Internet Engineering Task Force (IETF) Y. Sheffer
Request for Comments: 7525 Intuit
BCP: 195 R. Holz
Category: Best Current Practice NICTA
ISSN: 2070-1721 P. Saint-Andre
&yet
May 2015
Recommendations for Secure Use of Transport Layer Security (TLS)
and Datagram Transport Layer Security (DTLS)
Abstract
Transport Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) are widely used to protect data exchanged over application
protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the
last few years, several serious attacks on TLS have emerged,
including attacks on its most commonly used cipher suites and their
modes of operation. This document provides recommendations for
improving the security of deployed services that use TLS and DTLS.
The recommendations are applicable to the majority of use cases.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7525.
Sheffer, et al. Best Current Practice [Page 1]
RFC 7525 TLS Recommendations May 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Sheffer, et al. Best Current Practice [Page 2]
RFC 7525 TLS Recommendations May 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. General Recommendations . . . . . . . . . . . . . . . . . . . 5
3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 5
3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 5
3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 6
3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 7
3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8
3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 9
3.6. Server Name Indication . . . . . . . . . . . . . . . . . 9
4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 9
4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 9
4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 11
4.2.1. Implementation Details . . . . . . . . . . . . . . . 12
4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 12
4.4. Modular Exponential vs. Elliptic Curve DH Cipher Suites . 13
4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 14
5. Applicability Statement . . . . . . . . . . . . . . . . . . . 15
5.1. Security Services . . . . . . . . . . . . . . . . . . . . 15
5.2. Opportunistic Security . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6.1. Host Name Validation . . . . . . . . . . . . . . . . . . 17
6.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 18
6.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 19
6.5. Certificate Revocation . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Normative References . . . . . . . . . . . . . . . . . . 21
7.2. Informative References . . . . . . . . . . . . . . . . . 22
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
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RFC 7525 TLS Recommendations May 2015
1. Introduction
Transport Layer Security (TLS) [RFC5246] and Datagram Transport
Security Layer (DTLS) [RFC6347] are widely used to protect data
exchanged over application protocols such as HTTP, SMTP, IMAP, POP,
SIP, and XMPP. Over the last few years, several serious attacks on
TLS have emerged, including attacks on its most commonly used cipher
suites and their modes of operation. For instance, both the AES-CBC
[RFC3602] and RC4 [RFC7465] encryption algorithms, which together
have been the most widely deployed ciphers, have been attacked in the
context of TLS. A companion document [RFC7457] provides detailed
information about these attacks and will help the reader understand
the rationale behind the recommendations provided here.
Because of these attacks, those who implement and deploy TLS and DTLS
need updated guidance on how TLS can be used securely. This document
provides guidance for deployed services as well as for software
implementations, assuming the implementer expects his or her code to
be deployed in environments defined in Section 5. In fact, this
document calls for the deployment of algorithms that are widely
implemented but not yet widely deployed. Concerning deployment, this
document targets a wide audience -- namely, all deployers who wish to
add authentication (be it one-way only or mutual), confidentiality,
and data integrity protection to their communications.
The recommendations herein take into consideration the security of
various mechanisms, their technical maturity and interoperability,
and their prevalence in implementations at the time of writing.
Unless it is explicitly called out that a recommendation applies to
TLS alone or to DTLS alone, each recommendation applies to both TLS
and DTLS.
It is expected that the TLS 1.3 specification will resolve many of
the vulnerabilities listed in this document. A system that deploys
TLS 1.3 should have fewer vulnerabilities than TLS 1.2 or below.
This document is likely to be updated after TLS 1.3 gets noticeable
deployment.
These are minimum recommendations for the use of TLS in the vast
majority of implementation and deployment scenarios, with the
exception of unauthenticated TLS (see Section 5). Other
specifications that reference this document can have stricter
requirements related to one or more aspects of the protocol, based on
their particular circumstances (e.g., for use with a particular
application protocol); when that is the case, implementers are
advised to adhere to those stricter requirements. Furthermore, this
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document provides a floor, not a ceiling, so stronger options are
always allowed (e.g., depending on differing evaluations of the
importance of cryptographic strength vs. computational load).
Community knowledge about the strength of various algorithms and
feasible attacks can change quickly, and experience shows that a Best
Current Practice (BCP) document about security is a point-in-time
statement. Readers are advised to seek out any errata or updates
that apply to this document.
2. Terminology
A number of security-related terms in this document are used in the
sense defined in [RFC4949].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. General Recommendations
This section provides general recommendations on the secure use of
TLS. Recommendations related to cipher suites are discussed in the
following section.
3.1. Protocol Versions
3.1.1. SSL/TLS Protocol Versions
It is important both to stop using old, less secure versions of SSL/
TLS and to start using modern, more secure versions; therefore, the
following are the recommendations concerning TLS/SSL protocol
versions:
o Implementations MUST NOT negotiate SSL version 2.
Rationale: Today, SSLv2 is considered insecure [RFC6176].
o Implementations MUST NOT negotiate SSL version 3.
Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
plugged some significant security holes but did not support strong
cipher suites. SSLv3 does not support TLS extensions, some of
which (e.g., renegotiation_info [RFC5746]) are security-critical.
In addition, with the emergence of the POODLE attack [POODLE],
SSLv3 is now widely recognized as fundamentally insecure. See
[DEP-SSLv3] for further details.
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o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246];
the only exception is when no higher version is available in the
negotiation.
Rationale: TLS 1.0 (published in 1999) does not support many
modern, strong cipher suites. In addition, TLS 1.0 lacks a per-
record Initialization Vector (IV) for CBC-based cipher suites and
does not warn against common padding errors.
o Implementations SHOULD NOT negotiate TLS version 1.1 [RFC4346];
the only exception is when no higher version is available in the
negotiation.
Rationale: TLS 1.1 (published in 2006) is a security improvement
over TLS 1.0 but still does not support certain stronger cipher
suites.
o Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to
negotiate TLS version 1.2 over earlier versions of TLS.
Rationale: Several stronger cipher suites are available only with
TLS 1.2 (published in 2008). In fact, the cipher suites
recommended by this document (Section 4.2 below) are only
available in TLS 1.2.
This BCP applies to TLS 1.2 and also to earlier versions. It is not
safe for readers to assume that the recommendations in this BCP apply
to any future version of TLS.
3.1.2. DTLS Protocol Versions
DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS
1.1 was published. The following are the recommendations with
respect to DTLS:
o Implementations SHOULD NOT negotiate DTLS version 1.0 [RFC4347].
Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
o Implementations MUST support and MUST prefer to negotiate DTLS
version 1.2 [RFC6347].
Version 1.2 of DTLS correlates to version 1.2 of TLS (see above).
(There is no version 1.1 of DTLS.)
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3.1.3. Fallback to Lower Versions
Clients that "fall back" to lower versions of the protocol after the
server rejects higher versions of the protocol MUST NOT fall back to
SSLv3 or earlier.
Rationale: Some client implementations revert to lower versions of
TLS or even to SSLv3 if the server rejected higher versions of the
protocol. This fallback can be forced by a man-in-the-middle (MITM)
attacker. TLS 1.0 and SSLv3 are significantly less secure than TLS
1.2, the version recommended by this document. While TLS 1.0-only
servers are still quite common, IP scans show that SSLv3-only servers
amount to only about 3% of the current Web server population. (At
the time of this writing, an explicit method for preventing downgrade
attacks has been defined recently in [RFC7507].)
3.2. Strict TLS
The following recommendations are provided to help prevent SSL
Stripping (an attack that is summarized in Section 2.1 of [RFC7457]):
o In cases where an application protocol allows implementations or
deployments a choice between strict TLS configuration and dynamic
upgrade from unencrypted to TLS-protected traffic (such as
STARTTLS), clients and servers SHOULD prefer strict TLS
configuration.
o Application protocols typically provide a way for the server to
offer TLS during an initial protocol exchange, and sometimes also
provide a way for the server to advertise support for TLS (e.g.,
through a flag indicating that TLS is required); unfortunately,
these indications are sent before the communication channel is
encrypted. A client SHOULD attempt to negotiate TLS even if these
indications are not communicated by the server.
o HTTP client and server implementations MUST support the HTTP
Strict Transport Security (HSTS) header [RFC6797], in order to
allow Web servers to advertise that they are willing to accept
TLS-only clients.
o Web servers SHOULD use HSTS to indicate that they are willing to
accept TLS-only clients, unless they are deployed in such a way
that using HSTS would in fact weaken overall security (e.g., it
can be problematic to use HSTS with self-signed certificates, as
described in Section 11.3 of [RFC6797]).
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Rationale: Combining unprotected and TLS-protected communication
opens the way to SSL Stripping and similar attacks, since an initial
part of the communication is not integrity protected and therefore
can be manipulated by an attacker whose goal is to keep the
communication in the clear.
3.3. Compression
In order to help prevent compression-related attacks (summarized in
Section 2.6 of [RFC7457]), implementations and deployments SHOULD
disable TLS-level compression (Section 6.2.2 of [RFC5246]), unless
the application protocol in question has been shown not to be open to
such attacks.
Rationale: TLS compression has been subject to security attacks, such
as the CRIME attack.
Implementers should note that compression at higher protocol levels
can allow an active attacker to extract cleartext information from
the connection. The BREACH attack is one such case. These issues
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.
3.4. TLS Session Resumption
If TLS session resumption is used, care ought to be taken to do so
safely. In particular, when using session 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
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 6.3
for details on forward secrecy).
o For similar reasons, session ticket validity SHOULD be limited to
a reasonable duration (e.g., half as long as ticket key validity).
Rationale: session resumption is another kind of TLS handshake, and
therefore must be as secure as the initial handshake. This document
(Section 4) recommends the use of cipher suites that provide forward
secrecy, i.e. that prevent an attacker who gains momentary access to
the TLS endpoint (either client or server) and its secrets from
reading either past or future communication. The tickets must be
managed so as not to negate this security property.
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3.5. TLS Renegotiation
Where handshake renegotiation is implemented, both clients and
servers MUST implement the renegotiation_info extension, as defined
in [RFC5746].
The most secure option for countering the Triple Handshake attack is
to refuse any change of certificates during renegotiation. In
addition, TLS clients SHOULD apply the same validation policy for all
certificates received over a connection. The [triple-handshake]
document suggests several other possible countermeasures, such as
binding the master secret to the full handshake (see [SESSION-HASH])
and binding the abbreviated session resumption handshake to the
original full handshake. Although the latter two techniques are
still under development and thus do not qualify as current practices,
those who implement and deploy TLS are advised to watch for further
development of appropriate countermeasures.
3.6. Server Name Indication
TLS implementations MUST support the Server Name Indication (SNI)
extension defined in Section 3 of [RFC6066] for those higher-level
protocols that would benefit from it, including HTTPS. However, the
actual use of SNI in particular circumstances is a matter of local
policy.
Rationale: SNI supports deployment of multiple TLS-protected virtual
servers on a single address, and therefore enables fine-grained
security for these virtual servers, by allowing each one to have its
own certificate.
4. Recommendations: Cipher Suites
TLS and its implementations provide considerable flexibility in the
selection of cipher suites. Unfortunately, some available cipher
suites are insecure, some do not provide the targeted security
services, and some no longer provide enough security. Incorrectly
configuring a server leads to no or reduced security. This section
includes recommendations on the selection and negotiation of cipher
suites.
4.1. General Guidelines
Cryptographic algorithms weaken over time as cryptanalysis improves:
algorithms that were once considered strong become weak. Such
algorithms need to be phased out over time and replaced with more
secure cipher suites. This helps to ensure that the desired security
properties still hold. SSL/TLS has been in existence for almost 20
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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
as strong as desired. Therefore, this section modernizes the
recommendations concerning cipher suite selection.
o Implementations MUST NOT negotiate the cipher suites with NULL
encryption.
Rationale: The NULL cipher suites do not encrypt traffic and so
provide no confidentiality services. Any entity in the network
with access to the connection can view the plaintext of contents
being exchanged by the client and server. (Nevertheless, this
document does not discourage software from implementing NULL
cipher suites, since they can be useful for testing and
debugging.)
o Implementations MUST NOT negotiate RC4 cipher suites.
Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, as documented in [RFC7465]. Note that DTLS
specifically forbids the use of RC4 already.
o Implementations MUST NOT negotiate cipher suites offering less
than 112 bits of security, including so-called "export-level"
encryption (which provide 40 or 56 bits of security).
Rationale: Based on [RFC3766], at least 112 bits of security is
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.
o Implementations SHOULD NOT negotiate cipher suites that use
algorithms offering less than 128 bits of security.
Rationale: Cipher suites that offer between 112-bits and 128-bits
of security are not considered weak at this time; however, it is
expected that their useful lifespan is short enough to justify
supporting stronger cipher suites at this time. 128-bit ciphers
are expected to remain secure for at least several years, and
256-bit ciphers until the next fundamental technology
breakthrough. Note that, because of so-called "meet-in-the-
middle" attacks [Multiple-Encryption], some legacy cipher suites
(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).
Such cipher suites should be evaluated according to their
effective key length.
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o Implementations SHOULD NOT negotiate cipher suites based on RSA
key transport, a.k.a. "static RSA".
Rationale: These cipher suites, which have assigned values
starting with the string "TLS_RSA_WITH_*", have several drawbacks,
especially the fact that they do not support forward secrecy.
o Implementations MUST support and prefer to negotiate cipher suites
offering forward secrecy, such as those in the Ephemeral Diffie-
Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("DHE" and
"ECDHE") families.
Rationale: Forward secrecy (sometimes called "perfect forward
secrecy") prevents the recovery of information that was encrypted
with older session keys, thus limiting the amount of time during
which attacks can be successful. See Section 6.3 for a detailed
discussion.
4.2. Recommended Cipher Suites
Given the foregoing considerations, implementation and deployment of
the following cipher suites is RECOMMENDED:
o 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
authenticated encryption (AEAD) algorithms [RFC5116].
Typically, in order to prefer these suites, the order of suites needs
to be explicitly configured in server software. (See [BETTERCRYPTO]
for helpful deployment guidelines, but note that its recommendations
differ from the current document in some details.) It would be ideal
if server software implementations were to prefer these suites by
default.
Some devices have hardware support for AES-CCM but not AES-GCM, so
they are unable to follow the foregoing recommendations regarding
cipher suites. There are even devices that do not support public key
cryptography at all, but they are out of scope entirely.
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4.2.1. Implementation Details
Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
first proposal to any server, unless they have prior knowledge that
the server cannot respond to a TLS 1.2 client_hello message.
Servers MUST prefer this cipher suite over weaker cipher suites
whenever it is proposed, even if it is not the first proposal.
Clients are of course free to offer stronger cipher suites, e.g.,
using AES-256; when they do, the server SHOULD prefer the stronger
cipher suite unless there are compelling reasons (e.g., seriously
degraded performance) to choose otherwise.
This document does not change the mandatory-to-implement TLS cipher
suite(s) prescribed by TLS. To maximize interoperability, RFC 5246
mandates implementation of the TLS_RSA_WITH_AES_128_CBC_SHA cipher
suite, which is significantly weaker than the cipher suites
recommended here. (The GCM mode does not suffer from the same
weakness, caused by the order of MAC-then-Encrypt in TLS
[Krawczyk2001], since it uses an AEAD mode of operation.)
Implementers should consider the interoperability gain against the
loss in security when deploying the TLS_RSA_WITH_AES_128_CBC_SHA
cipher suite. Other application protocols specify other cipher
suites as mandatory to implement (MTI).
Note that some profiles of TLS 1.2 use different cipher suites. For
example, [RFC6460] defines a profile that uses the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
[RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves" extension [RFC4492]. For interoperability, clients
and servers SHOULD support the NIST P-256 (secp256r1) curve
[RFC4492]. In addition, clients SHOULD send an ec_point_formats
extension with a single element, "uncompressed".
4.3. Public Key Length
When using the cipher suites recommended in this document, two public
keys are normally used in the TLS handshake: one for the Diffie-
Hellman key agreement and one for server authentication. Where a
client certificate is used, a third public key is added.
With a key exchange based on modular exponential (MODP) Diffie-
Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048
bits are RECOMMENDED.
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Rationale: For various reasons, in practice, DH keys are typically
generated in lengths that are powers of two (e.g., 2^10 = 1024 bits,
2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 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"
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
bits might be sufficient for at least the next 10 years
[NIST.SP.800-56A]. See Section 4.4 for additional information on the
use of MODP Diffie-Hellman in TLS.
As noted in [RFC3766], correcting for the emergence of a TWIRL
machine would imply that 1024-bit DH keys yield about 65 bits of
equivalent strength and that a 2048-bit DH key would yield about 92
bits of equivalent strength.
With regard to ECDH keys, the IANA "EC Named Curve Registry" (within
the "Transport Layer Security (TLS) Parameters" registry [IANA-TLS])
contains 160-bit elliptic curves that are considered to be roughly
equivalent to only an 80-bit symmetric key [ECRYPT-II]. Curves of
less than 192 bits SHOULD NOT be used.
When using RSA, servers SHOULD authenticate using certificates with
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
more details). Clients SHOULD indicate to servers that they request
SHA-256, by using the "Signature Algorithms" extension defined in
TLS 1.2.
4.4. Modular Exponential vs. Elliptic Curve DH Cipher Suites
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
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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
Section 7 of [RFC6066].
Rationale: the extension does not apply to the AEAD cipher suites
recommended above. However it does apply to most other TLS cipher
suites. Its use has been shown to be insecure in [PatersonRS11].
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5. Applicability Statement
The recommendations of this document primarily apply to the
implementation and deployment of application protocols that are most
commonly used with TLS and DTLS on the Internet today. Examples
include, but are not limited to:
o Web software and services that wish to protect HTTP traffic with
TLS.
o Email software and services that wish to protect IMAP, POP3, or
SMTP traffic with TLS.
o Instant-messaging software and services that wish to protect
Extensible Messaging and Presence Protocol (XMPP) or Internet
Relay Chat (IRC) traffic with TLS.
o Realtime media software and services that wish to protect Secure
Realtime Transport Protocol (SRTP) traffic with DTLS.
This document does not modify the implementation and deployment
recommendations (e.g., mandatory-to-implement cipher suites)
prescribed by existing application protocols that employ TLS or DTLS.
If the community that uses such an application protocol wishes to
modernize its usage of TLS or DTLS to be consistent with the best
practices recommended here, it needs to explicitly update the
existing application protocol definition (one example is [TLS-XMPP],
which updates [RFC6120]).
Designers of new application protocols developed through the Internet
Standards Process [RFC2026] are expected at minimum to conform to the
best practices recommended here, unless they provide documentation of
compelling reasons that would prevent such conformance (e.g.,
widespread deployment on constrained devices that lack support for
the necessary algorithms).
5.1. Security Services
This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following:
o Confidentiality: all application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.
o Data integrity: any changes made to the communication in transit
are detectable by the receiver.
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o Authentication: an endpoint of the TLS communication is
authenticated as the intended entity to communicate with.
With regard to authentication, TLS enables authentication of one or
both endpoints in the communication. In the context of opportunistic
security [RFC7435], TLS is sometimes used without authentication. As
discussed in Section 5.2, considerations for opportunistic security
are not in scope for 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
foregoing security services.
This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that
enforce secrecy of the data in transit.
This document also assumes that data integrity protection is always
one of the goals of a deployment. In cases where integrity is not
required, it does not make sense to employ TLS in the first place.
There are attacks against confidentiality-only protection that
utilize the lack of integrity to also break confidentiality (see, for
instance, [DegabrieleP07] in the context of IPsec).
This document addresses itself to application protocols that are most
commonly used on the Internet with TLS and DTLS. Typically, all
communication between TLS clients and TLS servers requires all three
of the above security services. This is particularly true where TLS
clients are user agents like Web browsers or email software.
This document does not address the rarer deployment scenarios where
one of the above three properties is not desired, such as the use
case described in Section 5.2 below. As another scenario where
confidentiality is not needed, consider a monitored network where the
authorities in charge of the respective traffic domain require full
access to unencrypted (plaintext) traffic, and where users
collaborate and send their traffic in the clear.
5.2. Opportunistic Security
There are several important scenarios in which the use of TLS is
optional, i.e., the client decides dynamically ("opportunistically")
whether to use TLS with a particular server or to connect in the
clear. This practice, often called "opportunistic security", is
described at length in [RFC7435] and is often motivated by a desire
for backward compatibility with legacy deployments.
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In these scenarios, some of the recommendations in this document
might be too strict, since adhering to them could cause fallback to
cleartext, a worse outcome than using TLS with an outdated protocol
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
This entire document discusses the security practices directly
affecting applications using the TLS protocol. This section contains
broader security considerations related to technologies used in
conjunction with or by TLS.
6.1. Host Name Validation
Application authors should take note that some TLS implementations do
not validate host names. If the TLS implementation they are using
does not validate host names, authors might need to write their own
validation code or consider using a different TLS implementation.
It is noted that the requirements regarding host name validation
(and, in general, binding between the TLS layer and the protocol that
runs above it) vary between different protocols. For HTTPS, these
requirements are defined by Section 3 of [RFC2818].
Readers are referred to [RFC6125] for further details regarding
generic host name validation in the TLS context. In addition, that
RFC contains a long list of example protocols, some of which
implement a policy very different from HTTPS.
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
NOT be used as a reference identifier [RFC6125] even when it matches
the presented certificate. This proviso does not apply if the host
name is discovered securely (for further discussion, see [DANE-SRV]
and [DANE-SMTP]).
Host name validation typically applies only to the leaf "end entity"
certificate. Naturally, in order to ensure proper authentication in
the context of the PKI, application clients need to verify the entire
certification path in accordance with [RFC5280] (see also [RFC6125]).
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6.2. AES-GCM
Section 4.2 above recommends the use of the AES-GCM authenticated
encryption algorithm. Please refer to Section 11 of [RFC5246] for
general security considerations when using TLS 1.2, and to Section 6
of [RFC5288] for security considerations that apply specifically to
AES-GCM when used with TLS.
6.3. Forward Secrecy
Forward secrecy (also called "perfect forward secrecy" or "PFS" and
defined in [RFC4949]) is a defense against an attacker who records
encrypted conversations where the session keys are only encrypted
with the communicating parties' long-term keys. Should the attacker
be able to obtain these long-term keys at some point later in time,
the session keys and thus the entire conversation could be decrypted.
In the context of TLS and DTLS, such compromise of long-term keys is
not entirely implausible. It can happen, for example, due to:
o A client or server being attacked by some other attack vector, and
the private key retrieved.
o A long-term key retrieved from a device that has been sold or
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
retrieved from it either by extortion or compromise
[Soghoian2011].
o A cryptographic break-through, or the use of asymmetric keys with
insufficient length [Kleinjung2010].
o Social engineering attacks against system administrators.
o Collection of private keys from inadequately protected backups.
Forward secrecy ensures in such cases that it is not feasible for an
attacker to determine the session keys even if the attacker has
obtained the long-term keys some time after the conversation. It
also protects against an attacker who is in possession of the long-
term keys but remains passive during the conversation.
Forward secrecy is generally achieved by using the Diffie-Hellman
scheme to derive session keys. The Diffie-Hellman scheme has both
parties maintain private secrets and send parameters over the network
as modular powers over certain cyclic groups. The properties of the
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so-called Discrete Logarithm Problem (DLP) allow the parties to
derive the session keys without an eavesdropper being able to do so.
There is currently no known attack against DLP if sufficiently large
parameters are chosen. A variant of the Diffie-Hellman scheme uses
Elliptic Curves instead of the originally proposed modular
arithmetics.
Unfortunately, many TLS/DTLS cipher suites were defined that do not
feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This
document therefore advocates strict use of forward-secrecy-only
ciphers.
6.4. Diffie-Hellman Exponent Reuse
For performance reasons, many TLS implementations reuse Diffie-
Hellman and Elliptic Curve Diffie-Hellman exponents across multiple
connections. Such reuse can result in major security issues:
o 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
previous connections. In other words, exponent reuse negates the
effects of forward secrecy.
o TLS implementations that reuse exponents should test the DH public
key they receive for group membership, in order to avoid some
known attacks. These tests are not standardized in TLS at the
time of writing. See [RFC6989] for recipient tests required of
IKEv2 implementations that reuse DH exponents.
6.5. Certificate Revocation
The following considerations and recommendations represent the
current state of the art regarding certificate revocation, even
though no complete and efficient solution exists for the problem of
checking the revocation status of common public key certificates
[RFC5280]:
o Although Certificate Revocation Lists (CRLs) are the most widely
supported mechanism for distributing revocation information, they
have known scaling challenges that limit their usefulness (despite
workarounds such as partitioned CRLs and delta CRLs).
o Proprietary mechanisms that embed revocation lists in the Web
browser's configuration database cannot scale beyond a small
number of the most heavily used Web servers.
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o The On-Line Certification Status Protocol (OCSP) [RFC6960]
presents both scaling and privacy issues. In addition, clients
typically "soft-fail", meaning that they do not abort the TLS
connection if the OCSP server does not respond. (However, this
might be a workaround to avoid denial-of-service attacks if an
OCSP responder is taken offline.)
o The TLS Certificate Status Request extension (Section 8 of
[RFC6066]), commonly called "OCSP stapling", resolves the
operational issues with OCSP. However, it is still ineffective in
the presence of a MITM attacker because the attacker can simply
ignore the client's request for a stapled OCSP response.
o OCSP stapling as defined in [RFC6066] does not extend to
intermediate certificates used in a certificate chain. Although
the Multiple Certificate Status extension [RFC6961] addresses this
shortcoming, it is a recent addition without much deployment.
o Both CRLs and OCSP depend on relatively reliable connectivity to
the Internet, which might not be available to certain kinds of
nodes (such as newly provisioned devices that need to establish a
secure connection in order to boot up for the first time).
With regard to common public key certificates, servers SHOULD support
the following as a best practice given the current state of the art
and as a foundation for a possible future solution:
1. OCSP [RFC6960]
2. Both the status_request extension defined in [RFC6066] and the
status_request_v2 extension defined in [RFC6961] (This might
enable interoperability with the widest range of clients.)
3. The OCSP stapling extension defined in [RFC6961]
The considerations in this section do not apply to scenarios where
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
connections.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004,
<http://www.rfc-editor.org/info/rfc3766>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI
36, RFC 4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008, <http://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,
August 2008, <http://www.rfc-editor.org/info/rfc5289>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, January
2011, <http://www.rfc-editor.org/info/rfc6066>.
Sheffer, et al. Best Current Practice [Page 21]
RFC 7525 TLS Recommendations May 2015
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011,
<http://www.rfc-editor.org/info/rfc6125>.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, March 2011,
<http://www.rfc-editor.org/info/rfc6176>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012,
<http://www.rfc-editor.org/info/rfc6347>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
February 2015, <http://www.rfc-editor.org/info/rfc7465>.
7.2. Informative References
[BETTERCRYPTO]
bettercrypto.org, "Applied Crypto Hardening", April 2015,
<https://bettercrypto.org/static/
applied-crypto-hardening.pdf>.
[CAB-Baseline]
CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates Version
1.1.6", 2013, <https://www.cabforum.org/documents.html>.
[DANE-SMTP]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", Work in Progress, draft-ietf-
dane-smtp-with-dane-16, April 2015.
[DANE-SRV] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", Work in Progress,
draft-ietf-dane-srv-14, April 2015.
[DEP-SSLv3]
Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", Work in
Progress, draft-ietf-tls-sslv3-diediedie-03, April 2015.
Sheffer, et al. Best Current Practice [Page 22]
RFC 7525 TLS Recommendations May 2015
[DegabrieleP07]
Degabriele, J. and K. Paterson, "Attacking the IPsec
Standards in Encryption-only Configurations", IEEE
Symposium on Security and Privacy (SP '07), 2007,
<http://dx.doi.org/10.1109/SP.2007.8>.
[ECRYPT-II]
Smart, N., "ECRYPT II Yearly Report on Algorithms and
Keysizes (2011-2012)", 2012,
<http://www.ecrypt.eu.org/ecrypt2/>.
[Heninger2012]
Heninger, N., Durumeric, Z., Wustrow, E., and J.
Halderman, "Mining Your Ps and Qs: Detection of Widespread
Weak Keys in Network Devices", Usenix Security Symposium
2012, 2012.
[IANA-TLS] IANA, "Transport Layer Security (TLS) Parameters",
<http://www.iana.org/assignments/tls-parameters>.
[Kleinjung2010]
Kleinjung, T., "Factorization of a 768-Bit RSA modulus",
CRYPTO 10, 2010, <https://eprint.iacr.org/2010/006.pdf>.
[Krawczyk2001]
Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (Or: How Secure is SSL?)",
CRYPTO 01, 2001,
<https://www.iacr.org/archive/crypto2001/21390309.pdf>.
[Multiple-Encryption]
Merkle, R. and M. Hellman, "On the security of multiple
encryption", Communications of the ACM, Vol. 24, 1981,
<http://dl.acm.org/citation.cfm?id=358718>.
[NIST.SP.800-56A]
Barker, E., Chen, L., Roginsky, A., and M. Smid,
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", NIST Special
Publication 800-56A, 2013,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar2.pdf>.
[POODLE] US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE
Attack", Alert TA14-290A, October 2014,
<https://www.us-cert.gov/ncas/alerts/TA14-290A>.
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RFC 7525 TLS Recommendations May 2015
[PatersonRS11]
Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag size
does matter: attacks and proofs for the TLS record
protocol", 2011,
<http://dx.doi.org/10.1007/978-3-642-25385-0_20>.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996,
<http://www.rfc-editor.org/info/rfc2026>.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999,
<http://www.rfc-editor.org/info/rfc2246>.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602, September
2003, <http://www.rfc-editor.org/info/rfc3602>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006,
<http://www.rfc-editor.org/info/rfc4346>.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006,
<http://www.rfc-editor.org/info/rfc4347>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008,
<http://www.rfc-editor.org/info/rfc5077>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011,
<http://www.rfc-editor.org/info/rfc6090>.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011, <http://www.rfc-editor.org/info/rfc6101>.
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[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011,
<http://www.rfc-editor.org/info/rfc6120>.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
Layer Security (TLS)", RFC 6460, January 2012,
<http://www.rfc-editor.org/info/rfc6460>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012,
<http://www.rfc-editor.org/info/rfc6698>.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012,
<http://www.rfc-editor.org/info/rfc6797>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, June 2013,
<http://www.rfc-editor.org/info/rfc6960>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
June 2013, <http://www.rfc-editor.org/info/rfc6961>.
[RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013,
<http://www.rfc-editor.org/info/rfc6989>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, December 2014,
<http://www.rfc-editor.org/info/rfc7435>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, February 2015,
<http://www.rfc-editor.org/info/rfc7457>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, April 2015.
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[SESSION-HASH]
Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension", Work
in Progress, draft-ietf-tls-session-hash-05, April 2015.
[Smith2013]
Smith, B., "Proposal to Change the Default TLS
Ciphersuites Offered by Browsers.", 2013,
<https://briansmith.org/browser-ciphersuites-01.html>.
[Soghoian2011]
Soghoian, C. and S. Stamm, "Certified lies: Detecting and
defeating government interception attacks against SSL",
Proc. 15th Int. Conf. Financial Cryptography and Data
Security, 2011.
[TLS-XMPP] Saint-Andre, P. and a. alkemade, "Use of Transport Layer
Security (TLS) in the Extensible Messaging and Presence
Protocol (XMPP)", Work in Progress,
draft-ietf-uta-xmpp-07, April 2015.
[triple-handshake]
Delignat-Lavaud, A., Bhargavan, K., and A. Pironti,
"Triple Handshakes Considered Harmful: Breaking and Fixing
Authentication over TLS", 2014,
<https://secure-resumption.com/>.
Acknowledgments
Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen
Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson
Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller,
Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom
Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean
Turner, and Aaron Zauner for their feedback and suggested
improvements. Thanks also to Brian Smith, who has provided a great
resource in his "Proposal to Change the Default TLS Ciphersuites
Offered by Browsers" [Smith2013]. Finally, thanks to all others who
commented on the TLS, UTA, and other discussion lists but who are not
mentioned here by name.
Robert Sparks and Dave Waltermire provided helpful reviews on behalf
of the General Area Review Team and the Security Directorate,
respectively.
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During IESG review, Richard Barnes, Alissa Cooper, Spencer Dawkins,
Stephen Farrell, Barry Leiba, Kathleen Moriarty, and Pete Resnick
provided comments that led to further improvements.
Ralph Holz gratefully acknowledges the support by Technische
Universitaet Muenchen. The authors gratefully acknowledge the
assistance of Leif Johansson and Orit Levin as the working group
chairs and Pete Resnick as the sponsoring Area Director.
Authors' Addresses
Yaron Sheffer
Intuit
4 HaHarash St.
Hod HaSharon 4524075
Israel
EMail: [email protected]
Ralph Holz
NICTA
13 Garden St.
Eveleigh 2015 NSW
Australia
EMail: [email protected]
Peter Saint-Andre
&yet
EMail: [email protected]
URI: https://andyet.com/
Sheffer, et al. Best Current Practice [Page 27]
