10.6028/nist.sp.800-56ar3, April 2018,
NIST.SP.800-56Ar2.pdf>. <https://doi.org/10.6028/nist.sp.800-56ar3>.
[PatersonRS11] [PatersonRS11]
Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag Size Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag Size
Does Matter: Attacks and Proofs for the TLS Record Does Matter: Attacks and Proofs for the TLS Record
Protocol", Lecture Notes in Computer Science pp. 372-389, Protocol", Lecture Notes in Computer Science pp. 372-389,
DOI 10.1007/978-3-642-25385-0_20, 2011. DOI 10.1007/978-3-642-25385-0_20, 2011,
<https://doi.org/10.1007/978-3-642-25385-0_20>.
[POODLE] US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE [POODLE] US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE
Attack", October 2014, Attack", October 2014,
<https://www.us-cert.gov/ncas/alerts/TA14-290A>. <https://www.us-cert.gov/ncas/alerts/TA14-290A>.
[RACCOON] Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J.,
Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and
Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)",
30th USENIX Security Symposium (USENIX Security 21) ,
2021, <https://www.usenix.org/conference/usenixsecurity21/
presentation/merget>.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996, 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
<https://www.rfc-editor.org/info/rfc2026>. <https://www.rfc-editor.org/info/rfc2026>.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999, RFC 2246, DOI 10.17487/RFC2246, January 1999,
<https://www.rfc-editor.org/info/rfc2246>. <https://www.rfc-editor.org/info/rfc2246>.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher [RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602, Algorithm and Its Use with IPsec", RFC 3602,
skipping to change at page 25, line 24 skipping to change at page 29, line 49
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>. <https://www.rfc-editor.org/info/rfc5116>.
[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, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>. <https://www.rfc-editor.org/info/rfc5280>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/info/rfc6090>.
[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,
DOI 10.17487/RFC6101, August 2011, DOI 10.17487/RFC6101, August 2011,
<https://www.rfc-editor.org/info/rfc6101>. <https://www.rfc-editor.org/info/rfc6101>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>. March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport [RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
skipping to change at page 26, line 36 skipping to change at page 31, line 5
Known Attacks on Transport Layer Security (TLS) and Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457, Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/info/rfc7457>. February 2015, <https://www.rfc-editor.org/info/rfc7457>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer "Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>. 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
Nonce Misuse-Resistant Authenticated Encryption",
RFC 8452, DOI 10.17487/RFC8452, April 2019,
<https://www.rfc-editor.org/info/rfc8452>.
[RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.
[SESSION-HASH] [SESSION-HASH]
Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A., Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS) Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension", Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015, RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>. <https://www.rfc-editor.org/info/rfc7627>.
[Smith2013] [Smith2013]
Smith, B., "Proposal to Change the Default TLS Smith, B., "Proposal to Change the Default TLS
Ciphersuites Offered by Browsers.", 2013, Ciphersuites Offered by Browsers.", 2013,
<https://briansmith.org/browser-ciphersuites-01.html>. <https://briansmith.org/browser-ciphersuites-01.html>.
[Soghoian2011] [Soghoian2011]
Soghoian, C. and S. Stamm, "Certified Lies: Detecting and Soghoian, C. and S. Stamm, "Certified Lies: Detecting and
Defeating Government Interception Attacks Against SSL", Defeating Government Interception Attacks Against SSL",
SSRN Electronic Journal, DOI 10.2139/ssrn.1591033, 2010. SSRN Electronic Journal, DOI 10.2139/ssrn.1591033, 2010,
<https://doi.org/10.2139/ssrn.1591033>.
[TLS-XMPP] [Sy2018] Sy, E., Burkert, C., Federrath, H., and M. Fischer,
Saint-Andre, P. and T. Alkemade, "Use of Transport Layer "Tracking Users across the Web via TLS Session
Resumption", Proceedings of the 34th Annual Computer
Security Applications Conference,
DOI 10.1145/3274694.3274708, December 2018,
<https://doi.org/10.1145/3274694.3274708>.
[TLS-XMPP] Saint-Andre, P. and T. Alkemade, "Use of Transport Layer
Security (TLS) in the Extensible Messaging and Presence Security (TLS) in the Extensible Messaging and Presence
Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June
2015, <https://www.rfc-editor.org/info/rfc7590>. 2015, <https://www.rfc-editor.org/info/rfc7590>.
[triple-handshake] [triple-handshake]
Bhargavan, K., Lavaud, A., Fournet, C., Pironti, A., and Bhargavan, K., Lavaud, A., Fournet, C., Pironti, A., and
P. Strub, "Triple Handshakes and Cookie Cutters: Breaking P. Strub, "Triple Handshakes and Cookie Cutters: Breaking
and Fixing Authentication over TLS", 2014 IEEE Symposium and Fixing Authentication over TLS", 2014 IEEE Symposium
on Security and Privacy, DOI 10.1109/sp.2014.14, May 2014. on Security and Privacy, DOI 10.1109/sp.2014.14, May 2014,
<https://doi.org/10.1109/sp.2014.14>.
Appendix A. Differences from RFC 7525 Appendix A. Differences from RFC 7525
o Clarified some items (e.g. renegotiation) that only apply to TLS This revision of the Best Current Practices contains numerous
1.2 - many more TBD. changes, and this section is focused on the normative changes.
o Changed status of TLS 1.0 and 1.1 from SHOULD NOT to MUST NOT.
o Added TLS 1.3 at a SHOULD level. * High level differences:
o Similar changes to DTLS, pending publication of DTLS 1.3.
o Fallback SCSV as a MUST for TLS 1.2. - Clarified items (e.g. renegotiation) that only apply to TLS
o Added mention of TLS Encrypted Client Hello, but no recommendation 1.2.
to use yet.
- Changed status of TLS 1.0 and 1.1 from SHOULD NOT to MUST NOT.
- Added TLS 1.3 at a SHOULD level.
- Similar changes to DTLS, pending publication of DTLS 1.3.
- Specific guidance for multiplexed protocols.
- MUST-level implementation requirement for ALPN, and more
specific SHOULD-level guidance for ALPN and SNI.
- New attacks since [RFC7457]: ALPACA, Raccoon, Logjam, "Nonce-
Disrespecting Adversaries"
* Differences specific to TLS 1.2:
- Fallback SCSV as a MUST for TLS 1.2.
- SHOULD-level guidance on AES-GCM nonce generation in TLS 1.2.
- SHOULD NOT use static DH keys or reuse ephemeral DH keys across
multiple connections.
- 2048-bit DH now a MUST, ECDH minimal curve size is 224, vs. 192
previously.
* Differences specific to TLS 1.3:
- New TLS 1.3 capabilities: 0-RTT.
- Removed capabilities: renegotiation, compression.
- Added mention of TLS Encrypted Client Hello, but no
recommendation to use until it is finalized.
- SHOULD-level requirement for forward secrecy in TLS 1.3 session
resumption.
- Generic SHOULD-level guidance to avoid 0-RTT unless it is
documented for the particular protocol.
Appendix B. Document History Appendix B. Document History
[[Note to RFC Editor: please remove before publication.]] // Note to RFC Editor: please remove before publication.
B.1. draft-ietf-uta-rfc7525bis-00 B.1. draft-ietf-uta-rfc7525bis-01
o Renamed: WG document. * Many more changes, including:
o Started populating list of changes from RFC 7525.
o General rewording of abstract and intro for revised version.
o Protocol versions, fallback.
o Reference to ECHO.
B.2. draft-sheffer-uta-rfc7525bis-00 - SHOULD-level requirement for forward secrecy in TLS 1.3 session
resumption.
o Renamed, since the BCP number does not change. - Removed TLS 1.2 capabilities: renegotiation, compression.
o Added an empty "Differences from RFC 7525" section.
B.3. draft-sheffer-uta-bcp195bis-00 - Specific guidance for multiplexed protocols.
o Initial release, the RFC 7525 text as-is, with some minor - MUST-level implementation requirement for ALPN, and more
specific SHOULD-level guidance for ALPN and SNI.
- Generic SHOULD-level guidance to avoid 0-RTT unless it is
documented for the particular protocol.
- SHOULD-level guidance on AES-GCM nonce generation in TLS 1.2.
- SHOULD NOT use static DH keys or reuse ephemeral DH keys across
multiple connections.
- 2048-bit DH now a MUST, ECDH minimal curve size is 224, up from
192.
B.2. draft-ietf-uta-rfc7525bis-00
* Renamed: WG document.
* Started populating list of changes from RFC 7525.
* General rewording of abstract and intro for revised version.
* Protocol versions, fallback.
* Reference to ECHO.
B.3. draft-sheffer-uta-rfc7525bis-00
* Renamed, since the BCP number does not change.
* Added an empty "Differences from RFC 7525" section.
B.4. draft-sheffer-uta-bcp195bis-00
* Initial release, the RFC 7525 text as-is, with some minor
editorial changes to the references. editorial changes to the references.
Authors' Addresses Authors' Addresses
Yaron Sheffer Yaron Sheffer
Intuit Intuit
EMail: yaronf.ietf@gmail.com Email: yaronf.ietf@gmail.com
Ralph Holz Ralph Holz
University of Twente University of Twente
EMail: ralph.ietf@gmail.com Email: ralph.ietf@gmail.com
Peter Saint-Andre Peter Saint-Andre
Mozilla Mozilla
EMail: stpeter@mozilla.com Email: stpeter@mozilla.com
Thomas Fossati
arm
Email: thomas.fossati@arm.com
 End of changes. 124 change blocks. 
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