[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
[tor-commits] [torspec/master] WIP Migration to TLS 1.3 proposal.
commit fc561d2bb586141320306a86f17db9dac6ed0ce8
Author: Isis Lovecruft <isis@xxxxxxxxxxxxxx>
Date: Tue Jan 23 00:51:09 2018 +0000
WIP Migration to TLS 1.3 proposal.
---
proposals/xxx-tls-1.3.txt | 336 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 336 insertions(+)
diff --git a/proposals/xxx-tls-1.3.txt b/proposals/xxx-tls-1.3.txt
new file mode 100644
index 0000000..e89cf9f
--- /dev/null
+++ b/proposals/xxx-tls-1.3.txt
@@ -0,0 +1,336 @@
+Filename: xxx-tls-1.3.txt
+Title: TLS 1.3 Migration
+Authors: Isis Lovecruft
+Created: 11 December 2017
+Updated: 23 January 2018
+Status: Needs Research
+
+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 â??supported_groupsâ?? (Section 4.2.7) extension which indicates the
+ > (EC)DHE groups which the client supports and a â??key_shareâ?? (Section 4.2.8)
+ > extension which contains (EC)DHE shares for some or all of these groups.
+ > - A â??signature_algorithmsâ?? (Section 4.2.3) extension which indicates the
+ > signature algorithms which the client can accept.
+ > - A â??pre_shared_keyâ?? (Section 4.2.11) extension which contains a list of
+ > symmetric key identities known to the client and a â??psk_key_exchange_modesâ??
+ > (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 â??signature_algorithms_certâ?? 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 â??supported_versionsâ?? 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? â??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
+ > â??enable-tls1_3â?? option to â??configâ?? or â??Configureâ??.
+ >
+ > Currently OpenSSL has implemented the â??draft-20â?? version of
+ > TLSv1.3. Many other libraries are still using older draft versions in
+ > their implementations. Notably many popular browsers are using
+ > â??draft-18â??. 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:
+ > â??tls1.3-draft-18â?? and â??tls1.3-draft-19â??, 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 â??
+ including within threat models which include adversaries capable of recording
+ traffic now and decrypting with a potential quantum computer in the future â??
+ 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
_______________________________________________
tor-commits mailing list
tor-commits@xxxxxxxxxxxxxxxxxxxx
https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-commits