University of Auckland

Department of Computer Science University of Auckland Auckland New Zealand pgut001@cs.auckland.ac.nz

Security Area TLS Working Group Internet-Draft This document specifies a profile of TLS 1.2 for long-term support, one that represents what's already deployed for TLS 1.2 but with the security holes and bugs fixed. This represents a stable, known-good profile that can be deployed now to systems that can't can't roll out patches every month or two when the next attack on TLS is published.

TLS and DTLS, 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 simply too many of these to provide references for, and in any case more will have been published by the time you read this). This document presents a minimal, known-good profile 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. In particular it takes inspiration from numerous published analyses of TLS to select a standard interoperable feature set that provides the best chance of long-term stability and resistance to attack. This is intended for use in systems that need to run in a fixed configuration for a long 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 use of TLS, particularly in the embedded systems that are most in need of a long-term stable protocol profile.

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 .

The use of TLS-LTS is negotiated via TLS/DTLS extensions as defined in TLS Extensions. On connecting, the client includes the tls_lts extension in its client_hello if it wishes to use the TLS-LTS profile. If the server is capable of meeting this requirement, it responds with an tls_lts in its server_hello. The "extension_type" value for this extension SHALL be TBD (0xTBD) and the "extension_data" field of this extension SHALL be empty. The client and server MUST NOT use the TLS-LTS profile unless both sides have successfully exchanged tls_lts extensions.

The use of extensions precludes use with SSL 3.0, but then it's likely that anything still using this nearly two decades-old protocol will be vulnerable to any number of other attacks anyway, so there seems little point in bending over backwards to accomodate SSL 3.0.

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: 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_CBC_SHA256. As the use of SHA-256 with RSA or ECDSA is implicit in TLS-LTS, there is no need to signal it via the signature_algorithms extension. In addition the almost 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. As the use of P256 with uncompressed points is implicit in TLS-LTS, there is no need to signal it via the elliptic_curves and ec_point_formats extensions.

TLS-LTS only permits encrypt-then-MAC, not MAC-then-encrypt, fixing 20 years of attacks on this mechanism: TLS-LTS implementations MUST implement encrypt-then-MAC rather than the earlier MAC-then-encrypt. As the use of encrypt-then-MAC is implicit in TLS-LTS, there is no need to signal it via the encrypt_then_mac extension. TLS-LTS drops the IPsec cargo-cult MAC truncation, which serves no obvious purpose and leads to security concerns: TLS-LTS implementations MUST use full-length MAC values (for example 256 bits for SHA-256). In particular MAC values MUST NOT be truncated to 96 bits/12 bytes, removing the verify_data_length constraint in the Finished message. TLS-LTS recommends that implementations take measures to protect against side- channel attacks: 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. Implementations SHOULD take steps to protect against fault attacks, in particular for the extremely brittle ECC algorithms whose typical failure mode if a fault occurs is to leak the private key. One simple countermeasure is to use the public key to verify any signatures generated before they are sent over the wire.

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: TLS-LTS implementations MUST send the DH domain parameters as { p, g, q } rather than { p, g }. This makes the ServerDHParams field:

; opaque dh_g<1..2^16-1>; opaque dh_q<1..2^16-1>; opaque dh_Ys<1..2^16-1>; } ServerDHParams; /* Ephemeral DH parameters */ ]]>

The domain parameters MUST be verified as specified in FIPS 186. TLS-LTS adds a hash of the domain parameters into the master secret to protect against the use of manipulated curves/domain parameters: TLS-LTS implementations MUST include a SHA-256 hash of the EDH or ECDH parameters in the master secret computation by concatenating the hash to the pre_master_secret value. In the case of EDH, the value that's hashed is the ServerDHParams structure. In the case of ECDH the value that's hashed is the ServerECDHParams structure. This means that the master_secret computation becomes:

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: TLS-LTS implementations SHOULD NOT include the time in the Client/ServerHello random data. The data SHOULD consists entirely of random bytes. TLS-LTS drops compression and rehandshake, which have led to a number of attacks: TLS-LTS implementations MUST NOT implement compression or rehandshake.

TLS-LTS requires that RSA signature verification be done as encode-then-compare, which fixes all known padding-manipulation issues: TLS-LTS implementations MUST verify RSA signatures by using encode-then-compare, meaning that they encode the expected signature result and perform a constant-time compare against the recovered signature data. The TLS protocol has historically and somewhat arbitrarily been described as a state machine, which has led to a number of implementation flaws when state transitions weren't very carefully considered and enforced. A 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: Implementations SHOULD consider representing/implementing the protocol as a ladder diagram rather than a state machine, since the state-diagram form has led to a number of implementation errors in the past which are avoided through the use of the ladder diagram form.

A question that may be asked at this point is, why not use TLS 1.3 instead of creating a secure profile of TLS 1.2? The reason is that TLS 1.3 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 an almost entirely 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 of research published on them, and yet 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 profile uses the decades of experience we have with SSL and TLS to simplify TLS 1.2 into a known-good subset 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 bleeding-edge/experimental ideas, mechanisms, and algorithms. The intent is to create a long-term stable protocol profile that can be deployed once, not deployed and then patched, updated, and fixed constantly for the lifetime of the equipment that it's used with.

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 ones that remain to take advantage of many years of security research and implementation experience.

IANA has added the extension code point TBD (0xTBD) for the tls_lts extension to the TLS ExtensionType values registry as specified in TLS.

The author would like to thank the members of the TLS mailing list for their feedback on this document.

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