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Versions: 00

Network Working Group                                           E. Omara
Internet-Draft                                                 J. Uberti
Intended status: Informational                                    Google
Expires: November 20, 2020                                 A. GOUAILLARD
                                                              S. Murillo
                                                          CoSMo Software
                                                            May 19, 2020


                         Secure Frame (SFrame)
                         draft-omara-sframe-00

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 November 20, 2020.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of



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   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 Derivation  . . . . . . . . . . . . . . . . . . .   8
       4.3.2.  Encryption  . . . . . . . . . . . . . . . . . . . . .   9
       4.3.3.  Decryption  . . . . . . . . . . . . . . . . . . . . .  10
       4.3.4.  Duplicate Frames  . . . . . . . . . . . . . . . . . .  11
       4.3.5.  Key Rotation  . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Authentication  . . . . . . . . . . . . . . . . . . . . .  12
     4.5.  Ciphersuites  . . . . . . . . . . . . . . . . . . . . . .  14
       4.5.1.  SFrame  . . . . . . . . . . . . . . . . . . . . . . .  14
       4.5.2.  DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Key Management  . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  MLS-SFrame  . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Media Considerations  . . . . . . . . . . . . . . . . . . . .  16
     6.1.  SFU . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
       6.1.1.  LastN and RTP stream reuse  . . . . . . . . . . . . .  16
       6.1.2.  Simulcast . . . . . . . . . . . . . . . . . . . . . .  16
       6.1.3.  SVC . . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  Video Key Frames  . . . . . . . . . . . . . . . . . . . .  17
     6.3.  Partial Decoding  . . . . . . . . . . . . . . . . . . . .  17
   7.  Overhead  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Audio . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.2.  Video . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.3.  SFrame vs PERC-lite . . . . . . . . . . . . . . . . . . .  18
       7.3.1.  Audio . . . . . . . . . . . . . . . . . . . . . . . .  19
       7.3.2.  Video . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  Key Management  . . . . . . . . . . . . . . . . . . . . .  19
     8.2.  Authentication tag length . . . . . . . . . . . . . . . .  19
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20



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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.

     +-------------------------------+-------------------------------+^+
     |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 +--+

                           SRTP packet format




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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:

   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.



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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, so 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
   +-+-+-+-+-+-+-+-+
   |S|LEN  |X|  K  |
   +-+-+-+-+-+-+-+-+
   SFrame header metadata

   Signature flag (S): 1 bit This field indicates the payload contains a
   signature if set.  Counter Length (LEN): 3 bits This field indicates
   the length of the CTR fields in bytes.  Extended Key Id Flag (X): 1



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   bit Indicates if the key field contains 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
   +-+-+-+-+-+-+-+-+---------------------------------+
   |S|LEN  |0| KID |    CTR... (length=LEN)          |
   +-+-+-+-+-+-+-+-+---------------------------------+

   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
+-+-+-+-+-+-+-+-+---------------------------+---------------------------+
|S|LEN  |1|KLEN |   KID... (length=KLEN)    |    CTR... (length=LEN)    |
+-+-+-+-+-+-+-+-+---------------------------+---------------------------+

   Key length (KLEN): 3 bits The key length in bytes.  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

4.3.1.  Key Derivation

   Each client creates a 32 bytes secret key K and share it with with
   other participants via an E2EE channel.  From K, we derive 3 secrets:

   1- Salt key used to calculate the IV

   Key = HKDF(K, 'SFrameSaltKey', 16)

   2- Encryption key to encrypt the media frame

   Key = HKDF(K, 'SFrameEncryptionKey', 16)

   3- Authentication key to authenticate the encrypted frame and the
   media metadata

   Key = HKDF(K, 'SFrameAuthenticationKey', 32)




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   The IV is 128 bits long and calculated from the CTR field of the
   Frame header:

   IV = CTR XOR Salt key

4.3.2.  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.  The encryptor constructs SFrame header using frame
   counter and key id and derive the encryption IV.  The frame is
   encrypted using the encryption key and the header, encrypted frame,
   the media metadata and the header are authenticated using the
   authentication key.  The authentication tag is then truncated (If
   supported by the cipher suite) and prepended at the end of the
   ciphertext.

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





























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                             +---------------+  +---------------+
                             |               |  | frame metadata+----+
                             |               |  +---------------+    |
                             |     frame     |                       |
                             |               |                       |
                             |               |                       |
                             +-------+-------+                       |
                                     |                               |
            CTR +---------------> IV |Enc Key <----Master Key        |
                   derive IV         |                  |            |
             +                       |                  |            |
             |                       +                  v            |
             |                    encrypt           Auth Key         |
             |                       |                  +            |
             |                       |                  |            |
             |                       v                  |            |
             |               +-------+-------+          |            |
             |               |               |          |            |
             |               |   encrypted   |          v            |
             |               |     frame     +---->Authenticate<-----+
             +               |               |          +
         encode CTR          |               |          |
             +               +-------+-------+          |
             |                       |                  |
             |                       |                  |
             |                       |                  |
             |              generic RTP packetize       |
             |                       +                  |
             |                       |                  |
             |                       |                  +--------------+
  +----------+                       v                                 |
  |                                                                    |
  |   +---------------+      +---------------+     +---------------+   |
  +-> | SFrame header |      |               |     |               |   |
      +---------------+      |               |     |  payload N/N  |   |
      |               |      |  payload 2/N  |     |               |   |
      |  payload 1/N  |      |               |     +---------------+   |
      |               |      |               |     |    auth tag   | <-+
      +---------------+      +---------------+     +---------------+
                           Encryption flow

4.3.3.  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 Frame for decryption.  SFrame maintains multiple decryptor
   objects, one for each client in the call.  Initially the client might



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   not have the mapping between the incoming streams the user's keys, in
   this case SFrame tries all unmapped keys until it finds one that
   passes the authentication verification and use it to decrypt the
   frame.  If the client has the mapping ready, it can push it down to
   SFrame later.

   The KeyId field in the SFrame header is used to find the right key
   for that user, which is incremented by the sender when they switch to
   a new key.

   For frames that are failed to decrypt because there is not key
   available yet, SFrame will buffer them and retries to decrypt them
   once a key is received.

4.3.4.  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.

4.3.5.  Key Rotation

   Because the E2EE keys could be rotated during the call when people
   join and leave, these new keys are exchanged using the same E2EE
   secure channel used in the initial key negotiation.  Sending new
   fresh keys is an expensive operation, so the key management component
   might chose to send new keys only when other clients leave the call
   and use hash ratcheting for the join case, so no need to send a new
   key to the clients who are already on the call.  SFrame supports both
   modes

4.3.5.1.  Key Ratcheting

   When SFrame decryptor fails to decrypt one of the frames, it
   automatically ratchets the key forward and retries again until one
   ratchet succeed or it reaches the maximum allowed ratcheting window.
   If a new ratchet passed the decryption, all previous ratchets are
   deleted.

   K(i) = HKDF(K(i-1), 'SFrameRatchetKey', 32)







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4.3.5.2.  New Key

   SFrame will set the key immediately on the decrypts when it is
   received and destroys the old key material, so if the key manager
   sends a new key during the call, it is recommended not to start using
   it immediately and wait for a short time to make sure it is delivered
   to all other clients before using it to decrease the number of
   decryption failure.  It is up to the application and the key manager
   to define how long this period is.

4.4.  Authentication

   Every client in the call knows the secret key for all other clients
   so it can decrypt their traffic, it also means a malicious client can
   impersonate any other client in the call by using the victim key to
   encrypt their traffic.  This might not be a problem for consumer
   application where the number of clients in the call is small and
   users know each others, however for enterprise use case where large
   conference calls are common, an authentication mechanism is needed to
   protect against malicious users.  This authentication will come with
   extra cost.

   Adding a digital signature to each encrypted frame will be an
   overkill, instead we propose adding signature over multiple frames.

   The signature is calculated by concatenating the authentication tags
   of the frames that the sender wants to authenticate (in reverse sent
   order) and signing it with the signature key.  Signature keys are
   exchanged out of band along the encryption keys.

Signature = Sign(Key, AuthTag(Frame N) || AuthTag(Frame N-1) || ...|| AuthTag(Frame N-M))

   The authentication tags for the previous frames covered by the
   signature and the signature itself will be appended at end of the
   frame, after the current frame authentication tag, in the same order
   that the signature was calculated, and the SFrame header metadata
   signature bit (S) will be set to 1.














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       +^ +------------------+
       |  | SFrame header S=1|
       |  +------------------+
       |  |  Encrypted       |
       |  |  payload         |
       |  |                  |
       |^ +------------------+ ^+
       |  |  Auth Tag N      |  |
       |  +------------------+  |
       |  |  Auth Tag N-1    |  |
       |  +------------------+  |
       |  |    ........      |  |
       |  +------------------+  |
       |  |  Auth Tag N-M    |  |
       |  +------------------+ ^|
       |  | NUM | Signature  :  |
       |  +-----+            +  |
       |  :                  |  |
       |  +------------------+  |
       |                        |
       +-> Authenticated with   +-> Signed with
           Auth Tag N               Signature


       Encrypted Frame with Signature


   Note that the authentication tag for the current frame will only
   authenticate the SFrame header and the encrypted payload, ant not the
   signature nor the previous frames's authentication tags (N-1 to N-M)
   used to calculate the signature.

   The last byte (NUM) after the authentication tag list and before the
   signature indicates the number of the authentication tags from
   previous frames present in the current frame.  All the
   authentications tags MUST have the same size, which MUST be equal to
   the authentication tag size of the current frame.  The signature is
   fixed size depending on the signature algorithm used (for example, 64
   bytes for Ed25519).

   The receiver has to keep track of all the frames received but yet not
   verified, by storing the authentication tags of each received frame.
   When a signature is received, the receiver will verify it with the
   signature key associated to the key id of the frame the signature was
   sent in.  If the verification is successful, the received will mark
   the frames as authenticated and remove them from the list of the not
   verified frames.  It is up to the application to decide what to do
   when signature verification fails.



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   When using SVC, the hash will be calculated over all the frames of
   the different spatial layers within the same superframe/picture.
   However the SFU will be able to drop frames within the same stream
   (either spatial or temporal) to match target bitrate.

   If the signature is sent on a frame which layer that is dropped by
   the SFU, the receiver will not receive it and will not be able to
   perform the signature of the other received layers.

   An easy way of solving the issue would be to perform signature only
   on the base layer or take into consideration the frame dependency
   graph and send multiple signatures in parallel (each for a branch of
   the dependency graph).

   In case of simulcast or K-SVC, each spatial layer should be
   authenticated with different signatures to prevent the SFU to discard
   frames with the signature info.

   In any case, it is possible that the frame with the signature is lost
   or the SFU drops it, so the receiver MUST be prepared to not receive
   a signature for a frame and remove it from the pending to be verified
   list after a timeout.

4.5.  Ciphersuites

4.5.1.  SFrame

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

   o A hash function This is used for the Key derivation and frame
   hashes for signature.  We recommend using SHA256 hash function.

   o An AEAD encryption algorithm [RFC5116] While any AEAD algorithm can
   be used to encrypt the frame, we recommend using algorithms with safe
   MAC truncation like AES-CTR and HMAC to reduce the per-frame
   overhead.  In this case we can use 80 bits MAC for video frames and
   32 bits for audio frames similar to DTLS-SRTP cipher suites:

   1- AES_CM_128_HMAC_SHA256_80

   2- AES_CM_128_HMAC_SHA256_32

   o [Optional] A signature algorithm If signature is supported, we
   recommend using ed25519






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4.5.2.  DTLS-SRTP

   SRTP is used as an HBH encryption, since the media payload is already
   encrypted, and SRTP only protects the RTP headers, one implementation
   could use 4 bytes outer auth tag to decrease the overhead, however it
   is up to the application to use other ciphers like AES-128-GCM with
   full authentication tag.

5.  Key Management

   SFrame must be integrated with an E2EE key management framework to
   exchange and rotate the encryption keys.  This framework will
   maintain a group of participant endpoints who are in the call.  At
   call setup time, each endpoint will create a fresh key material and
   optionally signing key pair for that call and encrypt the key
   material and the public signing key to every other endpoints.  They
   encrypted keys are delivered by the messaging delivery server using a
   reliable channel.

   The KMS will monitor the group changes, and exchange new keys when
   necessary.  It is up to the application to define this group, for
   example one application could have ephemeral group for every call and
   keep rotating key when end points joins or leave the call, while
   another application could have a persisted group that can be used for
   multiple calls and exchange keys with all group endpoints for every
   call.

   When a new key material is created during the call, we recommend not
   to start using it immediately in SFrame to give time for the new keys
   to be delivered.  If the application supports delivery receipts, it
   can be used to track if the key is delivered to all other endpoints
   on the call before using it.

   Keys must have a sequential id starting from 0 and incremented eery
   time a new key is generated for this endpoint.  The key id will be
   added in the SFrame header during encryption, so the recipient know
   which key to use for the decryption.

5.1.  MLS-SFrame

   While any other E2EE KMS can be used with SFrame, there is a big
   advantage if it is used with [MLSARCH] which natively supports very
   large groups efficiently.  When [MLSPROTO] is used, the endpoints
   keys (AKA Application secret) can be used directly for SFrame without
   the need to exchange separate key material.  The application secret
   is rotated automatically by [MLSPROTO] when group membership changes.





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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.

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



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   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.

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.





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   +------------+-----------+-----------+----------+-----------+
   | 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 |
   +------------+--------- -+-----------+----------+-----------+

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 |
   +------------+-----------+------------+------------+------------+

7.3.  SFrame vs PERC-lite

   [PERC] 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.
   [PERCLITE] <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 [PERCLITE] 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 [PERCLITE] implementation





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7.3.1.  Audio

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

7.3.2.  Video

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

   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%.

8.  Security Considerations

8.1.  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".

8.2.  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.  References

10.1.  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>.




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   [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>.

10.2.  Informative References

   [MLSARCH]  Omara, E., Barnes, R., Rescorla, E., Inguva, S., Kwon, A.,
              and A. Duric, "Messaging Layer Security Architecture",
              2020.

   [MLSPROTO]
              Barnes, R., Millican, J., Omara, E., Cohn-Gordon, K., and
              R. Robert, "Messaging Layer Security Protocol", 2020.

   [PERC]     Jennings, C., Jones, P., Barnes, R., and A. Roach, "PERC",
              2020, <https://datatracker.ietf.org/doc/rfc8723/>.

   [PERCLITE]
              GOUAILLARD, A. and S. Murillo, "PERC-Lite", 2020,
              <https://tools.ietf.org/html/draft-murillo-perc-lite-01>.

Authors' Addresses

   Emad Omara
   Google

   Email: emadomara@google.com


   Justin Uberti
   Google

   Email: juberti@google.com


   Alexandre GOUAILLARD
   CoSMo Software

   Email: Alex.GOUAILLARD@cosmosoftware.io


   Sergio Garcia Murillo
   CoSMo Software

   Email: sergio.garcia.murillo@cosmosoftware.io






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