Network Working Group                                          R. Barnes
Internet-Draft                                                     Cisco
Intended status: Informational                                 R. Robert
Expires: 27 April 2023                                                 -
                                                           S. Nandakumar
                                                         24 October 2022

    Using Messaging Layer Security (MLS) to Provide Keys for SFrame


   Secure Frames (SFrame) defines a compact scheme for encrypting real-
   time media.  In order for SFrame to address cases where media are
   exchanged among many participants (e.g., real-time conferencing), it
   needs to be augmented with a group key management protocol.  The
   Messaging Layer Security (MLS) protocol provides continuous group
   authenticated key exchange, allowing a group of participants in a
   media session to authenticate each other and agree on a group key.
   This document defines how the group keys produced by MLS can be used
   with SFrame to secure real-time sessions for groups.

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Table of Contents

   1.  Introduction
   2.  SFrame Parameter Negotiation
     2.1.  SFrame Parameter Selection
   3.  SFrame Key Management
     3.1.  Multiple Sender Contexts
   4.  Security Considerations
   5.  IANA Considerations
   6.  Normative References
   Appendix A.  Acknowledgements
   Authors' Addresses

1.  Introduction

   Secure Frames (SFrame) defines a compact scheme for encrypting real-
   time media [I-D.omara-sframe].  In order for SFrame to address cases
   where media are exchanged among many participants (e.g., real-time
   conferencing), it needs to be augmented with a group key management
   protocol.  The Messaging Layer Security (MLS) protocol
   [I-D.ietf-mls-protocol] provides continuous group authenticated key
   exchange.  MLS provides several important security properties

   *  Group Key Exchange: All members of the group at a given time know
      a secret key that is inaccessible to parties outside the group.

   *  Authentication of group members: Each member of the group can
      authenticate the other members of the group.

   *  Group Agreement: The members of the group all agree on the
      identities of the participants in the group.

   *  Forward Secrecy: There are protocol events such that if a member's
      state is compromised after the event, group secrets created before
      the event are safe.

   *  Post-compromise Security: There are protocol events such that if a
      member's state is compromised before the event, the group secrets
      created after the event are safe.

   When a real-time session uses MLS as the basis for SFrame keys, these
   security properties apply to real-time media as well.  In the
   remainder of this document, we define how to use the secrets produced
   by MLS to generate the keys required by SFrame.

2.  SFrame Parameter Negotiation

   In order to interoperate, the sender and receiver(s) of an SFrame
   payload need to agree on two parameters:

   *  The SFrame ciphersuite

   *  The number of bits E used to signal the epoch

   These parameters can be negotiated in MLS using the sframe_parameters
   extension.  An MLS participant advertises its supported ciphersuites
   in its KeyPackage.  The creator of the group chooses the values of
   these parameters for the group (possibly based on a set of
   KeyPackages) and advises them to new joiners in Welcome messages.

   ``` uint16 SFrameCipherSuite;

   struct { SFrameCipherSuite cipher_suites<0..255>; }

   struct { SFrameCipherSuite cipher_suite; uint8 epoch_bits; }
   SFrameParameters; ```

   The values allowed for SFrameCipherSuite are defined in
   [I-D.omara-sframe] and the IANA registries it references.

   When an extension of type sframe_parameters appears in an MLS
   KeyPackage, the extension data field MUST contain an
   SFrameCapabilities object.  When such an extension appears in a
   Welcome message, it MUST contain an SFrameParameters object.  The
   ciphersuite values MUST represent valid SFrame ciphersuites.

   The SFrameParameters object for a group, if present, MUST be included
   in the GroupContext for the group, as an extension of type
   sframe_parameters.  This ensures that the members of the group agree
   on the SFrame parameters associated to the group.

2.1.  SFrame Parameter Selection

   Just as with MLS ciphersuite selection, the creator of an MLS group
   chooses the SFrame parameters to be used for the group.  The
   parameters are then fixed for the lifetime of the group.

   The creator of the group needs to choose a ciphersuite and an
   epoch_bits value.  The ciphersuite SHOULD be chosen from among those
   supported by the members of the group, as expressed by those members'
   key packages.  Members that don't support the chosen ciphersuite will
   not be able to send or receive SFrame-encrypted media.

   As discussed below, the epoch_bits field effectively bounds the rate
   at which the epoch can change, at the cost of possible growth in the
   KID field.  Applications SHOULD NOT use epoch_bits = 0, unless they
   have an external signal for which epoch's keys are in use.
   Otherwise, applications should choose a value for epoch_bits such
   that they expect to never have more than 2^epoch_bits epochs active
   at once.  That is, by the time the key for epoch k + 2^epoch_bits is
   distributed, all senders should have stopped sending with epoch k.

3.  SFrame Key Management

   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.

   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, 8), AEAD.Nk)

   [[ OPEN ISSUE: MLS has its own "secret tree" that provides better
   forward secrecy properties within an epoch.  (This scheme provides
   none.)  An alternative approach would be to re-use the MLS secret
   tree, either directly or as a data structure. ]]

   The Key ID (KID) field in the SFrame header provides the epoch and
   index values that are needed to generate the appropriate key from the
   MLS key schedule.

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

   For compactness, do not send the whole epoch number.  Instead, we
   send only its low-order E bits.  The participants in the group MUST
   agree on the value of E for a given session, through some negotiation
   not specified here.

   Note that E effectively defines a re-ordering window, since no more
   than 2^E epoch can be active at a given time.  The better the
   participants are in sync with regard to key roll-over, and the less
   reordering of SFrame-protected payloads by the network, the fewer
   bits of epoch are necessary.

   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

   [[ OPEN ISSUE: There might be some considerations for new joiners.
   Some trial decryption might be necessary to detect whether you're in
   epoch N or in epoch N + 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

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

3.1.  Multiple Sender Contexts

   Real-time media systems often have strong decoupling between media
   sources after initial setup, including things like providing separate
   SRTP contexts for different media sources.  In SRTP, this is safe
   because the RTP SSRC is included in the nonce computation.  With the
   system defined here, however, the key and nonce only depend on the
   secret and the sender ID in the KID, so having multiple contexts with
   the same secret and KID leads to nonce reuse.

   In order to support this situation, a sender MUST choose to use the
   high-order bits of the KID field to provide an additional context
   distinguisher.  These bits will be interpreted as part of the sender
   index by receivers, so different keys and independent nonce streams
   will be derived per context.

   A sender sending context IDs in this way MUST reserve S bits for the
   sender ID, such that any sender index in the group can be expressed
   in S bits.  That is, it MUST be the case that:

   group_size - 1 < (1 << S)

   Thus, for a group of a given size, the sender may use up to 64 - S -
   E bits of the KID field to provide additional context, at the expense
   of having to transmit a longer KID field.  The resulting KID value
   has the following form:

   S bits E bits <------> <------>
   +--------+------------+--------+-------+ | 000... | Context ID |
   Index | Epoch | +--------+------------+--------+-------+

4.  Security Considerations

   The security properties provided by MLS are discussed in detail in
   [I-D.ietf-mls-architecture] and [I-D.ietf-mls-protocol].  This
   document extends those guarantees to SFrame.

   It should be noted that the per-sender keys derived here do not
   provide per-sender authentication, since any member of the group
   could derive the same keys (as indeed they must in order to decrypt
   the protected payload).  Per-sender keys are derived only to avoid
   nonce collision among multiple unsynchronized senders.  So the
   authentication limitations of SFrame remain: There is per-sender
   authentication only when signatures are used.  Otherwise, SFrame only
   authenticates membership in the group, and members are free to
   impersonate each other.

   The Forward Secrecy and Post-compromise Security guarantees provided
   by an MLS group extend to a group of real time session participants,
   as long as all members of the MLS group are participants in the
   session.  It is recommended to keep the membership of the MLS group
   as tight as possible, i.e. members should only be added once they
   become session participants and evicted as soon as they drop off the
   session.  If the application already uses MLS groups that are more
   long term (e.g. chat groups), it is recommended to set up a new
   ephemeral MLS group for the session by using the sub-group branching
   mechanism provided by the MLS protocol to link the two groups

5.  IANA Considerations

   This document requests that IANA add an entry to the MLS Extension
   Types registry, with the following values:

   | Value | Name              | Message(s) | Recommended | Reference |
   | TBD   | sframe_parameters | KP, GI     | Y           | RFC XXXX  |

                                 Table 1

   RFC EDITOR: Please replace XXXX throughout with the RFC number
   assigned to this document.

6.  Normative References

              Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., Kwon,
              A., and A. Duric, "The Messaging Layer Security (MLS)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-mls-architecture-09, 19 August 2022,

              Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
              Omara, E., and K. Cohn-Gordon, "The Messaging Layer
              Security (MLS) Protocol", Work in Progress, Internet-
              Draft, draft-ietf-mls-protocol-16, 11 July 2022,

              Omara, E., Uberti, J., Gouaillard, A., and S. G. Murillo,
              "Secure Frame (SFrame)", Work in Progress, Internet-Draft,
              draft-omara-sframe-03, 17 September 2021,

Appendix A.  Acknowledgements


Authors' Addresses

   Richard Barnes

   Raphael Robert

   Suhas Nandakumar