TLS Working Group E. Rescorla
Internet-Draft Mozilla
Intended status: Informational R. Barnes
Expires: September 10, 2020 Cisco
H. Tschofenig
Arm Limited
March 09, 2020
Compact TLS 1.3
draft-rescorla-tls-ctls-04
Abstract
This document specifies a "compact" version of TLS 1.3. It is
isomorphic to TLS 1.3 but saves space by trimming obsolete material,
tighter encoding, and a template-based specialization technique. cTLS
is not directly interoperable with TLS 1.3, but it should eventually
be possible for a cTLS/TLS 1.3 server to exist and successfully
interoperate.
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
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This Internet-Draft will expire on September 10, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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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
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Common Primitives . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Varints . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Handshake Layer . . . . . . . . . . . . . . . . . . . . . 5
4. Handshake Messages . . . . . . . . . . . . . . . . . . . . . 6
4.1. ClientHello . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. ServerHello . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. HelloRetryRequest . . . . . . . . . . . . . . . . . . . . 7
5. Template-Based Specialization . . . . . . . . . . . . . . . . 7
5.1. Specifying a Specialization . . . . . . . . . . . . . . . 8
5.1.1. Requirements on the TLS Implementation . . . . . . . 9
5.1.2. Predefined Extensions . . . . . . . . . . . . . . . . 10
5.1.3. Known Certificates . . . . . . . . . . . . . . . . . 11
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Normative References . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Sample Transcripts . . . . . . . . . . . . . . . . . 13
A.1. ECDHE and Mutual Certificate-based Authentication . . . . 14
A.2. PSK . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
DISCLAIMER: This is a work-in-progress draft of cTLS and has not yet
seen significant security analysis, so could contain major errors.
It should not be used as a basis for building production systems.
This document specifies a "compact" version of TLS 1.3 [RFC8446]. It
is isomorphic to TLS 1.3 but designed to take up minimal bandwidth.
The space reduction is achieved by four basic techniques:
o Omitting unnecessary values that are a holdover from previous
versions of TLS.
o Omitting the fields and handshake messages required for preserving
backwards-compatibility with earlier TLS versions.
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o More compact encodings, omitting unnecessary values.
o A template-based specialization mechanism that allows for the
creation of application specific versions of TLS that omit
unnecessary values.
For the common (EC)DHE handshake with pre-established certificates,
cTLS achieves an overhead of 45 bytes over the minimum required by
the cryptovariables. For a PSK handshake, the overhead is 21 bytes.
Annotated handshake transcripts for these cases can be found in
Appendix A.
Because cTLS is semantically equivalent to TLS, it can be viewed
either as a related protocol or as a compression mechanism.
Specifically, it can be implemented by a layer between the TLS
handshake state machine and the record layer.
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.
Structure definitions listed below override TLS 1.3 definitions; any
PDU not internally defined is taken from TLS 1.3 except for replacing
integers with varints.
3. Common Primitives
3.1. Varints
cTLS makes use of variable-length integers in order to allow a wide
integer range while still providing for a minimal encoding. The
width of the integer is encoded in the first two bits of the field as
follows, with xs indicating bits that form part of the integer.
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+----------------------------+----------------+
| Bit pattern | Length (bytes) |
+----------------------------+----------------+
| 0xxxxxxx | 1 |
| | |
| | |
| | |
| 10xxxxxx xxxxxxxx | 2 |
| | |
| | |
| | |
| 11xxxxxx xxxxxxxx xxxxxxxx | 3 |
+----------------------------+----------------+
Thus, one byte can be used to carry values up to 127.
In the TLS syntax variable integers are denoted as "varint" and a
vector with a top range of a varint is denoted as:
opaque foo<1..V>;
cTLS replaces all integers in TLS with varints, including:
o Values of uint8, uint16, uint24, uint32, and uint64
o Vector length prefixes
o Enum / code point values
We do not show the structures which only change in this way.
This allows implementations' encoding and decoding logic to implement
cTLS simply by having a mode in which integers always use the varint
encoding. Note that if implementations treat opaque data in the same
way as "uint8" values, they MUST NOT convert the bytes of an opaque
value to varints.
As an example, suppose we are given the following struct:
struct {
uint32 FieldA;
opaque FieldB<0..2^16-1>;
} ExampleStruct;
Encoding a value of this type with values FieldA=0x0A and
FieldB=0x0B0B0B0B0B would result in the following octet strings in
"normal" (RFC 8446) and "compact" modes, respectively:
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Normal: 0000000A00050B0B0B0B0B
Compact: 0A050B0B0B0B0B
3.2. Record Layer
The cTLS Record Layer assumes that records are externally framed
(i.e., that the length is already known because it is carried in a
UDP datagram or the like). Depending on how this was carried, you
might need another byte or two for that framing. Thus, only the type
byte need be carried and TLSPlaintext becomes:
struct {
ContentType type;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
In addition, because the epoch is known in advance, the dummy content
type is not needed for the ciphertext, so TLSCiphertext becomes:
struct {
opaque content[TLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} TLSInnerPlaintext;
struct {
opaque encrypted_record[TLSCiphertext.length];
} TLSCiphertext;
Note: The user is responsible for ensuring that the sequence numbers/
nonces are handled in the usual fashion.
3.3. Handshake Layer
The cTLS handshake framing is same as the TLS 1.3 handshake framing,
except for two changes:
1. The length field is omitted
2. The HelloRetryRequest message is a true handshake message instead
of a specialization of ServerHello.
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struct {
HandshakeType msg_type; /* handshake type */
select (Handshake.msg_type) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case end_of_early_data: EndOfEarlyData;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case new_session_ticket: NewSessionTicket;
case key_update: KeyUpdate;
};
} Handshake;
4. Handshake Messages
In general, we retain the basic structure of each individual TLS
handshake message. However, the following handshake messages have
been modified for space reduction and cleaned up to remove pre TLS
1.3 baggage.
4.1. ClientHello
The cTLS ClientHello is as follows.
opaque Random[RandomLength]; // variable length
struct {
Random random;
CipherSuite cipher_suites<1..V>;
Extension extensions<1..V>;
} ClientHello;
4.2. ServerHello
We redefine ServerHello in a similar way:
struct {
Random random;
CipherSuite cipher_suite;
Extension extensions<1..V>;
} ServerHello;
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4.3. HelloRetryRequest
The HelloRetryRequest has the following format:
struct {
CipherSuite cipher_suite;
Extension extensions<2..V>;
} HelloRetryRequest;
It is the same as the ServerHello above but without the unnecessary
sentinel Random value.
5. Template-Based Specialization
The protocol in the previous section is fully general and isomorphic
to TLS 1.3; effectively it's just a small cleanup of the wire
encoding to match what we might have done starting from scratch. It
achieves some compaction, but only a modest amount. cTLS also
includes a mechanism for achieving very high compaction using
template-based specialization.
The basic idea is that we start with the basic TLS 1.3 handshake,
which is fully general and then remove degrees of freedom, eliding
parts of the handshake which are used to express those degrees of
freedom. For example, if we only support one version of TLS, then it
is not necessary to have version negotiation and the
supported_versions extension can be omitted.
Importantly, this process is performed only for the wire encoding but
not for the handshake transcript. The result is that the transcript
for a specialized cTLS handshake is the same as the transcript for a
TLS 1.3 handshake with the same features used.
One way of thinking of this is as if specialization is a stateful
compression layer between the handshake and the record layer:
+---------------+---------------+---------------+
| Handshake | Application | Alert |
+---------------+---------------+---------------+ +---------+
| cTLS Compression Layer |<---| Profile |
+---------------+---------------+---------------+ +---------+
| cTLS Record Layer / Application |
+---------------+---------------+---------------+
Specializations are defined by a "compression profile" that specifies
what features are to be optimized out of the handshake. In the
following subsections, we define the structure of these profiles, and
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how they are used in compressing and decompressing handshake
messages.
[[OPEN ISSUE: Do we want to have an explicit cTLS extension
indicating that cTLS is in use and which specialization is in use?
This goes back to whether we want the use of cTLS to be explicit.]]
5.1. Specifying a Specialization
A compression profile defining of a specialized version of TLS is
defined using a JSON dictionary. Each axis of specialization is a
key in the dictionary. [[OPEN ISSUE: If we ever want to serialize
this, we'll want to use a list instead.]].
For example, the following specialization describes a protocol with a
single fixed version (TLS 1.3) and a single fixed cipher suite
(TLS_AES_128_GCM_SHA256). On the wire, ClientHello.cipher_suites,
ServerHello.cipher_suites, and the supported_versions extensions in
the ClientHello and ServerHello would be omitted.
{
"version" : 772,
"cipherSuite" : "TLS_AES_128_GCM_SHA256"
}
cTLS allows specialization along the following axes:
version (integer): indicates that both sides agree to the single TLS
version specified by the given integer value (772 == 0x0304 for
TLS 1.3). The supported_versions extension is omitted from
ClientHello.extensions and reconstructed in the transcript as a
single-valued list with the specified value. The
supported_versions extension is omitted from
ClientHello.extensions and reconstructed in the transcript with
the specified value.
cipherSuite (string): indicates that both sides agree to the single
named cipher suite, using the "TLS_AEAD_HASH" syntax defined in
[RFC8446], Section 8.4. The ClientHello.cipher_suites field is
omitted and reconstructed in the transcript as a single-valued
list with the specified value. The server_hello.cipher_suite
field is omitted and reconstructed in the transcript as the
specified value.
dhGroup (string): specifies a single DH group to use for key
establishment. The group is listed by the code point name in
[RFC8446], Section 4.2.7. (e.g., x25519). This implies a literal
"supported_groups" extension consisting solely of this group.
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signatureAlgorithm (string): specifies a single signature scheme to
use for authentication. The group is listed by the code point
name in [RFC8446], Section 4.2.7. (e.g., ed25519). This implies a
literal "signature_algorithms" extension consisting solely of this
group.
randomSize (integer): indicates that the ClientHello.Random and
ServerHello.Random values are truncated to the given values. When
the transcript is reconstructed, the Random is padded to the right
with 0s and the anti-downgrade mechanism in {{RFC8446)},
Section 4.1.3 is disabled. IMPORTANT: Using short Random values
can lead to potential attacks. When Random values are shorter
than 8 bytes, PSK-only modes MUST NOT be used, and each side MUST
use fresh DH ephemerals. The Random length MUST be less than or
equal to 32 bytes.
clientHelloExtensions (predefined extensions): Predefined
ClientHello extensions, see {predefined-extensions}
serverHelloExtensions (predefined extensions): Predefined
ServerHello extensions, see {predefined-extensions}
encryptedExtensions (predefined extensions): Predefined
EncryptedExtensions extensions, see {predefined-extensions}
certRequestExtensions (predefined extensions): Predefined
CertificateRequest extensions, see {predefined-extensions}
knownCertificates (known certificates): A compression dictionary for
the Certificate message, see {known-certs}
finishedSize (integer): indicates that the Finished value is to be
truncated to the given length. When the transcript is
reconstructed, the remainder of the Finished value is filled in by
the receiving side. [[OPEN ISSUE: How short should we allow this
to be? TLS 1.3 uses the native hash and TLS 1.2 used 12 bytes.
More analysis is needed to know the minimum safe Finished size.
See [RFC8446]; Section E.1 for more on this, as well as
https://mailarchive.ietf.org/arch/msg/tls/
TugB5ddJu3nYg7chcyeIyUqWSbA.]]
5.1.1. Requirements on the TLS Implementation
To be compatible with the specializations described in this section,
a TLS stack needs to provide two key features:
If specialization of extensions is to be used, then the TLS stack
MUST order each vector of Extension values in ascending order
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according to the ExtensionType. This allows for a deterministic
reconstruction of the extension list.
If truncated Random values are to be used, then the TLS stack MUST be
configurable to set the remaining bytes of the random values to zero.
This ensures that the reconstructed, padded random value matches the
original.
If truncated Finished values are to be used, then the TLS stack MUST
be configurable so that only the provided bytes of the Finished are
verified, or so that the expected remaining values can be computed.
5.1.2. Predefined Extensions
Extensions used in the ClientHello, ServerHello, EncryptedExtensions,
and CertificateRequest messages can be "predefined" in a compression
profile, so that they do not have to be sent on the wire. A
predefined extensions object is a dictionary whose keys are extension
names specified in the TLS ExtensionTypeRegistry specified in
[RFC8446]. The corresponding value is a hex-encoded value for the
ExtensionData field of the extension.
When compressing a handshake message, the sender compares the
extensions in the message being compressed to the predefined
extensions object, applying the following rules:
o If the extensions list in the message is not sorted in ascending
order by extension type, it is an error, because the decompressed
message will not match.
o If there is no entry in the predefined extensions object for the
type of the extension, then the extension is included in the
compressed message
o If there is an entry:
* If the ExtensionData of the extension does not match the value
in the dictionary, it is an error, because decompression will
not produce the correct result.
* If the ExtensionData matches, then the extension is removed,
and not included in the compressed message.
When decompressing a handshake message the receiver reconstitutes the
original extensions list using the predefined extensions:
o If there is an extension in the compressed message with a type
that exists in the predefined extensions object, it is an error,
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because such an extension would not have been sent by a sender
with a compatible compression profile.
o For each entry in the predefined extensions dictionary, an
extension is added to the decompressed message with the specified
type and value.
o The resulting vector of extensions MUST be sorted in ascending
order by extension type.
Note that the "version", "dhGroup", and "signatureAlgorithm" fields
in the compression profile are specific instances of this algorithm
for the corresponding extensions.
[[OPEN ISSUE: Are there other extensions that would benefit from
special treatment, as opposed to hex values.]]
5.1.3. Known Certificates
Certificates are a major contributor to the size of a TLS handshake.
In order to avoid this overhead when the parties to a handshake have
already exchanged certificates, a compression profile can specify a
dictionary of "known certificates" that effectively acts as a
compression dictionary on certificates.
A known certificates object is a JSON dictionary whose keys are
strings containing hex-encoded compressed values. The corresponding
values are hex-encoded strings representing the uncompressed values.
For example:
{
"00": "3082...",
"01": "3082...",
}
When compressing a Certificate message, the sender examines the
cert_data field of each CertificateEntry. If the cert_data matches a
value in the known certificates object, then the sender replaces the
cert_data with the corresponding key. Decompression works the
opposite way, replacing keys with values.
Note that in this scheme, there is no signaling on the wire for
whether a given cert_data value is compressed or uncompressed. Known
certificates objects SHOULD be constructed in such a way as to avoid
a uncompressed object being mistaken for compressed one and
erroneously decompressed. For X.509, it is sufficient for the first
byte of the compressed value (key) to have a value other than 0x30,
since every X.509 certificate starts with this byte.
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6. Examples
The following section provides some example specializations.
TLS 1.3 only:
{
"Version" : 0x0304
}
TLS 1.3 with AES_GCM and X25519 and ALPN h2, short random values, and
everything else is ordinary TLS 1.3.
{
"Version" : 772,
"Random": 16,
"CipherSuite" : "TLS_AES_128_GCM_SHA256",
"DHGroup": "X25519",
"Extensions": {
"named_groups": 29,
"application_layer_protocol_negotiation" : "030016832",
"..." : null
}
}
Version 772 corresponds to the hex representation 0x0304, named group
"29" (0x001D) represents X25519.
[[OPEN ISSUE: Should we have a registry of well-known profiles?]]
7. Security Considerations
WARNING: This document is effectively brand new and has seen no
analysis. The idea here is that cTLS is isomorphic to TLS 1.3, and
therefore should provide equivalent security guarantees.
The use of key ids is a new feature introduced in this document,
which requires some analysis, especially as it looks like a potential
source of identity misbinding. This is, however, entirely separable
from the rest of the specification.
Transcript expansion also needs some analysis and we need to
determine whether we need an extension to indicate that cTLS is in
use and with which profile.
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8. IANA Considerations
This document has no IANA actions.
9. Normative References
[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/info/rfc2119>.
[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/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Appendix A. Sample Transcripts
In this section, we provide annotated example transcripts generated
using a draft implementation of this specification in the mint TLS
library. The transcripts shown are with the revised message formats
defined above, as well as specialization to the indicated cases,
using the aggressive compression profiles noted below. The resulting
byte counts are as follows:
ECDHE PSK
------------------ ------------------
TLS CTLS Overhead TLS CTLS Overhead
--- ---- -------- --- ---- --------
ClientHello 132 50 10 147 67 15
ServerHello 90 48 8 56 18 2
ServerFlight 478 104 16 42 12 3
ClientFlight 458 100 11 36 10 1
=====================================================
Total 1158 302 45 280 107 21
To increase legibility, we show the plaintext bytes of handshake
messages that would be encrypted and shorten some of the
cryptographic values (shown with "..."). The totals above include 9
bytes of encryption overhead for the client and server flights, which
would otherwise be encrypted (with a one-byte content type and an
8-byte tag).
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Obviously, these figures are very provisional, and as noted at
several points above, there are additional opportunities to reduce
overhead.
[[NOTE: We are using a shortened Finished message here. See
Section 5.1 for notes on Finished size. However, the overhead is
constant for all reasonable Finished sizes.]]
A.1. ECDHE and Mutual Certificate-based Authentication
Compression Profile:
{
"version": 772,
"cipherSuite": "TLS_AES_128_CCM_8_SHA256",
"dhGroup": "X25519",
"signatureAlgorithm": "ECDSA_P256_SHA256",
"randomSize": 8,
"finishedSize": 8,
"clientHelloExtensions": {
"server_name": "000e00000b6578616d706c652e636f6d",
},
"certificateRequestExtensions": {
"signature_algorithms": "00020403"
},
"knownCertificates": {
"61": "3082...",
"62": "3082..."
}
}
ClientHello: 50 bytes = RANDOM(8) + DH(32) + Overhead(10)
01 // ClientHello
2ef16120dd84a721 // Random
28 // Extensions.length
33 26 // KeyShare
0024 // client_shares.length
001d // KeyShareEntry.group
0020 a690...af948 // KeyShareEntry.key_exchange
ServerHello: 48 = RANDOM(8) + DH(32) + Overhead(8)
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02 // ServerHello
962547bba5e00973 // Random
26 // Extensions.length
33 24 // KeyShare
001d // KeyShareEntry.group
0020 9fbc...0f49 // KeyShareEntry.key_exchange
Server Flight: 96 = SIG(71) + MAC(8) + CERTID(1) + Overhead(16)
08 // EncryptedExtensions
00 // Extensions.length
0d // CertificateRequest
00 // CertificateRequestContext.length
00 // Extensions.length
0b // Certificate
00 // CertificateRequestContext
03 // CertificateList
01 // CertData.length
61 // CertData = 'a'
00 // Extensions.length
0f // CertificateVerify
0403 // SignatureAlgorithm
4047 3045...10ce // Signature
14 // Finished
bfc9d66715bb2b04 // VerifyData
Client Flight: 91 bytes = SIG(71) + MAC(8) + CERTID(1) + Overhead(11)
0b // Certificate
00 // CertificateRequestContext
03 // CertificateList
01 // CertData.length
62 // CertData = 'b'
00 // Extensions.length
0f // CertificateVerify
0403 // SignatureAlgorithm
4047 3045...f60e // Signature.length
14 // Finished
35e9c34eec2c5dc1 // VerifyData
A.2. PSK
Compression Profile:
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{
"version": 772,
"cipherSuite": "TLS_AES_128_CCM_8_SHA256",
"signatureAlgorithm": "ECDSA_P256_SHA256",
"randomSize": 16,
"finishedSize": 0,
"clientHelloExtensions": {
"server_name": "000e00000b6578616d706c652e636f6d",
"psk_key_exchange_modes": "0100"
},
"serverHelloExtensions": {
"pre_shared_key": "0000"
}
}
ClientHello: 67 bytes = RANDOM(16) + PSKID(4) + BINDER(32) +
Overhead(15)
01 // ClientHello
e230115e62d9a3b58f73e0f2896b2e35 // Random
2d // Extensions.length
29 2b // PreSharedKey
000a // identities.length
0004 00010203 // identity
7bd05af6 // obfuscated_ticket_age
0021 // binders.length
20 2428...bb3f // binder
ServerHello: 18 bytes = RANDOM(16) + 2
02 // ServerHello
7232e2d3e61e476b844d9c1f6a4c868f // Random
00 // Extensions.length
Server Flight: 3 bytes = Overhead(3)
08 // EncryptedExtensions
00 // Extensions.length
14 // Finished
Client Flight: 1 byte = Overhead(3)
14 // Finished
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Acknowledgments
We would like to thank Karthikeyan Bhargavan, Owen Friel, Sean
Turner, Martin Thomson and Chris Wood.
Authors' Addresses
Eric Rescorla
Mozilla
Email: ekr@rtfm.com
Richard Barnes
Cisco
Email: rlb@ipv.sx
Hannes Tschofenig
Arm Limited
Email: hannes.tschofenig@arm.com
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