TLS Working Group                                            E. Rescorla
Internet-Draft                                                   Mozilla
Intended status: Standards Track                               R. Barnes
Expires: 28 April 2022                                             Cisco
                                                           H. Tschofenig
                                                             Arm Limited
                                                         25 October 2021

                            Compact TLS 1.3


   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, a template-based specialization technique, and
   alternative cryptographic techniques. 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 28 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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

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   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
     2.1.  Template-based Specialization . . . . . . . . . . . . . .   3
       2.1.1.  Requirements on TLS Implementations . . . . . . . . .   6
       2.1.2.  Predefined Extensions . . . . . . . . . . . . . . . .   7
       2.1.3.  Known Certificates  . . . . . . . . . . . . . . . . .   8
     2.2.  Record Layer  . . . . . . . . . . . . . . . . . . . . . .   9
     2.3.  Handshake Layer . . . . . . . . . . . . . . . . . . . . .  10
   3.  Handshake Messages  . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  ClientHello . . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  ServerHello . . . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  HelloRetryRequest . . . . . . . . . . . . . . . . . . . .  12
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  13
   Appendix A.  Example Exchange . . . . . . . . . . . . . . . . . .  14
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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 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

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   *  Alternative cryptographic techniques, such as semi-static Diffie-

   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",
   "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.

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.

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   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      |

   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 specified via a JSON dictionary.

   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"

   The following elements are defined:

   profile (integer):  identifies the profile being defined.

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   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.

   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

   random (integer):  indicates that the ClientHello.Random and
      ServerHello.Random values are truncated to the given length.  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.  The Random length MUST be less
      than or equal to 32 bytes.

   [[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.

   mutualAuth (boolean):  if set to true, 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.]]

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   extension_order:  indicates in what order extensions appear in
      respective messages.  This allows to omit sending the type.  If
      there is only a single extension to be transmitted, then the
      extension length field can also be omitted.  For example, imagine
      that only the KeyShare extension needs to be sent in the
      ClientHello as the only extension.  Then, the following structure

      28                    // Extensions.length
      33 26                 // KeyShare
        0024                // client_shares.length
          001d              //
          0020 a690...af948 // KeyShareEntry.key_exchange

   is compressed down to (assuming the KeyShare group has been pre-

      0020 a690...af948 // KeyShareEntry.key_exchange

   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

2.1.1.  Requirements on TLS Implementations

   To be compatible with the specializations described in this section,
   a TLS stack needs to provide the following features:

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   *  If specialization of extensions is to be used, then the TLS stack
      MUST order each vector of Extension values in ascending order
      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.

2.1.2.  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:

   *  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.

   *  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

   *  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:

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   *  If there is an extension in the compressed message with a type
      that exists in the predefined extensions object, it is an error,
      because such an extension would not have been sent by a sender
      with a compatible compression profile.

   *  For each entry in the predefined extensions dictionary, an
      extension is added to the decompressed message with the specified
      type and value.

   *  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.]]

2.1.3.  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.

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   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.

2.2.  Record Layer

   The only cTLS records that are sent in plaintext are handshake
   records (ClientHello and ServerHello/HRR).  The content type is
   therefore constant (it is always handshake), so we instead 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.

   The profile_id field allows the client and server to agree on which
   compression profile should be used for this session (see
   Section 2.1).  This field MUST be set to zero if and only if no
   compression profile is used.  Non-zero values are negotiated out of
   band between the client and server, as part of the specification of
   the compression profile.

         struct {
             ContentType content_type = ctls_handshake;
             opaque profile_id<0..2^8-1>;
             opaque fragment<0..V>;
         } CTLSPlaintext;

   [[OPEN ISSUE: The profile_id is needed in the ClientHello to inform
   the server what compression profile to use.  For a ServerHello this
   field is not required.  Should we make this field optional?]]

   Encrypted records use DTLS 1.3 record framing, comprising a
   configuration octet followed by optional connection ID, sequence
   number, and length fields.

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         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

   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.  In cases where a sequence number is not
   required (e.g., when a reliable transport is in use), a cTLS
   implementation may suppress it by setting the suppressSequenceNumber
   flag in the compression profile being used (see Section 2.1).  When
   this flag is enabled, the S bit in the configuration octet MUST be

2.3.  Handshake Layer

   The cTLS handshake framing is same as the TLS 1.3 handshake framing,
   except for two changes:

   *  The length field is omitted.

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   *  The HelloRetryRequest message is a true handshake message instead
      of a specialization of ServerHello.

         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;

3.  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.

3.1.  ClientHello

   The cTLS ClientHello is defined as follows.

         opaque Random[RandomLength];      // variable length

         struct {
             Random random;
             CipherSuite cipher_suites<1..V>;
             Extension extensions<1..V>;
         } ClientHello;

3.2.  ServerHello

   We redefine ServerHello in the following way.

         struct {
             Random random;
             CipherSuite cipher_suite;
             Extension extensions<1..V>;
         } ServerHello;

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3.3.  HelloRetryRequest

   The HelloRetryRequest has the following format.

         struct {
             CipherSuite cipher_suite;
             Extension extensions<2..V>;
         } HelloRetryRequest;

   The HelloRetryRequest is the same as the ServerHello above but
   without the unnecessary sentinel Random value.

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.

      "Version" : 0x0304
      "Profile" : 1,
      "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?]]

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.

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   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

   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

               | Value | Description | DTLS-OK | Reference |
               | TBD   | ctls        | N       | 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.]]

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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

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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:

                 TLS  CTLS  Overhead
                 ---  ----  --------
   ClientHello   132   36       4
   ServerHello    90   36       4
   ServerFlight  478   80       7
   ClientFlight  458   80       7
   Total        1158  232      22

   The following compression profile was used in this example:

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     "profile": 1,
     "version": 772,
     "cipherSuite": "TLS_AES_128_CCM_8_SHA256",
     "dhGroup": "X25519",
     "signatureAlgorithm": "ECDSA_P256_SHA256",
     "finishedSize": 8,
     "clientHelloExtensions": {
       "server_name": "000e00000b6578616d706c652e636f6d",
     "certificateRequestExtensions": {
       "certificate_request_context": 0,
       "signature_algorithms": "00020403"
     "mutualAuth": true,
     "extension-order": {
          "clientHelloExtensions": {
          "ServerHelloExtensions": {

     "knownCertificates": {
       "61": "3082...",
       "62": "3082...",
       "63": "...",
       "64": "...",

   ClientHello: 36 bytes = DH(32) + Overhead(4)

   01                    // ClientHello
   01                    // Profile ID
   0020 a690...af948     // KeyShareEntry.key_exchange

   ServerHello: 36 = DH(32) + Overhead(4)

   02                 // ServerHello
   26                 // Extensions.length
   0020 9fbc...0f49   // KeyShareEntry.key_exchange

   Server Flight: 80 = SIG(64) + MAC(8) + CERTID(1) + Overhead(7)

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   The EncryptedExtensions, and the CertificateRequest messages are
   omitted because they are empty.

   0b                 // Certificate
     03               //   CertificateList
       01             //     CertData.length
         61           //       CertData = 'a'

   0f                 // CertificateVerify
     4064             //   Signature.length
          3045...10ce //   Signature

   14                 // Finished
     bfc9d66715bb2b04 //   VerifyData

   Client Flight: 80 bytes = SIG(64) + MAC(8) + CERTID(1) + Overhead(7)

   0b                 // Certificate
     03               //   CertificateList
       01             //     CertData.length
         62           //       CertData = 'b'

   0f                 // CertificateVerify
     4064             //   Signature.length
          3045...f60e //   Signature

   14                 // Finished
     35e9c34eec2c5dc1 //   VerifyData


   We would like to thank Karthikeyan Bhargavan, Owen Friel, Sean
   Turner, Martin Thomson and Chris Wood.

Authors' Addresses

   Eric Rescorla


   Richard Barnes


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Internet-Draft                  cTLS 1.3                    October 2021

   Hannes Tschofenig
   Arm Limited


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