Recommendations for Secure Use of TLS and DTLS
draft-ietf-uta-tls-bcp-05
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (uta WG) | |
|---|---|---|---|
| Authors | Yaron Sheffer , Ralph Holz , Peter Saint-Andre | ||
| Last updated | 2014-10-13 | ||
| Replaces | draft-sheffer-tls-bcp | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
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draft-ietf-uta-tls-bcp-05
UTA Y. Sheffer
Internet-Draft Porticor
Intended status: Best Current Practice R. Holz
Expires: April 17, 2015 TUM
P. Saint-Andre
&yet
October 14, 2014
Recommendations for Secure Use of TLS and DTLS
draft-ietf-uta-tls-bcp-05
Abstract
Transport Layer Security (TLS) and Datagram Transport Security Layer
(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 April 17, 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. Intended Audience and Applicability Statement . . . . . . . . 4
2.1. Security Services . . . . . . . . . . . . . . . . . . . . 4
2.2. Unauthenticated TLS . . . . . . . . . . . . . . . . . . . 5
3. Conventions used in this document . . . . . . . . . . . . . . 5
4. General Recommendations . . . . . . . . . . . . . . . . . . . 6
4.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 6
4.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 6
4.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 7
4.1.3. Fallback to Earlier Versions . . . . . . . . . . . . 7
4.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8
4.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8
4.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 9
4.6. Server Name Indication . . . . . . . . . . . . . . . . . 9
5. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 9
5.1. General Guidelines . . . . . . . . . . . . . . . . . . . 10
5.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 11
5.3. Cipher Suite Negotiation Details . . . . . . . . . . . . 11
5.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 12
5.5. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 12
5.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 14
7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 14
7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 15
7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 20
A.1. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 20
A.2. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 20
A.3. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 20
A.4. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 20
A.5. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 21
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A.6. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 21
A.7. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 21
A.8. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 21
A.9. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Transport Layer Security (TLS) and Datagram Transport Security Layer
(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. For instance, both the AES-CBC and RC4 encryption
algorithms, which together comprise most current usage, 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. Note that
this document provides guidance for deployed services as well as
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 confidentiality and data integrity
protection to their communications. In many (but not all) cases
authentication is also desired. This document does not address the
rare deployment scenarios where no confidentiality is desired.
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 noted otherwise, these recommendations apply to both TLS and
DTLS. TLS 1.3, when it is standardized and deployed in the field,
should resolve the current vulnerabilities while providing
significantly better functionality and will very likely obsolete this
document.
These are minimum recommendations for the use of TLS for the
specified audience. Individual specifications may have stricter
requirements related to one or more aspects of the protocol, based on
their particular circumstances. When that is the case, implementers
MUST adhere to those stricter requirements.
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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. Intended Audience and Applicability Statement
The deployment recommendations address the operators of application
layer services that are most commonly used on the Internet,
including, but not limited to:
o Operators of WWW servers that wish to protect HTTP with TLS.
o Operators of email servers who wish to protect the application-
layer protocols with TLS (e.g., IMAP, POP3 or SMTP).
o Operators of instant-messaging services who wish to protect their
application-layer protocols with TLS (e.g. XMPP or IRC).
2.1. Security Services
This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following:
o Confidentiality: all (payload) communication is encrypted with the
goal that no party should be able to decrypt it except the
intended receiver.
o Data integrity: any changes made to the communication in transit
are detectable by the receiver.
o Authentication: this means that an end-point of the TLS
communication is authenticated as the intended entity to
communicate with. TLS allows to authenticate one or both end-
points in the communication. Some TLS usage scenarios do not
require authentication, and are further discussed in Section 2.2.
Deployers MUST verify that they do not need one of the above security
services if they deviate from the recommendations given in this
document.
This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that
enforce secrecy of the data-in-transit. While this includes the
majority of the TLS use cases, there are some notable exceptions.
This document assumes that data integrity protection is always one of
the goals of a deployment. In cases when integrity is not required,
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it does not make sense to employ TLS in the first place. There are
attacks against confidentiality-only protection that utilize the lack
of integrity to also break confidentiality (see e.g. [DegabrieleP07]
in the context of IPsec).
The intended audience covers those services that are most commonly
used on the Internet. Typically, all communication between clients
and servers requires all three of the above security services. This
is particularly true where clients are user agents like Web browsers
or email software.
This document does not address the rare deployment scenarios where
one of the above three properties is not desired, with the exception
of the use case described in Section 2.2 below. An example of an
audience not needing confidentiality is the following: a monitored
network where the authorities in charge of the respective traffic
domain require full access to unencrypted (plaintext) traffic, and
where users collaborate and send their traffic in the clear.
2.2. Unauthenticated TLS
Several important applications use TLS to protect data between a
client and a server, but do so without the client verifying the
server's certificate. The reader is referred to
[I-D.dukhovni-smtp-opportunistic-tls] for additional details and an
explanation why this insecure practice is still common and likely to
remain so for a while.
In many of these scenarios the actual use of TLS is optional, i.e.
the client decides dynamically ("opportunistically") whether to use
TLS with a particular server or to connect in the clear.
Opportunistic encryption is described at length in Sec. 2 of
[I-D.farrelll-mpls-opportunistic-encrypt].
Despite the threat model differing from "standard" authenticated
usage of TLS, the recommendations in this document are applicable to
unauthenticated uses of TLS, with the obvious exception of peer
authentication.
3. Conventions used in this document
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].
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4. General Recommendations
This section provides general recommendations on the secure use of
TLS. Recommendations related to cipher suites are discussed in the
following section.
4.1. Protocol Versions
4.1.1. 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. In addition, SSLv3 does not support TLS
extensions, some of which (e.g. renegotiation_info) are security-
critical.
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.
o Implementations MAY 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, and prefer to negotiate, TLS version
1.2 [RFC5246].
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 5.2 below) are only
available in TLS 1.2.
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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.
4.1.2. DTLS Protocol Versions
DTLS is an adaptation of TLS for UDP datagrams.
The following are the recommendations with respect to DTLS:
o Implementations MAY negotiate DTLS version 1.0 [RFC4347].
o Implementations MUST negotiate DTLS version 1.2 [RFC6347].
Rationale: DTLS is an adaptation of TLS for UDP that was introduced
when TLS 1.1 was published. Version 1.0 correlates to TLS 1.1 and
Version 1.2 correlates to TLS 1.2. There is no Version 1.1.
Note: DTLS and TLS are nearly identical. The most notable exception
is that RC4, which is a stream-based bulk encryption algorithm,
cannot be supported by DTLS.
4.1.3. Fallback to Earlier Versions
Clients that "fallback" to lower versions of the protocol after the
server rejects higher versions of the protocol MUST NOT fallback to
SSLv3.
Rationale: Some client implementations revert to lower versions of
TLS or even to SSLv3 if the server rejected higher versions of the
protocol. This fallback can be forced by a man in the middle (MITM)
attacker. TLS 1.0 and SSLv3 are significantly less secure than TLS
1.2, the version recommended by this document. While TLS 1.0-only
servers are still quite common, IP scans show that SSLv3-only servers
amount to only about 3% of the current Web server population.
4.2. 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 even if the server has not advertised that TLS is mandatory or
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even supported (e.g., this is often the case in messaging
protocols such as IMAP and XMPP). Application clients SHOULD use
TLS by default, and disable this default only through explicit
configration by the user.
o HTTP client and server implementations MUST support the HTTP
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.
4.3. Compression
Implementations and deployments SHOULD disable TLS-level compression
([RFC5246], Sec. 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 Sec. 2.5 of [I-D.ietf-uta-tls-attacks] for
further details.
4.4. 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).
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o Session ticket validity SHOULD be limited to a reasonable duration
(e.g. 1 day), for similar reasons.
Rationale: session resumption is another kind of TLS handshake, and
therefore must be as secure as the initial handshake. This document
(Section 5) recommends the use of cipher suites that provide forward
secrecy, i.e. that prevent an attacker who gains momentary access to
the TLS endpoint (either client or server) and its secrets from
reading either past or future communication. The tickets must be
managed so as not to negate this security property.
4.5. TLS Renegotiation
Where handshake renegotiation is implemented, both clients and
servers MUST implement the renegotiation_info extension, as defined
in [RFC5746].
To counter the Triple Handshake attack, we adopt the recommendation
from [triple-handshake]: TLS clients SHOULD ensure that all
certificates received over a connection are valid for the current
server endpoint, and abort the handshake if they are not. In some
usages, it may be simplest to refuse any change of certificates
during renegotiation.
4.6. 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 grain
security for these virtual servers, by allowing each one to have its
own certificate.
5. Recommendations: Cipher Suites
TLS and its implementations provide considerable flexibility in the
selection of cipher suites. Unfortunately many available cipher
suites are insecure, and so misconfiguration can easily result in
reduced security. This section includes recommendations on the
selection and negotiation of cipher suites.
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5.1. 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.
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 only so-
called "export-level" encryption (including algorithms with 40
bits or 56 bits of security).
Rationale: These cipher suites are deliberately "dumbed down" and
are very easy to break.
o Applications MUST NOT negotiate cipher suites of less than 112
bits of security.
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
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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.
5.2. 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
It is noted that those cipher suites are supported only in TLS 1.2
since they are authenticated encryption (AEAD) algorithms [RFC5116].
[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".
5.3. Cipher Suite Negotiation 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
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cipher suite unless there are compelling reasons (e.g., seriously
degraded performance) to choose otherwise.
Note that other profiles of TLS 1.2 exist that use different cipher
suites. For example, [RFC6460] defines a profile that uses the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
This document is not an application profile standard, in the sense of
Sec. 9 of [RFC5246]. As a result, clients and servers are still
REQUIRED to support the mandatory TLS cipher suite,
TLS_RSA_WITH_AES_128_CBC_SHA.
5.4. Public Key Length
When using the cipher suites recommended in this document, two public
keys are normally used in the TLS handshake: one for the Diffie-
Hellman key agreement and one for server authentication. Where a
client certificate is used, a third one is added.
With a key exchange based on modular Diffie-Hellman ("DHE" cipher
suites), DH key lengths of at least 2048 bits are RECOMMENDED.
Rationale: because Diffie-Hellman keys of 1024 bits are estimated to
be roughly equivalent to 80-bit symmetric keys, it is better to use
longer keys for the "DHE" family of cipher suites. Key lengths of at
least 2048 bits are estimated to be roughly equivalent to 112-bit
symmetric keys and might be sufficient for at least the next
10 years. See Section 5.5 for additional information on the use of
modular Diffie-Hellman in TLS.
Servers SHOULD authenticate using 2048-bit certificates. In
addition, the use of SHA-256 fingerprints 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.
5.5. Modular vs. Elliptic Curve DH Cipher Suites
Not all TLS implementations support both modular and EC Diffie-
Hellman groups, as required by Section 5.2. Some implementations are
severely limited in the length of DH values. When such
implementations need to be accommodated, we recommend using (in
priority order):
1. Elliptic Curve DHE with negotiated parameters [RFC5289]
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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: Elliptic Curve Cryptography is not universally deployed
for several reasons, including its complexity compared to modular
arithmetic and longstanding IPR concerns. On the other hand, there
are two related issues hindering effective use of modular Diffie-
Hellman cipher suites in TLS:
o There are no protocol mechanisms to negotiate the DH groups or
parameter lengths supported by client and server.
o There are widely deployed client implementations that reject
received DH parameters if they are longer than 1024 bits.
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.
5.6. Truncated HMAC
Implementations MUST NOT use the Truncated HMAC extension, defined in
Sec. 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].
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.
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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 Sec. 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]).
7.2. AES-GCM
Section 5.2 above recommends the use of the AES-GCM authenticated
encryption algorithm. Please refer to [RFC5246], Sec. 11 for general
security considerations when using TLS 1.2, and to [RFC5288], Sec. 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.
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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].
PFS 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.
PFS 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 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 PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate
strict use of PFS-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, 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.
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7.5. Certificate Revocation
Unfortunately there is currently no effective, Internet-scale
mechanism to effect certificate revocation:
o Certificate Revocation Lists (CRLs) are non-scalable and therefore
rarely used.
o The On-Line Certification Status Protocol (OCSP) presents both
scaling and privacy issues when used for heavy traffic Web
servers. In addition, clients typically "soft-fail", meaning they
do not abort the TLS connection if the OCSP server does not
respond.
o OCSP stapling (Sec. 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. [RFC6961]
addresses this shortcoming, but is a recent addition without much
deployment.
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.
The current consensus appears to be that OCSP stapling, combined with
a "must staple" mechanism similar to HSTS, would finally resolve this
problem; in particular when used together with the extension defined
in [RFC6961]. But such a mechanism has not been standardized yet.
8. Acknowledgments
We would like to thank Uri Blumenthal, Viktor Dukhovni, Stephen
Farrell, Simon Josefsson, Watson Ladd, Orit Levin, Johannes Merkle,
Bodo Moeller, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom
Ritter, Rich Salz, Sean Turner, Aaron Zauner for their review and
improvements. Thanks to Brian Smith whose "browser cipher suites"
page is a great resource. Finally, thanks to all others who
commented on the TLS, UTA and other lists and 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.
[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.
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>.
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[DegabrieleP07]
Degabriele, J. and K. Paterson, "Attacking the IPsec
standards in encryption-only configurations", 2007,
<http://dx.doi.org/10.1109/SP.2007.8>.
[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.dukhovni-smtp-opportunistic-tls]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-dukhovni-smtp-
opportunistic-tls-01 (work in progress), July 2013.
[I-D.farrelll-mpls-opportunistic-encrypt]
Farrel, A. and S. Farrell, "Opportunistic Encryption in
MPLS Networks", draft-farrelll-mpls-opportunistic-
encrypt-02 (work in progress), February 2014.
[I-D.ietf-dane-smtp]
Finch, T., "Secure SMTP using DNS-Based Authentication of
Named Entities (DANE) TLSA records.", draft-ietf-dane-
smtp-01 (work in progress), February 2013.
[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-07 (work in
progress), July 2014.
[I-D.ietf-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf-
tls-prohibiting-rc4-00 (work in progress), July 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>.
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[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.
[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.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 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.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012.
[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.
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[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.
[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-05
o Lots of comments by Sean Turner.
o Unauthenticated TLS, following a long thread on the list.
A.2. draft-ietf-uta-tls-bcp-04
o Some cleanup, and input from TLS WG discussion on applicability.
A.3. 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.
o HSTS: MUST implement, SHOULD deploy.
o Session identities are not protected, only tickets are.
o Clarified the target audience.
A.4. 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.
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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.5. 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.6. draft-ietf-tls-bcp-00
o Initial WG version, with only updated references.
A.7. 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.8. draft-sheffer-tls-bcp-01
o Clarified our motivation in the introduction.
o Added a section justifying the need for PFS.
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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.9. 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
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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