Internet DRAFT - draft-gutmann-tls-lts
draft-gutmann-tls-lts
TLS Working Group P. Gutmann
Internet-Draft University of Auckland
Intended status: Standards Track June 12, 2019
Expires: December 14, 2019
TLS 1.2 Update for Long-term Support
draft-gutmann-tls-lts-12
Abstract
This document specifies an update of TLS 1.2 for long-term support on
systems that can have multi-year or even decade-long update cycles,
one that incoporates as far as possible what's already deployed for
TLS 1.2 but with the security holes and bugs fixed. This document
also recognises the fact that there is a huge amount of TLS use
outside the web content-delivery environment with its resource-rich
hardware and software that can be updated whenever required and
provides a long-term stable, known-good version that can be deployed
to systems that can't roll out ongoing changes on a continuous basis.
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
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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 December 14, 2019.
Copyright Notice
Copyright (c) 2019 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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. TLS-LTS Negotiation . . . . . . . . . . . . . . . . . . . . . 4
3. TLS-LTS . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Encryption/Authentication . . . . . . . . . . . . . . . . 4
3.2. Message Formats . . . . . . . . . . . . . . . . . . . . . 7
3.3. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Implementation Issues . . . . . . . . . . . . . . . . . . 9
3.5. Use of TLS Extensions . . . . . . . . . . . . . . . . . . 12
3.6. Downgrade Attack Prevention . . . . . . . . . . . . . . . 13
3.7. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 14
4. Implementer's Checklist . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
5.1. Security Properties Provided by TLS-LTS . . . . . . . . . 16
5.2. Security Properties Not Provided by TLS-LTS . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
TLS [2] and DTLS [5], by nature of their enormous complexity and the
inclusion of large amounts of legacy material, contain numerous
security issues that have been known to be a problem for many years
and that keep coming up again and again in attacks (there are too
many of these to provide references for in the standard manner, and
in any case more will have been published by the time you read this).
This document presents a minimal, known-good set of mechanisms that
defend against all currently-known weaknesses in TLS, that would have
defended against them ten years ago, and that have a good chance of
defending against them ten years from now, providing the long-term
stability that's required by many systems in the field. This long-
term stability is particularly important in light of the fact that
widespread mainstream adoption of new versions of TLS has been shown
to take 15 years or more [29], with adoption in embedded environments
taking even longer.
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In particular, this document takes inspiration from numerous
published analyses of TLS [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] along with two
decades of implementation and deployment experience to select a
standard interoperable feature set that provides the best chance of
long-term stability and resistance to attack, as well as guidance on
implementing this feature set in a sound manner. This is intended
for use in systems that need to run in a fixed configuration for a
long period of time after they're deployed, with little or no ability
to roll out patches every month or two when the next attack on TLS is
published.
Unlike the full TLS 1.2, TLS-LTS is not meant to be all things to all
people. It represents a fixed, safe solution that's appropriate for
users who require a simple, secure, and long-term stable means of
getting data from A to B. This represents the majority of the non-
browser uses of TLS, particularly for embedded systems that are most
in need of a long-term stable protocol definition.
[Note: Although this specification is present as a draft, it
has been stable since -03 and is already supported in a
number of deployed implementations. The specification is
unlikely to change before its final publication, and may be
regarded as stable and representative of the final
published form.
There is currently a TLS 1.2 LTS test server running at
either https://82.94.251.205:8443 or 82.94.251.197:8443
depending on the load balance. This uses the extension
value 26 until a value is permanently assigned for LTS use.
To connect, your implementation should accept whatever
certificate is presented by the server or use PSK with
name = "plc", password = "test". For embedded systems
testing, note that the this is a conventional web server,
not an IED/RTU/PLC, that talks HTTP and not DNP3 or
ICCP/TASE.2, so you'll get an error if you try and connect
with a PLC control centre that expects one of those
protocols].
1.1. 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 [1].
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2. TLS-LTS Negotiation
The use of TLS-LTS is negotiated via TLS/DTLS extensions as defined
in TLS Extensions [4]. On connecting, the client includes the
tls_lts extension in its Client Hello if it wishes to use TLS-LTS.
If the server is capable of meeting this requirement, it responds
with a tls_lts extension in its Server Hello. The "extension_type"
value for this extension MUST be 26 (0x1A, see IANA Considerations
below) and the "extension_data" field of this extension is empty.
The client and server MUST NOT use TLS-LTS unless both sides have
successfully exchanged tls_lts extensions.
Note that the TLS-LTS extension applies to TLS 1.2, not to any
earlier version of TLS. If a client requests the use of TLS-LTS in
its client_hello but the server falls back to TLS 1.1 or earlier, it
MUST NOT indicate the use of TLS-LTS in its server_hello.
In the case of session resumption, once TLS-LTS has been negotiated
implementations MUST retain the use of TLS-LTS across all subsequent
resumed sessions. In other words if TLS-LTS is enabled for the
current session then the resumed session MUST also use TLS-LTS. If a
client or server attempts to resume a TLS-LTS session as a non-TLS-
LTS session then the peer MUST abort the handshake.
3. TLS-LTS
TLS-LTS specifies a few simple restrictions on the huge range of TLS
suites, options and parameters to limit the protocol to a known-good
subset, as well as making minor corrections to prevent or at least
limit various attacks.
3.1. Encryption/Authentication
TLS-LTS restricts the more or less unlimited TLS 1.2 with its more
than three hundred cipher suites, over forty ECC parameter sets, and
zoo of supplementary algorithms, parameters, and parameter formats,
to just two, one traditional one with DHE + AES-CBC + HMAC-SHA-256 +
RSA-SHA-256/PSK and one ECC one with ECDHE-P256 + AES-GCM + HMAC-
SHA-256 + ECDSA-P256-SHA-256/PSK with uncompressed points:
o TLS-LTS implementations MUST support
TLS_DHE_RSA_WITH_AES_128_CBC_SHA256,
TLS_DHE_PSK_WITH_AES_128_CBC_SHA256,
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_PSK_WITH_AES_128_GCM_SHA256. For these suites, SHA-256
is used in all locations in the protocol where a hash function is
required, specifically in the PRF and per-packet MAC calculations
(as indicated by the _SHA256 in the suite) and also in the client
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and server signatures in the CertificateVerify and
ServerKeyExchange messages.
[Note: TLS_ECDHE_PSK_WITH_AES_128_GCM_SHA256 is based on
draft-ietf-tls-ecdhe-psk-aead, currently still
progressing as an IETF draft, the reference will be
updated to the full RFC once it's published].
TLS-LTS only permits encrypt-then-MAC, not MAC-then-encrypt, fixing
20 years of attacks on this mechanism:
o TLS-LTS implementations MUST implement encrypt-then-MAC [6] rather
than the earlier MAC-then-encrypt.
TLS-LTS adds a hash of all messages leading up to the calculation of
the master secret into the master secret to protect against the use
of manipulated handshake parameters:
o TLS-LTS implementations MUST implement extended master secret [8]
to protect handshake and crypto parameters.
In several locations TLS modifies or truncates the output of
cryptographic operations so that the original security guarantees
associated with them may no longer be valid. TLS-LTS utilises the
full cryptographic parameters rather than partial, truncated, or
otherwise modified forms. In particular, TLS-LTS drops the MAC
truncation of the Finished message contents and uses the full
elliptic curve point Q output from the ECDH key agreement mechanism
rather than the point's x coordinate by itself:
o The length of verify_data (verify_data_length) in the Finished
message MUST be equal to the length of the output of the hash
function used for the PRF. For the mandatory TLS-LTS cipher
suites this hash is always SHA-256, so the value of
verify_data_length will be 32 bytes. For other suites, the size
depends on the hash algorithm associated with the suite. For
example for SHA-512 it would be 64 bytes.
o When ECDH is used to establish the premaster secret, the premaster
secret value is the full elliptic curve point Q as output from the
ECDH key agreement mechanism rather than the x coordinate of the
point Q by itself. In other words for the uncompressed point
format used in TLS-LTS, the premaster secret would be 04 || qx ||
qy rather than qx by itself.
TLS-LTS signs a hash of the client and server hello messages for the
ServerKeyExchange rather than signing just the client and server
nonces, avoiding various attacks that build on the fact that standard
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TLS doesn't authenticate previously-exchanged parameters when the
ServerKeyExchange message is sent:
o When generating the ServerKeyExchange signature, the signed_params
value is updated to replace the client_random and server_random
with a hash of the full Client Hello and Server Hello using the
hash algorithm for the chosen cipher suite. In other words the
value being signed is changed from:
digitally-signed struct {
opaque client_random[32];
opaque server_random[32];
ServerDHParams params;
} signed_params;
to:
digitally-signed struct {
opaque client_server_hello_hash;
ServerDHParams params;
} signed_params;
For the mandatory TLS-LTS cipher suites the hash algorithm is
always SHA-256, so the length of the client_server_hello_hash is
32 bytes. For other suites, the size depends on the hash
algorithm associated with the suite. For example for SHA-512 it
would be 64 bytes.
(In terms of side-channel attack prevention, it would be preferable
to include a non-public quantity into the data being signed since
this reduces the scope of attack from a passive to an active one,
with the attacker needing to initiate their own handshakes in order
to carry out their attack. However no shared secret value has been
established at this point so only public data can be signed).
The choice of key sizes is something that will never get any
consensus because there are so many different worldviews involved.
TLS-LTS makes only general recommendations on best practices and
leaves the choice of which key sizes are appropriate to implementers
and policy makers:
o Implementations SHOULD choose public-key algorithm key sizes that
are appropriate for the situation, weighted by the value of the
information being protected, the probability of attack and
capabilities of the attacker(s), any relevant security policies,
and the ability of the system running the TLS implementation to
deal with the computational load of large keys. For example a
SCADA system being used to switch a ventilator on and off doesn't
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require anywhere near the keysize-based security of a system used
to transfer classified data.
One way to avoid having to use very large public keys is to switch
the keys periodically. For example for DH keys this can be done by
regenerating DH parameters in a background thread and rolling them
over from time to time. If this isn't possible, an alternative
option is to pre-generate a selection of DH parameters and choose one
set at random for each new handshake, or again roll them over from
time to time from the pre-generated selection, so that an attacker
has to attack multiple sets of parameters rather than just one.
3.2. Message Formats
TLS-LTS sends the full set of DH parameters, X9.42/FIPS 186 style,
not p and g only, PKCS #3 style. This allows verification of the DH
parameters, which the current format doesn't allow:
o TLS-LTS implementations MUST send the DH domain parameters as { p,
q, g } rather than { p, g }. This makes the ServerDHParams field:
struct {
opaque dh_p<1..2^16-1>;
opaque dh_q<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} ServerDHParams; /* Ephemeral DH parameters */
Note that this uses the standard DLP parameter order { p, q, g },
not the erroneous { p, g, q } order from the X9.42 DH
specification.
o The domain parameters MUST either be compared for equivalence to a
set of known-good parameters provided by an appropriate standards
body or they MUST be verified as specified in FIPS 186 [9].
Examples of the former may be found in RFC 3526 [32].
Note that while other sources of DH parameters exist, these should be
treated with a great deal of caution. For example RFC 5114 [33]
provides no source for the values used, leading to suspicions that
they may be trapdoored, and RFC 7919 [34] mandates fallback to RSA if
the sole DH parameter set for each key size specified in the standard
isn't automatically chosen by both client and server.
Industry standards bodies may consider restricting domain parameters
to only allow known-good values such as those referenced in the above
standard, or ones generated by the standards body. This makes
checking easier, but has the downside that restricting the choice to
a small set of values makes them a more tempting target for well-
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resourced attackers. In addition it requires that the values be
carefully generated, and the generation process be well-documented,
to produce a so-called NUMS (Nothing Up My Sleeve) number that avoids
any suspicion of it having undesirable hidden properties (the
standard mentioned above, RFC 5114 [33], does not contain NUMS
values).
In any case signing the Client/Server Hello messages and the use of
Extended Master Secret makes active attacks that manipulate the
domain parameters on the fly far more difficult than they would be
for standard TLS.
3.3. Miscellaneous
TLS-LTS drops the need to send the current time in the random data,
which serves no obvious purpose and leaks the client/server's time to
attackers:
o TLS-LTS implementations SHOULD NOT include the time in the Client/
Server Hello random data. The data SHOULD consist entirely of
random bytes.
[Note: A proposed downgrade-attack prevention mechanism
may make use of these bytes, see section 3.6].
TLS-LTS drops compression and rehandshake, which have led to a number
of attacks:
o TLS-LTS implementations MUST NOT implement compression or
rehandshake.
TLS-LTS drops the requirement to generate the Client.random and
Server.random using "a secure random number generator", typically the
one used to generate encryption keys. The use of a secure/
cryptographic random number generator serves no obvious purpose (all
that's required is a unique value), but exposes 224 bits of the
cryptographic RNG output to an attacker, allowing them to analyse and
potentially attack the RNG, and by extension any crypto keys that it
generates:
o Implementations SHOULD NOT use the raw output from a
cryptographic/secure RNG that's used to generate keying material
to generate the Client.random and Server.random values. Instead,
they SHOULD employ a mechanism that doesn't directly expose
cryptographic RNG output to attackers, for example by using the
crypto RNG to seed a hash-based PRF such as the TLS PRF and using
the output of that for the values.
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3.4. Implementation Issues
TLS-LTS requires that RSA signature verification be done as encode-
then-compare, which fixes all known padding-manipulation issues:
o TLS-LTS implementations MUST verify RSA signatures by using
encode-then-compare as described in PKCS #1 [10], meaning that
they encode the expected signature result and perform a constant-
time compare against the recovered signature data.
The constant-time compare isn't strictly necessary for security in
this case, but it's generally good hygiene and is explicitly required
when comparing secret data values:
o All operations on crypto- or security-related values SHOULD be
performed in a manner that's as timing-independent as possible.
For example compares of MAC values such as those used in the
Finished message and data packets SHOULD be performed using a
constant-time memcmp() or equivalent so as not to leak timing data
to an attacker.
TLS-LTS recommends that implementations take measures to protect
against side-channel attacks:
o Implementations SHOULD take steps to protect against timing
attacks, for example by using constant-time implementations of
algorithms and by using blinding for non-randomised algorithms
like RSA.
TLS uses a number of crypto mechanisms, some of which are more
brittle than others. The ECC algorithms used in are quite vulnerable
to faults, with RSA significantly less so. Conversely, the PSK
mechanisms are essentially immune to key compromise induced by
faults. In terms of bulk encryption mechanisms, AES-GCM is far more
vulnerable to faults than AES-CBC:
o Implementations SHOULD take steps to protect against fault
attacks. One simple countermeasure for the public-key signature
mechanisms is to use the public key to verify any signatures
generated before they are sent over the wire. Other protection
measures include checksumming key data held in memory,
particularly where the key is stored over an extended period of
time. Implementations intended to be used in harsh environments
where faults are expected SHOULD consider the use of TLS-PSK in
place of any of the mechanisms using public/private-key
authentication, for which key compromise in the presence of faults
is unlikely.
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Authentication mechanisms for protocols run over TLS typically have
separate authentication procedures for the tunnelled protocol and the
encapsulating TLS session. The leads to an issue known as the
channel binding problem in which the tunnelled protocol isn't tied to
the encapsulating TLS session and can be manipulated by an attacker
once it passes the TLS endpoint. Channel binding ties the
cryptographic protection offered by TLS to the protocol that's being
run over the TLS tunnel:
o Implementations that require authentication for protocols run over
TLS SHOULD consider using channel bindings to tie the application-
level protocol to the TLS session, specifically the tls_unique
binding, which makes use of the contents of the first TLS Finished
message sent in an exchange to bind to the tunneled application-
level protocol [3].
The original description of the tls_unique binding contains a long
note detailing problems that arise due to rehandshake issues and how
to deal with them. Since TLS-LTS doesn't allow rehandshakes, these
problems don't exist, so no special handling is required.
The TLS protocol has historically and somewhat arbitrarily been
described as a state machine, which has led to numerous
implementation flaws when state transitions weren't very carefully
considered and enforced [20][23] [25] [26]. A safer and more logical
means of representing the protocol is as a ladder diagram, which
hardcodes the transitions into the diagram and removes the need to
juggle a large amount of state:
o Implementations SHOULD consider representing/implementing the
protocol as a ladder diagram rather than a state machine, since
the state-diagram form has led to numerous implementation errors
in the past which are avoided through the use of the ladder
diagram form.
TLS-LTS mandates the use of cipher suites that provide so-called
Perfect Forward Secrecy (PFS), in which an attacker can't record
sessions and decrypt them at a later date. The PFS property is
however impacted by the TLS session cache and session tickets, which
allow an attacker to decrypt old sessions. The session cache is
relatively short-term and only allows decryption while a session is
held in the cache, but the use of long-term keys in combination with
session tickets means that an attacker can decrypt any session used
with that key, defeating PFS:
o Implementations SHOULD consider the impact of using session caches
and session tickets on PFS. Security issues in this area can be
mitigated by using short session cache expiry times, and avoiding
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session tickets or changing the key used to encrypt them
periodically.
Another form of cacheing that can affect security is the reuse of the
supposedly-ephemeral DH value y = g^x mod p or its elliptic curve
equivalent. Instead of computing a fresh value for each session,
some servers for performance reasons compute the y value once and
then reuse it across multiple TLS sessions. If this is done then an
attacker can compute the discrete log value from one TLS session and
reuse it to attack later sessions:
o Implementations SHOULD consider the impact of reusing the DH y =
g^x mod p value across multiple TLS sessions, and avoid this reuse
if possible. Where the reuse of y is unavoidable, it SHOULD be
refreshed as often as is feasible. One way to do this is to
compute it as a background task so that a fresh value is available
when required.
TLS-LTS protects its handshake by including cryptographic integrity
checks of preceding messages in subsequent messages, defeating
attacks that build on the ability to manipulate handshake messages to
compromise security. What's authenticated at various stages is a log
of preceding messages in the exchange. The simplest way to implement
this, if the underlying API supports it, is to keep a running hash of
all messages (which will be required for the final Finished
computation) and peel off a copy of the current hash state to
generate the hash value required at various stages during the
handshake. If only the traditional { Begin, [ Update, Update, ... ],
Final } hash API interface is available then several parallel chains
of hashing will need to be run in order to terminate the hashing at
different points during the handshake.
Cryptographic protocol implementations rely critically on the
implementation performing extensive checking of all crypto operations
to ensure that problems are detected and caught. Testing for the
failure of these checks is rarely performed in implementations and
test suites, and the problem is not picked up by normal testing. To
deal with this issue, this specification recommends that
implementations test their cryptographic mechanisms to ensure that
crypto failures are detected and caught:
o Implementations SHOULD apply fault-injection testing to ensure
that cryptographic failures are correctly caught. At a minimum,
test suites SHOULD be capable of inducing faults in the
client_random/server_random, the ServerDHParams/ServerECDHParams
in the ServerKeyExchange, the signature value for the server key,
the MAC value in the finished message, and the IV, payload data,
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and MAC values for messages, and the implementation MUST be able
to detect these faults.
One way to induce such a fault is to flip a bit in the appropriate
data value in a location where the problem must be detected by
cryptographic means, for example in the binary payload data rather
than in an identifier or length field where it would be picked up
as a decoding error.
o If certificate-based authentication is used, implementations
SHOULD apply fault-injection testing to ensure that cryptographic
failures in the certificate processing are correctly caught. At a
minimum, test suites SHOULD be capable of inducing faults in the
signed certificate content and the certificate signature data, and
the implementation MUST be able to detect these faults.
PKI provides near-unlimited scope for further checking,
implementations MAY apply additional testing as required.
o If PSK-based authentication is used, implementations SHOULD apply
fault-injection testing to ensure that failures in the PSK
authentication are correctly caught. At a minimum, test suites
SHOULD be capable of inducing faults in the psk_identity and the
psk, and the implementation MUST be able to detect these faults.
3.5. Use of TLS Extensions
TLS-LTS is inspired by Grigg's Law that "there is only one mode and
that is secure". Because it mandates the use of known-good
mechanisms, much of the signalling and negotiation that's required in
standard TLS to reach the same state becomes redundant. In
particular, TLS-LTS removes the need to use the following extensions:
o The signature_algorithms extension, since the use of SHA-256 with
RSA or ECDSA is implicit in TLS-LTS.
o The elliptic_curves and ec_point_formats extensions, since the use
of P256 with uncompressed points is implicit in TLS-LTS.
o The universally-ignored requirement that all certificates provided
by the server must be signed by the algorithm(s) specified in the
signature_algorithms extension is removed both implicitly by not
sending the extension and explicitly by removing this requirement.
o The encrypt_then_mac extension, since the use of encrypt-then-MAC
is implicit in TLS-LTS.
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o The extended_master_secret extension, since the use of extended
Master Secret is implicit in TLS-LTS.
TLS-LTS implementations that wish to communicate only with other TLS-
LTS implementations MAY omit these extensions, with the presence of
tls_lts implying signature_algorithms = RSA/ECDSA + SHA-256,
elliptic_curves = P256, ec_point_formats = uncompressed,
encrypt_then_mac = TRUE, and extended_master_secret = TRUE.
Implementations that wish to communicate with legacy implementations
and wish to use the capabilities described by the extensions outside
of TLS-LTS MUST include these extensions in their Client Hello.
Conversely, although all of the above extensions are implied by TLS-
LTS, if a client requests TLS-LTS in its Client Hello then it doesn't
expect to see them returned in the Server Hello if TLS-LTS is
indicated. The handling of extensions during the Client/Server Hello
exchange is therefore as follows:
+-------------------------+--------------------+--------------------+
| Client Hello | Server Chooses | Server Hello |
+-------------------------+--------------------+--------------------+
| TLS-LTS | TLS-LTS | TLS-LTS |
| | | |
| TLS-LTS, | TLS-LTS | TLS-LTS |
| EMS/EncThenMAC/... | | |
| | | |
| TLS-LTS, | EMS/EncThenMAC/... | EMS/EncThenMAC/... |
| EMS/EncThenMAC/... | | |
+-------------------------+--------------------+--------------------+
Table 1: Use of TLS-LTS Extensions
TLS-LTS capabilities are indicated purely by the presence of the
tls_lts extension, not the plethora of other extensions that it's
comprised of. This allows an implementation that needs to be
backwards-compatible with legacy implementations to specify
individual options for use with non-TLS-LTS implementations via a
range of extensions, and specify the use of TLS-LTS via the tls_lts
extension.
3.6. Downgrade Attack Prevention
The use of the TLS-LTS improvements relies on an attacker not being
able to delete the TLS-LTS extension from the Client/Server Hello
messages. This is achieved through the SCSV [7] signalling
mechanism.
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[If SCSV is used then insert required boilerplate here, however this
will also require banning weak cipher suites like export ones, which
is a bit interesting in that it'll required banning something that in
theory has already been extinct for 15 years. A better option is to
refer to Karthikeyan Bhargavan's rather clever idea on anti-downgrade
signalling, which is a more reliable mechanism than SCSV].
3.7. Rationale
This section addresses the question of why this document specifies a
long-term support profile for TLS 1.2 rather than going to TLS 1.3.
The reason for this is twofold. Firstly, we know that TLS, which has
become more or less the universal substrate for secure communications
over the Internet, has extremely long deployment times. Much of this
information is anecdotal (although there are a large number of these
anecdotes), however one survey carried out in 2015 and 2016
illustrates the scope of the problem. This study found that the most
frequently-encountered protocol (in terms of use in observed Internet
connections) was the fifteen-year-old TLS 1.0, with the next most
common, TLS 1.2, lagging well behind [29]. This was on the public
Internet, in the non-public arena (where much of the anecdotal
evidence comes from, since it's not possible to perform a public
scan) the most common protocol appears to be TLS 1.0 (which includes
it being hardcoded into specifications like the widely-used DPWS [30]
and IEC 62351 [31]), with significant numbers of systems still using
the twenty-year-old SSLv3.
Given that TLS 1.3 is almost a completely new protocol compared to
the incremental changes from SSLv3 to TLS 1.2, and that the most
widely-encountered protocol version from that branch is more than
fifteen years old, it's likely that TLS 1.3 deployment outside of
constantly-updated web browsers may take one to two decades, or may
never happen at all given that a move to TLS 1.2 is an incremental
change from TLS 1.0 while TLS 1.3 requires the implementation of a
new protocol. This document takes the position that if a protocol
from the TLS 1.0 - 1.2 branch will remain in use for decades to come,
it should be the best form of TLS 1.2 available.
The second reason why this document exists has already been mentioned
above, that while TLS 1.0 - 1.2 are all from the same fairly similar
family, TLS 1.3 is an almost entirely new protocol. As such, it
rolls back the 20 years of experience that we have with all the
things that can go wrong in TLS and starts again from scratch with a
new protocol based on bleeding-edge/experimental ideas, mechanisms,
and algorithms. When SSLv3 was introduced, it used ideas that were
10-20 years old (DH, RSA, DES, and so on were all long-established
algorithms, only SHA-1 was relatively new). These were mature
algorithms with large amounts of research published on them, and yet
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we're still fixing issues with them 20 years later (the DH algorithm
was published in 1976, SSLv3 dates from 1996, and the latest DH
issue, Logjam, dates from 2015). With TLS 1.3 we currently have zero
implementation and deployment experience, which means that we're
likely to have another 10-20 years of patching holes and fixing
protocol and implementation problems ahead of us.
It's for this reason that this specification uses the decades of
experience we have with SSL and TLS and the huge deployed base of TLS
1.0 - 1.2 implementations to update TLS 1.2 into a known-good form
that leverages about 15 years of analysis and 20 years of
implementation experience, rather than betting on what's almost an
entirely new protocol based on experimental ideas, mechanisms, and
algorithms, and hoping that it can be deployed in less than a decade-
or multi-decade time frame. The intent is to create a long-term
stable protocol specification that can be deployed once as a minor
update to existing TLS implementations, not deployed as a new from-
scratch implementation and then patched, updated, and fixed
constantly for the lifetime of the equipment that it's used with.
4. Implementer's Checklist
This section provides an implementer's checklist of the core features
that are required for a TLS-LTS implementation. This doesn't cover
all of the requirements in this document, merely the minimum ones
required for an interoperable implementation. See the remainder of
this document for the full set of requirements.
[ ] Client sends TLS-LTS extension and checks for returned extension
from server.
[ ] Server accepts TLS-LTS extension and returns it to client.
[ ] Once TLS-LTS is negotiated, it persists across session
resumptions.
[ ] Implementation of Encrypt-then-MAC.
[ ] Implementation of Extended Master Secret.
[ ] Use of full-length MAC values rather than their truncated form.
[ ] Use of the full Q value rather than only the x coordinate qx.
[ ] Signing of the full client and server hello rather than only the
nonces.
[ ] Server sends and client checks the full DH parameter set { p, q,
g }, not just { p, g }.
[ ] Compression and rehandshake are disabled.
5. Security Considerations
This document defines a minimal, known-good subset of TLS 1.2 that
attempts to address all known weaknesses in the protocol, mostly by
simply removing known-insecure mechanisms but also by updating the
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ones that remain to take advantage of many years of security research
and implementation experience. As an example of its efficacy,
several attacks on standard TLS that emerged after this document was
first published were countered by the mechanisms specified here, with
no updates or changes to TLS-LTS implementations being necessary to
deal with them.
5.1. Security Properties Provided by TLS-LTS
If implemented correctly, TLS will provide confidentiality and
integrity protection of traffic, and guarantees liveness of the
communications. In some circumstances it also provides
authentication, see below. Apart from that, it provides no other
guarantees.
5.2. Security Properties Not Provided by TLS-LTS
TLS does not in general protect against spoofing (most commonly
encountered on the web as phishing). The one exception is when one
of the PSK mechanisms, which provides mutual cryptographic
authentication of client and server, is used. PKI, a mechanism
outside of TLS, is expected to provide protection against spoofing,
but in practice rarely does so.
Unless implemented very carefully, TLS does not provide strong
protection against side-channel attacks. While this document
specifies countermeasures against timing and oracle side-channels
that should be employed, these are very difficult to get right and
not always effective.
TLS provides no real protection against traffic analysis. While the
protocol specification contains provisions for message padding, this
has little effect on attackers in practice.
In the presence of implementation flaws (bugs) or hardware or
software errors, some TLS mechanisms may fail catastrophically. AES-
GCM is fatally vulnerable to nonce reuse or repeated counter/IV
values. AES-CBC in contrast can be arbitrarily abused, for example
by setting the IV to the constant value zero, with at most a slight
degradation in security (reduction to ECB mode) rather than a
complete loss of security.
TLS provides no availability guarantees. In fact since it increases
susceptibility to failures, either genuine or artificially-induced
(for example due to an expired certificate that's otherwise fully
valid), it reduces overall availability.
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TLS provides no guarantees of non-repudiation, access control, or
authorisation. These services must be provided by external
mechanisms.
In short, TLS provides confidentiality (if the crypto is implemented
properly and steps are taken to protect against faults and failures),
integrity protection, and in some limited cases authentication. It
does not provide any other service. If further security services are
required, these must be provided through additional, external
mechanisms.
TLS is a cryptographic protocol, not security pixie dust. Before
deciding to employ it, you should evaluate whether it actually
provides the security services that you think it does.
6. IANA Considerations
IANA has added the extension code point 26 (0x1A) for the tls_lts
extension to the TLS ExtensionType values registry as specified in
TLS [2].
7. Acknowledgements
The author would like to thank contributors from various embedded
system vendors for their feedback on this document.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[3] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
[4] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions", RFC 6066, January 2011.
[5] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[6] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, September 2014.
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[7] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, April 2015.
[8] Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
A., and M. Ray, "Transport Layer Security (TLS) Session
Hash and Extended Master Secret Extension", RFC 7627,
September 2015.
[9] NIST, "Digital Signature Standard (DSS)", FIPS 186, July
2013.
[10] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
8.2. Informative References
[11] Gajek, S., Manulis, M., Pereira, O., Sadeghi, A., and J.
Schwenk, "Universally Composable Security Analysis of
TLS", Springer-Verlag LNCS 5324, November 2008.
[12] Morrissey, P., Smart, N., and B. Warinschi, "A Modular
Security Analysis of the TLS Handshake Protocol",
Springer-Verlag LNCS 5350, December 2008.
[13] Firing, T., "Analysis of the Transport Layer Security
protocol", June 2010.
[14] Shrimpton, T., "A long answer to the simple question, "Is
TLS provably secure?"", Workshop on Theory and Practice in
Cryptography 2012, January 2012.
[15] Brzuska, C., Fischlin, M., Smart, N., Warinschi, B., and
S. Williams, "Less is more: relaxed yet compatible
security notions for key exchange", IACR ePrint
archive 2012/242, April 2012.
[16] Jager, T., Kohlar, F., Schaege, S., and J. Schwenk, "On
the security of TLS-DHE in the standard model", Springer-
Verlag LNCS 7417, August 2012.
[17] Meyer, C. and J. Schwenk, "Lessons Learned From Previous
SSL/TLS Attacks - A Brief Chronology Of Attacks And
Weaknesses", Cryptology ePrint Archive 2013/049, January
2013.
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[18] Bhargavan, K., Fournet, C., Kohlweiss, M., Pironti, A.,
and P. Strub, "Implementing TLS with Verified
Cryptographic Security", IEEE Security and Privacy 2013,
May 2013.
[19] Krawczyk, H., Paterson, K., and H. Wee, "On the security
of the TLS protocol", Springer-Verlag LNCS 8042, August
2013.
[20] Giesen, F., Kohlar, F., and D. Stebila, "On the security
of TLS renegotiation", ACM CCS 2013, November 2013.
[21] Wee, H., "On the Security of SSL/TLS", Workshop on Theory
and Practice in Cryptography 2014, January 2014.
[22] Stebila, D., "Provable security of advanced properties of
TLS and SSH", Workshop on Theory and Practice in
Cryptography 2014, January 2014.
[23] Bhargavan, K., Fournet, C., Kohlweiss, M., Pironti, A.,
Strub, P., and S. Zanella-Beguelin, "Proving the TLS
handshake secure (as is)", Springer-Verlag LNCS 8617,
August 2014.
[24] Bhargavan, K. and M. Kohlweiss, "Triple Handshake: Can
cryptography, formal methods, and applied security be
friends?", Workshop on Theory and Practice in
Cryptography 2015, January 2015.
[25] Beurdouche, B., Bhargavan, K., Delignat-Lavaud, A.,
Fournet, C., Kohlweiss, M., Pironti, A., Strub, P., and J.
Zinzindohoue, "A Messy State of the Union: Taming the
Composite State Machines of TLS", IEEE Symposium on
Security and Privacy 2015, May 2015.
[26] Dowling, B. and D. Stebila, "Modelling ciphersuite and
version negotiation in the TLS protocol", Springer-Verlag
LNCS 9144, June 2015.
[27] Beurdouche, B., Delignat-Lavaud, A., Kobeissi, N.,
Pironti, A., and K. Bhargavan, "FLEXTLS: A Tool for
Testing TLS Implementations", Workshop on Offensive
Technologies 2015, August 2015.
[28] Somorovsky, J., "Systematic Fuzzing and Testing of TLS
Libraries", Proceedings of the Conference on Computer and
Communications Security 2016, October 2016.
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[29] Holz, R., Amann, J., Mehani, O., Wachs, M., and M. Kaafar,
"TLS in the Wild: An Internet-Wide Analysis of TLS-Based
Protocols for Electronic Communication", Network and
Distributed System Security Symposium 2016, February 2016.
[30] OASIS, "Devices Profile for Web Services Version 1.1",
OASIS Standard wsdd-dpws-1.1-spec-os, July 2009.
[31] IEC, "Power systems management and associated information
exchange - Data and communications security - Part 3:
Communication network and system security - Profiles
including TCP/IP", IEC Standard 62351-3, 2007.
[32] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[33] Lepinski, M. and S. Kent, "Additional Diffie-Hellman
Groups for Use with IETF Standards", RFC 5114, January
2008.
[34] Gillmor, D., "Negotiated Finite Field Diffie-Hellman
Ephemeral Parameters for Transport Layer Security (TLS)",
RFC 7919, August 2016.
Author's Address
Peter Gutmann
University of Auckland
Department of Computer Science
University of Auckland
New Zealand
Email: pgut001@cs.auckland.ac.nz
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