Transport Layer Security R. Salz
Internet-Draft Akamai Technologies
Intended status: Informational N. Aviram
Expires: 21 December 2023 19 June 2023
TLS 1.2 is Frozen
draft-rsalz-tls-tls12-frozen-01
Abstract
TLS 1.2 is in widespread use and can be configured such that it
provides good security properties. TLS 1.3 is also in widespread use
and fixes some known deficiencies with TLS 1.2, such as removing
error-prone cryptographic primitives and encrypting more of the
traffic so that it is not readable by outsiders.
Both versions have several extension points, so items like new
cryptographic algorithms, new supported groups (formerly "named
curves"), etc., can be added without defining a new protocol. This
document specifies that TLS 1.2 is frozen: no new algorithms or
extensions will be approved.
Further, TLS 1.3 use is widespread, and new protocols should require
and assume its existence.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-rsalz-tls-tls12-frozen/.
Discussion of this document takes place on the Transport Layer
Security Working Group mailing list (mailto:tls@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/tls/. Subscribe
at https://www.ietf.org/mailman/listinfo/tls/.
Source for this draft and an issue tracker can be found at
https://github.com/richsalz/tls12-frozen.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Implications for post-quantum cryptography . . . . . . . . . 3
4. TLS Use by Other Protocols . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
TLS 1.2 [TLS12] is in widespread use and can be configured such that
it provides good security properties. However, this protocol version
suffers from several deficiencies:
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1. While application layer traffic is always encrypted, most of the
handshake messages are not encrypted. Therefore, the privacy
provided is suboptimal.
2. The list of cryptographic primitives specified for the protocol,
both in-use primitives and deprecated ones, includes several
primitives that were a source for vulnerabilities throughout the
years, such as RSA key exchange, CBC cipher suites, and
problematic finite-field Diffie-Hellman group negotiation. See
Section 5 for elaboration.
3. The original protocol, as-is, does not provide security (cite
Renegotiation attack). Rather, some extensions are required to
provide security.
In contrast, TLS 1.3 [TLS13] is also in widespread use and fixes most
known deficiencies with TLS 1.2, such as encrypting more of the
traffic so that it is not readable by outsiders and removing most
cryptographic primitives considered dangerous. Importantly, TLS 1.3
enjoys robust security proofs and provides excellent security as-is.
Both versions have several extension points, so items like new
cryptographic algorithms, new supported groups (formerly "named
curves"), etc., can be added without defining a new protocol. This
document specifies that TLS 1.2 is frozen: no new algorithms or
extensions will be approved.
Further, TLS 1.3 use is widespread, and new protocols should require
and assume its existence.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Implications for post-quantum cryptography
Quantum computers, once available, will have a huge impact on TLS.
In 2016, the US National Institute of Standards and Technology
started a multi-year effort to standardize algorithms that will be
"safe" once quantum computers are feasible [PQC]. First IETF
discussions happened around the same time [CFRGSLIDES].
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While the industry is waiting for NIST to finish standardization, the
IETF has several efforts underway. A working group was formed in
early 2013 to work on use of PQC in IETF protocols, [PQUIPWG].
Several other working groups, including TLS [TLSWG], are working on
drafts to support hybrid algorithms and identifiers, for use during a
transition from classic to a post-quantum world.
For TLS it is important to note that the focus of these efforts is
TLS 1.3 or later. TLS 1.2 is WILL NOT be supported (see Section 6).
4. TLS Use by Other Protocols
Any new protocol that uses TLS MUST specify TLS 1.3 as its default.
For example, QUIC [QUICTLS] requires TLS 1.3 and specifies that
endpoints MUST terminate the connection if an older version is used.
If deployment considerations are a concern, the protocol MAY specify
TLS 1.2 as an additional, non-default option. As a counter example,
the Usage Profile for DNS over TLS [DNSTLS] specifies TLS 1.2 as the
default, while also allowing TLS 1.3. For newer specifications that
choose to support TLS 1.2, those preferences are to be reversed.
5. Security Considerations
TLS 1.2 was specified with several cryptographic primitives and
design choices that have historically hindered its security. The
purpose of this section is to briefly survey several such prominent
problems that have affected the protocol. It should be noted,
however, that TLS 1.2 can be configured securely; it is merely much
more difficult to configure it securely as opposed to using its
modern successor, TLS 1.3. See [RFC9325] for a more thorough guide
on the secure deployment of TLS 1.2.
Firstly, the TLS 1.2 protocol, without any extension points, is
vulnerable to the renegotiation attack and the Triple Handshake
attack. Broadly, these attacks exploit the protocol's support for
renegotiation in order to inject a prefix chosen by the attacker into
the plaintext stream. This is usually a devastating threat in
practice, that allows e.g. obtaining secret cookies in a web setting.
Refer to [RENEG1], [RENEG2], [TRIPLESHAKE] for elaboration. In light
of the above problems, [RFC5746] specifies an extension that prevents
this category of attacks. To securely deploy TLS 1.2, either
renegotiation must be disabled entirely, or this extension must be
present. Additionally, clients must not allow servers to renegotiate
the certificate during a connection.
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Secondly, the original key exchange methods specified for the
protocol, namely RSA key exchange and finite field Diffie-Hellman,
suffer from several weaknesses. As before, to securely deploy the
protocol, these key exchange methods must be disabled. Refer to
draft-obsolete-kex for elaboration (TODO I guess we will anyway wait
for WGLC for draft-obsolete-kex, so no sense to temporarily refer to
the draft.)
Thirdly, symmetric ciphers which were widely-used in the protocol,
namely RC4 and CBC cipher suites, suffer from several weaknesses.
RC4 suffers from exploitable biases in its key stream; see [RFC7465].
CBC cipher suites have been a source of vulnerabilities throughout
the years. A straightforward implementation of these cipher suites
inherently suffers from the Lucky13 timing attack [LUCKY13],
[LUCKY13FIX]. The first attempt to implement the cipher suites in
constant time introduced an even more severe vulnerability. There
have been further similar vulnerabilities throughout the years
exploiting CBC cipher suites; refer to e.g. [CBCSCANNING] for an
example and a survey of similar works.
And lastly, historically the protocol was affected by several other
attacks that TLS 1.3 is immune to: BEAST [BEAST], Logjam [WEAKDH],
FREAK [FREAK], and SLOTH [SLOTH].
6. IANA Considerations
IANA will stop accepting registrations for any TLS parameters
[TLS13REG] except for the following:
* TLS Exporter Labels
* TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs
Entries in any other TLS protocol registry should have an indication
like "For TLS 1.3 or later" in their entry.
7. References
7.1. Normative References
[DNSTLS] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/rfc/rfc8310>.
[QUICTLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/rfc/rfc5746>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015,
<https://www.rfc-editor.org/rfc/rfc7465>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/rfc/rfc9325>.
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/rfc/rfc5246>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[TLS13REG] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/rfc/rfc8447>.
7.2. Informative References
[BEAST] Duong, T. and J. Rizzo, "Here come the xor ninjas", n.d.,
<http://www.hpcc.ecs.soton.ac.uk/dan/talks/bullrun/
Beast.pdf>.
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[CBCSCANNING]
Merget, R., Somorovsky, J., Aviram, N., Young, C.,
Fliegenschmidt, J., Schwenk, J., and Y. Shavitt, "Scalable
Scanning and Automatic Classification of TLS Padding
Oracle Vulnerabilities", n.d.,
<https://www.usenix.org/system/files/sec19-merget.pdf>.
[CFRGSLIDES]
McGrew, D., "Post Quantum Secure Cryptography Discussion",
n.d., <https://www.ietf.org/proceedings/95/slides/slides-
95-cfrg-4.pdf>.
[FREAK] Beurdouche, B., Bhargavan, K., Delignat-Lavaud, A.,
Fournet, C., Kohlweiss, M., Pironti, A., Strub, P.-Y., and
J. K. Zinzindohoue, "A messy state of the union: Taming
the composite state machines of TLS", n.d.,
<https://inria.hal.science/hal-01114250/file/messy-state-
of-the-union-oakland15.pdf>.
[LUCKY13] Al Fardan, N. J. and K. G. Paterson, "Lucky Thirteen:
Breaking the TLS and DTLS record protocols", n.d.,
<http://www.isg.rhul.ac.uk/tls/TLStiming.pdf>.
[LUCKY13FIX]
Somorovsky, J., "Systematic fuzzing and testing of TLS
libraries", n.d., <https://nds.rub.de/media/nds/
veroeffentlichungen/2016/10/19/tls-attacker-ccs16.pdf>.
[PQC] "Post=Quantum Cryptography", January 2017,
<https://csrc.nist.gov/projects/post-quantum-
cryptography>.
[PQUIPWG] "Post-Quantum Use in Protocols", n.d.,
<https://datatracker.ietf.org/wg/pquip/about/>.
[RENEG1] Rescorla, E., "Understanding the TLS Renegotiation
Attack", n.d.,
<https://web.archive.org/web/20091231034700/
http://www.educatedguesswork.org/2009/11/
understanding_the_tls_renegoti.html>.
[RENEG2] Ray, M., "Authentication Gap in TLS Renegotiation", n.d.,
<https://web.archive.org/web/20091228061844/
http://extendedsubset.com/?p=8>.
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[SLOTH] Bhargavan, K. and G. Leurent, "Transcript collision
attacks: Breaking authentication in TLS, IKE, and SSH",
n.d., <https://inria.hal.science/hal-01244855/file/
SLOTH_NDSS16.pdf>.
[TLSWG] "Transport Layer Security", n.d.,
<https://datatracker.ietf.org/wg/tls/about/>.
[TRIPLESHAKE]
"Triple Handshakes Considered Harmful Breaking and Fixing
Authentication over TLS", n.d.,
<https://mitls.org/pages/attacks/3SHAKE>.
[WEAKDH] Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
Green, M., Halderman, J. A., Heninger, N., Springall, D.,
Thomé, E., Valenta, L., and B. VanderSloot, "Imperfect
forward secrecy: How Diffie-Hellman fails in practice",
n.d.,
<https://dl.acm.org/doi/pdf/10.1145/2810103.2813707>.
Acknowledgments
None yet.
Authors' Addresses
Rich Salz
Akamai Technologies
Email: rsalz@akamai.com
Nimrod Aviram
Email: nimrod.aviram@gmail.com
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