UTA Y. Sheffer
Internet-Draft Porticor
Intended status: Best Current Practice R. Holz
Expires: June 10, 2015 TUM
P. Saint-Andre
&yet
December 7, 2014
Recommendations for Secure Use of TLS and DTLS
draft-ietf-uta-tls-bcp-08
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 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 10, 2015.
Copyright Notice
Copyright (c) 2014 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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. General Recommendations . . . . . . . . . . . . . . . . . . . 4
3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 4
3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 4
3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 5
3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 6
3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 7
3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7
3.6. Server Name Indication . . . . . . . . . . . . . . . . . 8
4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8
4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8
4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 9
4.2.1. Implementation Details . . . . . . . . . . . . . . . 10
4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 11
4.4. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 11
4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 12
5. Applicability Statement . . . . . . . . . . . . . . . . . . . 13
5.1. Security Services . . . . . . . . . . . . . . . . . . . . 13
5.2. Unauthenticated TLS and Opportunistic Security . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 15
7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 16
7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 17
7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 17
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23
A.1. draft-ietf-uta-tls-bcp-08 . . . . . . . . . . . . . . . . 23
A.2. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 23
A.3. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 23
A.4. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 23
A.5. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 23
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A.6. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 23
A.7. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 24
A.8. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 24
A.9. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 24
A.10. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 25
A.11. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 25
A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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 modes of operation. For instance, both the AES-CBC
[RFC3602] and RC4 [I-D.ietf-tls-prohibiting-rc4] encryption
algorithms, which together are the most widely deployed ciphers, have
been attacked in the context of TLS. A companion document
[I-D.ietf-uta-tls-attacks] provides detailed information about these
attacks.
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 the following section. 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 will have fewer vulnerabilities than TLS 1.2 or below. This
document is likely to be updated after TLS 1.3 gets noticeable
deployment.
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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
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
security BCP 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.
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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) are security-critical. In
addition, with the emergence of the POODLE attack [POODLE], SSLv3
is now widely recognized as fundamentally insecure.
o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246].
Rationale: TLS 1.0 (published in 1999) does not support many
modern, strong cipher suites. In addition, TLS 1.0 lacks a per-
record IV for CBC-based cipher suites and does not warn against
common padding errors.
o Implementations SHOULD NOT negotiate TLS version 1.1 [RFC4346].
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. 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 MAY 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 prefer to negotiate, DTLS
version 1.2 [RFC6347].
Version 1.2 of DTLS correlates to Version 1.2 of TLS 1.2 (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.
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 is being defined in [I-D.ietf-tls-downgrade-scsv].)
3.2. Strict TLS
To prevent SSL Stripping:
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 In many application protocols, clients can be configured to use
TLS no matter whether the server offers TLS during a protocol
exchange or advertises support for TLS (e.g., through a flag
indicating that TLS is required). Application clients SHOULD use
TLS by default, and disable this default only through explicit
configuration by the user.
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 When applicable, Web servers SHOULD use HSTS to indicate that they
are willing to accept TLS-only clients.
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.
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3.3. Compression
Implementations and deployments SHOULD disable TLS-level compression
([RFC5246], Section 6.2.2).
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 out of scope of the
current document. See Section 2.6 of [I-D.ietf-uta-tls-attacks] 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 7.3
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.
3.5. TLS Renegotiation
Where handshake renegotiation is implemented, both clients and
servers MUST implement the renegotiation_info extension, as defined
in [RFC5746].
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To counter the Triple Handshake attack, we adopt the recommended
countermeasures from [triple-handshake]: TLS clients SHOULD apply the
same validation policy for all certificates received over a
connection, bind the master secret to the full handshake, and bind
the abbreviated session resumption handshake to the original full
handshake. In some usages, it may be simplest to refuse any change
of certificates during renegotiation.
3.6. Server Name Indication
TLS implementations MUST support the Server Name Indication (SNI)
extension for those higher level protocols which would benefit from
it, including HTTPS. However, unlike implementation, the 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.
In other words, as time progresses, algorithms that were once
considered strong but are now weak, need to be phased out over time
and replaced with more secure cipher suites to ensure that desired
security properties still hold. SSL/TLS has been in existence for
almost 20 years at this point and this section provides some much
needed recommendations concerning cipher suite selection:
o Implementations MUST NOT negotiate the 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.
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o Implementations MUST NOT negotiate RC4 cipher suites.
Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. We
note that this guideline does not apply to DTLS, which
specifically forbids the use of RC4.
o Implementations MUST NOT negotiate cipher suites offering less
than 112 bits of security, including the 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 some legacy cipher suites (e.g., 168-bit
3DES) have an effective key length which is smaller than their
nominal key length (112 bits in the case of 3DES). Such cipher
suites should be evaluated according to their effective key
length.
o Implementations MUST support, and SHOULD prefer to negotiate,
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 7.3 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
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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.
Some devices have hardware support for AES-CCM but not AES-GCM.
There are even devices that do not support public key cryptography at
all. This BCP does not cover such devices.
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 SHOULD prefer this cipher suite 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 or application protocols using 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. Implementers should
consider the interoperability gain against the loss in security when
deploying that 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
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[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 Diffie-Hellman ("DHE" cipher
suites), DH key lengths of at least 2048 bits are RECOMMENDED.
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. See
Section 4.4 for additional information on the use of modular 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 named curve registry contains
160-bit elliptic curves which are considered to be roughly equivalent
to only an 80-bit symmetric key [ECRYPT-II]. The use of curves of
less than 192-bits is NOT RECOMMENDED.
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 vs. Elliptic Curve DH Cipher Suites
Not all TLS implementations support both modular and elliptic curve
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, we recommend using
(in priority order):
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1. Elliptic Curve DHE with negotiated parameters [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 uptake 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 modular 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 [I-D.ietf-uta-tls-attacks]
We note that with DHE and ECDHE cipher suites, the TLS master key
only depends on the Diffie-Hellman parameters and not on the strength
of the RSA certificate; moreover, 1024 bit modular DH parameters are
generally considered insufficient at this time.
With modular ephemeral DH, deployers SHOULD 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 deployment recommendations of this document address the operators
of application layer services that are most commonly used on the
Internet, including, but not limited to:
o Operators of web servers that wish to protect HTTP with TLS.
o Operators of email servers who wish to protect the application-
layer protocols with TLS (e.g., IMAP, POP3 or SMTP).
o Operators of instant-messaging services who wish to protect their
application-layer protocols with TLS (e.g., XMPP or IRC).
5.1. Security Services
This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following:
o Confidentiality: all application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.
o Data integrity: any changes made to the communication in transit
are detectable by the receiver.
o Authentication: an end-point of the TLS communication is
authenticated as the intended entity to communicate with.
With regard to authentication, TLS enables authentication of one or
both end-points in the communication. Although some TLS usage
scenarios do not require authentication, those scenarios are not in
scope for this document (a rationale for this decision is provided
under Section 5.2).
If deployers deviate from the recommendations given in this document,
they MUST verify that they do not need one of the foregoing security
services.
This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that
enforce secrecy of the data-in-transit.
This document also assumes that data integrity protection is always
one of the goals of a deployment. In cases where integrity is not
required, it does not make sense to employ TLS in the first place.
There are attacks against confidentiality-only protection that
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utilize the lack of integrity to also break confidentiality (see for
instance [DegabrieleP07] in the context of IPsec).
The intended audience covers those services that are most commonly
used on the Internet. Typically, all communication between TLS
clients and TLS servers requires all three of the above security
services. This is particularly true where TLS clients are user
agents like Web browsers or email software.
This document does not address the rarer deployment scenarios where
one of the above three properties is not desired, such as the use
case described under Section 5.2 below. Another example of an
audience not needing confidentiality is the following: a monitored
network where the authorities in charge of the respective traffic
domain require full access to unencrypted (plaintext) traffic, and
where users collaborate and send their traffic in the clear.
5.2. Unauthenticated TLS and Opportunistic Security
Several important applications use TLS to protect data between a TLS
client and a TLS server, but do so without the TLS client necessarily
verifying the server's certificate. This practice is often called
"unauthenticated TLS". The reader is referred to
[I-D.ietf-dane-smtp-with-dane] for an example and an explanation of
why this less secure practice will likely remain common in the
context of SMTP (especially for MTA-to-MTA communications). The
practice is also encountered in similar contexts such as server-to-
server traffic on the XMPP network (where multi-tenant hosting
environments make it difficult for operators to obtain proper
certificates for all of the domains they service).
Furthermore, in some scenarios the use of TLS itself is optional,
i.e. the client decides dynamically ("opportunistically") whether to
use TLS with a particular server or to connect in the clear. This
practice, often called "opportunistic security", and is described at
length in Section 2 of [I-D.farrelll-mpls-opportunistic-encrypt].
It can be argued that the recommendations provided in this document
ought to apply equally to unauthenticated TLS as well as
authenticated TLS. That would keep TLS implementations and
deployments in sync, which is a desirable property given that servers
can be used simultaneously for unauthenticated TLS and for
authenticated TLS (indeed, a server cannot know whether a client
might attempt authenticated or unauthenticated TLS). On the other
hand, it has been argued that some of the recommendations in this
document might be too strict for unauthenticated scenarios and that
any security is better than no security at all (i.e., sending traffic
in the clear), even if it means deploying outdated protocol versions
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and ciphers in unauthenticated scenarios. The sense of the UTA
Working Group was to complete work on this document about
authenticated TLS and to initiate work on a separate document about
unauthenticated TLS.
In summary: this document does not apply to unauthenticated TLS use
cases.
6. IANA Considerations
This document requests no actions of IANA. [Note to RFC Editor:
please remove this whole section before publication.]
7. 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.
7.1. Host Name Validation
Application authors should take note that TLS implementations
frequently do not validate host names and must therefore determine if
the TLS implementation they are using does and, if not, write their
own validation code or consider changing the 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, the
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 for example
[I-D.ietf-dane-srv] and [I-D.ietf-dane-smtp-with-dane]).
Host name validation typically applies only to the leaf "end entity"
certificate. Naturally, in order to ensure proper authentication in
the context of the PKI, application clients need to verify the entire
certification path in accordance with [RFC5280] (see also [RFC6125]).
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7.2. AES-GCM
Section 4.2 above recommends the use of the AES-GCM authenticated
encryption algorithm. Please refer to [RFC5246], Section 11 for
general security considerations when using TLS 1.2, and to [RFC5288],
Section 6 for security considerations that apply specifically to AES-
GCM when used with TLS.
7.3. Forward Secrecy
Forward secrecy (also often 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, he will be able to decrypt the session keys and thus the
entire conversation. 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 the session keys cannot be
determined even by an attacker who obtains 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
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as modular powers over certain cyclic groups. The properties of the
so-called Discrete Logarithm Problem (DLP) allow 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. We
thus advocate strict use of forward-secrecy-only ciphers.
7.4. 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 a long time (e.g., 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.
7.5. Certificate Revocation
Unfortunately, no mechanism exists at this time that we can recommend
as a complete and efficient solution for the problem of checking the
revocation status of common public key certificates (a.k.a. PKIX
certificates, [RFC5280]). The current state of the art is as
follows:
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.
o The On-Line Certification Status Protocol (OCSP) [RFC6960]
presents both scaling and privacy issues. In addition, clients
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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
OSCP responder is taken offline).
o OCSP stapling (Section 8 of [RFC6066]) resolves the operational
issues with OCSP, but 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
[RFC6961] addresses this shortcoming, it is a recent addition
without much deployment.
o Both CRLs and OSCP 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 PKIX certificates, servers SHOULD support both OCSP
[RFC6960] and OCSP stapling. To enable interoperability with the
widest range of clients, servers SHOULD support both the
status_request extension defined in [RFC6066] and the
status_request_v2 extension defined in [RFC6961]. Servers also
SHOULD support the OCSP stapling extension defined in [RFC6961] as a
best practice given the current state of the art and as a foundation
for a possible future solution.
The foregoing considerations 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.
8. Acknowledgments
We would like to thank 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 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.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with
SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
August 2008.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
[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.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, March 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
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9.2. Informative References
[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>.
[DegabrieleP07]
Degabriele, J. and K. Paterson, "Attacking the IPsec
standards in encryption-only configurations", 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/documents/D.SPA.20.pdf>.
[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.
[I-D.farrelll-mpls-opportunistic-encrypt]
Farrel, A. and S. Farrell, "Opportunistic Encryption in
MPLS Networks", draft-farrelll-mpls-opportunistic-
encrypt-02 (work in progress), February 2014.
[I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-10
(work in progress), May 2014.
[I-D.ietf-dane-srv]
Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", draft-ietf-dane-srv-06 (work in
progress), June 2014.
[I-D.ietf-tls-downgrade-scsv]
Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", draft-ietf-tls-downgrade-scsv-02 (work in
progress), November 2014.
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[I-D.ietf-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf-
tls-prohibiting-rc4-01 (work in progress), October 2014.
[I-D.ietf-uta-tls-attacks]
Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Current Attacks on TLS and DTLS", draft-ietf-uta-tls-
attacks-04 (work in progress), September 2014.
[Kleinjung2010]
Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",
CRYPTO 10, 2010, <https://eprint.iacr.org/2010/006.pdf>.
[POODLE] Moeller, B., Duong, T., and K. Kotowicz, "This POODLE
Bites: Exploiting the SSL 3.0 Fallback", 2014, <https://
www.openssl.org/~bodo/ssl-poodle.pdf>.
[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>.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602, September
2003.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
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[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.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
Layer Security (TLS)", RFC 6460, January 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012.
[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.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
June 2013.
[RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013.
[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.
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[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/>.
Appendix A. Change Log
Note to RFC Editor: please remove this section before publication.
A.1. draft-ietf-uta-tls-bcp-08
o More WGLC feedback.
o TLS 1.1 is now SHOULD NOT, just like TLS 1.0.
o SHOULD NOT use curves of less than 192 bits for ECDH.
o Clarification regarding OCSP and OSCP stapling.
A.2. draft-ietf-uta-tls-bcp-07
o WGLC feedback.
A.3. draft-ietf-uta-tls-bcp-06
o Undo unauthenticated TLS, following another long thread on the
list.
A.4. draft-ietf-uta-tls-bcp-05
o Lots of comments by Sean Turner.
o Unauthenticated TLS, following a long thread on the list.
A.5. draft-ietf-uta-tls-bcp-04
o Some cleanup, and input from TLS WG discussion on applicability.
A.6. draft-ietf-uta-tls-bcp-03
o Disallow truncated HMAC.
o Applicability to DTLS.
o Some more text restructuring.
o Host name validation is sometimes irrelevant.
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o HSTS: MUST implement, SHOULD deploy.
o Session identities are not protected, only tickets are.
o Clarified the target audience.
A.7. draft-ietf-uta-tls-bcp-02
o Rearranged some sections for clarity and re-styled the text so
that normative text is followed by rationale where possible.
o Removed the recommendation to use Brainpool curves.
o Triple Handshake mitigation.
o MUST NOT negotiate algorithms lower than 112 bits of security.
o MUST implement SNI, but use per local policy.
o Changed SHOULD NOT negotiate or fall back to SSLv3 to MUST NOT.
o Added hostname validation.
o Non-normative discussion of DH exponent reuse.
A.8. draft-ietf-tls-bcp-01
o Clarified that specific TLS-using protocols may have stricter
requirements.
o Changed TLS 1.0 from MAY to SHOULD NOT.
o Added discussion of "optional TLS" and HSTS.
o Recommended use of the Signature Algorithm and Renegotiation Info
extensions.
o Use of a strong cipher for a resumption ticket: changed SHOULD to
MUST.
o Added an informational discussion of certificate revocation, but
no recommendations.
A.9. draft-ietf-tls-bcp-00
o Initial WG version, with only updated references.
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A.10. draft-sheffer-tls-bcp-02
o Reorganized the content to focus on recommendations.
o Moved description of attacks to a separate document (draft-
sheffer-uta-tls-attacks).
o Strengthened recommendations regarding session resumption.
A.11. draft-sheffer-tls-bcp-01
o Clarified our motivation in the introduction.
o Added a section justifying the need for forward secrecy.
o Added recommendations for RSA and DH parameter lengths. Moved
from DHE to ECDHE, with a discussion on whether/when DHE is
appropriate.
o Recommendation to avoid fallback to SSLv3.
o Initial information about browser support - more still needed!
o More clarity on compression.
o Client can offer stronger cipher suites.
o Discussion of the regular TLS mandatory cipher suite.
A.12. draft-sheffer-tls-bcp-00
o Initial version.
Authors' Addresses
Yaron Sheffer
Porticor
29 HaHarash St.
Hod HaSharon 4501303
Israel
Email: yaronf.ietf@gmail.com
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Ralph Holz
Technische Universitaet Muenchen
Boltzmannstr. 3
Garching 85748
Germany
Email: ralph.ietf@gmail.com
Peter Saint-Andre
&yet
Email: peter@andyet.com
URI: https://andyet.com/
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