TLS Working Group E. Rescorla
Internet-Draft Mozilla
Intended status: Standards Track R. Barnes
Expires: 10 January 2023 Cisco
H. Tschofenig
Arm Limited
B. Schwartz
Google
9 July 2022
Compact TLS 1.3
draft-ietf-tls-ctls-06
Abstract
This document specifies a "compact" version of TLS and DTLS. It is
logically isomorphic to ordinary TLS, but saves space by trimming
obsolete material, tighter encoding, a template-based specialization
technique, and alternative cryptographic techniques. cTLS is not
directly interoperable with TLS or DTLS, but it should eventually be
possible for a single server port to offer cTLS alongside TLS or
DTLS.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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 10 January 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
Rescorla, et al. Expires 10 January 2023 [Page 1]
Internet-Draft cTLS 1.3 July 2022
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
2.1. Template-based Specialization . . . . . . . . . . . . . . 4
2.1.1. Initial template elements . . . . . . . . . . . . . . 6
2.1.2. Static vector compression . . . . . . . . . . . . . . 11
2.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 12
2.3. cTLS Handshake Layer . . . . . . . . . . . . . . . . . . 14
2.3.1. The Transport layer . . . . . . . . . . . . . . . . . 14
2.3.2. The Transcript layer . . . . . . . . . . . . . . . . 15
2.3.3. The Logical layer . . . . . . . . . . . . . . . . . . 16
3. Handshake Messages . . . . . . . . . . . . . . . . . . . . . 16
3.1. ClientHello . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. ServerHello . . . . . . . . . . . . . . . . . . . . . . . 16
3.3. HelloRetryRequest . . . . . . . . . . . . . . . . . . . . 17
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6.1. Adding a ContentType . . . . . . . . . . . . . . . . . . 18
6.2. Template Keys . . . . . . . . . . . . . . . . . . . . . . 18
6.3. Adding a cTLS Template message type . . . . . . . . . . . 19
6.4. Activating the HelloRetryRequest MessageType . . . . . . 20
6.5. Reserved profiles . . . . . . . . . . . . . . . . . . . . 20
7. Normative References . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Example Exchange . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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.
Rescorla, et al. Expires 10 January 2023 [Page 2]
Internet-Draft cTLS 1.3 July 2022
This document specifies "compact" versions of TLS [RFC8446] and DTLS
[RFC9147], respectively known as "Stream cTLS" and "Datagram cTLS".
cTLS provides equivalent security and functionality to TLS and DTLS,
but it is designed to take up minimal bandwidth. The space reduction
is achieved by five basic techniques:
* Omitting unnecessary values that are a holdover from previous
versions of TLS.
* Omitting the fields and handshake messages required for preserving
backwards-compatibility with earlier TLS versions.
* More compact encodings, for example point compression.
* A template-based specialization mechanism that allows pre-
populating information at both endpoints without the need for
negotiation.
* Alternative cryptographic techniques, such as semi-static Diffie-
Hellman.
OPEN ISSUE: Semi-static and point compression are never mentioned
again.
For the common (EC)DHE handshake with pre-established certificates,
Stream 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.
TODO: Update these values.
cTLS supports the functionality of TLS and DTLS 1.3, and is forward-
compatible to future versions of TLS and DTLS. cTLS itself is
versioned by CTLSTemplate.version (currently zero).
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.
Rescorla, et al. Expires 10 January 2023 [Page 3]
Internet-Draft cTLS 1.3 July 2022
2.1. Template-based Specialization
A significant transmission overhead in TLS 1.3 is contributed to by
two factors:
* the negotiation of algorithm parameters, and extensions, as well
as
* the exchange of certificates.
TLS 1.3 supports different credential types and modes that are
impacted differently by a compression scheme. For example, TLS
supports certificate-based authentication, raw public key-based
authentication as well as pre-shared key (PSK)-based authentication.
PSK-based authentication can be used with externally configured PSKs
or with PSKs established through tickets.
The basic idea of template-based specialization 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.
Thus, each specialization produces a new protocol that preserves the
security guarantees of TLS, but has its own unique handshake.
By assuming that out-of-band agreements took place already prior to
the start of the cTLS protocol exchange, the amount of data exchanged
can be radically reduced. Because different clients may use
different compression templates and because multiple compression
templates may be available for use in different deployment
environments, a client needs to inform the server about the profile
it is planning to use. The profile field in the ClientHello serves
this purpose.
Although the template-based specialization mechanisms described here
are general, we also include specific mechanism for certificate-based
exchanges because those are where the most complexity and size
reduction can be obtained. Most of the other exchanges in TLS 1.3
are highly optimized and do not require compression to be used.
The compression profile defining the use of algorithms, algorithm
parameters, and extensions is represented by the CTLSTemplate
structure:
Rescorla, et al. Expires 10 January 2023 [Page 4]
Internet-Draft cTLS 1.3 July 2022
enum {
profile(0),
version(1),
cipher_suite(2),
dh_group(3),
signature_algorithm(4),
random(5),
mutual_auth(6),
handshake_framing(7),
client_hello_extensions(8),
server_hello_extensions(9),
encrypted_extensions(10),
cert_request_extensions(11),
known_certificates(12),
finished_size(13),
optional(65535)
} CTLSTemplateElementType;
struct {
CTLSTemplateElementType type;
opaque data<0..2^32-1>;
} CTLSTemplateElement;
struct {
uint16 ctls_version = 0;
CTLSTemplateElement elements<0..2^32-1>
} CTLSTemplate;
TODO: Reorder enum.
Elements in a CTLSTemplate MUST appear sorted by the type field in
strictly ascending order. The initial elements are defined in the
subsections below. Future elements can be added via an IANA registry
(Section 6.2). When generating a template, all elements are OPTIONAL
to include. When processing a template, all elements are mandatory
to understand (but see discussion of optional in Section 2.1.1.11).
For ease of configuration, an equivalent JSON dictionary format is
also defined. It consists of a dictionary whose keys are the name of
each element type (converted from snake_case to camelCase), and whose
values are a type-specific representation of the element intended to
maximize legibility. The cTLS version is represented by the key
"ctlsVersion", whose value is an integer, defaulting to 0 if omitted.
OPEN ISSUE: Is it really worth converting snake_case to camelCase?
camelCase is slightly more traditional in JSON, and saves one
byte, but it seems annoying to implement.
Rescorla, et al. Expires 10 January 2023 [Page 5]
Internet-Draft cTLS 1.3 July 2022
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.
{
"ctlsVersion": 0,
"profile": "0001020304050607",
"version": 772,
"cipherSuite": "TLS_AES_128_GCM_SHA256"
}
2.1.1. Initial template elements
TODO: Reorder section.
2.1.1.1. profile
This element identifies the profile being defined. Its binary value
is:
opaque ProfileID<1..2^8-1>
This encodes the profile ID, if one is specified. IDs whose decoded
length is 4 bytes or less are reserved (see Section 6.5). When a
reserved value is used (including the default value), other keys MUST
NOT appear in the template, and a client MUST NOT accept the template
unless it recognizes the ID.
In JSON, the profile ID is represented as a hexadecimal-encoded
string.
2.1.1.2. version
Value: a single ProtocolVersion ([RFC8446], Section 4.1.2) that both
parties agree to use. For TLS 1.3, the ProtocolVersion is 0x0304.
When this element is included, the supported_versions extension is
omitted from ClientHello.extensions.
In JSON, the version is represented as an integer (772 = 0x0304 for
TLS 1.3).
2.1.1.3. cipher_suite
Value: a single CipherSuite ([RFC8446], Section 4.1.2) that both
parties agree to use.
Rescorla, et al. Expires 10 January 2023 [Page 6]
Internet-Draft cTLS 1.3 July 2022
When this element is included, the ClientHello.cipher_suites and
ServerHello.cipher_suite fields are omitted.
In JSON, the cipher suite is represented using the "TLS_AEAD_HASH"
syntax defined in [RFC8446], Section 8.4.
2.1.1.4. dh_group
Value: a single NamedGroup ([RFC8446], Section 4.2.7) to use for key
establishment.
This is equivalent to a literal "supported_groups" extension
consisting solely of this group.
Static vectors (see Section 2.1.2):
* KeyShareClientHello.client_shares
* KeyShareEntry.key_exchange, if the NamedGroup uses fixed-size key
shares.
In JSON, the group is listed by the code point name in [RFC8446],
Section 4.2.7 (e.g., "x25519").
2.1.1.5. signature_algorithm
Value: a single SignatureScheme ([RFC8446], Section 4.2.3) to use for
authentication.
This is equivalent to a literal "signature_algorithms" extension
consisting solely of this group.
In JSON, the signature algorithm is listed by the code point name in
[RFC8446], Section 4.2.3. (e.g., ecdsa_secp256r1_sha256).
2.1.1.6. random
Value: a single uint8.
The ClientHello.Random and ServerHello.Random values are truncated to
the given length. Where a 32-byte Random is required, 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. The Random length MUST be less
than or equal to 32 bytes.
Rescorla, et al. Expires 10 January 2023 [Page 7]
Internet-Draft cTLS 1.3 July 2022
OPEN ISSUE: Karthik Bhargavan suggested the idea of hashing
ephemeral public keys and to use the result (truncated to 32
bytes) as random values. Such a change would require a security
analysis.
In JSON, the length is represented as an integer.
2.1.1.7. mutual_auth
Value: a single uint8, with 1 representing "true" and 0 representing
"false". All other values are forbidden.
If set to true, this element indicates that the client must
authenticate with a certificate by sending Certificate and a
CertificateVerify message. The server MUST omit the
CertificateRequest message, as its contents are redundant.
OPEN ISSUE: We don't actually say that you can omit empty
messages, so we need to add that somewhere.
In JSON, this value is represented as true or false.
2.1.1.8. client_hello_extensions, server_hello_extensions,
encrypted_extensions, and cert_request_extensions
Value: a single CTLSExtensionTemplate struct:
struct {
Extension predefined_extensions<0..2^16-1>;
ExtensionType expected_extensions<0..2^16-1>;
uint8 allow_additional;
} CTLSExtensionTemplate;
The predefined_extensions field indicates extensions that should be
treated as if they were included in the corresponding message. This
allows these extensions to be omitted entirely.
The expected_extensions field indicates extensions that must be
included in the corresponding message, at the beginning of its
extensions field. The types of these extensions are omitted when
serializing the extensions field of the corresponding message.
The allow_additional field MUST be 0 (false) or 1 (true), indicating
whether additional extensions are allowed here.
Rescorla, et al. Expires 10 January 2023 [Page 8]
Internet-Draft cTLS 1.3 July 2022
predefined_extensions and expected_extensions MUST be in strictly
ascending order by ExtensionType, and a single ExtensionType MUST NOT
appear in both lists. If the version, dh_group, or
signature_algorithm element appears in the template, the
corresponding ExtensionType MUST NOT appear here.
OPEN ISSUE: Are there other extensions that would benefit from
special treatment, as opposed to hex values.
Static vectors (see Section 2.1.2):
* Extension.extension_data for any extension in expected_extensions
whose value has fixed length. This applies only to the
corresponding message.
* The extensions field of the corresponding message, if
allow_additional is false.
In JSON, this value is represented as a dictionary with three keys:
* predefinedExtensions: a dictionary mapping ExtensionType names
([RFC8446], Section 4.2) to values encoded as hexadecimal strings.
* expectedExtensions: an array of ExtensionType names.
* allowAdditional: true or false.
If predefinedExtensions or expectedExtensions is empty, it MAY be
omitted.
OPEN ISSUE: Should we have a certificate_entry_extensions element?
2.1.1.9. finished_size
Value: uint8, indicating that the Finished value is to be truncated
to the given length.
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],
Appendix E.1 for more on this, as well as
https://mailarchive.ietf.org/arch/msg/tls/
TugB5ddJu3nYg7chcyeIyUqWSbA.
In JSON, this length is represented as an integer.
Rescorla, et al. Expires 10 January 2023 [Page 9]
Internet-Draft cTLS 1.3 July 2022
2.1.1.10. handshake_framing
Value: uint8, with 0 indicating "false" and 1 indicating "true". If
true, handshake messages MUST be conveyed inside a Handshake
([RFC8446], Section 4) struct on stream transports, or a
DTLSHandshake ([RFC9147], Section 5.2) struct on datagram transports,
and MAY be broken into multiple records as in TLS and DTLS.
Otherwise, each handshake message is conveyed in a CTLSHandshake or
CTLSDatagramHandshake struct (Section 2.3), which MUST be the payload
of a single record.
In JSON, this value is represented as true or false.
2.1.1.11. optional
Value: a CTLSTemplate containing elements that are not required to be
understood by the client. Server operators MUST NOT place an element
in this section unless the server is able to determine whether the
client is using it from the client data it receives. A key MUST NOT
appear in both the main template and the optional section.
In JSON, this value is represented in the same way as the
CTLSTemplate itself.
2.1.1.12. known_certificates
Value: a CertificateMap struct:
struct {
opaque id<1..2^8-1>;
opaque cert_data<1..2^16-1>;
} CertificateMapEntry;
struct {
CertificateMapEntry entries<2..2^24-1>;
} CertificateMap;
Entries in the certificate map must appear in strictly ascending
lexicographic order by ID.
In JSON, CertificateMap is represented as a dictionary from id to
cert_data, which are both represented as hexademical strings:
{
"00": "3082...",
"01": "3082...",
}
Rescorla, et al. Expires 10 January 2023 [Page 10]
Internet-Draft cTLS 1.3 July 2022
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.
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.
2.1.2. Static vector compression
Some cTLS template elements imply that certain vectors (as defined in
[RFC8446], Section 3.4) have a fixed number of elements during the
handshake. These template elements note these "static vectors" in
their definition. When encoding a "static vector", its length prefix
is omitted.
For example, suppose that the cTLS template is:
{
"ctlsVersion": 0,
"version": 772,
"dhGroup": "x25519",
"clientHelloExtensions": {
"expectedExtensions": ["key_share"],
"allowAdditional": false
}
}
Then, the following structure:
28 // length(extensions)
33 26 // extension_type = KeyShare
0024 // length(client_shares)
001d // KeyShareEntry.group
0020 // length(KeyShareEntry.key_exchange)
a690...af948 // KeyShareEntry.key_exchange
Rescorla, et al. Expires 10 January 2023 [Page 11]
Internet-Draft cTLS 1.3 July 2022
is compressed down to:
a690...af948 // KeyShareEntry.key_exchange
according to the following rationale:
* The length of extensions is omitted because allowAdditional is
false, so the number of items in extensions (i.e., 1) is known in
advance.
* extension_type is omitted because it is specified by
expected_extensions.
* The length of client_shares is omitted because the use of dhGroup
implies that there can only be one KeyShareEntry.
* KeyShareEntry.group is omitted because it is specified by dhGroup.
* The length of the key_exchange is omitted because the "x25519" key
share has a fixed size (32 bytes).
2.2. Record Layer
The only cTLS records that are sent in plaintext are handshake
records (ClientHello and ServerHello/HRR) and alerts. cTLS alerts are
the same as TLS/DTLS alerts and use the same content types. For
handshake records, we set the content_type field to a fixed cTLS-
specific value to distinguish cTLS plaintext records from encrypted
records, TLS/DTLS records, and other protocols using the same
5-tuple.
struct {
ContentType content_type = ctls_handshake;
opaque profile_id<0..2^8-1>;
opaque fragment<0..2^16-1>;
} CTLSClientPlaintext;
The profile_id field MUST identify the profile that is in use. A
zero-length ID corresponds to the cTLS default protocol. The
server's reply does not include the profile_id, because the server
must be using the same profile indicated by the client.
struct {
ContentType content_type = ctls_handshake;
opaque fragment<0..2^16-1>;
} CTLSServerPlaintext;
Rescorla, et al. Expires 10 January 2023 [Page 12]
Internet-Draft cTLS 1.3 July 2022
Encrypted records use DTLS 1.3 [RFC9147] record framing, comprising a
configuration octet followed by optional connection ID, sequence
number, and length fields. The encryption process and additional
data are also as described in DTLS.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0|0|1|C|S|L|E E|
+-+-+-+-+-+-+-+-+
| Connection ID | Legend:
| (if any, |
/ length as / C - Connection ID (CID) present
| negotiated) | S - Sequence number length
+-+-+-+-+-+-+-+-+ L - Length present
| 8 or 16 bit | E - Epoch
|Sequence Number|
| (if present) |
+-+-+-+-+-+-+-+-+
| 16 bit Length |
| (if present) |
+-+-+-+-+-+-+-+-+
struct {
opaque unified_hdr[variable];
opaque encrypted_record[length];
} CTLSCiphertext;
The presence and size of the connection ID field is negotiated as in
DTLS.
As with DTLS, the length field MAY be omitted by clearing the L bit,
which means that the record consumes the entire rest of the data in
the lower level transport. In this case it is not possible to have
multiple DTLSCiphertext format records without length fields in the
same datagram. In stream-oriented transports (e.g., TCP), the length
field MUST be present. For use over other transports length
information may be inferred from the underlying layer.
Normal DTLS does not provide a mechanism for suppressing the sequence
number field entirely. When a reliable, ordered transport (e.g.,
TCP) is in use, the S bit in the configuration octet MUST be cleared
and the sequence number MUST be omitted. When an unreliable
transport is in use, the S bit has its usual meaning and the sequence
number MUST be included.
Rescorla, et al. Expires 10 January 2023 [Page 13]
Internet-Draft cTLS 1.3 July 2022
2.3. cTLS Handshake Layer
The cTLS handshake is modeled in three layers:
1. The Transport layer
2. The Transcript layer
3. The Logical layer
2.3.1. The Transport layer
When template.handshake_framing is false, the cTLS transport layer
uses a custom handshake framing that saves space by relying on the
record layer for message lengths. (This saves 3 bytes per message
compared to TLS, or 9 bytes compared to DTLS.) This compact framing
is defined by the CTLSHandshake and CTLSDatagramHandshake structs.
Any handshake type registered in the IANA TLS HandshakeType Registry
can be conveyed in a CTLS[Datagram]Handshake, but not all messages
are actually allowed on a given connection. This definition shows
the messages types supported in CTLSHandshake as of TLS 1.3 and DTLS
1.3, but any future message types are also permitted.
Rescorla, et al. Expires 10 January 2023 [Page 14]
Internet-Draft cTLS 1.3 July 2022
struct {
HandshakeType msg_type; /* handshake type */
select (CTLSHandshake.msg_type) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest; /* New */
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;
case request_connection_id: RequestConnectionId;
case new_connection_id: NewConnectionId;
};
} CTLSHandshake;
struct {
HandshakeType msg_type; /* handshake type */
uint16 message_seq; /* DTLS-required field */
select (CTLSDatagramHandshake.msg_type) {
... /* same as CTLSHandshake */
};
} CTLSDatagramHandshake;
Each CTLSHandshake or CTLSDatagramHandshake MUST be conveyed as a
single CTLSClientPlaintext.fragment, CTLSServerPlaintext.fragment, or
CTLSCiphertext.encrypted_record, and is therefore limited to a
maximum length of 2^16-1 or less. When operating over UDP, large
CTLSDatagramHandshake messages will also require the use of IP
fragmentation, which is sometimes undesirable. Operators can avoid
these concerns by setting template.handshakeFraming = true.
2.3.2. The Transcript layer
TLS and DTLS start the handshake with an empty transcript. cTLS is
different: it starts the transcript with a "virtual message" whose
HandshakeType is ctls_template (Section 6.3) containing the
CTLSTemplate used for this connection. This message is included in
the transcript even though it is not exchanged during connection
setup, in order to ensure that both parties are using the same
template. Subsequent messages are appended to the transcript as
usual.
Rescorla, et al. Expires 10 January 2023 [Page 15]
Internet-Draft cTLS 1.3 July 2022
When computing the handshake transcript, all handshake messages are
represented in TLS Handshake messages, as in DTLS 1.3 ([RFC9147],
Section 5.2), regardless of template.handshake_framing.
To ensure that all parties agree about what protocol is in use, the
Cryptographic Label Prefix used for the handshake SHALL be "Sctls "
for Stream cTLS and "Dctls " for Datagram cTLS. (This is similar to
the prefix substitution in Section 5.9 of [RFC9147]).
2.3.3. The Logical layer
The logical handshake layer consists of handshake messages that are
reconstructed following the instructions in the template. At this
layer, predefined extensions are reintroduced, truncated Random
values are extended, and all information is prepared to enable the
cryptographic handshake and any import or export of key material and
configuration.
There is no obligation to reconstruct logical handshake messages in
any specific format, and client and server do not need to agree on
the precise representation of these messages, so long as they agree
on their logical contents.
3. Handshake Messages
In general, we retain the basic structure of each individual TLS or
DTLS handshake message. However, the following handshake messages
have been modified for space reduction and cleaned up to remove pre-
TLS 1.3 baggage.
3.1. ClientHello
The cTLS ClientHello is defined as follows.
opaque Random[RandomLength]; // variable length
struct {
Random random;
CipherSuite cipher_suites<1..2^16-1>;
Extension extensions<1..2^16-1>;
} ClientHello;
3.2. ServerHello
We redefine ServerHello in the following way.
Rescorla, et al. Expires 10 January 2023 [Page 16]
Internet-Draft cTLS 1.3 July 2022
struct {
Random random;
CipherSuite cipher_suite;
Extension extensions<1..2^16-1>;
} ServerHello;
3.3. HelloRetryRequest
In cTLS, the HelloRetryRequest message is a true handshake message
instead of a specialization of ServerHello. The HelloRetryRequest
has the following format.
struct {
CipherSuite cipher_suite;
Extension extensions<2..2^16-1>;
} HelloRetryRequest;
The HelloRetryRequest is the same as the ServerHello above but
without the unnecessary sentinel Random value.
OPEN ISSUE: Does server_hello_extensions apply to
HelloRetryRequest?
4. Examples
This section provides some example specializations.
For this example we use TLS 1.3 only with AES_GCM, x25519, ALPN h2,
short random values, and everything else is ordinary TLS 1.3.
{
"ctlsVersion": 0,
"profile": "0504030201",
"version" : 772,
"random": 16,
"cipherSuite" : "TLS_AES_128_GCM_SHA256",
"dhGroup": "x25519",
"clientHelloExtensions": {
"predefinedExtensions": {
"application_layer_protocol_negotiation" : "030016832",
},
"allowAdditional": true
}
}
Version 772 corresponds to the hex representation 0x0304 (i.e. 1.3).
Rescorla, et al. Expires 10 January 2023 [Page 17]
Internet-Draft cTLS 1.3 July 2022
5. 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.
6. IANA Considerations
6.1. Adding a ContentType
This document requests that a code point be allocated from the "TLS
ContentType registry. This value must be in the range 0-31
(inclusive). The row to be added in the registry has the following
form:
+=======+================+=========+===========+
| Value | Description | DTLS-OK | Reference |
+=======+================+=========+===========+
| TBD | ctls | Y | RFCXXXX |
+-------+----------------+---------+-----------+
| TBD | ctls_handshake | Y | RFCXXXX |
+-------+----------------+---------+-----------+
Table 1
RFC EDITOR: Please replace the value TBD with the value assigned
by IANA, and the value XXXX to the RFC number assigned for this
document.
OPEN ISSUE: Should we require standards action for all profile IDs
that would fit in 2 octets.
6.2. Template Keys
This document requests that IANA open a new registry entitled "cTLS
Template Keys", on the Transport Layer Security (TLS) Parameters
page, with a "Specification Required" registration policy and the
following initial contents:
Rescorla, et al. Expires 10 January 2023 [Page 18]
Internet-Draft cTLS 1.3 July 2022
+=========================+=======+=================+
| Name | Value | Reference |
+=========================+=======+=================+
| profile | 0 | (This document) |
+-------------------------+-------+-----------------+
| version | 1 | (This document) |
+-------------------------+-------+-----------------+
| cipher_suite | 2 | (This document) |
+-------------------------+-------+-----------------+
| dh_group | 3 | (This document) |
+-------------------------+-------+-----------------+
| signature_algorithm | 4 | (This document) |
+-------------------------+-------+-----------------+
| random | 5 | (This document) |
+-------------------------+-------+-----------------+
| mutual_auth | 6 | (This document) |
+-------------------------+-------+-----------------+
| handshake_framing | 7 | (This document) |
+-------------------------+-------+-----------------+
| client_hello_extensions | 8 | (This document) |
+-------------------------+-------+-----------------+
| server_hello_extensions | 9 | (This document) |
+-------------------------+-------+-----------------+
| encrypted_extensions | 10 | (This document) |
+-------------------------+-------+-----------------+
| cert_request_extensions | 11 | (This document) |
+-------------------------+-------+-----------------+
| known_certificates | 12 | (This document) |
+-------------------------+-------+-----------------+
| finished_size | 13 | (This document) |
+-------------------------+-------+-----------------+
| optional | 65535 | (This document) |
+-------------------------+-------+-----------------+
Table 2
6.3. Adding a cTLS Template message type
IANA is requested to add the following entry to the TLS HandshakeType
registry.
* Value: TBD
* Description: ctls_template
* DTLS-OK: ??? Not clear what to put here.
* Reference: (This document)
Rescorla, et al. Expires 10 January 2023 [Page 19]
Internet-Draft cTLS 1.3 July 2022
* Comment: Virtual message used in cTLS.
6.4. Activating the HelloRetryRequest MessageType
This document requests that IANA change the name of entry 6 in the
TLS HandshakeType Registry from "hello_retry_request_RESERVED" to
"hello_retry_request", and set its Reference field to this document.
6.5. Reserved profiles
This document requests that IANA open a new registry entitled "Well-
known cTLS Profile IDs", on the Transport Layer Security (TLS)
Parameters page, with the following columns:
* ID value: A sequence of 1-4 octets.
* Template: A JSON object.
* Note: An explanation or reference.
The ID values of length 1 are subject to a "Standards Action"
registry policy. Values of length 2 are subject to an "RFC Required"
policy. Values of length 3 and 4 are subject to a "First Come First
Served" policy. Values longer than 4 octets are not subject to
registration and MUST NOT appear in this registry.
The initial registry contents are:
+==========+==================+===============+
| ID value | Template | Note |
+==========+==================+===============+
| [0x00] | {"version": 772} | cTLS 1.3-only |
+----------+------------------+---------------+
Table 3
7. 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/rfc/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/rfc/rfc8174>.
Rescorla, et al. Expires 10 January 2023 [Page 20]
Internet-Draft cTLS 1.3 July 2022
[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/rfc/rfc8446>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
Appendix A. Example Exchange
The follow exchange illustrates a complete cTLS-based exchange
supporting mutual authentication using certificates. The digital
signatures use ECDSA with SHA256 and NIST P256r1. The ephemeral
Diffie-Hellman uses the FX25519 curve and the exchange negotiates
TLS-AES-128-CCM8-SHA256. The certificates are exchanged using
certificate identifiers.
The resulting byte counts are as follows:
ECDHE
------------------
TLS CTLS Overhead
--- ---- --------
ClientHello 132 69 2
ServerHello 90 64 2
ServerFlight 478 73 5
ClientFlight 458 73 5
==================================
Total 1158 279 14
The following compression profile was used in this example:
Rescorla, et al. Expires 10 January 2023 [Page 21]
Internet-Draft cTLS 1.3 July 2022
{
"ctlsVersion": 0,
"profile": "abcdef1234",
"version": 772,
"cipherSuite": "TLS_AES_128_CCM_8_SHA256",
"dhGroup": "x25519",
"signatureAlgorithm": "ecdsa_secp256r1_sha256",
"finishedSize": 8,
"clientHelloExtensions": {
"predefinedExtensions": {
"server_name": "000e00000b6578616d706c652e636f6d"
},
"expectedExtensions": ["key_share"],
"allowAdditional": false
},
"serverHelloExtensions": {
"expectedExtensions": ["key_share"],
"allowAdditional": false
},
"certificateRequestExtensions": {
"predefinedExtensions": {
"certificate_request_context": "00",
"signature_algorithms": "00020403"
},
"allowAdditional": false
},
"mutualAuth": true,
"knownCertificates": {
"61": "3082...",
"62": "3082...",
"63": "...",
"64": "...",
...
}
}
ClientHello: 71 bytes = Profile ID(5) + Random(32) + DH(32) +
Overhead(2)
01 // Handshake.msg_type = ClientHello
05 abcdef1234 // ClientHello.profile_id
5856a1...43168c130 // ClientHello.random
a690...af948 // KeyShareEntry.key_exchange
ServerHello: 65 bytes = Random(32) + DH(32) + Overhead(1)
Rescorla, et al. Expires 10 January 2023 [Page 22]
Internet-Draft cTLS 1.3 July 2022
02 // Handshake.msg_type = ServerHello
cff4c0...684c859ca8 // ServerHello.random
9fbc...0f49 // KeyShareEntry.key_exchange
Server Flight: 78 = SIG(64) + MAC(8) + CERTID(1) + Overhead(5)
The EncryptedExtensions, and the CertificateRequest messages are
omitted because they are empty.
0b // Certificate
03 // CertificateList
01 // CertData.length
61 // CertData = 'a'
0f // CertificateVerify
3045...10ce // signature
14 // Finished
bfc9d66715bb2b04 // VerifyData
Client Flight: 78 bytes = SIG(64) + MAC(8) + CERTID(1) + Overhead(5)
0b // Certificate
03 // CertificateList
01 // CertData.length
62 // CertData = 'b'
0f // CertificateVerify
3045...f60e // signature
14 // Finished
35e9c34eec2c5dc1 // VerifyData
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
Rescorla, et al. Expires 10 January 2023 [Page 23]
Internet-Draft cTLS 1.3 July 2022
Email: rlb@ipv.sx
Hannes Tschofenig
Arm Limited
Email: hannes.tschofenig@arm.com
Benjamin M. Schwartz
Google
Email: bemasc@google.com
Rescorla, et al. Expires 10 January 2023 [Page 24]