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torspec/proposals/294-tls-1.3.txt

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Filename: 294-tls-1.3.txt
Title: TLS 1.3 Migration
Authors: Isis Lovecruft
Created: 11 December 2017
Updated: 23 January 2018
Status: Draft
This proposal is currently in draft state and should be periodically
revised as we research how much of our idiosyncratic older TLS uses
can be removed.
1. Motivation
TLS 1.3 is a substantial redesign over previous versions of TLS, with several
significant protocol changes which should likely provide Tor implementations
with not only greater security but an improved ability to blend into "normal"
TLS traffic on the internet, due to its improvements in encrypting more
portions of the handshake.
Tor implementations may utilise the new TLS 1.3 EncryptedExtensions feature
to define arbitrary encrypted TLS extensions to encompass our less standard
(ab)uses of TLS. Additionally, several new Elliptic Curve (EC) based
signature algorithms, including Ed25519 and Ed448, are included within the
base specification including a single specification for EC point compression
for each supported curve, further decreasing our reliance on
Tor-protocol-specific uses and extensions (and implementation details).
Other new features which Tor implementations might take advantage of include
improved (server-side) stateless session resumption, which might be usable
for OPs to resume sessions with their guards, for example after network
disconnection or router IP address reassignment.
2. Summary
Everything that's currently TLS 1.2: make it use TLS 1.3. KABLAM. DONE.
For an excellent summary of differences between TLS 1.2 and TLS 1.3, see
[TLS-1.3-DIFFERENCES].
3. Specification
3.1. Link Subprotocol 4
(We call it "Link v4" here, but reserve whichever is the subsequently
available subprotocol version at the time.)
3.2. TLS Session Resumption & Compression
As before, implementations MUST NOT allow TLS session resumption. In the
event that it might be decided in the future that OR implementations would
benefit from 0-RTT, we can re-evaluate this decision and its security
considerations in a separate proposal.
Compression has been removed from TLS in version 1.3, so we no longer need to
make recommendations against its usage.
3.3. Handshake Protocol
3.3.1. Negotiation
The initiator sends the following four sets of options, as defined in §4.1.1
of [TLS-1.3-NEGOTIATION]:
>
> - A list of cipher suites which indicates the AEAD algorithm/HKDF hash
> pairs which the client supports.
> - A “supported_groups” (Section 4.2.7) extension which indicates the
> (EC)DHE groups which the client supports and a “key_share” (Section 4.2.8)
> extension which contains (EC)DHE shares for some or all of these groups.
> - A “signature_algorithms” (Section 4.2.3) extension which indicates the
> signature algorithms which the client can accept.
> - A “pre_shared_key” (Section 4.2.11) extension which contains a list of
> symmetric key identities known to the client and a “psk_key_exchange_modes”
> (Section 4.2.9) extension which indicates the key exchange modes that may be
> used with PSKs.
In our case, the initiator MUST leaave the PSK section blank and MUST include
the "key_share" extension, and the responder proceeds to select a ECDHE
group, including its "key_share" in the response ServerHello.
3.3.2. ClientHello
To initiate a v4 handshake, the client sends a TLS1.3 ClientHello with the
following options:
- The "legacy_version" field MUST be set to "TLS 1.2 (0x0303)". TLS 1.3
REQUIRES this. (Actual version negotiation is done via the
"supported_versions" extension. See §5.1 of this proposal for details of
the case where a TLS-1.3 capable initiator finds themself talking to a
node which does not support TLS 1.3 and/or doesn't support v4.)
- The "random" field MUST be filled with 32 bytes of securely generated
randomness.
- The "legacy_session_id" MUST be set to a new pseudorandom value each
time, regardless of whether the initiator has previously opened either a
TLS1.2 or TLS1.3 connection to the other side.
- The "legacy_compression_methods" MUST be set to a single null byte,
indicating no compression is supported. (This is the only valid setting
for this field in TLS1.3, since there is no longer any compression
support.)
- The "cipher_suites" should be set to
"TLS13-CHACHA20-POLY1305-SHA256:TLS13-AES-128-GCM-SHA256:TLS13-AES-256-GCM-SHA384:"
This is the DEFAULT cipher suite list for OpenSSL 1.1.1. While an
argument could be made for customisation to remove the AES-128 option, we
choose to attempt to blend in which the majority of other TLS-1.3
clients, since this portion of the handshake is unencrypted.
(If the initiator actually means to begin a v3 protocol connection, they
send these ciphersuites anyway, cf. §5.2 of this proposal.)
- The "supported_groups" MUST include "X25519" and SHOULD NOT include any
of the NIST P-* groups.
- The "signature_algorithms" MUST include "ed25519 (0x0807)".
Implementations MAY advertise support for other signature schemes,
including "ed448 (0x0808)", however they MUST NOT advertise support for
ECDSA schemes due to the perils of secure implementation.
The initiator MUST NOT send any "pre_shared_key" or "psk_key_exchange_modes"
extensions.
The details of the "signature_algorithms" choice depends upon the final
standardisation of PKIX. [IETF-PKIX]
3.3.2.1. ClientHello Extensions
From [TLS-1.3_SIGNATURE_ALGOS]:
>
> The “signature_algorithms_cert” extension was added to allow implementatations
> which supported different sets of algorithms for certificates and in TLS itself
> to clearly signal their capabilities. TLS 1.2 implementations SHOULD also
> process this extension.
In order to support cross-certification, the initiator's ClientHello MUST
include the "signature_algorithms_cert" extension, in order to signal that
some certificate chains (one in particular) will include a certificate signed
using RSA-PKCSv1-SHA1:
- The "signature_algorithms_cert" MUST include the legacy algorithm
"rsa_pkcs1_sha1(0x0201)".
3.3.3. ServerHello
To respond to a TLS 1.3 ClientHello which supports the v4 link handshake
protocol, the responder sends a ServerHello with the following options:
- The "legacy_version" field MUST be set to "TLS 1.2 (0x0303)". TLS 1.3
REQUIRES this. (Actual version negotiation is done via the
"supported_versions" extension. See §5.1 of this proposal for details of
the case where a TLS-1.3 capable initiator finds themself talking to a
node which does not support TLS 1.3 and/or doesn't support v4.)
- The "random" field MUST be filled with 32 bytes of securely generated
randomness.
- The "legacy_session_id_echo" field MUST be filled with the contents of
the "legacy_session_id" from the initiator's ClientHello.
- The "cipher_suite" field MUST be set to "TLS13-CHACHA20-POLY1305-SHA256".
- The "legacy_compression_method" MUST be set to a single null byte,
indicating no compression is supported. (This is the only valid setting
for this field in TLS1.3, since there is no longer any compression
support.)
XXX structure and "key_share" response (XXX can we pre-generate a cache of
XXX key_shares?)
3.3.3.1 ServerHello Extensions
XXX what extensions do we need?
4. Implementation Details
4.1. Certificate Chains and Cross-Certifications
TLS 1.3 specifies that a certificate in a chain SHOULD be directly certified by
the preceding certificate in the chain. This seems to imply that OR
implementations SHOULD NOT do the DAG-like construction normally implied by
cross-certification between the master Ed25519 identity key and the master
RSA-1024 identity key.
Instead, since certificate chains are expected to be linear, we'll need three
certificate chains included in the same handshake:
1. EdMaster->EdSigning, EdSigning->Link
2. EdMaster->RSALegacy
3. RSALegacy->EdMaster
where A->B denotes that the certificate containing B has been signed with key A.
4.2. Removal of AUTHENTICATE, CLIENT_AUTHENTICATE, and CERTS cells
XXX see prop#224 and RFC5705 and compare
XXX when can we remove our "renegotiation" handshake completely?
5. Compatibility
5.1. TLS 1.2 version negotiation
From [TLS-1.3-DIFFERENCES]:
>
> The “supported_versions” ClientHello extension can be used to
> negotiate the version of TLS to use, in preference to the
> legacy_version field of the ClientHello.
If an OR does not receive a ClientHello with a "supported_versions"
extenstion, it MUST fallback to using the Tor Link subprotocols v3. That is,
the OR MUST immediately fallback to TLS 1.2 (or v3 with TLS 1.3, cf. the next
section) and, following both Tor's "renegotiation" and "in-protocol" version
negotiation mechanisms, immediately send a VERSIONS cell.
Otherwise, upon seeing a "supported_versions" in the ClientHello set to
0x0304, the OR should procede with Tor's Link subprotocol 4.
5.2. Preparing Tor's v3 Link Subprotocol for TLS 1.3
Some changes to the current v3 Link protocol are required, and these MUST
be backported, since implementations which are currently compiled against
TLS1.3-supporting OpenSSLs fail to establish any connections due to:
- failing to include any ciphersuite candidates which are TLS1.3 compatible
This is likely to be accomplished by:
1. Prefacing our v3 ciphersuite lists with
TLS13-CHACHA20-POLY1305-SHA256:TLS13-AES-128-GCM-SHA256:TLS13-AES-256-GCM-SHA384:
(We could also retroactively change our custom cipher suite list to be
the HIGH cipher suites, since this includes all TLS 1.3 suites.)
2. Calling SSL_CTX_set1_groups() to set the supported groups (should be set
to "X25519:P-256"). [TLS-1.3_SET1_GROUPS]
3. Taking care that older OpenSSLs, which instead have the concept of
"curves" not groups, should have their equivalent TLS context settings
in place. [TLS-1.3_SET1_GROUPS] mentions that "The curve functions were
first added to OpenSSL 1.0.2. The equivalent group functions were first
added to OpenSSL 1.1.1".
However more steps may need to be taken.
[XXX are there any more steps necessary? —isis]
6. Security Implications
XXX evaluate the static RSA attack and its effects on TLS1.2/TLS1.3
XXX dual-operable protocols and determine if they apply
XXX
XXX Jager, T., Schwenk, J. and J. Somorovsky, "On the Security
XXX of TLS 1.3 and QUIC Against Weaknesses in PKCS#1 v1.5 Encryption",
XXX Proceedings of ACM CCS 2015 , 2015.
XXX https://www.nds.rub.de/media/nds/veroeffentlichungen/2015/08/21/Tls13QuicAttacks.pdf
7. Performance and Scalability
8. Availability and External Deployment
8.1. OpenSSL Availability and Interoperability
Implementation should be delayed until the stable release of OpenSSL 1.1.1.
OpenSSL 1.1.1 will be binary and API compatible with OpenSSL 1.1.0, so in
preparation we might wish to revise our current usage to OpenSSL 1.1.0 to be
prepared.
From Matt Caswell in [OPENSSL-BLOG-TLS-1.3]:
>
> OpenSSL 1.1.1 will not be released until (at least) TLSv1.3 is
> finalised. In the meantime the OpenSSL git master branch contains
> our development TLSv1.3 code which can be used for testing purposes
> (i.e. it is not for production use). You can check which draft
> TLSv1.3 version is implemented in any particular OpenSSL checkout
> by examining the value of the TLS1_3_VERSION_DRAFT_TXT macro in the
> tls1.h header file. This macro will be removed when the final
> version of the standard is released.
>
> In order to compile OpenSSL with TLSv1.3 support you must use the
> “enable-tls1_3” option to “config” or “Configure”.
>
> Currently OpenSSL has implemented the “draft-20” version of
> TLSv1.3. Many other libraries are still using older draft versions in
> their implementations. Notably many popular browsers are using
> “draft-18”. This is a common source of interoperability
> problems. Interoperability of the draft-18 version has been tested
> with BoringSSL, NSS and picotls.
>
> Within the OpenSSL git source code repository there are two branches:
> “tls1.3-draft-18” and “tls1.3-draft-19”, which implement the older
> TLSv1.3 draft versions. In order to test interoperability with other
> TLSv1.3 implementations you may need to use one of those
> branches. Note that those branches are considered temporary and are
> likely to be removed in the future when they are no longer needed.
At the time of its release, we may wish to test interoperability with other
implementation(s).
9. Future Directions
The implementation of this proposal would greatly ease the
implementation difficulty and maintenance requirements for some
other possible future beneficial areas of work.
9.1. TLS Handshake Composability
Handshake composition (i.e. hybrid handshakes) in TLS 1.3 is incredibly
straightforward.
For example, provided we had a Supersingular Isogeny Diffie-Hellman (SIDH)
based implementation with a sane API, composition of Elliptic Curve
Diffie-Hellman (ECDH) and SIDH handshakes would be a trivial codebase
addition (~10-20 lines of code, for others who have implemented this).
Our current circuit-layer protocol safeguards the majority of our security
and anonymity guarantees, while our TLS layer has historically been either a
stop-gap and/or an attempted (albeit usually not-so-successful) obfuscation
mechanism. However, our TLS usage has, in many cases, successfully, through
combination with the circuit layer cryptography, prevented more then a few
otherwise horrendous bugs. After our circuit-layer protocol is upgraded to a
hybrid post-quantum secure protocol (prop#269 and prop#XXX), and in order to
ensure that our TLS layer continues to act in this manner as a stop gap —
including within threat models which include adversaries capable of recording
traffic now and decrypting with a potential quantum computer in the future —
our TLS layer should also provide safety against such a quantum-capable
adversary.
A. References
[TLS-1.3-DIFFERENCES]:
https://tlswg.github.io/tls13-spec/draft-ietf-tls-tls13.html#rfc.section.1.3
[OPENSSL-BLOG-TLS-1.3]:
https://www.openssl.org/blog/blog/2017/05/04/tlsv1.3/
[TLS-1.3-NEGOTIATION]:
https://tlswg.github.io/tls13-spec/draft-ietf-tls-tls13.html#rfc.section.4.1.1
[IETF-PKIX]:
https://datatracker.ietf.org/doc/draft-ietf-curdle-pkix/
[TLS-1.3_SET1_GROUPS]:
https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set1_groups.html
[TLS-1.3_SIGNATURE_ALGOS]:
https://tlswg.github.io/tls13-spec/draft-ietf-tls-tls13.html#signature-algorithms
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