wdiff draft-sheffer-tls-bcp-01.txt draft-sheffer-tls-bcp-02.txt

tls
UTA Y. Sheffer
Internet-Draft Porticor
Intended status: BCP Best Current Practice R. Holz
Expires: March 24, August 17, 2014 TUM
September 20, 2013
P. Saint-Andre
&yet
February 13, 2014

Recommendations for Secure Use of TLS and DTLS
draft-sheffer-tls-bcp-01
draft-sheffer-tls-bcp-02

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 there have been years, several serious attacks on
TLS, TLS have emerged,
including attacks on its most commonly used ciphers cipher suites and modes
of operation. This document offers provides recommendations on securely using for improving
the TLS security of both software implementations and DTLS protocols, given existing standards deployed services
that use TLS and
implementations. DTLS.

Status of this 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 March 24, August 17, 2014.

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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . 3
1.1. . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
2. Attacks on TLS
3. Recommendations . . . . . . . . . . . . . . . . . . . . 3
2.1. BEAST . . . 3
3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 3
3.2. Fallback to SSL . 4
2.2. Lucky Thirteen . . . . . . . . . . . . . . . . . . . . 4
2.3. Attacks on RC4
3.3. Cipher Suites . . . . . . . . . . . . . . . . . . . . 4
2.4. Compression Attacks: CRIME and BREACH . . 4
3.4. Public Key Length . . . . . . 4
3. Selection Criteria . . . . . . . . . . . . . . 6
3.5. Compression . . . . 4
4. Recommendations . . . . . . . . . . . . . . . . . . . 5
4.1. Summary 6
3.6. Session Resumption . . . . . . . . . . . . . . . . . . . 6
4. Detailed Guidelines . . . . 5
4.2. Cipher Suite Negotiation Details . . . . . . . . . . . 6
4.3. Downgrade Attacks . . . . . . 6
4.1. Cipher Suite Negotiation Details . . . . . . . . . . . . 6
4.4. Alternatives 7
4.2. Alternative Cipher Suites . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . 6
5. Implementation Status . . . . . . . . . . . . . . . . 7 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Perfect Forward Secrecy (PFS) . . . . . . . . . . . . 8
6.3. Session Resumption . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . 9
8. References . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . 9
9.
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . 9
9.1. Normative References . . . . . . . . . . . . 10
Appendix A. Appendix: Change Log . . . . . 9
9.2. Informative References . . . . . . . . . . . 11
A.1. -02 . . . . . 10
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . . 12
A.1. . . . . . 11
A.2. -01 . . . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. . . 11
A.3. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12

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 there have been years, several major serious attacks on TLS
[RFC5246], have emerged,
including attacks on its most commonly used ciphers cipher suites and modes
of operation. Details are given in Section 2, but suffice it
to say that For instance, both AES-CBC and RC4, which together make up for
comprise most current usage, have been seriously attacked in the context of
TLS.

Given A companion document [I-D.sheffer-uta-tls-attacks] provides
detailed information about these issues, there is attacks.

Because of these attacks, those who implement and deploy TLS and DTLS
need for IETF updated guidance on how TLS can be used securely. Unlike most IETF documents, Note that
this is document provides guidance for
deployers, deployed services, as well as for implementers.
software implementations. In fact the recommendations
below call fact, this document calls for the use
deployment of algorithms that are widely implemented algorithms, which are but not seeing widespread use today.

Rather than standardizing new mechanisms in TLS, our goal is to
recommend a few already-specified mechanisms and cipher suites, and
to encourage the industry to use them in order to improve yet
widely deployed.

The recommendations herein take into consideration the overall security of TLS-protected network traffic. When picking these
various mechanisms, we consider their security, their technical maturity and interoperability, as well as
and their prevalence in implementatios at the time of writing.

This recommendation applies 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 the current document.

Our

Community knowledge about the strength of various algorithms and
feasible attacks can change quickly, and experience shows that a crypto
security BCP is a point-in-time statement more than other BCPs. statement. Readers are advised to
seek out any errata or udpates updates that apply to this document.

1.1.

2. Conventions used in this document

[[Are we normative? Currently we're not and this section might go
away.]]

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. Recommendations

3.1. 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:

o Implementations MUST NOT negotiate SSL version 2. Attacks on

Rationale: SSLv2 has serious security vulnerabilities [RFC6176].

o Implementations SHOULD 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.

o Implementations MAY negotiate TLS

This section lists version 1.0 [RFC2246].

Rationale: TLS 1.0 (published in 1999) includes a way to downgrade
the connection to SSLv3 and does not support more modern, strong
cipher suites.

o Implementations MAY negotiate TLS version 1.1 [RFC4346].

Rationale: TLS 1.1 (published in 2006) prevents downgrade attacks that motivated the current
recommendations. This is
to SSL, but does not intended support certain stronger cipher suites.

o Implementations MUST support, and prefer to be an extensive survey of
TLS's security.

While there negotiate, TLS version
1.2 [RFC5246].

Rationale: Several stronger cipher suites are widely deployed mitigations for some of the attacks
listed below, we believe that their root causes necessitate a more
systemic solution.

2.1. BEAST

The BEAST attack [BEAST] uses issues available only with the
TLS 1.0 implementation 1.2 (published in 2008).

As of CBC (that is, predictable IV) to decrypt parts the date of a packet, and
specifically shows how this can be used to decrypt HTTP cookies when
run over TLS.

2.2. Lucky Thirteen

A consequence of writing, the MAC-then-encrypt design in all current versions latest version of TLS is the existence of padding oracle attacks [Padding-Oracle].
A recent incarnation of these attacks 1.2.
When TLS is the Lucky Thirteen attack
[CBC-Attack], a timing side-channel attack that allows the attacker updated to decrypt arbitrary ciphertext.

2.3. Attacks on RC4

The RC4 algorithm [RC4] has been used with TLS (and previously, SSL)
for many years. Attacks have also been known for a long time, e.g.
[RC4-Attack-FMS]. But recent attacks ([RC4-Attack],
[RC4-Attack-AlF]) have weakened newer version, this algorithm even more. See
[I-D.popov-tls-prohibiting-rc4] for more details.

2.4. Compression Attacks: CRIME and BREACH

The CRIME attack [CRIME] allows an active attacker document will be updated
to decrypt
cyphertext (specifically, cookies) when TLS is used with protocol-
level compression.

The BREACH attack [BREACH] makes similar use of HAdded TTP-level
compression, which is much more prevalent than compression at recommend support for the TLS
level, to decrypt secret data passed latest version. If this document is not
updated in the HTTP response.

The former attack a timely manner, it can be mitigated by disabling TLS compression, as
recommended below. We are not aware of mitigations at assumed that support for the protocol
level
latest version of TLS is recommended.

3.2. Fallback to the latter attack, and so application-level mitigations are
needed (see [BREACH]). For example, SSL

Some client implementations revert to SSLv3 if the server rejected
higher versions of HTTP SSL/TLS. This fallback can be forced by a MITM
attacker. Moreover, IP scans [[reference?]] show that use
CSRF tokens will need SSLv3-only
servers amount to randomize them even when the recommendations only about 3% of the current document are adopted.

3. Selection Criteria

Given the above attacks, we are proposing that deployers opt for a
specific web server population.
Therefore, by default clients SHOULD NOT fall back from TLS to SSLv3.

3.3. Cipher Suites

It is important both to stop using old, insecure cipher suite when negotiating TLS. We have used the
following criteria when framing our recommendations: suites and to
start using modern, more secure cipher suites. Therefore:

o Implementations MUST NOT negotiate the NULL cipher suites.

Rationale: The NULL cipher suite must be secure in default use, suites offer no encryption whatsoever
and should not
require any additional security measures beyond those defined in
the standard. thus are completely insecure.

o Implementations MUST NOT negotiate RC4 cipher suites

Rationale: The RC4 stream cipher suite must be widely implemented, i.e. available in has a
large percentage variety of popular cryptographic libraries.
weaknesses, as documented in [I-D.popov-tls-prohibiting-rc4].

o The Implementations MUST NOT negotiate cipher suite must have undergone a significant amount of
analysis, and the algorithm and mode suites offering only so-
called "export-level" encryption (including algorithms with 40
bits or 56 bits of operation must both be
standardized by relevant organizations.
o We prefer security).

Rationale: These cipher suites that provide client-side privacy and
perfect forward secrecy, i.e. those that use ephemeral Diffie-
Hellman. See Section 6.2 for more details.
o As currently specified and implemented, elliptic curve groups are
preferable over modular DH groups: they are easier deliberately "dumbed down" and safer
are very easy to
use within TLS. break.

o When there are multiple key sizes available, we have chosen the
current industry standard, Implementations SHOULD NOT negotiate cipher suites that use
algorithms offering less than 128 bits of strength. Of course
deployers security (even if they
advertise more bits, such as the 168-bit 3DES cipher suites).

Rationale: Although these cipher suites are free not actively subject
to opt for a breakage, their useful life is short enough that stronger
cipher suite.

4. Recommendations

Following suites are recommendations desirable.

o Implementations SHOULD prefer cipher suites that use algorithms
with at least 128 (and, if possible, 256) bits of security.

Rationale: Although the useful life of such cipher suites is
unknown, it is probably at least several years for people implementing and deploying
client the 128-bit
ciphers and server-side TLS.

4.1. Summary

Based on "until the criteria above, we recommend using as a preferred next fundamental technology breakthrough"
for 256-bit ciphers.

o Implementations MUST support, and SHOULD prefer to negotiate,
cipher
suite suites offering forward secrecy, such as those in the following:

o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5829]

It is noted
"EDH", "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 above amount of time during
which attacks can be successful.

Given the foregoing considerations, implementation of the following
cipher suite suites is an RECOMMENDED (see [RFC5289] for details):

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

We suggest that TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 be preferred in
general.

Unfortunately, those cipher suites are supported only in TLS 1.2
since they are authenticated encryption (AEAD) algorithm [RFC5116], and therefore requires the use algorithms [RFC5116].
A future version of TLS 1.2.

We this document might recommend using 2048-bit server certificates, with a SHA-256
fingerprint. See [CAB-Baseline] cipher suites for more details.
earlier versions of TLS.

[RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). We recommend that clients Clients and servers SHOULD prefer verifiably random curves
(specifically Brainpool P-256, brainpoolp256r1
[I-D.merkle-tls-brainpool]), [RFC7027]), and fall
back to the commonly used NIST P-256 (secp256r1) curve [RFC4492]. In
addition, clients should SHOULD send an ec_point_formats extension with a
single element, "uncompressed".

We recommend

3.4. Public Key Length

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 "DH" family of cipher suites. Unfortunately, some existing
software cannot handle (or cannot easily handle) key lengths greater
than 1024 bits. The most common workaround for these systems is to always
prefer the "ECDHE" family of cipher suites instead of the "DH"
family, then use longer keys. Key lengths of at least 2048 bits are
RECOMMENDED, since they are estimated to be roughly equivalent to
112-bit symmetric keys and might be sufficient for at least the next
10 years. In addition to 2048-bit server certificates, the use of
SHA-256 fingerprints is RECOMMENDED (see [CAB-Baseline] for more
details).

Note: The foregoing recommendations are preliminary and will likely
be corrected and enhanced in a future version of this document.

3.5. Compression

Implementations and deployments SHOULD disable TLS-level compression
([RFC5246], Sec. 6.2.2).

Finally, we recommend that clients disable fallback

3.6. Session Resumption

If TLS session resumption is used, care ought to SSLv3 (see
Section 4.3).

4.2. be taken to do so
safely. In particular, the resumption information (either session
IDs [RFC5246] or session tickets [RFC5077]) needs to be authenticated
and encrypted to prevent modification or eavesdropping by an
attacker. For session tickets, a strong cipher suite SHOULD be used
when encrypting the ticket (as least as strong as the main TLS cipher
suite); ticket keys MUST be changed regularly, e.g. once every week,
so as not to negate the effect of forward secrecy. Session ticket
validity SHOULD be limited to a reasonable duration (e.g. 1 day), so
as not to negate the benefits of forward secrecy.

4. Detailed Guidelines

The following sections provide more detailed information about the
recommendations listed above.

4.1. Cipher Suite Negotiation Details

We recommend that clients

Clients SHOULD include the above cipher suite 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.

We recommend that servers

Servers SHOULD prefer this cipher suite (or a similar but stronger
one) whenever it is proposed, even if it is not the first proposal.

Both clients and servers should SHOULD include the "Supported Elliptic
Curves" extension [RFC4492].

Clients are of course free to offer stronger cipher suites, e.g.
using AES-256; when they do, the server should SHOULD prefer the stronger
cipher suite unless there are compelling reasons (e.g. (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 TLS mandatory cipher suite,
TLS_RSA_WITH_AES_128_CBC_SHA.

4.3. Downgrade Attacks

Some client implementations revert to SSLv3 if the server rejected
higher versions of SSL/TLS. This fallback can be forced by a MITM
attacker. Moreover, IP scans [[reference?]] show that SSLv3-only
servers amount to about 3% of the current server population. As a
result, we recommend that by default, clients should avoid falling
back to SSLv3.

4.4. Alternatives

4.2. Alternative Cipher Suites

Elliptic Curves 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
the the RSA certificate; moreover, 1024 bits DH parameters are
generally considered insufficient at this time.

Because of the above, we recommend using (in priority order):

1. Elliptic Curve DHE with negotiated parameters, as described in
Section 4.1. parameters [RFC5289]

2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
Diffie-Hellman parameters. parameters

3. The same cipher suite, with 1024-bit parameters.

With modular ephemeral DH, deployers should SHOULD carefully evaluate
interoperability vs. security considerations when configuring their
TLS endpoints.

5. Implementation Status

Since this IANA Considerations

This document does not propose a new protocol or a new cipher
suite, we do not provide a full implementation status, as per
[RFC6982]. However it is useful to list some known existing
implementations of the recommended cipher suite(s).

6. Security Considerations

6.1. AES-GCM

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.

6.2. Forward Secrecy

Forward secrecy (also often called Perfect Forward Secrecy (PFS)

PFS or "PFS")
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 the future, 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].

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.

Unfortunately, many TLS/DTLS cipher suites were defined that do not
enable PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate
strict use of PFS-only ciphers. These are listed in Section
Section 4.1.

6.3. Session Resumption

TBD, https://www.imperialviolet.org/2013/06/27/botchingpfs.html.

7. IANA Considerations

[Note to RFC Editor: please remove this section before publication.]

This document requires no IANA actions.

8. Acknowledgements

We would like to thank Stephen Farrell, Simon Josefsson, Yoav Nir,
Kenny Paterson, Patrick Pelletier, and Rich Salz for their review.
Thanks to Brian Smith whose "browser cipher suites" page is a great
resource. Finally, Thanks thanks to all others who commented on the TLS and
other lists and are not mentioned here by name.

The document was prepared using the lyx2rfc tool, created by Nico
Williams.

8. References

9.1.

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[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.

[RFC5829] Brown, A., Clemm, G.,

[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with
SHA-256/384 and J. Reschke, "Link Relation Types
for Simple AES Galois Counter Mode (GCM)", RFC 5289,
August 2008.

[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version Navigation between Web Resources", 2.0", RFC 5829, April 2010.

[I-D.merkle-tls-brainpool] 6176, March 2011.

[RFC7027] Merkle, J. and M. Lochter, "ECC "Elliptic Curve Cryptography
(ECC) Brainpool Curves for Transport Layer Security
(TLS)",
draft-merkle-tls-brainpool-04 (work in progress),
July RFC 7027, October 2013.

9.2.

8.2. Informative References

[I-D.popov-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites",
draft-popov-tls-prohibiting-rc4-00 (work in progress),
August 2013.

[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.

[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
Layer Security (TLS)", RFC 6460, January 2012.

[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
July 2013.

[CBC-Attack]
AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
the TLS and DTLS Record Protocols", IEEE Symposium on
Security and Privacy , 2013.

[BEAST] Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
2011, <http://packetstormsecurity.com/files/105499/
Browser-Exploit-Against-SSL-TLS.html>.

[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty
Security Conference 2012, 2012.

[BREACH] Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
2013, <http://breachattack.com/>.

[RC4] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C, 2nd Ed.", 1996.

[RC4-Attack-FMS]
Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
Key Scheduling Algorithm of RC4", Selected Areas in
Cryptography , 2001.

[RC4-Attack]
ISOBE, T., OHIGASHI, T., WATANABE, Y., and M. MORII, "Full
Plaintext Recovery Attack on Broadcast RC4", International
Workshop on Fast Software Encryption , 2013.

[RC4-Attack-AlF]
AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
and J. Schuldt, "On the Security of RC4 in TLS", Usenix
Security Symposium 2013, 2013, <https://www.usenix.org/
conference/usenixsecurity13/security-rc4-tls>.

[Padding-Oracle]
Vaudenay, S., "Security Flaws Induced by CBC Padding
Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
2002, <http://www.iacr.org/cryptodb/archive/2002/
EUROCRYPT/2850/2850.pdf>.

[CAB-Baseline]
"Baseline Requirements for the Issuance and Management of
Publicly-Trusted Certificates Version 1.1.6", 2013,
<https://www.cabforum.org/documents.html>.

[TLS-IANA]
"Transport Layer Security (TLS) Parameters - TLS Cipher
Suite Registry", <https://www.iana.org/assignments/
tls-parameters/tls-parameters.xhtml#tls-parameters-4>.

[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.popov-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-popov-
tls-prohibiting-rc4-01 (work in progress), October 2013.

[I-D.sheffer-uta-tls-attacks]
Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Current Attacks on TLS and DTLS", draft-sheffer-uta-tls-
attacks-00 (work in progress), February 2014.

[Kleinjung2010]
Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",
CRYPTO 10, 2010.

[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.

[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.

[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.

[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.

Appendix A. Appendix: Change Log

Note to RFC Editor: please remove this section before publication.

A.1. -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.2. -01

o Clarified our motivation in the introduction.

o Added a section justifying the need for PFS.

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.2.

A.3. -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: holz@net.in.tum.de

Peter Saint-Andre
&yet

Email: ietf@stpeter.im