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Versions: 00 01 02 03                                                   
Network Working Group                                           E. Omara
Internet-Draft                                                     Apple
Intended status: Informational                                 J. Uberti
Expires: 17 February 2022                                         Google
                                                           A. GOUAILLARD
                                                              S. Murillo
                                                          CoSMo Software
                                                          16 August 2021


                         Secure Frame (SFrame)
                         draft-omara-sframe-03

Abstract

   This document describes the Secure Frame (SFrame) end-to-end
   encryption and authentication mechanism for media frames in a
   multiparty conference call, in which central media servers (SFUs) can
   access the media metadata needed to make forwarding decisions without
   having access to the actual media.  The proposed mechanism differs
   from other approaches through its use of media frames as the
   encryptable unit, instead of individual RTP packets, which makes it
   more bandwidth efficient and also allows use with non-RTP transports.

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 17 February 2022.

Copyright Notice

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






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   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 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  SFrame  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  SFrame Format . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  SFrame Header . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Encryption Schema . . . . . . . . . . . . . . . . . . . .   8
       4.3.1.  Key Selection . . . . . . . . . . . . . . . . . . . .   9
       4.3.2.  Key Derivation  . . . . . . . . . . . . . . . . . . .   9
       4.3.3.  Encryption  . . . . . . . . . . . . . . . . . . . . .  10
       4.3.4.  Decryption  . . . . . . . . . . . . . . . . . . . . .  12
       4.3.5.  Duplicate Frames  . . . . . . . . . . . . . . . . . .  12
     4.4.  Ciphersuites  . . . . . . . . . . . . . . . . . . . . . .  12
       4.4.1.  AES-CM with SHA2  . . . . . . . . . . . . . . . . . .  13
   5.  Key Management  . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Sender Keys . . . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  MLS . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Media Considerations  . . . . . . . . . . . . . . . . . . . .  17
     6.1.  SFU . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
       6.1.1.  LastN and RTP stream reuse  . . . . . . . . . . . . .  17
       6.1.2.  Simulcast . . . . . . . . . . . . . . . . . . . . . .  17
       6.1.3.  SVC . . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.2.  Video Key Frames  . . . . . . . . . . . . . . . . . . . .  18
     6.3.  Partial Decoding  . . . . . . . . . . . . . . . . . . . .  18
   7.  Overhead  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Audio . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  Video . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.3.  SFrame vs PERC-lite . . . . . . . . . . . . . . . . . . .  20
       7.3.1.  Audio . . . . . . . . . . . . . . . . . . . . . . . .  20
       7.3.2.  Video . . . . . . . . . . . . . . . . . . . . . . . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
     8.1.  No Per-Sender Authentication  . . . . . . . . . . . . . .  21
     8.2.  Key Management  . . . . . . . . . . . . . . . . . . . . .  21
     8.3.  Authentication tag length . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. Test Vectors  . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  AES_CM_128_HMAC_SHA256_4 . . . . . . . . . . . . . . . .  22



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     10.2.  AES_CM_128_HMAC_SHA256_8 . . . . . . . . . . . . . . . .  23
     10.3.  AES_GCM_128_SHA256 . . . . . . . . . . . . . . . . . . .  25
     10.4.  AES_GCM_256_SHA512 . . . . . . . . . . . . . . . . . . .  27
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     11.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   Modern multi-party video call systems use Selective Forwarding Unit
   (SFU) servers to efficiently route RTP streams to call endpoints
   based on factors such as available bandwidth, desired video size,
   codec support, and other factors.  In order for the SFU to work
   properly though, it needs to be able to access RTP metadata and RTCP
   feedback messages, which is not possible if all RTP/RTCP traffic is
   end-to-end encrypted.

   As such, two layers of encryptions and authentication are required:

   1.  Hop-by-hop (HBH) encryption of media, metadata, and feedback
       messages between the the endpoints and SFU

   2.  End-to-end (E2E) encryption of media between the endpoints

   While DTLS-SRTP can be used as an efficient HBH mechanism, it is
   inherently point-to-point and therefore not suitable for a SFU
   context.  In addition, given the various scenarios in which video
   calling occurs, minimizing the bandwidth overhead of end-to-end
   encryption is also an important goal.

   This document proposes a new end-to-end encryption mechanism known as
   SFrame, specifically designed to work in group conference calls with
   SFUs.

















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     +-------------------------------+-------------------------------+^+
     |V=2|P|X|  CC   |M|     PT      |       sequence number         | |
     +-------------------------------+-------------------------------+ |
     |                           timestamp                           | |
     +---------------------------------------------------------------+ |
     |           synchronization source (SSRC) identifier            | |
     |=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=| |
     |            contributing source (CSRC) identifiers             | |
     |                               ....                            | |
     +---------------------------------------------------------------+ |
     |                   RTP extension(s) (OPTIONAL)                 | |
   +^---------------------+------------------------------------------+ |
   | |   payload header   |                                          | |
   | +--------------------+     payload  ...                         | |
   | |                                                               | |
   +^+---------------------------------------------------------------+^+
   | :                       authentication tag                      : |
   | +---------------------------------------------------------------+ |
   |                                                                   |
   ++ Encrypted Portion                       Authenticated Portion +--+

                        Figure 1: SRTP packet format

2.  Terminology

   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.

   SFU:  Selective Forwarding Unit (AKA RTP Switch)

   IV:  Initialization Vector

   MAC:  Message Authentication Code

   E2EE:  End to End Encryption

   HBH:  Hop By Hop

   KMS:  Key Management System

3.  Goals

   SFrame is designed to be a suitable E2EE protection scheme for
   conference call media in a broad range of scenarios, as outlined by
   the following goals:



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   1.  Provide an secure E2EE mechanism for audio and video in
       conference calls that can be used with arbitrary SFU servers.

   2.  Decouple media encryption from key management to allow SFrame to
       be used with an arbitrary KMS.

   3.  Minimize packet expansion to allow successful conferencing in as
       many network conditions as possible.

   4.  Independence from the underlying transport, including use in non-
       RTP transports, e.g., WebTransport.

   5.  When used with RTP and its associated error resilience
       mechanisms, i.e., RTX and FEC, require no special handling for
       RTX and FEC packets.

   6.  Minimize the changes needed in SFU servers.

   7.  Minimize the changes needed in endpoints.

   8.  Work with the most popular audio and video codecs used in
       conferencing scenarios.

4.  SFrame

   We propose a frame level encryption mechanism that provides effective
   end-to-end encryption, is simple to implement, has no dependencies on
   RTP, and minimizes encryption bandwidth overhead.  Because SFrame
   encrypts the full frame, rather than individual packets, bandwidth
   overhead is reduced by having a single IV and authentication tag for
   each media frame.

   Also, because media is encrypted prior to packetization, the
   encrypted frame is packetized using a generic RTP packetizer instead
   of codec-dependent packetization mechanisms.  With this move to a
   generic packetizer, media metadata is moved from codec-specific
   mechanisms to a generic frame RTP header extension which, while
   visible to the SFU, is authenticated end-to-end.  This extension
   includes metadata needed for SFU routing such as resolution, frame
   beginning and end markers, etc.

   The generic packetizer splits the E2E encrypted media frame into one
   or more RTP packets and adds the SFrame header to the beginning of
   the first packet and an auth tag to the end of the last packet.







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      +-------------------------------------------------------+
      |                                                       |
      |  +----------+      +------------+      +-----------+  |
      |  |          |      |   SFrame   |      |Packetizer |  |       DTLS+SRTP
      |  | Encoder  +----->+    Enc     +----->+           +-------------------------+
 ,+.  |  |          |      |            |      |           |  |   +--+  +--+  +--+   |
 `|'  |  +----------+      +-----+------+      +-----------+  |   |  |  |  |  |  |   |
 /|\  |                          ^                            |   |  |  |  |  |  |   |
  +   |                          |                            |   |  |  |  |  |  |   |
 / \  |                          |                            |   +--+  +--+  +--+   |
Alice |                    +-----+------+                     |   Encrypted Packets  |
      |                    |Key Manager |                     |                      |
      |                    +------------+                     |                      |
      |                         ||                            |                      |
      |                         ||                            |                      |
      |                         ||                            |                      |
      +-------------------------------------------------------+                      |
                                ||                                                   |
                                ||                                                   v
                           +------------+                                      +-----+------+
            E2EE channel   |  Messaging |                                      |   Media    |
              via the      |  Server    |                                      |   Server   |
          Messaging Server |            |                                      |            |
                           +------------+                                      +-----+------+
                                ||                                                   |
                                ||                                                   |
      +-------------------------------------------------------+                      |
      |                         ||                            |                      |
      |                         ||                            |                      |
      |                         ||                            |                      |
      |                    +------------+                     |                      |
      |                    |Key Manager |                     |                      |
 ,+.  |                    +-----+------+                     |   Encrypted Packets  |
 `|'  |                          |                            |   +--+  +--+  +--+   |
 /|\  |                          |                            |   |  |  |  |  |  |   |
  +   |                          v                            |   |  |  |  |  |  |   |
 / \  |  +----------+      +-----+------+      +-----------+  |   |  |  |  |  |  |   |
 Bob  |  |          |      |   SFrame   |      |   De+     |  |   +--+  +--+  +--+   |
      |  | Decoder  +<-----+    Dec     +<-----+Packetizer +<------------------------+
      |  |          |      |            |      |           |  |        DTLS+SRTP
      |  +----------+      +------------+      +-----------+  |
      |                                                       |
      +-------------------------------------------------------+

   The E2EE keys used to encrypt the frame are exchanged out of band
   using a secure E2EE channel.





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4.1.  SFrame Format

     +------------+------------------------------------------+^+
     |S|LEN|X|KID |         Frame Counter                    | |
   +^+------------+------------------------------------------+ |
   | |                                                       | |
   | |                                                       | |
   | |                                                       | |
   | |                                                       | |
   | |                  Encrypted Frame                      | |
   | |                                                       | |
   | |                                                       | |
   | |                                                       | |
   | |                                                       | |
   +^+-------------------------------------------------------+^+
   | |                 Authentication Tag                    | |
   | +-------------------------------------------------------+ |
   |                                                           |
   |                                                           |
   +----+Encrypted Portion            Authenticated Portion+---+

4.2.  SFrame Header

   Since each endpoint can send multiple media layers, each frame will
   have a unique frame counter that will be used to derive the
   encryption IV.  The frame counter must be unique and monotonically
   increasing to avoid IV reuse.

   As each sender will use their own key for encryption, so the SFrame
   header will include the key id to allow the receiver to identify the
   key that needs to be used for decrypting.

   Both the frame counter and the key id are encoded in a variable
   length format to decrease the overhead.  The length is up to 8 bytes
   and is represented in 3 bits in the SFrame header: 000 represents a
   length of 1, 001 a length of 2... The first byte in the SFrame header
   is fixed and contains the header metadata with the following format:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |R|LEN  |X|  K  |
   +-+-+-+-+-+-+-+-+
   SFrame header metadata

   Reserved (R): 1 bit This field MUST be set to zero on sending, and
   MUST be ignored by receivers.  Counter Length (LEN): 3 bits This
   field indicates the length of the CTR fields in bytes (1-8).
   Extended Key Id Flag (X): 1 bit Indicates if the key field contains



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   the key id or the key length.  Key or Key Length: 3 bits This field
   contains the key id (KID) if the X flag is set to 0, or the key
   length (KLEN) if set to 1.

   If X flag is 0 then the KID is in the range of 0-7 and the frame
   counter (CTR) is found in the next LEN bytes:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+---------------------------------+
   |R|LEN  |0| KID |    CTR... (length=LEN)          |
   +-+-+-+-+-+-+-+-+---------------------------------+

   Frame counter byte length (LEN): 3bits The frame counter length in
   bytes (1-8).  Key id (KID): 3 bits The key id (0-7).  Frame counter
   (CTR): (Variable length) Frame counter value up to 8 bytes long.

   if X flag is 1 then KLEN is the length of the key (KID), that is
   found after the SFrame header metadata byte.  After the key id (KID),
   the frame counter (CTR) will be found in the next LEN bytes:

 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+---------------------------+---------------------------+
|R|LEN  |1|KLEN |   KID... (length=KLEN)    |    CTR... (length=LEN)    |
+-+-+-+-+-+-+-+-+---------------------------+---------------------------+

   Frame counter byte length (LEN): 3bits The frame counter length in
   bytes (1-8).  Key length (KLEN): 3 bits The key length in bytes
   (1-8).  Key id (KID): (Variable length) The key id value up to 8
   bytes long.  Frame counter (CTR): (Variable length) Frame counter
   value up to 8 bytes long.

4.3.  Encryption Schema

   SFrame encryption uses an AEAD encryption algorithm and hash function
   defined by the ciphersuite in use (see Section 4.4).  We will refer
   to the following aspects of the AEAD algorithm below:

   *  "AEAD.Encrypt" and "AEAD.Decrypt" - The encryption and decryption
      functions for the AEAD.  We follow the convention of RFC 5116
      [RFC5116] and consider the authentication tag part of the
      ciphertext produced by "AEAD.Encrypt" (as opposed to a separate
      field as in SRTP [RFC3711]).

   *  "AEAD.Nk" - The size of a key for the encryption algorithm, in
      bytes

   *  "AEAD.Nn" - The size of a nonce for the encryption algorithm, in
      bytes



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4.3.1.  Key Selection

   Each SFrame encryption or decryption operation is premised on a
   single secret "base\_key", which is labeled with an integer KID value
   signaled in the SFrame header.

   The sender and receivers need to agree on which key should be used
   for a given KID.  The process for provisioning keys and their KID
   values is beyond the scope of this specification, but its security
   properties will bound the assurances that SFrame provides.  For
   example, if SFrame is used to provide E2E security against
   intermediary media nodes, then SFrame keys MUST be negotiated in a
   way that does not make them accessible to these intermediaries.

   For each known KID value, the client stores the corresponding
   symmetric key "base\_key".  For keys that can be used for encryption,
   the client also stores the next counter value CTR to be used when
   encrypting (initially 0).

   When encrypting a frame, the application specifies which KID is to be
   used, and the counter is incremented after successful encryption.
   When decrypting, the "base\_key" for decryption is selected from the
   available keys using the KID value in the SFrame Header.

   A given key MUST NOT be used for encryption by multiple senders.
   Such reuse would result in multiple encrypted frames being generated
   with the same (key, nonce) pair, which harms the protections provided
   by many AEAD algorithms.  Implementations SHOULD mark each key as
   usable for encryption or decryption, never both.

   Note that the set of available keys might change over the lifetime of
   a real-time session.  In such cases, the client will need to manage
   key usage to avoid media loss due to a key being used to encrypt
   before all receivers are able to use it to decrypt.  For example, an
   application may make decryption-only keys available immediately, but
   delay the use of encryption-only keys until (a) all receivers have
   acknowledged receipt of the new key or (b) a timeout expires.

4.3.2.  Key Derivation

   SFrame encrytion and decryption use a key and salt derived from the
   "base\_key" associated to a KID.  Given a "base\_key" value, the key
   and salt are derived using HKDF [RFC5869] as follows:

   sframe_secret = HKDF-Extract(K, 'SFrame10')
   sframe_key = HKDF-Expand(sframe_secret, 'key', AEAD.Nk)
   sframe_salt = HKDF-Expand(sframe_secret, 'salt', AEAD.Nn)




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   The hash function used for HKDF is determined by the ciphersuite in
   use.

4.3.3.  Encryption

   After encoding the frame and before packetizing it, the necessary
   media metadata will be moved out of the encoded frame buffer, to be
   used later in the RTP generic frame header extension.  The encoded
   frame, the metadata buffer and the frame counter are passed to SFrame
   encryptor.

   SFrame encryption uses the AEAD encryption algorithm for the
   ciphersuite in use.  The key for the encryption is the "sframe\_key"
   and the nonce is formed by XORing the "sframe\_salt" with the current
   counter, encoded as a big-endian integer of length "AEAD.Nn".

   The encryptor forms an SFrame header using the S, CTR, and KID values
   provided.  The encoded header is provided as AAD to the AEAD
   encryption operation, with any frame metadata appended.

def encrypt(S, CTR, KID, frame_metadata, frame):
  sframe_key, sframe_salt = key_store[KID]

  frame_ctr = encode_big_endian(CTR, AEAD.Nn)
  frame_nonce = xor(sframe_salt, frame_ctr)

  header = encode_sframe_header(S, CTR, KID)
  frame_aad = header + frame_metadata

  encrypted_frame = AEAD.Encrypt(sframe_key, frame_nonce, frame_aad, frame)
  return header + encrypted_frame

   The encrypted payload is then passed to a generic RTP packetized to
   construct the RTP packets and encrypt it using SRTP keys for the HBH
   encryption to the media server.
















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      +----------------+  +---------------+
      | frame metadata |  |               |
      +-------+--------+  |               |
              |           |     frame     |
              |           |               |
              |           |               |
              |           +-------+-------+
              |                   |
   header ----+------------------>| AAD
   +-----+                        |
   |  S  |                        |
   +-----+                        |
   | KID +--+--> sframe_key ----->| Key
   |     |  |                     |
   |     |  +--> sframe_salt -+   |
   +-----+                    |   |
   | CTR +--------------------+-->| Nonce
   |     |                        |
   |     |                        |
   +-----+                        |
      |                       AEAD.Encrypt
      |                           |
      |                           V
      |                   +-------+-------+
      |                   |               |
      |                   |               |
      |                   |   encrypted   |
      |                   |     frame     |
      |                   |               |
      |                   |               |
      |                   +-------+-------+
      |                           |
      |                  generic RTP packetize
      |                           |
      |                           v
      V
   +---------------+      +---------------+     +---------------+
   | SFrame header |      |               |     |               |
   +---------------+      |               |     |               |
   |               |      |  payload 2/N  |     |  payload N/N  |
   |  payload 1/N  |      |               |     |               |
   |               |      |               |     |               |
   +---------------+      +---------------+     +---------------+

                         Figure 2: Encryption flow






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4.3.4.  Decryption

   The receiving clients buffer all packets that belongs to the same
   frame using the frame beginning and ending marks in the generic RTP
   frame header extension, and once all packets are available, it passes
   it to SFrame for decryption.  The KID field in the SFrame header is
   used to find the right key for the encrypted frame.

def decrypt(frame_metadata, sframe):
  header, encrypted_frame = split_header(sframe)
  S, CTR, KID = parse_header(header)

  sframe_key, sframe_salt = key_store[KID]

  frame_ctr = encode_big_endian(CTR, AEAD.Nn)
  frame_nonce = xor(sframe_salt, frame_ctr)
  frame_aad = header + frame_metadata

  return AEAD.Decrypt(sframe_key, frame_nonce, frame_aad, encrypted_frame)

   For frames that are failed to decrypt because there is key available
   for the KID in the SFrame header, the client MAY buffer the frame and
   retry decryption once a key with that KID is received.

4.3.5.  Duplicate Frames

   Unlike messaging application, in video calls, receiving a duplicate
   frame doesn't necessary mean the client is under a replay attack,
   there are other reasons that might cause this, for example the sender
   might just be sending them in case of packet loss.  SFrame decryptors
   use the highest received frame counter to protect against this.  It
   allows only older frame pithing a short interval to support out of
   order delivery.

4.4.  Ciphersuites

   Each SFrame session uses a single ciphersuite that specifies the
   following primitives:

   o A hash function used for key derivation and hashing signature
   inputs

   o An AEAD encryption algorithm [RFC5116] used for frame encryption,
   optionally with a truncated authentication tag

   o [Optional] A signature algorithm

   This document defines the following ciphersuites:



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     +========+==========================+====+====+====+===========+
     | Value  | Name                     | Nh | Nk | Nn | Reference |
     +========+==========================+====+====+====+===========+
     | 0x0001 | AES_CM_128_HMAC_SHA256_8 | 32 | 16 | 12 | RFC XXXX  |
     +--------+--------------------------+----+----+----+-----------+
     | 0x0002 | AES_CM_128_HMAC_SHA256_4 | 32 | 16 | 12 | RFC XXXX  |
     +--------+--------------------------+----+----+----+-----------+
     | 0x0003 | AES_GCM_128_SHA256       | 32 | 16 | 12 | RFC XXXX  |
     +--------+--------------------------+----+----+----+-----------+
     | 0x0004 | AES_GCM_256_SHA512       | 64 | 32 | 12 | RFC XXXX  |
     +--------+--------------------------+----+----+----+-----------+

                                 Table 1

   In the "AES_CM" suites, the length of the authentication tag is
   indicated by the last value: "_8" indicates an eight-byte tag and
   "_4" indicates a four-byte tag.

   In a session that uses multiple media streams, different ciphersuites
   might be configured for different media streams.  For example, in
   order to conserve bandwidth, a session might use a ciphersuite with
   80-bit tags for video frames and another ciphersuite with 32-bit tags
   for audio frames.

4.4.1.  AES-CM with SHA2

   In order to allow very short tag sizes, we define a synthetic AEAD
   function using the authenticated counter mode of AES together with
   HMAC for authentication.  We use an encrypt-then-MAC approach as in
   SRTP [RFC3711].

   Before encryption or decryption, encryption and authentication
   subkeys are derived from the single AEAD key using HKDF.  The subkeys
   are derived as follows, where "Nk" represents the key size for the
   AES block cipher in use and "Nh" represents the output size of the
   hash function:

   def derive_subkeys(sframe_key):
     aead_secret = HKDF-Extract(sframe_key, 'SFrame10 AES CM AEAD')
     enc_key = HKDF-Expand(aead_secret, 'enc', Nk)
     auth_key = HKDF-Expand(aead_secret, 'auth', Nh)
     return enc_key, auth_key

   The AEAD encryption and decryption functions are then composed of
   individual calls to the CM encrypt function and HMAC.  The resulting
   MAC value is truncated to a number of bytes "tag_len" fixed by the
   ciphersuite.




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   def compute_tag(auth_key, nonce, aad, ct):
     aad_len = encode_big_endian(len(aad), 8)
     ct_len = encode_big_endian(len(ct), 8)
     auth_data = aad_len + ct_len + nonce + aad + ct
     tag = HMAC(auth_key, auth_data)
     return truncate(tag, tag_len)

   def AEAD.Encrypt(key, nonce, aad, pt):
     enc_key, auth_key = derive_subkeys(key)
     ct = AES-CM.Encrypt(enc_key, nonce, pt)
     tag = compute_tag(auth_key, nonce, aad, ct)
     return ct + tag

   def AEAD.Decrypt(key, nonce, aad, ct):
     inner_ct, tag = split_ct(ct, tag_len)

     enc_key, auth_key = derive_subkeys(key)
     candidate_tag = compute_tag(auth_key, nonce, aad, inner_ct)
     if !constant_time_equal(tag, candidate_tag):
       raise Exception("Authentication Failure")

     return AES-CM.Decrypt(enc_key, nonce, inner_ct)

5.  Key Management

   SFrame must be integrated with an E2E key management framework to
   exchange and rotate the keys used for SFrame encryption and/or
   signing.  The key management framework provides the following
   functions:

   *  Provisioning KID/"base\_key" mappings to participating clients

   *  (optional) Provisioning clients with a list of trusted signing
      keys

   *  Updating the above data as clients join or leave

   It is up to the application to define a rotation schedule for keys.
   For example, one application might have an ephemeral group for every
   call and keep rotating key when end points joins or leave the call,
   while another application could have a persistent group that can be
   used for multiple calls and simply derives ephemeral symmetric keys
   for a specific call.








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5.1.  Sender Keys

   If the participants in a call have a pre-existing E2E-secure channel,
   they can use it to distribute SFrame keys.  Each client participating
   in a call generates a fresh encryption key and optionally a signing
   key pair.  The client then uses the E2E-secure channel to send their
   encryption key and signing public key to the other participants.

   In this scheme, it is assumed that receivers have a signal outside of
   SFrame for which client has sent a given frame, for example the RTP
   SSRC.  SFrame KID values are then used to distinguish generations of
   the sender's key.  At the beginning of a call, each sender encrypts
   with KID=0.  Thereafter, the sender can ratchet their key forward for
   forward secrecy:

   sender_key[i+1] = HKDF-Expand(
                       HKDF-Extract(sender_key[i], 'SFrame10 ratchet'),
                         '', AEAD.Nk)

   The sender signals such an update by incrementing their KID value.  A
   receiver who receives from a sender with a new KID computes the new
   key as above.  The old key may be kept for some time to allow for
   out-of-order delivery, but should be deleted promptly.

   If a new participant joins mid-call, they will need to receive from
   each sender (a) the current sender key for that sender, (b) the
   signing key for the sender, if used, and (c) the current KID value
   for the sender.  Evicting a participant requires each sender to send
   a fresh sender key to all receivers.

5.2.  MLS

   The Messaging Layer Security (MLS) protocol provides group
   authenticated key exchange [I-D.ietf-mls-architecture]
   [I-D.ietf-mls-protocol].  In principle, it could be used to
   instantiate the sender key scheme above, but it can also be used more
   efficiently directly.

   MLS creates a linear sequence of keys, each of which is shared among
   the members of a group at a given point in time.  When a member joins
   or leaves the group, a new key is produced that is known only to the
   augmented or reduced group.  Each step in the lifetime of the group
   is know as an "epoch", and each member of the group is assigned an
   "index" that is constant for the time they are in the group.







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   In SFrame, we derive per-sender "base\_key" values from the group
   secret for an epoch, and use the KID field to signal the epoch and
   sender index.  First, we use the MLS exporter to compute a shared
   SFrame secret for the epoch.

   sframe_epoch_secret = MLS-Exporter("SFrame 10 MLS", "", AEAD.Nk)

   sender_base_key[index] = HKDF-Expand(sframe_epoch_secret,
                              encode_big_endian(index, 4), AEAD.Nk)

   For compactness, do not send the whole epoch number.  Instead, we
   send only its low-order E bits.  Note that E effectively defines a
   re-ordering window, since no more than 2^E epoch can be active at a
   given time.  Receivers MUST be prepared for the epoch counter to roll
   over, removing an old epoch when a new epoch with the same E lower
   bits is introduced.  (Sender indices cannot be similarly compressed.)

   KID = (sender_index << E) + (epoch % (1 << E))

   Once an SFrame stack has been provisioned with the
   "sframe_epoch_secret" for an epoch, it can compute the required KIDs
   and "sender_base_key" values on demand, as it needs to encrypt/
   decrypt for a given member.

           ...
            |
   Epoch 17 +--+-- index=33 -> KID = 0x211
            |  |
            |  +-- index=51 -> KID = 0x331
            |
            |
   Epoch 16 +--+-- index=2 --> KID = 0x20
            |
            |
   Epoch 15 +--+-- index=3 --> KID = 0x3f
            |  |
            |  +-- index=5 --> KID = 0x5f
            |
            |
   Epoch 14 +--+-- index=3 --> KID = 0x3e
            |  |
            |  +-- index=7 --> KID = 0x7e
            |  |
            |  +-- index=20 -> KID = 0x14e
            |
           ...





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   MLS also provides an authenticated signing key pair for each
   participant.  When SFrame uses signatures, these are the keys used to
   generate SFrame signatures.

6.  Media Considerations

6.1.  SFU

   Selective Forwarding Units (SFUs) as described in
   https://tools.ietf.org/html/rfc7667#section-3.7 receives the RTP
   streams from each participant and selects which ones should be
   forwarded to each of the other participants.  There are several
   approaches about how to do this stream selection but in general, in
   order to do so, the SFU needs to access metadata associated to each
   frame and modify the RTP information of the incoming packets when
   they are transmitted to the received participants.

   This section describes how this normal SFU modes of operation
   interacts with the E2EE provided by SFrame

6.1.1.  LastN and RTP stream reuse

   The SFU may choose to send only a certain number of streams based on
   the voice activity of the participants.  To reduce the number of SDP
   O/A required to establish a new RTP stream, the SFU may decide to
   reuse previously existing RTP sessions or even pre-allocate a
   predefined number of RTP streams and choose in each moment in time
   which participant media will be sending through it.  This means that
   in the same RTP stream (defined by either SSRC or MID) may carry
   media from different streams of different participants.  As different
   keys are used by each participant for encoding their media, the
   receiver will be able to verify which is the sender of the media
   coming within the RTP stream at any given point if time, preventing
   the SFU trying to impersonate any of the participants with another
   participant's media.  Note that in order to prevent impersonation by
   a malicious participant (not the SFU) usage of the signature is
   required.  In case of video, the a new signature should be started
   each time a key frame is sent to allow the receiver to identify the
   source faster after a switch.

6.1.2.  Simulcast

   When using simulcast, the same input image will produce N different
   encoded frames (one per simulcast layer) which would be processed
   independently by the frame encryptor and assigned an unique counter
   for each.





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6.1.3.  SVC

   In both temporal and spatial scalability, the SFU may choose to drop
   layers in order to match a certain bitrate or forward specific media
   sizes or frames per second.  In order to support it, the sender MUST
   encode each spatial layer of a given picture in a different frame.
   That is, an RTP frame may contain more than one SFrame encrypted
   frame with an incrementing frame counter.

6.2.  Video Key Frames

   Forward and Post-Compromise Security requires that the e2ee keys are
   updated anytime a participant joins/leave the call.

   The key exchange happens async and on a different path than the SFU
   signaling and media.  So it may happen that when a new participant
   joins the call and the SFU side requests a key frame, the sender
   generates the e2ee encrypted frame with a key not known by the
   receiver, so it will be discarded.  When the sender updates his
   sending key with the new key, it will send it in a non-key frame, so
   the receiver will be able to decrypt it, but not decode it.

   Receiver will re-request an key frame then, but due to sender and sfu
   policies, that new key frame could take some time to be generated.

   If the sender sends a key frame when the new e2ee key is in use, the
   time required for the new participant to display the video is
   minimized.

6.3.  Partial Decoding

   Some codes support partial decoding, where it can decrypt individual
   packets without waiting for the full frame to arrive, with SFrame
   this won't be possible because the decoder will not access the
   packets until the entire frame is arrived and decrypted.

7.  Overhead

   The encryption overhead will vary between audio and video streams,
   because in audio each packet is considered a separate frame, so it
   will always have extra MAC and IV, however a video frame usually
   consists of multiple RTP packets.  The number of bytes overhead per
   frame is calculated as the following 1 + FrameCounter length + 4 The
   constant 1 is the SFrame header byte and 4 bytes for the HBH
   authentication tag for both audio and video packets.






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7.1.  Audio

   Using three different audio frame durations 20ms (50 packets/s) 40ms
   (25 packets/s) 100ms (10 packets/s) Up to 3 bytes frame counter (3.8
   days of data for 20ms frame duration) and 4 bytes fixed MAC length.

       +=============+===========+==========+==========+===========+
       | Counter len |  Packets  | Overhead | Overhead |  Overhead |
       +=============+===========+==========+==========+===========+
       |             |           | bps@20ms | bps@40ms | bps@100ms |
       +-------------+-----------+----------+----------+-----------+
       |      1      |   0-255   |   2400   |   1200   |    480    |
       +-------------+-----------+----------+----------+-----------+
       |      2      | 255 - 65K |   2800   |   1400   |    560    |
       +-------------+-----------+----------+----------+-----------+
       |      3      | 65K - 16M |   3200   |   1600   |    640    |
       +-------------+-----------+----------+----------+-----------+

                                  Table 2

7.2.  Video

   The per-stream overhead bits per second as calculated for the
   following video encodings: 30fps@1000Kbps (4 packets per frame)
   30fps@512Kbps (2 packets per frame) 15fps@200Kbps (2 packets per
   frame) 7.5fps@30Kbps (1 packet per frame) Overhead bps = (Counter
   length + 1 + 4 ) * 8 * fps

     +=============+===========+===========+===========+============+
     | Counter len |   Frames  |  Overhead |  Overhead |  Overhead  |
     +=============+===========+===========+===========+============+
     |             |           | bps@30fps | bps@15fps | bps@7.5fps |
     +-------------+-----------+-----------+-----------+------------+
     |      1      |   0-255   |    1440   |    1440   |    720     |
     +-------------+-----------+-----------+-----------+------------+
     |      2      | 256 - 65K |    1680   |    1680   |    840     |
     +-------------+-----------+-----------+-----------+------------+
     |      3      | 56K - 16M |    1920   |    1920   |    960     |
     +-------------+-----------+-----------+-----------+------------+
     |      4      |  16M - 4B |    2160   |    2160   |    1080    |
     +-------------+-----------+-----------+-----------+------------+

                                 Table 3








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7.3.  SFrame vs PERC-lite

   [RFC8723] has significant overhead over SFrame because the overhead
   is per packet, not per frame, and OHB (Original Header Block) which
   duplicates any RTP header/extension field modified by the SFU.
   [I-D.murillo-perc-lite] https://mailarchive.ietf.org/arch/msg/perc/
   SB0qMHWz6EsDtz3yIEX0HWp5IEY/ is slightly better because it doesn't
   use the OHB anymore, however it still does per packet encryption
   using SRTP.  Below the the overheard in [I-D.murillo-perc-lite]
   implemented by Cosmos Software which uses extra 11 bytes per packet
   to preserve the PT, SEQ_NUM, TIME_STAMP and SSRC fields in addition
   to the extra MAC tag per packet.

   OverheadPerPacket = 11 + MAC length Overhead bps = PacketPerSecond *
   OverHeadPerPacket * 8

   Similar to SFrame, we will assume the HBH authentication tag length
   will always be 4 bytes for audio and video even though it is not the
   case in this [I-D.murillo-perc-lite] implementation

7.3.1.  Audio

      +===================+===================+====================+
      | Overhead bps@20ms | Overhead bps@40ms | Overhead bps@100ms |
      +===================+===================+====================+
      |        6000       |        3000       |        1200        |
      +-------------------+-------------------+--------------------+

                                 Table 4

7.3.2.  Video

   +=======================+====================+=====================+
   |   Overhead bps@30fps  | Overhead bps@15fps | Overhead bps@7.5fps |
   +=======================+====================+=====================+
   | (4 packets per frame) |   (2 packets per   |    (1 packet per    |
   |                       |       frame)       |        frame)       |
   +-----------------------+--------------------+---------------------+
   |         14400         |        7200        |         3600        |
   +-----------------------+--------------------+---------------------+

                                 Table 5

   For a conference with a single incoming audio stream (@ 50 pps) and 4
   incoming video streams (@200 Kbps), the savings in overhead is 34800
   - 9600 = ~25 Kbps, or ~3%.





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8.  Security Considerations

8.1.  No Per-Sender Authentication

   SFrame does not provide per-sender authentication of media data.  Any
   sender in a session can send media that will be associated with any
   other sender.  This is because SFrame uses symmetric encryption to
   protect media data, so that any receiver also has the keys required
   to encrypt packets for the sender.

8.2.  Key Management

   Key exchange mechanism is out of scope of this document, however
   every client MUST change their keys when new clients joins or leaves
   the call for "Forward Secrecy" and "Post Compromise Security".

8.3.  Authentication tag length

   The cipher suites defined in this draft use short authentication tags
   for encryption, however it can easily support other ciphers with full
   authentication tag if the short ones are proved insecure.

9.  IANA Considerations

   This document makes no requests of IANA.

10.  Test Vectors

   This section provides a set of test vectors that implementations can
   use to verify that they correctly implement SFrame encryption and
   decryption.  For each ciphersuite, we provide:

   *  [in] The "base_key" value (hex encoded)

   *  [out] The "secret", "key", and "salt" values derived from the
      "base_key" (hex encoded)

   *  A plaintext value that is encrypted in the following encryption
      cases

   *  A sequence of encryption cases, including:

      -  [in] The "KID" and "CTR" values to be included in the header

      -  [out] The resulting encoded header (hex encoded)

      -  [out] The nonce computed from the "salt" and "CTR" values




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      -  The ciphertext resulting from encrypting the plaintext with
         these parameters (hex encoded)

   An implementation should reproduce the output values given the input
   values: * An implementation should be able to encrypt with the input
   values and the plaintext to produce the ciphertext. * An
   implementation must be able to decrypt with the input values and the
   ciphertext to generate the plaintext.

   Line breaks and whitespace within values are inserted to conform to
   the width requirements of the RFC format.  They should be removed
   before use.  These test vectors are also available in JSON format at
   [TestVectors].

10.1.  AES_CM_128_HMAC_SHA256_4

   CipherSuite:    0x01
   Base Key:       101112131415161718191a1b1c1d1e1f
   Key:            343d3290f5c0b936415bea9a43c6f5a2
   Salt:           42d662fbad5cd81eb3aad79a
   Plaintext:      46726f6d2068656176656e6c79206861
                   726d6f6e79202f2f205468697320756e
                   6976657273616c206672616d65206265
                   67616e

   KID:            0x7
   CTR:            0x0
   Header:         1700
   Nonce:          42d662fbad5cd81eb3aad79a
   Ciphertext:     170065c67c6fb784631a7db1b589ffb6
                   2d75b78e28b0899e632fbbee3b944747
                   a6382d75b6bd3788dc7b71b9295c7fb9
                   0b5098f7add14ef329

   KID:            0x7
   CTR:            0x1
   Header:         1701
   Nonce:          42d662fbad5cd81eb3aad79b
   Ciphertext:     1701ec742e98d667be810f153ff0d4da
                   d7969f69b310aa7c6b9cb911e83af09b
                   0f0a6d74772d8195c8c9dae3878fd1cb
                   10edb4176d12e2387a









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   KID:            0x7
   CTR:            0x2
   Header:         1702
   Nonce:          42d662fbad5cd81eb3aad798
   Ciphertext:     1702ac9b495d37a1e48c712ade5cba72
                   df0bf90f24aa022a454cfb92d8b87cd5
                   4335fb6b9eeded6a5aa4e2643d7a0994
                   6646001d0a41b09557

   KID:            0xf
   CTR:            0xaa
   Header:         190faa
   Nonce:          42d662fbad5cd81eb3aad730
   Ciphertext:     190faaeaa5adc70cae0d6ebd36805fa8
                   7d2351dd02c55c751cd351a7fdb7f092
                   7b474eae3e800033e08100a440002da1
                   7579678b36dc275789d5

   KID:            0x1ff
   CTR:            0xaa
   Header:         1a01ffaa
   Nonce:          42d662fbad5cd81eb3aad730
   Ciphertext:     1a01ffaaeaa5adc70cae0d6ebd36805f
                   a87d2351dd02c55c751cd351a7fdb7f0
                   927b474eae3e800033e08100a440002d
                   a17579678b36dc9bbe558b

   KID:            0x1ff
   CTR:            0xaaaa
   Header:         2a01ffaaaa
   Nonce:          42d662fbad5cd81eb3aa7d30
   Ciphertext:     2a01ffaaaa170500225053f1a044e51c
                   4e91a6b783f69b1714fb31531d95d5b8
                   dd7926c2d43405b4f32b9b49dd6e0aa5
                   aba2427a94ff97f81dcd2826

   KID:            0xffffffffffffff
   CTR:            0xffffffffffffff
   Header:         7fffffffffffffffffffffffffffff
   Nonce:          42d662fbada327e14c552865
   Ciphertext:     7fffffffffffffffffffffffffffffdc
                   a3655d5117bc838d6f4382ca468a4f99
                   2ff77bfd1d2f4391be6b33e8fb638dc4
                   8aa82f57fd91430c714def0b2089c8bf
                   b2ac9da92415

10.2.  AES_CM_128_HMAC_SHA256_8




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   CipherSuite:    0x02
   Base Key:       202122232425262728292a2b2c2d2e2f
   Key:            3fce747d505e46ec9b92d9f58ee7a5d4
   Salt:           77fbf5f1d82c73f6d2b353c9
   Plaintext:      46726f6d2068656176656e6c79206861
                   726d6f6e79202f2f205468697320756e
                   6976657273616c206672616d65206265
                   67616e

   KID:            0x7
   CTR:            0x0
   Header:         1700
   Nonce:          77fbf5f1d82c73f6d2b353c9
   Ciphertext:     1700647513fce71aab7fed1e904fd924
                   0343d77092c831f0d58fde0985a0f3e5
                   ba4020e87a7b9c870b5f8f7f628d2769
                   0cc1e571e4d391da5fbf428433

   KID:            0x7
   CTR:            0x1
   Header:         1701
   Nonce:          77fbf5f1d82c73f6d2b353c8
   Ciphertext:     17019e1bdf713b0d4c02f3dbf50a72ea
                   773286e7da38f3872cc734f3e1b1448a
                   ab5009b424e05495214f96d02e4e8f8d
                   a975cc808f40f67cafead7cffd

   KID:            0x7
   CTR:            0x2
   Header:         1702
   Nonce:          77fbf5f1d82c73f6d2b353cb
   Ciphertext:     170220ad36fd9191453ace2d36a175ad
                   8a69c1f16b8613d14b4f7ef30c68bc56
                   09e349df38155cc1544d7dbfa079e3fa
                   ae3c7883b448e75047caafe05b

   KID:            0xf
   CTR:            0xaa
   Header:         190faa
   Nonce:          77fbf5f1d82c73f6d2b35363
   Ciphertext:     190faadab9b284a4b9e3aea36b9cdcae
                   4a58e141d3f0f52f240ef80a93dbb8d8
                   09ede01b05b2cace18a22fb39c032724
                   481c5baa181d6b793458355b0f30







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   KID:            0x1ff
   CTR:            0xaa
   Header:         1a01ffaa
   Nonce:          77fbf5f1d82c73f6d2b35363
   Ciphertext:     1a01ffaadab9b284a4b9e3aea36b9cdc
                   ae4a58e141d3f0f52f240ef80a93dbb8
                   d809ede01b05b2cace18a22fb39c0327
                   24481c5baa181dad5ad0f89a1cfb58

   KID:            0x1ff
   CTR:            0xaaaa
   Header:         2a01ffaaaa
   Nonce:          77fbf5f1d82c73f6d2b3f963
   Ciphertext:     2a01ffaaaae0f2384e4dc472cb92238b
                   5b722159205c4481665484de66985f15
                   5071655ca4e9d1c998781f8c7d439f8d
                   1eb6f6071cd80fd22f7e8846ba91036a

   KID:            0xffffffffffffff
   CTR:            0xffffffffffffff
   Header:         7fffffffffffffffffffffffffffff
   Nonce:          77fbf5f1d8d38c092d4cac36
   Ciphertext:     7fffffffffffffffffffffffffffff4b
                   8c7429d7ee83eec5e53808b80555b1f8
                   0b1df9d97877575fa1c7fa35b6119c68
                   ed6543020075959dcc4ca6900a7f9cf1
                   d936b640bba41ca62f6c

10.3.  AES_GCM_128_SHA256

   CipherSuite:    0x03
   Base Key:       303132333435363738393a3b3c3d3e3f
   Key:            2ea2e8163ff56c0613e6fa9f20a213da
   Salt:           a80478b3f6fba19983d540d5
   Plaintext:      46726f6d2068656176656e6c79206861
                   726d6f6e79202f2f205468697320756e
                   6976657273616c206672616d65206265
                   67616e

   KID:            0x7
   CTR:            0x0
   Header:         1700
   Nonce:          a80478b3f6fba19983d540d5
   Ciphertext:     17000e426255e47ed70dd7d15d69d759
                   bf459032ca15f5e8b2a91e7d348aa7c1
                   86d403f620801c495b1717a35097411a
                   a97cbb140671eb3b49ac3775926db74d
                   57b91e8e6c



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   KID:            0x7
   CTR:            0x1
   Header:         1701
   Nonce:          a80478b3f6fba19983d540d4
   Ciphertext:     170103bbafa34ada8a6b9f2066bc34a1
                   959d87384c9f4b1ce34fed58e938bde1
                   43393910b1aeb55b48d91d5b0db3ea67
                   e3d0e02b843afd41630c940b1948e72d
                   d45396a43a

   KID:            0x7
   CTR:            0x2
   Header:         1702
   Nonce:          a80478b3f6fba19983d540d7
   Ciphertext:     170258d58adebd8bf6f3cc0c1fcacf34
                   ba4d7a763b2683fe302a57f1be7f2a27
                   4bf81b2236995fec1203cadb146cd402
                   e1c52d5e6a10989dfe0f4116da1ee4c2
                   fad0d21f8f

   KID:            0xf
   CTR:            0xaa
   Header:         190faa
   Nonce:          a80478b3f6fba19983d5407f
   Ciphertext:     190faad0b1743bf5248f90869c945636
                   6d55724d16bbe08060875815565e90b1
                   14f9ccbdba192422b33848a1ae1e3bd2
                   66a001b2f5bb727112772e0072ea8679
                   ca1850cf11d8

   KID:            0x1ff
   CTR:            0xaa
   Header:         1a01ffaa
   Nonce:          a80478b3f6fba19983d5407f
   Ciphertext:     1a01ffaad0b1743bf5248f90869c9456
                   366d55724d16bbe08060875815565e90
                   b114f9ccbdba192422b33848a1ae1e3b
                   d266a001b2f5bbc9c63bd3973c19bd57
                   127f565380ed4a












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   KID:            0x1ff
   CTR:            0xaaaa
   Header:         2a01ffaaaa
   Nonce:          a80478b3f6fba19983d5ea7f
   Ciphertext:     2a01ffaaaa9de65e21e4f1ca2247b879
                   43c03c5cb7b182090e93d508dcfb76e0
                   8174c6397356e682d2eaddabc0b3c101
                   8d2c13c3570f61c1beaab805f27b565e
                   1329a823a7a649b6

   KID:            0xffffffffffffff
   CTR:            0xffffffffffffff
   Header:         7fffffffffffffffffffffffffffff
   Nonce:          a80478b3f6045e667c2abf2a
   Ciphertext:     7fffffffffffffffffffffffffffff09
                   981bdcdad80e380b6f74cf6afdbce946
                   839bedadd57578bfcd809dbcea535546
                   cc24660613d2761adea852155785011e
                   633534f4ecc3b8257c8d34321c27854a
                   1422

10.4.  AES_GCM_256_SHA512

   CipherSuite:    0x04
   Base Key:       404142434445464748494a4b4c4d4e4f
                   505152535455565758595a5b5c5d5e5f
   Key:            436774b0b5ae45633d96547f8f3cb06c
                   8e6628eff2e4255b5c4d77e721aa3355
   Salt:           31ed26f90a072e6aee646298
   Plaintext:      46726f6d2068656176656e6c79206861
                   726d6f6e79202f2f205468697320756e
                   6976657273616c206672616d65206265
                   67616e

   KID:            0x7
   CTR:            0x0
   Header:         1700
   Nonce:          31ed26f90a072e6aee646298
   Ciphertext:     1700f3e297c1e95207710bd31ccc4ba3
                   96fbef7b257440bde638ff0f3c891154
                   0136df61b26220249d6c432c245ae8d5
                   5ef45bfccf32530a15aeaaf313a03838
                   e51bd45652








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   KID:            0x7
   CTR:            0x1
   Header:         1701
   Nonce:          31ed26f90a072e6aee646299
   Ciphertext:     170193268b0bf030071bff443bb6b447
                   1bdfb1cc81bc9625f4697b0336ff4665
                   d15f152f02169448d8a967fb06359a87
                   d2145398de0ce3fbe257b0992a3da153
                   7590459f3c

   KID:            0x7
   CTR:            0x2
   Header:         1702
   Nonce:          31ed26f90a072e6aee64629a
   Ciphertext:     1702649691ba27c4c01a41280fba4657
                   c03fa7fe21c8f5c862e9094227c3ca3e
                   c0d9468b1a2cb060ff0978f25a24e6b1
                   06f5a6e1053c1b8f5fce794d88a0e481
                   8c081e18ea

   KID:            0xf
   CTR:            0xaa
   Header:         190faa
   Nonce:          31ed26f90a072e6aee646232
   Ciphertext:     190faa2858c10b5ddd231c1f26819490
                   521678603a050448d563c503b1fd890d
                   02ead01d754f074ecb6f32da9b2f3859
                   f380b4f47d4edd1e15f42f9a2d7ecfac
                   99067e238321

   KID:            0x1ff
   CTR:            0xaa
   Header:         1a01ffaa
   Nonce:          31ed26f90a072e6aee646232
   Ciphertext:     1a01ffaa2858c10b5ddd231c1f268194
                   90521678603a050448d563c503b1fd89
                   0d02ead01d754f074ecb6f32da9b2f38
                   59f380b4f47d4e3bf7040eb10ec25b81
                   26b2ce7b1d9d31












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   KID:            0x1ff
   CTR:            0xaaaa
   Header:         2a01ffaaaa
   Nonce:          31ed26f90a072e6aee64c832
   Ciphertext:     2a01ffaaaad9bc6a258a07d210a814d5
                   45eca70321c0e87498ada6e5c708b7ea
                   d162ffcf4fbaba1eb82650590a87122b
                   4d95fe36bd88b278812166d26e046ed0
                   a530b7ee232ee0f2

   KID:            0xffffffffffffff
   CTR:            0xffffffffffffff
   Header:         7fffffffffffffffffffffffffffff
   Nonce:          31ed26f90af8d195119b9d67
   Ciphertext:     7fffffffffffffffffffffffffffffaf
                   480d4779ce0c02b5137ee6a61e026c04
                   ac999cb0c97319feceeb258d58df23bc
                   e14979e5c67a431777b34498062e72f9
                   39ca42ec84ffbc7b50eff923f515a2df
                   760c

11.  References

11.1.  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>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [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>.

11.2.  Informative References

   [I-D.ietf-mls-architecture]
              Omara, E., Beurdouche, B., Rescorla, E., Inguva, S., Kwon,
              A., and A. Duric, "The Messaging Layer Security (MLS)



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              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-mls-architecture-06, 8 March 2021,
              <https://www.ietf.org/archive/id/draft-ietf-mls-
              architecture-06.txt>.

   [I-D.ietf-mls-protocol]
              Barnes, R., Beurdouche, B., Millican, J., Omara, E., Cohn-
              Gordon, K., and R. Robert, "The Messaging Layer Security
              (MLS) Protocol", Work in Progress, Internet-Draft, draft-
              ietf-mls-protocol-11, 22 December 2020,
              <https://www.ietf.org/archive/id/draft-ietf-mls-protocol-
              11.txt>.

   [I-D.murillo-perc-lite]
              Murillo, S. G. and A. Gouaillard, "End to End Media
              Encryption Procedures", Work in Progress, Internet-Draft,
              draft-murillo-perc-lite-01, 12 May 2020,
              <https://www.ietf.org/archive/id/draft-murillo-perc-lite-
              01.txt>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC8723]  Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
              "Double Encryption Procedures for the Secure Real-Time
              Transport Protocol (SRTP)", RFC 8723,
              DOI 10.17487/RFC8723, April 2020,
              <https://www.rfc-editor.org/info/rfc8723>.

   [TestVectors]
              "SFrame Test Vectors", 2021,
              <https://github.com/eomara/sframe/blob/master/test-
              vectors.json>.

Authors' Addresses

   Emad Omara
   Apple

   Email: eomara@apple.com


   Justin Uberti
   Google

   Email: juberti@google.com



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   Alexandre GOUAILLARD
   CoSMo Software

   Email: Alex.GOUAILLARD@cosmosoftware.io


   Sergio Garcia Murillo
   CoSMo Software

   Email: sergio.garcia.murillo@cosmosoftware.io









































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