



Internet Engineering Task Force (IETF)                          E. Omara
Request for Comments: 9605                                         Apple
Category: Standards Track                                      J. Uberti
ISSN: 2070-1721                                                 Fixie.ai
                                                           S. G. Murillo
                                                          CoSMo Software
                                                          R. Barnes, Ed.
                                                                   Cisco
                                                               Y. Fablet
                                                                   Apple
                                                             August 2024


    Secure Frame (SFrame): Lightweight Authenticated Encryption for
                            Real-Time Media

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 (Selective
   Forwarding Units or SFUs) can access the media metadata needed to
   make forwarding decisions without having access to the actual media.

   This mechanism differs from the Secure Real-Time Protocol (SRTP) in
   that it is independent of RTP (thus compatible with non-RTP media
   transport) and can be applied to whole media frames in order to be
   more bandwidth efficient.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9605.

Copyright Notice

   Copyright (c) 2024 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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Goals
   4.  SFrame
     4.1.  Application Context
     4.2.  SFrame Ciphertext
     4.3.  SFrame Header
     4.4.  Encryption Schema
       4.4.1.  Key Selection
       4.4.2.  Key Derivation
       4.4.3.  Encryption
       4.4.4.  Decryption
     4.5.  Cipher Suites
       4.5.1.  AES-CTR with SHA2
   5.  Key Management
     5.1.  Sender Keys
     5.2.  MLS
   6.  Media Considerations
     6.1.  Selective Forwarding Units
       6.1.1.  RTP Stream Reuse
       6.1.2.  Simulcast
       6.1.3.  Scalable Video Coding (SVC)
     6.2.  Video Key Frames
     6.3.  Partial Decoding
   7.  Security Considerations
     7.1.  No Header Confidentiality
     7.2.  No Per-Sender Authentication
     7.3.  Key Management
     7.4.  Replay
     7.5.  Risks Due to Short Tags
   8.  IANA Considerations
     8.1.  SFrame Cipher Suites
   9.  Application Responsibilities
     9.1.  Header Value Uniqueness
     9.2.  Key Management Framework
     9.3.  Anti-Replay
     9.4.  Metadata
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  Example API
   Appendix B.  Overhead Analysis
     B.1.  Assumptions
     B.2.  Audio
     B.3.  Video
     B.4.  Conferences
     B.5.  SFrame over RTP
   Appendix C.  Test Vectors
     C.1.  Header Encoding/Decoding
     C.2.  AEAD Encryption/Decryption Using AES-CTR and HMAC
     C.3.  SFrame Encryption/Decryption
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Modern multiparty video call systems use Selective Forwarding Unit
   (SFU) servers to efficiently route media streams to call endpoints
   based on factors such as available bandwidth, desired video size,
   codec support, and other factors.  An SFU typically does not need
   access to the media content of the conference, which allows the media
   to be encrypted "end to end" so that it cannot be decrypted by the
   SFU.  In order for the SFU to work properly, though, it usually 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 encryption and authentication are required:

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

   2.  End-to-end (E2E) encryption (E2EE) of media between the endpoints

   The Secure Real-Time Protocol (SRTP) is already widely used for HBH
   encryption [RFC3711].  The SRTP "double encryption" scheme defines a
   way to do E2E encryption in SRTP [RFC8723].  Unfortunately, this
   scheme has poor efficiency and high complexity, and its entanglement
   with RTP makes it unworkable in several realistic SFU scenarios.

   This document proposes a new E2EE protection scheme known as SFrame,
   specifically designed to work in group conference calls with SFUs.
   SFrame is a general encryption framing that can be used to protect
   media payloads, agnostic of transport.

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.

   MAC:  Message Authentication Code

   E2EE:  End-to-End Encryption

   HBH:  Hop-by-Hop

   We use "Selective Forwarding Unit (SFU)" and "media stream" in a less
   formal sense than in [RFC7656].  An SFU is a selective switching
   function for media payloads, and a media stream is a sequence of
   media payloads, regardless of whether those media payloads are
   transported over RTP or some other protocol.

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 a 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 key management system.

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

   4.  Decouple the media encryption framework from the underlying
       transport, allowing use in non-RTP scenarios, e.g., WebTransport
       [WEBTRANSPORT].

   5.  When used with RTP and its associated error-resilience
       mechanisms, i.e., RTX and Forward Error Correction (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

   This document defines an encryption mechanism that provides effective
   E2EE, is simple to implement, has no dependencies on RTP, and
   minimizes encryption bandwidth overhead.  This section describes how
   the mechanism works and includes details of how applications utilize
   SFrame for media protection as well as the actual mechanics of E2EE
   for protecting media.

4.1.  Application Context

   SFrame is a general encryption framing, intended to be used as an
   E2EE layer over an underlying HBH-encrypted transport such as SRTP or
   QUIC [RFC3711][MOQ-TRANSPORT].

   The scale at which SFrame encryption is applied to media determines
   the overall amount of overhead that SFrame adds to the media stream
   as well as the engineering complexity involved in integrating SFrame
   into a particular environment.  Two patterns are common: using SFrame
   to encrypt either whole media frames (per frame) or individual
   transport-level media payloads (per packet).

   For example, Figure 1 shows a typical media sender stack that takes
   media from some source, encodes it into frames, divides those frames
   into media packets, and then sends those payloads in SRTP packets.
   The receiver stack performs the reverse operations, reassembling
   frames from SRTP packets and decoding.  Arrows indicate two different
   ways that SFrame protection could be integrated into this media
   stack: to encrypt whole frames or individual media packets.

   Applying SFrame per frame in this system offers higher efficiency but
   may require a more complex integration in environments where
   depacketization relies on the content of media packets.  Applying
   SFrame per packet avoids this complexity at the cost of higher
   bandwidth consumption.  Some quantitative discussion of these trade-
   offs is provided in Appendix B.

   As noted above, however, SFrame is a general media encapsulation and
   can be applied in other scenarios.  The important thing is that the
   sender and receivers of an SFrame-encrypted object agree on that
   object's semantics.  SFrame does not provide this agreement; it must
   be arranged by the application.

      +------------------------------------------------------+
      |                                                      |
      |  +--------+      +-------------+      +-----------+  |
 .-.  |  |        |      |             |      |    HBH    |  |
|   | |  | Encode |----->|  Packetize  |----->|  Protect  |----------+
 '+'  |  |        |   ^  |             |  ^   |           |  |       |
 /|\  |  +--------+   |  +-------------+  |   +-----------+  |       |
/ + \ |               |                   |         ^        |       |
 / \  |            SFrame              SFrame       |        |       |
/   \ |            Protect             Protect      |        |       |
Alice |          (per frame)         (per packet)   |        |       |
      |               ^                   ^         |        |       |
      |               |                   |         |        |       |
      +---------------|-------------------|---------|--------+       |
                      |                   |         |                v
                      |                   |         |         +------+-+
                      |      E2E Key      |       HBH Key     | Media  |
                      +---- Management ---+      Management   | Server |
                      |                   |         |         +------+-+
                      |                   |         |                |
      +---------------|-------------------|---------|--------+       |
      |               |                   |         |        |       |
      |               V                   V         |        |       |
 .-.  |            SFrame               SFrame      |        |       |
|   | |           Unprotect            Unprotect    |        |       |
 '+'  |          (per frame)          (per packet)  |        |       |
 /|\  |               |                    |        V        |       |
/ + \ |  +--------+   |  +-------------+   |  +-----------+  |       |
 / \  |  |        |   V  |             |   V  |    HBH    |  |       |
/   \ |  | Decode |<-----| Depacketize |<-----| Unprotect |<---------+
 Bob  |  |        |      |             |      |           |  |
      |  +--------+      +-------------+      +-----------+  |
      |                                                      |
      +------------------------------------------------------+

Figure 1: Two Options for Integrating SFrame in a Typical Media Stack

   Like SRTP, SFrame does not define how the keys used for SFrame are
   exchanged by the parties in the conference.  Keys for SFrame might be
   distributed over an existing E2E-secure channel (see Section 5.1) or
   derived from an E2E-secure shared secret (see Section 5.2).  The key
   management system MUST ensure that each key used for encrypting media
   is used by exactly one media sender in order to avoid reuse of
   nonces.

4.2.  SFrame Ciphertext

   An SFrame ciphertext comprises an SFrame header followed by the
   output of an Authenticated Encryption with Associated Data (AEAD)
   encryption of the plaintext [RFC5116], with the header provided as
   additional authenticated data (AAD).

   The SFrame header is a variable-length structure described in detail
   in Section 4.3.  The structure of the encrypted data and
   authentication tag are determined by the AEAD algorithm in use.

      +-+----+-+----+--------------------+--------------------+<-+
      |K|KLEN|C|CLEN|       Key ID       |      Counter       |  |
   +->+-+----+-+----+--------------------+--------------------+  |
   |  |                                                       |  |
   |  |                                                       |  |
   |  |                                                       |  |
   |  |                                                       |  |
   |  |                   Encrypted Data                      |  |
   |  |                                                       |  |
   |  |                                                       |  |
   |  |                                                       |  |
   |  |                                                       |  |
   +->+-------------------------------------------------------+<-+
   |  |                 Authentication Tag                    |  |
   |  +-------------------------------------------------------+  |
   |                                                             |
   |                                                             |
   +--- Encrypted Portion               Authenticated Portion ---+

                Figure 2: Structure of an SFrame Ciphertext

   When SFrame is applied per packet, the payload of each packet will be
   an SFrame ciphertext.  When SFrame is applied per frame, the SFrame
   ciphertext representing an encrypted frame will span several packets,
   with the header appearing in the first packet and the authentication
   tag in the last packet.  It is the responsibility of the application
   to reassemble an encrypted frame from individual packets, accounting
   for packet loss and reordering as necessary.

4.3.  SFrame Header

   The SFrame header specifies two values from which encryption
   parameters are derived:

   *  A Key ID (KID) that determines which encryption key should be used

   *  A Counter (CTR) that is used to construct the nonce for the
      encryption

   Applications MUST ensure that each (KID, CTR) combination is used for
   exactly one SFrame encryption operation.  A typical approach to
   achieve this guarantee is outlined in Section 9.1.

      Config Byte
           |
    .-----' '-----.
   |               |
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+------------+------------+
   |X|  K  |Y|  C  |   KID...   |   CTR...   |
   +-+-+-+-+-+-+-+-+------------+------------+

                          Figure 3: SFrame Header

   The SFrame header has the overall structure shown in Figure 3.  The
   first byte is a "config byte", with the following fields:

   Extended KID Flag (X, 1 bit):  Indicates if the K field contains the
      KID or the KID length.

   KID or KID Length (K, 3 bits):  If the X flag is set to 0, this field
      contains the KID.  If the X flag is set to 1, then it contains the
      length of the KID, minus one.

   Extended CTR Flag (Y, 1 bit):  Indicates if the C field contains the
      CTR or the CTR length.

   CTR or CTR Length (C, 3 bits):  This field contains the CTR if the Y
      flag is set to 0, or the CTR length, minus one, if set to 1.

   The KID and CTR fields are encoded as compact unsigned integers in
   network (big-endian) byte order.  If the value of one of these fields
   is in the range 0-7, then the value is carried in the corresponding
   bits of the config byte (K or C) and the corresponding flag (X or Y)
   is set to zero.  Otherwise, the value MUST be encoded with the
   minimum number of bytes required and appended after the config byte,
   with the KID first and CTR second.  The header field (K or C) is set
   to the number of bytes in the encoded value, minus one.  The value
   000 represents a length of 1, 001 a length of 2, etc.  This allows a
   3-bit length field to represent the value lengths 1-8.

   The SFrame header can thus take one of the four forms shown in
   Figure 4, depending on which of the X and Y flags are set.

   KID < 8, CTR < 8:
   +-+-----+-+-----+
   |0| KID |0| CTR |
   +-+-----+-+-----+

   KID < 8, CTR >= 8:
   +-+-----+-+-----+------------------------+
   |0| KID |1|CLEN |  CTR... (length=CLEN)  |
   +-+-----+-+-----+------------------------+

   KID >= 8, CTR < 8:
   +-+-----+-+-----+------------------------+
   |1|KLEN |0| CTR |  KID... (length=KLEN)  |
   +-+-----+-+-----+------------------------+

   KID >= 8, CTR >= 8:
   +-+-----+-+-----+------------------------+------------------------+
   |1|KLEN |1|CLEN |  KID... (length=KLEN)  |  CTR... (length=CLEN)  |
   +-+-----+-+-----+------------------------+------------------------+

                  Figure 4: Forms of Encoded SFrame Header

4.4.  Encryption Schema

   SFrame encryption uses an AEAD encryption algorithm and hash function
   defined by the cipher suite in use (see Section 4.5).  We will refer
   to the following aspects of the AEAD and the hash 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 in bytes of a key for the encryption algorithm

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

   *  AEAD.Nt - The overhead in bytes of the encryption algorithm
      (typically the size of a "tag" that is added to the plaintext)

   *  AEAD.Nka - For cipher suites using the compound AEAD described in
      Section 4.5.1, the size in bytes of a key for the underlying
      encryption algorithm

   *  Hash.Nh - The size in bytes of the output of the hash function

4.4.1.  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 base_key should be
   used for a given KID.  Moreover, senders and receivers need to agree
   on whether a base_key will be used for encryption or decryption only.
   The process for provisioning base_key values 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 need to 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 CTR value to be used when encrypting
   (initially 0).

   When encrypting a plaintext, the application specifies which KID is
   to be used, and the CTR value 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 base_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 MUST mark each base_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 keys for encryption until (a) all receivers have
   acknowledged receipt of the new key, or (b) a timeout expires.

4.4.2.  Key Derivation

   SFrame encryption and decryption use a key and salt derived from the
   base_key associated with a KID.  Given a base_key value, the key and
   salt are derived using HMAC-based Key Derivation Function (HKDF)
   [RFC5869] as follows:

   def derive_key_salt(KID, base_key):
     sframe_secret = HKDF-Extract("", base_key)

     sframe_key_label = "SFrame 1.0 Secret key " + KID + cipher_suite
     sframe_key =
       HKDF-Expand(sframe_secret, sframe_key_label, AEAD.Nk)

     sframe_salt_label = "SFrame 1.0 Secret salt " + KID + cipher_suite
     sframe_salt =
       HKDF-Expand(sframe_secret, sframe_salt_label, AEAD.Nn)

     return sframe_key, sframe_salt

   In the derivation of sframe_secret:

   *  The + operator represents concatenation of byte strings.

   *  The KID value is encoded as an 8-byte big-endian integer, not the
      compressed form used in the SFrame header.

   *  The cipher_suite value is a 2-byte big-endian integer representing
      the cipher suite in use (see Section 8.1).

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

4.4.3.  Encryption

   SFrame encryption uses the AEAD encryption algorithm for the cipher
   suite in use.  The key for the encryption is the sframe_key.  The
   nonce is formed by first XORing the sframe_salt with the current CTR
   value, and then encoding the result as a big-endian integer of length
   AEAD.Nn.

   The encryptor forms an SFrame header using the CTR and KID values
   provided.  The encoded header is provided as AAD to the AEAD
   encryption operation, together with application-provided metadata
   about the encrypted media (see Section 9.4).

   def encrypt(CTR, KID, metadata, plaintext):
     sframe_key, sframe_salt = key_store[KID]

     # encode_big_endian(x, n) produces an n-byte string encoding the
     # integer x in big-endian byte order.
     ctr = encode_big_endian(CTR, AEAD.Nn)
     nonce = xor(sframe_salt, CTR)

     # encode_sframe_header produces a byte string encoding the
     # provided KID and CTR values into an SFrame header.
     header = encode_sframe_header(CTR, KID)
     aad = header + metadata

     ciphertext = AEAD.Encrypt(sframe_key, nonce, aad, plaintext)
     return header + ciphertext

   For example, the metadata input to encryption allows for frame
   metadata to be authenticated when SFrame is applied per frame.  After
   encoding the frame and before packetizing it, the necessary media
   metadata will be moved out of the encoded frame buffer to be sent in
   some channel visible to the SFU (e.g., an RTP header extension).

                                     +---------------+
                                     |               |
                                     |               |
                                     |   plaintext   |
                                     |               |
                                     |               |
                                     +-------+-------+
                                             |
           .- +-----+                        |
          |   |     +--+--> sframe_key ----->| Key
   Header |   | KID |  |                     |
          |   |     |  +--> sframe_salt --+  |
       +--+   +-----+                     |  |
       |  |   |     +---------------------+->| Nonce
       |  |   | CTR |                        |
       |  |   |     |                        |
       |   '- +-----+                        |
       |                                     |
       |          +----------------+         |
       |          |    metadata    |         |
       |          +-------+--------+         |
       |                  |                  |
       +------------------+----------------->| AAD
       |                                     |
       |                                AEAD.Encrypt
       |                                     |
       |               SFrame Ciphertext     |
       |               +---------------+     |
       +-------------->| SFrame Header |     |
                       +---------------+     |
                       |               |     |
                       |               |<----+
                       |   ciphertext  |
                       |               |
                       |               |
                       +---------------+

                 Figure 5: Encrypting an SFrame Ciphertext

4.4.4.  Decryption

   Before decrypting, a receiver needs to assemble a full SFrame
   ciphertext.  When an SFrame ciphertext is fragmented into multiple
   parts for transport (e.g., a whole encrypted frame sent in multiple
   SRTP packets), the receiving client collects all the fragments of the
   ciphertext, using appropriate sequencing and start/end markers in the
   transport.  Once all of the required fragments are available, the
   client reassembles them into the SFrame ciphertext and passes the
   ciphertext to SFrame for decryption.

   The KID field in the SFrame header is used to find the right key and
   salt for the encrypted frame, and the CTR field is used to construct
   the nonce.  The SFrame decryption procedure is as follows:

   def decrypt(metadata, sframe_ciphertext):
     KID, CTR, header, ciphertext = parse_ciphertext(sframe_ciphertext)

     sframe_key, sframe_salt = key_store[KID]

     ctr = encode_big_endian(CTR, AEAD.Nn)
     nonce = xor(sframe_salt, ctr)
     aad = header + metadata

     return AEAD.Decrypt(sframe_key, nonce, aad, ciphertext)

   If a ciphertext fails to decrypt because there is no key available
   for the KID in the SFrame header, the client MAY buffer the
   ciphertext and retry decryption once a key with that KID is received.
   If a ciphertext fails to decrypt for any other reason, the client
   MUST discard the ciphertext.  Invalid ciphertexts SHOULD be discarded
   in a way that is indistinguishable (to an external observer) from
   having processed a valid ciphertext.  In other words, the SFrame
   decrypt operation should take the same amount of time regardless of
   whether decryption succeeds or fails.

                       SFrame Ciphertext
                       +---------------+
       +---------------| SFrame Header |
       |               +---------------+
       |               |               |
       |               |               |-----+
       |               |   ciphertext  |     |
       |               |               |     |
       |               |               |     |
       |               +---------------+     |
       |                                     |
       |   .- +-----+                        |
       |  |   |     +--+--> sframe_key ----->| Key
       |  |   | KID |  |                     |
       |  |   |     |  +--> sframe_salt --+  |
       +->+   +-----+                     |  |
       |  |   |     +---------------------+->| Nonce
       |  |   | CTR |                        |
       |  |   |     |                        |
       |   '- +-----+                        |
       |                                     |
       |          +----------------+         |
       |          |    metadata    |         |
       |          +-------+--------+         |
       |                  |                  |
       +------------------+----------------->| AAD
                                             |
                                        AEAD.Decrypt
                                             |
                                             V
                                     +---------------+
                                     |               |
                                     |               |
                                     |   plaintext   |
                                     |               |
                                     |               |
                                     +---------------+

                 Figure 6: Decrypting an SFrame Ciphertext

4.5.  Cipher Suites

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

   *  A hash function used for key derivation

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

   This document defines the following cipher suites, with the constants
   defined in Section 4.4:

         +============================+====+=====+====+====+====+
         | Name                       | Nh | Nka | Nk | Nn | Nt |
         +============================+====+=====+====+====+====+
         | AES_128_CTR_HMAC_SHA256_80 | 32 | 16  | 48 | 12 | 10 |
         +----------------------------+----+-----+----+----+----+
         | AES_128_CTR_HMAC_SHA256_64 | 32 | 16  | 48 | 12 | 8  |
         +----------------------------+----+-----+----+----+----+
         | AES_128_CTR_HMAC_SHA256_32 | 32 | 16  | 48 | 12 | 4  |
         +----------------------------+----+-----+----+----+----+
         | AES_128_GCM_SHA256_128     | 32 | n/a | 16 | 12 | 16 |
         +----------------------------+----+-----+----+----+----+
         | AES_256_GCM_SHA512_128     | 64 | n/a | 32 | 12 | 16 |
         +----------------------------+----+-----+----+----+----+

                  Table 1: SFrame Cipher Suite Constants

   Numeric identifiers for these cipher suites are defined in the IANA
   registry created in Section 8.1.

   In the suite names, the length of the authentication tag is indicated
   by the last value: "_128" indicates a 128-bit tag, "_80" indicates an
   80-bit tag, "_64" indicates a 64-bit tag, and "_32" indicates a
   32-bit tag.

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

4.5.1.  AES-CTR 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.  The overall length of
   the AEAD key is Nka + Nh, where Nka represents the key size for the
   AES block cipher in use and Nh represents the output size of the hash
   function (as in Section 4.4).  The encryption subkey comprises the
   first Nka bytes and the authentication subkey comprises the remaining
   Nh bytes.

   def derive_subkeys(sframe_key):
     # The encryption key comprises the first Nka bytes
     enc_key = sframe_key[..Nka]

     # The authentication key comprises Nh remaining bytes
     auth_key = sframe_key[Nka..]

     return enc_key, auth_key

   The AEAD encryption and decryption functions are then composed of
   individual calls to the CTR encrypt function and HMAC.  The resulting
   MAC value is truncated to a number of bytes Nt fixed by the cipher
   suite.

   def truncate(tag, n):
     # Take the first `n` bytes of `tag`
     return tag[..n]

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

   def AEAD.Encrypt(key, nonce, aad, pt):
     enc_key, auth_key = derive_subkeys(key)
     initial_counter = nonce + 0x00000000 # append four zero bytes
     ct = AES-CTR.Encrypt(enc_key, initial_counter, 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")

     initial_counter = nonce + 0x00000000 # append four zero bytes
     return AES-CTR.Decrypt(enc_key, initial_counter, 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.  The key
   management framework provides the following functions:

   *  Provisioning KID / base_key mappings to participating clients

   *  Updating the above data as clients join or leave

   It is the responsibility of the application to provide the key
   management framework, as described in Section 9.2.

5.1.  Sender Keys

   If the participants in a call have a preexisting E2E-secure channel,
   they can use it to distribute SFrame keys.  Each client participating
   in a call generates a fresh base_key value that it will use to
   encrypt media.  The client then uses the E2E-secure channel to send
   their encryption 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 (e.g., an RTP
   synchronization source (SSRC)).  SFrame KID values are then used to
   distinguish between versions of the sender's base_key.

   KID values in this scheme have two parts: a "key generation" and a
   "ratchet step".  Both are unsigned integers that begin at zero.  The
   key generation increments each time the sender distributes a new key
   to receivers.  The ratchet step is incremented each time the sender
   ratchets their key forward for forward secrecy:

   base_key[i+1] = HKDF-Expand(
                     HKDF-Extract("", base_key[i]),
                     "SFrame 1.0 Ratchet", CipherSuite.Nh)

   For compactness, we do not send the whole ratchet step.  Instead, we
   send only its low-order R bits, where R is a value set by the
   application.  Different senders may use different values of R, but
   each receiver of a given sender needs to know what value of R is used
   by the sender so that they can recognize when they need to ratchet
   (vs. expecting a new key).  R effectively defines a reordering
   window, since no more than 2^R ratchet steps can be active at a given
   time.  The key generation is sent in the remaining 64 - R bits of the
   KID.

   KID = (key_generation << R) + (ratchet_step % (1 << R))

        64-R bits         R bits
    <---------------> <------------>
   +-----------------+--------------+
   | Key Generation  | Ratchet Step |
   +-----------------+--------------+

           Figure 7: Structure of a KID in the Sender Keys Scheme

   The sender signals such a ratchet step update by sending with a KID
   value in which the ratchet step has been incremented.  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 in the middle of a session, they will need
   to receive from each sender (a) the current sender key for that
   sender and (b) the current KID value for the sender.  Evicting a
   participant requires each sender to send a fresh sender key to all
   receivers.

   It is the application's responsibility to decide when sender keys are
   updated.  A sender key may be updated by sending a new base_key
   (updating the key generation) or by hashing the current base_key
   (updating the ratchet step).  Ratcheting the key forward is useful
   when adding new receivers to an SFrame-based interaction, since it
   ensures that the new receivers can't decrypt any media encrypted
   before they were added.  If a sender wishes to assure the opposite
   property when removing a receiver (i.e., ensuring that the receiver
   can't decrypt media after they are removed), then the sender will
   need to distribute a new sender key.

5.2.  MLS

   The Messaging Layer Security (MLS) protocol provides group
   authenticated key exchange [MLS-ARCH] [MLS-PROTO].  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 known as an "epoch", and each member of the group is assigned an
   "index" that is constant for the time they are in the group.

   To generate keys and nonces for SFrame, we use the MLS exporter
   function to generate a base_key value for each MLS epoch.  Each
   member of the group is assigned a set of KID values so that each
   member has a unique sframe_key and sframe_salt that it uses to
   encrypt with.  Senders may choose any KID value within their assigned
   set of KID values, e.g., to allow a single sender to send multiple,
   uncoordinated outbound media streams.

   base_key = MLS-Exporter("SFrame 1.0 Base Key", "", AEAD.Nk)

   For compactness, we do not send the whole epoch number.  Instead, we
   send only its low-order E bits, where E is a value set by the
   application.  E effectively defines a reordering window, since no
   more than 2^E epochs can be active at a given time.  To handle
   rollover of the epoch counter, receivers MUST remove an old epoch
   when a new epoch with the same low-order E bits is introduced.

   Let S be the number of bits required to encode a member index in the
   group, i.e., the smallest value such that group_size <= (1 << S).
   The sender index is encoded in the S bits above the epoch.  The
   remaining 64 - S - E bits of the KID value are a context value chosen
   by the sender (context value 0 will produce the shortest encoded
   KID).

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

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

               Figure 8: Structure of a KID for an MLS Sender

   Once an SFrame stack has been provisioned with the
   sframe_epoch_secret for an epoch, it can compute the required KID
   values on demand (as well as the resulting SFrame keys/nonces derived
   from the base_key and KID) as it needs to encrypt or decrypt for a
   given member.

     ...
            |
            |
   Epoch 14 +--+-- index=3 ---> KID = 0x3e
            |  |
            |  +-- index=7 ---> KID = 0x7e
            |  |
            |  +-- index=20 --> KID = 0x14e
            |
            |
   Epoch 15 +--+-- index=3 ---> KID = 0x3f
            |  |
            |  +-- index=5 ---> KID = 0x5f
            |
            |
   Epoch 16 +----- index=2 --+--> context = 2 --> KID = 0x820
            |                |
            |                +--> context = 3 --> KID = 0xc20
            |
            |
   Epoch 17 +--+-- index=33 --> KID = 0x211
            |  |
            |  +-- index=51 --> KID = 0x331
            |
            |
     ...

       Figure 9: An Example Sequence of KIDs for an MLS-based SFrame
             Session (E=4; S=6, Allowing for 64 Group Members)

6.  Media Considerations

6.1.  Selective Forwarding Units

   SFUs (e.g., those described in Section 3.7 of [RFC7667]) receive the
   media streams from each participant and select which ones should be
   forwarded to each of the other participants.  There are several
   approaches for stream selection, but in general, the SFU needs to
   access metadata associated with each frame and modify the RTP
   information of the incoming packets when they are transmitted to the
   received participants.

   This section describes how these normal SFU modes of operation
   interact with the E2EE provided by SFrame.

6.1.1.  RTP Stream Reuse

   The SFU may choose to send only a certain number of streams based on
   the voice activity of the participants.  To avoid the overhead
   involved in establishing new transport streams, the SFU may decide to
   reuse previously existing streams or even pre-allocate a predefined
   number of streams and choose in each moment in time which participant
   media will be sent through it.

   This means that the same transport-level stream (e.g., an RTP stream
   defined by either SSRC or Media Identification (MID)) may carry media
   from different streams of different participants.  Because each
   participant uses a different key to encrypt their media, the receiver
   will be able to verify the sender of the media within the RTP stream
   at any given point in time.  Thus the receiver will correctly
   associate the media with the sender indicated by the authenticated
   SFrame KID value, irrespective of how the SFU transmits the media to
   the client.

   Note that in order to prevent impersonation by a malicious
   participant (not the SFU), a mechanism based on digital signature
   would be required.  SFrame does not protect against such attacks.

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 CTR value
   for each.

6.1.3.  Scalable Video Coding (SVC)

   In both temporal and spatial scalability, the SFU may choose to drop
   layers in order to match a certain bitrate or to forward specific
   media sizes or frames per second.  In order to support the SFU
   selectively removing layers, the sender MUST encapsulate each layer
   in a different SFrame ciphertext.

6.2.  Video Key Frames

   Forward security and post-compromise security require that the E2EE
   keys (base keys) are updated any time a participant joins or leaves
   the call.

   The key exchange happens asynchronously 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 frame with a key that is 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.

   The new receiver will then re-request a key frame, 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 after the new E2EE key is in use, the
   time required for the new participant to display the video is
   minimized.

   Note that this issue does not arise for media streams that do not
   have dependencies among frames, e.g., audio streams.  In these
   streams, each frame is independently decodable, so a frame never
   depends on another frame that might be on the other side of a key
   rotation.

6.3.  Partial Decoding

   Some codecs support partial decoding, where individual packets can be
   decoded without waiting for the full frame to arrive.  When SFrame is
   applied per frame, partial decoding is not possible because the
   decoder cannot access data until an entire frame has arrived and has
   been decrypted.

7.  Security Considerations

7.1.  No Header Confidentiality

   SFrame provides integrity protection to the SFrame header (the KID
   and CTR values), but it does not provide confidentiality protection.
   Parties that can observe the SFrame header may learn, for example,
   which parties are sending SFrame payloads (from KID values) and at
   what rates (from CTR values).  In cases where SFrame is used for end-
   to-end security on top of hop-by-hop protections (e.g., running over
   SRTP as described in Appendix B.5), the hop-by-hop security
   mechanisms provide confidentiality protection of the SFrame header
   between hops.

7.2.  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.

7.3.  Key Management

   The specifics of key management are beyond the scope of this
   document.  However, every client SHOULD change their keys when new
   clients join or leave the call for forward secrecy and post-
   compromise security.

7.4.  Replay

   The handling of replay is out of the scope of this document.
   However, senders MUST reject requests to encrypt multiple times with
   the same key and nonce since several AEAD algorithms fail badly in
   such cases (see, e.g., Section 5.1.1 of [RFC5116]).

7.5.  Risks Due to Short Tags

   The SFrame cipher suites based on AES-CTR allow for the use of short
   authentication tags, which bring a higher risk that an attacker will
   be able to cause an SFrame receiver to accept an SFrame ciphertext of
   the attacker's choosing.

   Assuming that the authentication properties of the cipher suite are
   robust, the only attack that an attacker can mount is an attempt to
   find an acceptable (ciphertext, tag) combination through brute force.
   Such a brute-force attack will have an expected success rate of the
   following form:

   attacker_success_rate = attempts_per_second / 2^(8*Nt)

   For example, a gigabit Ethernet connection is able to transmit
   roughly 2^20 packets per second.  If an attacker saturated such a
   link with guesses against a 32-bit authentication tag (Nt=4), then
   the attacker would succeed on average roughly once every 2^12
   seconds, or about once an hour.

   In a typical SFrame usage in a real-time media application, there are
   a few approaches to mitigating this risk:

   *  Receivers only accept SFrame ciphertexts over HBH-secure channels
      (e.g., SRTP security associations or QUIC connections).  If this
      is the case, only an entity that is part of such a channel can
      mount the above attack.

   *  The expected packet rate for a media stream is very predictable
      (and typically far lower than the above example).  On the one
      hand, attacks at this rate will succeed even less often than the
      high-rate attack described above.  On the other hand, the
      application may use an elevated packet arrival rate as a signal of
      a brute-force attack.  This latter approach is common in other
      settings, e.g., mitigating brute-force attacks on passwords.

   *  Media applications typically do not provide feedback to media
      senders as to which media packets failed to decrypt.  When media-
      quality feedback mechanisms are used, decryption failures will
      typically appear as packet losses, but only at an aggregate level.

   *  Anti-replay mechanisms (see Section 7.4) prevent the attacker from
      reusing valid ciphertexts (either observed or guessed by the
      attacker).  A receiver applying anti-replay controls will only
      accept one valid plaintext per CTR value.  Since the CTR value is
      covered by SFrame authentication, an attacker has to do a fresh
      search for a valid tag for every forged ciphertext, even if the
      encrypted content is unchanged.  In other words, when the above
      brute-force attack succeeds, it only allows the attacker to send a
      single SFrame ciphertext; the ciphertext cannot be reused because
      either it will have the same CTR value and be discarded as a
      replay, or else it will have a different CTR value and its tag
      will no longer be valid.

   Nonetheless, without these mitigations, an application that makes use
   of short tags will be at heightened risk of forgery attacks.  In many
   cases, it is simpler to use full-size tags and tolerate slightly
   higher bandwidth usage rather than to add the additional defenses
   necessary to safely use short tags.

8.  IANA Considerations

   IANA has created a new registry called "SFrame Cipher Suites"
   (Section 8.1) under the "SFrame" group registry heading.

8.1.  SFrame Cipher Suites

   The "SFrame Cipher Suites" registry lists identifiers for SFrame
   cipher suites as defined in Section 4.5.  The cipher suite field is
   two bytes wide, so the valid cipher suites are in the range 0x0000 to
   0xFFFF.  Except as noted below, assignments are made via the
   Specification Required policy [RFC8126].

   The registration template is as follows:

   *  Value: The numeric value of the cipher suite

   *  Name: The name of the cipher suite

   *  Recommended: Whether support for this cipher suite is recommended
      by the IETF.  Valid values are "Y", "N", and "D" as described in
      Section 17.1 of [MLS-PROTO].  The default value of the
      "Recommended" column is "N".  Setting the Recommended item to "Y"
      or "D", or changing an item whose current value is "Y" or "D",
      requires Standards Action [RFC8126].

   *  Reference: The document where this cipher suite is defined

   *  Change Controller: Who is authorized to update the row in the
      registry

   Initial contents:

   +========+============================+===+===========+============+
   | Value  | Name                       | R | Reference | Change     |
   |        |                            |   |           | Controller |
   +========+============================+===+===========+============+
   | 0x0000 | Reserved                   | - | RFC 9605  | IETF       |
   +--------+----------------------------+---+-----------+------------+
   | 0x0001 | AES_128_CTR_HMAC_SHA256_80 | Y | RFC 9605  | IETF       |
   +--------+----------------------------+---+-----------+------------+
   | 0x0002 | AES_128_CTR_HMAC_SHA256_64 | Y | RFC 9605  | IETF       |
   +--------+----------------------------+---+-----------+------------+
   | 0x0003 | AES_128_CTR_HMAC_SHA256_32 | Y | RFC 9605  | IETF       |
   +--------+----------------------------+---+-----------+------------+
   | 0x0004 | AES_128_GCM_SHA256_128     | Y | RFC 9605  | IETF       |
   +--------+----------------------------+---+-----------+------------+
   | 0x0005 | AES_256_GCM_SHA512_128     | Y | RFC 9605  | IETF       |
   +--------+----------------------------+---+-----------+------------+
   | 0xF000 | Reserved for Private Use   | - | RFC 9605  | IETF       |
   | -      |                            |   |           |            |
   | 0xFFFF |                            |   |           |            |
   +--------+----------------------------+---+-----------+------------+

                      Table 2: SFrame Cipher Suites

9.  Application Responsibilities

   To use SFrame, an application needs to define the inputs to the
   SFrame encryption and decryption operations, and how SFrame
   ciphertexts are delivered from sender to receiver (including any
   fragmentation and reassembly).  In this section, we lay out
   additional requirements that an application must meet in order for
   SFrame to operate securely.

   In general, an application using SFrame is responsible for
   configuring SFrame.  The application must first define when SFrame is
   applied at all.  When SFrame is applied, the application must define
   which cipher suite is to be used.  If new versions of SFrame are
   defined in the future, it will be the application's responsibility to
   determine which version should be used.

   This division of responsibilities is similar to the way other media
   parameters (e.g., codecs) are typically handled in media
   applications, in the sense that they are set up in some signaling
   protocol and not described in the media.  Applications might find it
   useful to extend the protocols used for negotiating other media
   parameters (e.g., Session Description Protocol (SDP) [RFC8866]) to
   also negotiate parameters for SFrame.

9.1.  Header Value Uniqueness

   Applications MUST ensure that each (base_key, KID, CTR) combination
   is used for at most one SFrame encryption operation.  This ensures
   that the (key, nonce) pairs used by the underlying AEAD algorithm are
   never reused.  Typically this is done by assigning each sender a KID
   or set of KIDs, then having each sender use the CTR field as a
   monotonic counter, incrementing for each plaintext that is encrypted.
   In addition to its simplicity, this scheme minimizes overhead by
   keeping CTR values as small as possible.

   In applications where an SFrame context might be written to
   persistent storage, this context needs to include the last-used CTR
   value.  When the context is used later, the application should use
   the stored CTR value to determine the next CTR value to be used in an
   encryption operation, and then write the next CTR value back to
   storage before using the CTR value for encryption.  Storing the CTR
   value before usage (vs. after) helps ensure that a storage failure
   will not cause reuse of the same (base_key, KID, CTR) combination.

9.2.  Key Management Framework

   The application is responsible for provisioning SFrame with a mapping
   of KID values to base_key values and the resulting keys and salts.
   More importantly, the application specifies which KID values are used
   for which purposes (e.g., by which senders).  An application's KID
   assignment strategy MUST be structured to assure the non-reuse
   properties discussed in Section 9.1.

   The application is also responsible for defining a rotation schedule
   for keys.  For example, one application might have an ephemeral group
   for every call and keep rotating keys when endpoints join 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.

   It should be noted that KID values are not encrypted by SFrame and
   are thus visible to any application-layer intermediaries that might
   handle an SFrame ciphertext.  If there are application semantics
   included in KID values, then this information would be exposed to
   intermediaries.  For example, in the scheme of Section 5.1, the
   number of ratchet steps per sender is exposed, and in the scheme of
   Section 5.2, the number of epochs and the MLS sender ID of the SFrame
   sender are exposed.

9.3.  Anti-Replay

   It is the responsibility of the application to handle anti-replay.
   Replay by network attackers is assumed to be prevented by network-
   layer facilities (e.g., TLS, SRTP).  As mentioned in Section 7.4,
   senders MUST reject requests to encrypt multiple times with the same
   key and nonce.

   It is not mandatory to implement anti-replay on the receiver side.
   Receivers MAY apply time- or counter-based anti-replay mitigations.
   For example, Section 3.3.2 of [RFC3711] specifies a counter-based
   anti-replay mitigation, which could be adapted to use with SFrame,
   using the CTR field as the counter.

9.4.  Metadata

   The metadata input to SFrame operations is an opaque byte string
   specified by the application.  As such, the application needs to
   define what information should go in the metadata input and ensure
   that it is provided to the encryption and decryption functions at the
   appropriate points.  A receiver MUST NOT use SFrame-authenticated
   metadata until after the SFrame decrypt function has authenticated
   it, unless the purpose of such usage is to prepare an SFrame
   ciphertext for SFrame decryption.  Essentially, metadata may be used
   "upstream of SFrame" in a processing pipeline, but only to prepare
   for SFrame decryption.

   For example, consider an application where SFrame is used to encrypt
   audio frames that are sent over SRTP, with some application data
   included in the RTP header extension.  Suppose the application also
   includes this application data in the SFrame metadata, so that the
   SFU is allowed to read, but not modify, the application data.  A
   receiver can use the application data in the RTP header extension as
   part of the standard SRTP decryption process since this is required
   to recover the SFrame ciphertext carried in the SRTP payload.
   However, the receiver MUST NOT use the application data for other
   purposes before SFrame decryption has authenticated the application
   data.

10.  References

10.1.  Normative References

   [MLS-PROTO]
              Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
              Omara, E., and K. Cohn-Gordon, "The Messaging Layer
              Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
              July 2023, <https://www.rfc-editor.org/info/rfc9420>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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

   [MLS-ARCH] Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., and
              A. Duric, "The Messaging Layer Security (MLS)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-mls-architecture-15, 3 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
              architecture-15>.

   [MOQ-TRANSPORT]
              Curley, L., Pugin, K., Nandakumar, S., Vasiliev, V., and
              I. Swett, Ed., "Media over QUIC Transport", Work in
              Progress, Internet-Draft, draft-ietf-moq-transport-05, 8
              July 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-moq-transport-05>.

   [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>.

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
              September 2012, <https://www.rfc-editor.org/info/rfc6716>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <https://www.rfc-editor.org/info/rfc7656>.

   [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
              DOI 10.17487/RFC7667, November 2015,
              <https://www.rfc-editor.org/info/rfc7667>.

   [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>.

   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,
              <https://www.rfc-editor.org/info/rfc8866>.

   [RTP-PAYLOAD]
              Murillo, S. G., Fablet, Y., and A. Gouaillard, "Codec
              agnostic RTP payload format for video", Work in Progress,
              Internet-Draft, draft-gouaillard-avtcore-codec-agn-rtp-
              payload-01, 9 March 2021,
              <https://datatracker.ietf.org/doc/html/draft-gouaillard-
              avtcore-codec-agn-rtp-payload-01>.

   [TestVectors]
              "SFrame Test Vectors", commit 025d568, September 2023,
              <https://github.com/sframe-wg/sframe/blob/025d568/test-
              vectors/test-vectors.json>.

   [WEBTRANSPORT]
              Vasiliev, V., "The WebTransport Protocol Framework", Work
              in Progress, Internet-Draft, draft-ietf-webtrans-overview-
              08, 25 August 2024,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              ietf-webtrans-overview/>.

Appendix A.  Example API

   *This section is not normative.*

   This section describes a notional API that an SFrame implementation
   might expose.  The core concept is an "SFrame context", within which
   KID values are meaningful.  In the key management scheme described in
   Section 5.1, each sender has a different context; in the scheme
   described in Section 5.2, all senders share the same context.

   An SFrame context stores mappings from KID values to "key contexts",
   which are different depending on whether the KID is to be used for
   sending or receiving (an SFrame key should never be used for both
   operations).  A key context tracks the key and salt associated to the
   KID, and the current CTR value.  A key context to be used for sending
   also tracks the next CTR value to be used.

   The primary operations on an SFrame context are as follows:

   *  *Create an SFrame context:* The context is initialized with a
      cipher suite and no KID mappings.

   *  *Add a key for sending:* The key and salt are derived from the
      base key and used to initialize a send context, together with a
      zero CTR value.

   *  *Add a key for receiving:* The key and salt are derived from the
      base key and used to initialize a send context.

   *  *Encrypt a plaintext:* Encrypt a given plaintext using the key for
      a given KID, including the specified metadata.

   *  *Decrypt an SFrame ciphertext:* Decrypt an SFrame ciphertext with
      the KID and CTR values specified in the SFrame header, and the
      provided metadata.

   Figure 10 shows an example of the types of structures and methods
   that could be used to create an SFrame API in Rust.

   type KeyId = u64;
   type Counter = u64;
   type CipherSuite = u16;

   struct SendKeyContext {
     key: Vec<u8>,
     salt: Vec<u8>,
     next_counter: Counter,
   }

   struct RecvKeyContext {
     key: Vec<u8>,
     salt: Vec<u8>,
   }

   struct SFrameContext {
     cipher_suite: CipherSuite,
     send_keys: HashMap<KeyId, SendKeyContext>,
     recv_keys: HashMap<KeyId, RecvKeyContext>,
   }

   trait SFrameContextMethods {
     fn create(cipher_suite: CipherSuite) -> Self;
     fn add_send_key(&self, kid: KeyId, base_key: &[u8]);
     fn add_recv_key(&self, kid: KeyId, base_key: &[u8]);
     fn encrypt(&mut self, kid: KeyId, metadata: &[u8],
                plaintext: &[u8]) -> Vec<u8>;
     fn decrypt(&self, metadata: &[u8], ciphertext: &[u8]) -> Vec<u8>;
   }

                      Figure 10: An Example SFrame API

Appendix B.  Overhead Analysis

   Any use of SFrame will impose overhead in terms of the amount of
   bandwidth necessary to transmit a given media stream.  Exactly how
   much overhead will be added depends on several factors:

   *  The number of senders involved in a conference (length of KID)

   *  The duration of the conference (length of CTR)

   *  The cipher suite in use (length of authentication tag)

   *  Whether SFrame is used to encrypt packets, whole frames, or some
      other unit

   Overall, the overhead rate in kilobits per second can be estimated
   as:

   OverheadKbps = (1 + |CTR| + |KID| + |TAG|) * 8 * CTPerSecond / 1024

   Here the constant value 1 reflects the fixed SFrame header; |CTR|
   and |KID| reflect the lengths of those fields; |TAG| reflects the
   cipher overhead; and CTPerSecond reflects the number of SFrame
   ciphertexts sent per second (e.g., packets or frames per second).

   In the remainder of this section, we compute overhead estimates for a
   collection of common scenarios.

B.1.  Assumptions

   In the below calculations, we make conservative assumptions about
   SFrame overhead so that the overhead amounts we compute here are
   likely to be an upper bound of those seen in practice.

           +==============+=======+============================+
           | Field        | Bytes | Explanation                |
           +==============+=======+============================+
           | Config byte  |     1 | Fixed                      |
           +--------------+-------+----------------------------+
           | Key ID (KID) |     2 | >255 senders; or MLS epoch |
           |              |       | (E=4) and >16 senders      |
           +--------------+-------+----------------------------+
           | Counter      |     3 | More than 24 hours of      |
           | (CTR)        |       | media in common cases      |
           +--------------+-------+----------------------------+
           | Cipher       |    16 | Full authentication tag    |
           | overhead     |       | (longest defined here)     |
           +--------------+-------+----------------------------+

                   Table 3: Overhead Analysis Assumptions

   In total, then, we assume that each SFrame encryption will add 22
   bytes of overhead.

   We consider two scenarios: applying SFrame per frame and per packet.
   In each scenario, we compute the SFrame overhead in absolute terms
   (kbps) and as a percentage of the base bandwidth.

B.2.  Audio

   In audio streams, there is typically a one-to-one relationship
   between frames and packets, so the overhead is the same whether one
   uses SFrame at a per-packet or per-frame level.

   Table 4 considers three scenarios that are based on recommended
   configurations of the Opus codec [RFC6716] (where "fps" stands for
   "frames per second"):

    +==============+==============+=====+======+==========+==========+
    | Scenario     | Frame length | fps | Base | Overhead | Overhead |
    |              |              |     | kbps |   kbps   |    %     |
    +==============+==============+=====+======+==========+==========+
    | Narrow-band  |    120 ms    | 8.3 |  8   |   1.4    |  17.9%   |
    | speech       |              |     |      |          |          |
    +--------------+--------------+-----+------+----------+----------+
    | Full-band    |    20 ms     |  50 |  32  |   8.6    |  26.9%   |
    | speech       |              |     |      |          |          |
    +--------------+--------------+-----+------+----------+----------+
    | Full-band    |    10 ms     | 100 | 128  |   17.2   |  13.4%   |
    | stereo music |              |     |      |          |          |
    +--------------+--------------+-----+------+----------+----------+

                Table 4: SFrame Overhead for Audio Streams

B.3.  Video

   Video frames can be larger than an MTU and thus are commonly split
   across multiple frames.  Tables 5 and 6 show the estimated overhead
   of encrypting a video stream, where SFrame is applied per frame and
   per packet, respectively.  The choices of resolution, frames per
   second, and bandwidth roughly reflect the capabilities of modern
   video codecs across a range from very low to very high quality.

      +=============+=====+===========+===============+============+
      | Scenario    | fps | Base kbps | Overhead kbps | Overhead % |
      +=============+=====+===========+===============+============+
      | 426 x 240   | 7.5 |     45    |      1.3      |    2.9%    |
      +-------------+-----+-----------+---------------+------------+
      | 640 x 360   |  15 |    200    |      2.6      |    1.3%    |
      +-------------+-----+-----------+---------------+------------+
      | 640 x 360   |  30 |    400    |      5.2      |    1.3%    |
      +-------------+-----+-----------+---------------+------------+
      | 1280 x 720  |  30 |    1500   |      5.2      |    0.3%    |
      +-------------+-----+-----------+---------------+------------+
      | 1920 x 1080 |  60 |    7200   |      10.3     |    0.1%    |
      +-------------+-----+-----------+---------------+------------+

        Table 5: SFrame Overhead for a Video Stream Encrypted per
                                  Frame

      +==========+=====+==============+======+==========+==========+
      | Scenario | fps | Packets per  | Base | Overhead | Overhead |
      |          |     | Second (pps) | kbps |   kbps   |    %     |
      +==========+=====+==============+======+==========+==========+
      | 426 x    | 7.5 |     7.5      |  45  |   1.3    |   2.9%   |
      | 240      |     |              |      |          |          |
      +----------+-----+--------------+------+----------+----------+
      | 640 x    |  15 |      30      | 200  |   5.2    |   2.6%   |
      | 360      |     |              |      |          |          |
      +----------+-----+--------------+------+----------+----------+
      | 640 x    |  30 |      60      | 400  |   10.3   |   2.6%   |
      | 360      |     |              |      |          |          |
      +----------+-----+--------------+------+----------+----------+
      | 1280 x   |  30 |     180      | 1500 |   30.9   |   2.1%   |
      | 720      |     |              |      |          |          |
      +----------+-----+--------------+------+----------+----------+
      | 1920 x   |  60 |     780      | 7200 |  134.1   |   1.9%   |
      | 1080     |     |              |      |          |          |
      +----------+-----+--------------+------+----------+----------+

        Table 6: SFrame Overhead for a Video Stream Encrypted per
                                  Packet

   In the per-frame case, the SFrame percentage overhead approaches zero
   as the quality of the video improves since bandwidth is driven more
   by picture size than frame rate.  In the per-packet case, the SFrame
   percentage overhead approaches the ratio between the SFrame overhead
   per packet and the MTU (here 22 bytes of SFrame overhead divided by
   an assumed 1200-byte MTU, or about 1.8%).

B.4.  Conferences

   Real conferences usually involve several audio and video streams.
   The overhead of SFrame in such a conference is the aggregate of the
   overhead across all the individual streams.  Thus, while SFrame
   incurs a large percentage overhead on an audio stream, if the
   conference also involves a video stream, then the audio overhead is
   likely negligible relative to the overall bandwidth of the
   conference.

   For example, Table 7 shows the overhead estimates for a two-person
   conference where one person is sending low-quality media and the
   other is sending high-quality media.  (And we assume that SFrame is
   applied per frame.)  The video streams dominate the bandwidth at the
   SFU, so the total bandwidth overhead is only around 1%.

     +=====================+===========+===============+============+
     | Stream              | Base Kbps | Overhead Kbps | Overhead % |
     +=====================+===========+===============+============+
     | Participant 1 audio |     8     |      1.4      |   17.9%    |
     +---------------------+-----------+---------------+------------+
     | Participant 1 video |     45    |      1.3      |    2.9%    |
     +---------------------+-----------+---------------+------------+
     | Participant 2 audio |     32    |       9       |   26.9%    |
     +---------------------+-----------+---------------+------------+
     | Participant 2 video |    1500   |       5       |    0.3%    |
     +---------------------+-----------+---------------+------------+
     | Total at SFU        |    1585   |      16.5     |    1.0%    |
     +---------------------+-----------+---------------+------------+

           Table 7: SFrame Overhead for a Two-Person Conference

B.5.  SFrame over RTP

   SFrame is a generic encapsulation format, but many of the
   applications in which it is likely to be integrated are based on RTP.
   This section discusses how an integration between SFrame and RTP
   could be done, and some of the challenges that would need to be
   overcome.

   As discussed in Section 4.1, there are two natural patterns for
   integrating SFrame into an application: applying SFrame per frame or
   per packet.  In RTP-based applications, applying SFrame per packet
   means that the payload of each RTP packet will be an SFrame
   ciphertext, starting with an SFrame header, as shown in Figure 11.
   Applying SFrame per frame means that different RTP payloads will have
   different formats: The first payload of a frame will contain the
   SFrame headers, and subsequent payloads will contain further chunks
   of the ciphertext, as shown in Figure 12.

   In order for these media payloads to be properly interpreted by
   receivers, receivers will need to be configured to know which of the
   above schemes the sender has applied to a given sequence of RTP
   packets.  SFrame does not provide a mechanism for distributing this
   configuration information.  In applications that use SDP for
   negotiating RTP media streams [RFC8866], an appropriate extension to
   SDP could provide this function.

   Applying SFrame per frame also requires that packetization and
   depacketization be done in a generic manner that does not depend on
   the media content of the packets, since the content being packetized
   or depacketized will be opaque ciphertext (except for the SFrame
   header).  In order for such a generic packetization scheme to work
   interoperably, one would have to be defined, e.g., as proposed in
   [RTP-PAYLOAD].

      +---+-+-+-------+-+-----------+------------------------------+<-+
      |V=2|P|X|  CC   |M|     PT    |       sequence number        |  |
      +---+-+-+-------+-+-----------+------------------------------+  |
      |                          timestamp                         |  |
      +------------------------------------------------------------+  |
      |          synchronization source (SSRC) identifier          |  |
      +============================================================+  |
      |           contributing source (CSRC) identifiers           |  |
      |                              ....                          |  |
      +------------------------------------------------------------+  |
      |                  RTP extension(s) (OPTIONAL)               |  |
   +->+-------------------+----------------------------------------+  |
   |  |  SFrame header    |                                        |  |
   |  +-------------------+                                        |  |
   |  |                                                            |  |
   |  |         SFrame encrypted and authenticated payload         |  |
   |  |                                                            |  |
   +->+------------------------------------------------------------+<-+
   |  |                   SRTP authentication tag                  |  |
   |  +------------------------------------------------------------+  |
   |                                                                  |
   +--- SRTP Encrypted Portion          SRTP Authenticated Portion ---+

            Figure 11: SRTP Packet with SFrame-Protected Payload

      +----------------+  +---------------+
      | frame metadata |  |               |
      +-------+--------+  |               |
              |           |     frame     |
              |           |               |
              |           |               |
              |           +-------+-------+
              |                   |
              |                   |
              V                   V
   +--------------------------------------+
   |            SFrame Encrypt            |
   +--------------------------------------+
      |                           |
      |                           |
      |                           V
      |                   +-------+-------+
      |                   |               |
      |                   |               |
      |                   |   encrypted   |
      |                   |     frame     |
      |                   |               |
      |                   |               |
      |                   +-------+-------+
      |                           |
      |                  generic RTP packetize
      |                           |
      |    +----------------------+--------.....--------+
      |    |                      |                     |
      V    V                      V                     V
   +---------------+      +---------------+     +---------------+
   | SFrame header |      |               |     |               |
   +---------------+      |               |     |               |
   |               |      |  payload 2/N  | ... |  payload N/N  |
   |  payload 1/N  |      |               |     |               |
   |               |      |               |     |               |
   +---------------+      +---------------+     +---------------+

        Figure 12: Encryption Flow with per-Frame Encryption for RTP

Appendix C.  Test Vectors

   This section provides a set of test vectors that implementations can
   use to verify that they correctly implement SFrame encryption and
   decryption.  In addition to test vectors for the overall process of
   SFrame encryption/decryption, we also provide test vectors for header
   encoding/decoding, and for AEAD encryption/decryption using the AES-
   CTR construction defined in Section 4.5.1.

   All values are either numeric or byte strings.  Numeric values are
   represented as hex values, prefixed with 0x.  Byte strings are
   represented in hex encoding.

   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].  In the JSON test vectors, numeric values are JSON
   numbers and byte string values are JSON strings containing the hex
   encoding of the byte strings.

C.1.  Header Encoding/Decoding

   For each case, we provide:

   *  kid: A KID value

   *  ctr: A CTR value

   *  header: An encoded SFrame header

   An implementation should verify that:

   *  Encoding a header with the KID and CTR results in the provided
      header value

   *  Decoding the provided header value results in the provided KID and
      CTR values

   kid: 0x0000000000000000
   ctr: 0x0000000000000000
   header: 00

   kid: 0x0000000000000000
   ctr: 0x0000000000000001
   header: 01

   kid: 0x0000000000000000
   ctr: 0x00000000000000ff
   header: 08ff

   kid: 0x0000000000000000
   ctr: 0x0000000000000100
   header: 090100

   kid: 0x0000000000000000
   ctr: 0x000000000000ffff
   header: 09ffff

   kid: 0x0000000000000000
   ctr: 0x0000000000010000
   header: 0a010000

   kid: 0x0000000000000000
   ctr: 0x0000000000ffffff
   header: 0affffff

   kid: 0x0000000000000000
   ctr: 0x0000000001000000
   header: 0b01000000

   kid: 0x0000000000000000
   ctr: 0x00000000ffffffff
   header: 0bffffffff

   kid: 0x0000000000000000
   ctr: 0x0000000100000000
   header: 0c0100000000

   kid: 0x0000000000000000
   ctr: 0x000000ffffffffff
   header: 0cffffffffff

   kid: 0x0000000000000000
   ctr: 0x0000010000000000
   header: 0d010000000000

   kid: 0x0000000000000000
   ctr: 0x0000ffffffffffff
   header: 0dffffffffffff

   kid: 0x0000000000000000
   ctr: 0x0001000000000000
   header: 0e01000000000000

   kid: 0x0000000000000000
   ctr: 0x00ffffffffffffff
   header: 0effffffffffffff

   kid: 0x0000000000000000
   ctr: 0x0100000000000000
   header: 0f0100000000000000

   kid: 0x0000000000000000
   ctr: 0xffffffffffffffff
   header: 0fffffffffffffffff

   kid: 0x0000000000000001
   ctr: 0x0000000000000000
   header: 10

   kid: 0x0000000000000001
   ctr: 0x0000000000000001
   header: 11

   kid: 0x0000000000000001
   ctr: 0x00000000000000ff
   header: 18ff

   kid: 0x0000000000000001
   ctr: 0x0000000000000100
   header: 190100

   kid: 0x0000000000000001
   ctr: 0x000000000000ffff
   header: 19ffff

   kid: 0x0000000000000001
   ctr: 0x0000000000010000
   header: 1a010000

   kid: 0x0000000000000001
   ctr: 0x0000000000ffffff
   header: 1affffff

   kid: 0x0000000000000001
   ctr: 0x0000000001000000
   header: 1b01000000

   kid: 0x0000000000000001
   ctr: 0x00000000ffffffff
   header: 1bffffffff

   kid: 0x0000000000000001
   ctr: 0x0000000100000000
   header: 1c0100000000

   kid: 0x0000000000000001
   ctr: 0x000000ffffffffff
   header: 1cffffffffff

   kid: 0x0000000000000001
   ctr: 0x0000010000000000
   header: 1d010000000000

   kid: 0x0000000000000001
   ctr: 0x0000ffffffffffff
   header: 1dffffffffffff

   kid: 0x0000000000000001
   ctr: 0x0001000000000000
   header: 1e01000000000000

   kid: 0x0000000000000001
   ctr: 0x00ffffffffffffff
   header: 1effffffffffffff

   kid: 0x0000000000000001
   ctr: 0x0100000000000000
   header: 1f0100000000000000

   kid: 0x0000000000000001
   ctr: 0xffffffffffffffff
   header: 1fffffffffffffffff

   kid: 0x00000000000000ff
   ctr: 0x0000000000000000
   header: 80ff

   kid: 0x00000000000000ff
   ctr: 0x0000000000000001
   header: 81ff

   kid: 0x00000000000000ff
   ctr: 0x00000000000000ff
   header: 88ffff

   kid: 0x00000000000000ff
   ctr: 0x0000000000000100
   header: 89ff0100

   kid: 0x00000000000000ff
   ctr: 0x000000000000ffff
   header: 89ffffff

   kid: 0x00000000000000ff
   ctr: 0x0000000000010000
   header: 8aff010000

   kid: 0x00000000000000ff
   ctr: 0x0000000000ffffff
   header: 8affffffff

   kid: 0x00000000000000ff
   ctr: 0x0000000001000000
   header: 8bff01000000

   kid: 0x00000000000000ff
   ctr: 0x00000000ffffffff
   header: 8bffffffffff

   kid: 0x00000000000000ff
   ctr: 0x0000000100000000
   header: 8cff0100000000

   kid: 0x00000000000000ff
   ctr: 0x000000ffffffffff
   header: 8cffffffffffff

   kid: 0x00000000000000ff
   ctr: 0x0000010000000000
   header: 8dff010000000000

   kid: 0x00000000000000ff
   ctr: 0x0000ffffffffffff
   header: 8dffffffffffffff

   kid: 0x00000000000000ff
   ctr: 0x0001000000000000
   header: 8eff01000000000000

   kid: 0x00000000000000ff
   ctr: 0x00ffffffffffffff
   header: 8effffffffffffffff

   kid: 0x00000000000000ff
   ctr: 0x0100000000000000
   header: 8fff0100000000000000

   kid: 0x00000000000000ff
   ctr: 0xffffffffffffffff
   header: 8fffffffffffffffffff

   kid: 0x0000000000000100
   ctr: 0x0000000000000000
   header: 900100

   kid: 0x0000000000000100
   ctr: 0x0000000000000001
   header: 910100

   kid: 0x0000000000000100
   ctr: 0x00000000000000ff
   header: 980100ff

   kid: 0x0000000000000100
   ctr: 0x0000000000000100
   header: 9901000100

   kid: 0x0000000000000100
   ctr: 0x000000000000ffff
   header: 990100ffff

   kid: 0x0000000000000100
   ctr: 0x0000000000010000
   header: 9a0100010000

   kid: 0x0000000000000100
   ctr: 0x0000000000ffffff
   header: 9a0100ffffff

   kid: 0x0000000000000100
   ctr: 0x0000000001000000
   header: 9b010001000000

   kid: 0x0000000000000100
   ctr: 0x00000000ffffffff
   header: 9b0100ffffffff

   kid: 0x0000000000000100
   ctr: 0x0000000100000000
   header: 9c01000100000000

   kid: 0x0000000000000100
   ctr: 0x000000ffffffffff
   header: 9c0100ffffffffff

   kid: 0x0000000000000100
   ctr: 0x0000010000000000
   header: 9d0100010000000000

   kid: 0x0000000000000100
   ctr: 0x0000ffffffffffff
   header: 9d0100ffffffffffff

   kid: 0x0000000000000100
   ctr: 0x0001000000000000
   header: 9e010001000000000000

   kid: 0x0000000000000100
   ctr: 0x00ffffffffffffff
   header: 9e0100ffffffffffffff

   kid: 0x0000000000000100
   ctr: 0x0100000000000000
   header: 9f01000100000000000000

   kid: 0x0000000000000100
   ctr: 0xffffffffffffffff
   header: 9f0100ffffffffffffffff

   kid: 0x000000000000ffff
   ctr: 0x0000000000000000
   header: 90ffff

   kid: 0x000000000000ffff
   ctr: 0x0000000000000001
   header: 91ffff

   kid: 0x000000000000ffff
   ctr: 0x00000000000000ff
   header: 98ffffff

   kid: 0x000000000000ffff
   ctr: 0x0000000000000100
   header: 99ffff0100

   kid: 0x000000000000ffff
   ctr: 0x000000000000ffff
   header: 99ffffffff

   kid: 0x000000000000ffff
   ctr: 0x0000000000010000
   header: 9affff010000

   kid: 0x000000000000ffff
   ctr: 0x0000000000ffffff
   header: 9affffffffff

   kid: 0x000000000000ffff
   ctr: 0x0000000001000000
   header: 9bffff01000000

   kid: 0x000000000000ffff
   ctr: 0x00000000ffffffff
   header: 9bffffffffffff

   kid: 0x000000000000ffff
   ctr: 0x0000000100000000
   header: 9cffff0100000000

   kid: 0x000000000000ffff
   ctr: 0x000000ffffffffff
   header: 9cffffffffffffff

   kid: 0x000000000000ffff
   ctr: 0x0000010000000000
   header: 9dffff010000000000

   kid: 0x000000000000ffff
   ctr: 0x0000ffffffffffff
   header: 9dffffffffffffffff

   kid: 0x000000000000ffff
   ctr: 0x0001000000000000
   header: 9effff01000000000000

   kid: 0x000000000000ffff
   ctr: 0x00ffffffffffffff
   header: 9effffffffffffffffff

   kid: 0x000000000000ffff
   ctr: 0x0100000000000000
   header: 9fffff0100000000000000

   kid: 0x000000000000ffff
   ctr: 0xffffffffffffffff
   header: 9fffffffffffffffffffff

   kid: 0x0000000000010000
   ctr: 0x0000000000000000
   header: a0010000

   kid: 0x0000000000010000
   ctr: 0x0000000000000001
   header: a1010000

   kid: 0x0000000000010000
   ctr: 0x00000000000000ff
   header: a8010000ff

   kid: 0x0000000000010000
   ctr: 0x0000000000000100
   header: a90100000100

   kid: 0x0000000000010000
   ctr: 0x000000000000ffff
   header: a9010000ffff

   kid: 0x0000000000010000
   ctr: 0x0000000000010000
   header: aa010000010000

   kid: 0x0000000000010000
   ctr: 0x0000000000ffffff
   header: aa010000ffffff

   kid: 0x0000000000010000
   ctr: 0x0000000001000000
   header: ab01000001000000

   kid: 0x0000000000010000
   ctr: 0x00000000ffffffff
   header: ab010000ffffffff

   kid: 0x0000000000010000
   ctr: 0x0000000100000000
   header: ac0100000100000000

   kid: 0x0000000000010000
   ctr: 0x000000ffffffffff
   header: ac010000ffffffffff

   kid: 0x0000000000010000
   ctr: 0x0000010000000000
   header: ad010000010000000000

   kid: 0x0000000000010000
   ctr: 0x0000ffffffffffff
   header: ad010000ffffffffffff

   kid: 0x0000000000010000
   ctr: 0x0001000000000000
   header: ae01000001000000000000

   kid: 0x0000000000010000
   ctr: 0x00ffffffffffffff
   header: ae010000ffffffffffffff

   kid: 0x0000000000010000
   ctr: 0x0100000000000000
   header: af0100000100000000000000

   kid: 0x0000000000010000
   ctr: 0xffffffffffffffff
   header: af010000ffffffffffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0000000000000000
   header: a0ffffff

   kid: 0x0000000000ffffff
   ctr: 0x0000000000000001
   header: a1ffffff

   kid: 0x0000000000ffffff
   ctr: 0x00000000000000ff
   header: a8ffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0000000000000100
   header: a9ffffff0100

   kid: 0x0000000000ffffff
   ctr: 0x000000000000ffff
   header: a9ffffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0000000000010000
   header: aaffffff010000

   kid: 0x0000000000ffffff
   ctr: 0x0000000000ffffff
   header: aaffffffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0000000001000000
   header: abffffff01000000

   kid: 0x0000000000ffffff
   ctr: 0x00000000ffffffff
   header: abffffffffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0000000100000000
   header: acffffff0100000000

   kid: 0x0000000000ffffff
   ctr: 0x000000ffffffffff
   header: acffffffffffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0000010000000000
   header: adffffff010000000000

   kid: 0x0000000000ffffff
   ctr: 0x0000ffffffffffff
   header: adffffffffffffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0001000000000000
   header: aeffffff01000000000000

   kid: 0x0000000000ffffff
   ctr: 0x00ffffffffffffff
   header: aeffffffffffffffffffff

   kid: 0x0000000000ffffff
   ctr: 0x0100000000000000
   header: afffffff0100000000000000

   kid: 0x0000000000ffffff
   ctr: 0xffffffffffffffff
   header: afffffffffffffffffffffff

   kid: 0x0000000001000000
   ctr: 0x0000000000000000
   header: b001000000

   kid: 0x0000000001000000
   ctr: 0x0000000000000001
   header: b101000000

   kid: 0x0000000001000000
   ctr: 0x00000000000000ff
   header: b801000000ff

   kid: 0x0000000001000000
   ctr: 0x0000000000000100
   header: b9010000000100

   kid: 0x0000000001000000
   ctr: 0x000000000000ffff
   header: b901000000ffff

   kid: 0x0000000001000000
   ctr: 0x0000000000010000
   header: ba01000000010000

   kid: 0x0000000001000000
   ctr: 0x0000000000ffffff
   header: ba01000000ffffff

   kid: 0x0000000001000000
   ctr: 0x0000000001000000
   header: bb0100000001000000

   kid: 0x0000000001000000
   ctr: 0x00000000ffffffff
   header: bb01000000ffffffff

   kid: 0x0000000001000000
   ctr: 0x0000000100000000
   header: bc010000000100000000

   kid: 0x0000000001000000
   ctr: 0x000000ffffffffff
   header: bc01000000ffffffffff

   kid: 0x0000000001000000
   ctr: 0x0000010000000000
   header: bd01000000010000000000

   kid: 0x0000000001000000
   ctr: 0x0000ffffffffffff
   header: bd01000000ffffffffffff

   kid: 0x0000000001000000
   ctr: 0x0001000000000000
   header: be0100000001000000000000

   kid: 0x0000000001000000
   ctr: 0x00ffffffffffffff
   header: be01000000ffffffffffffff

   kid: 0x0000000001000000
   ctr: 0x0100000000000000
   header: bf010000000100000000000000

   kid: 0x0000000001000000
   ctr: 0xffffffffffffffff
   header: bf01000000ffffffffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0000000000000000
   header: b0ffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0000000000000001
   header: b1ffffffff

   kid: 0x00000000ffffffff
   ctr: 0x00000000000000ff
   header: b8ffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0000000000000100
   header: b9ffffffff0100

   kid: 0x00000000ffffffff
   ctr: 0x000000000000ffff
   header: b9ffffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0000000000010000
   header: baffffffff010000

   kid: 0x00000000ffffffff
   ctr: 0x0000000000ffffff
   header: baffffffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0000000001000000
   header: bbffffffff01000000

   kid: 0x00000000ffffffff
   ctr: 0x00000000ffffffff
   header: bbffffffffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0000000100000000
   header: bcffffffff0100000000

   kid: 0x00000000ffffffff
   ctr: 0x000000ffffffffff
   header: bcffffffffffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0000010000000000
   header: bdffffffff010000000000

   kid: 0x00000000ffffffff
   ctr: 0x0000ffffffffffff
   header: bdffffffffffffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0001000000000000
   header: beffffffff01000000000000

   kid: 0x00000000ffffffff
   ctr: 0x00ffffffffffffff
   header: beffffffffffffffffffffff

   kid: 0x00000000ffffffff
   ctr: 0x0100000000000000
   header: bfffffffff0100000000000000

   kid: 0x00000000ffffffff
   ctr: 0xffffffffffffffff
   header: bfffffffffffffffffffffffff

   kid: 0x0000000100000000
   ctr: 0x0000000000000000
   header: c00100000000

   kid: 0x0000000100000000
   ctr: 0x0000000000000001
   header: c10100000000

   kid: 0x0000000100000000
   ctr: 0x00000000000000ff
   header: c80100000000ff

   kid: 0x0000000100000000
   ctr: 0x0000000000000100
   header: c901000000000100

   kid: 0x0000000100000000
   ctr: 0x000000000000ffff
   header: c90100000000ffff

   kid: 0x0000000100000000
   ctr: 0x0000000000010000
   header: ca0100000000010000

   kid: 0x0000000100000000
   ctr: 0x0000000000ffffff
   header: ca0100000000ffffff

   kid: 0x0000000100000000
   ctr: 0x0000000001000000
   header: cb010000000001000000

   kid: 0x0000000100000000
   ctr: 0x00000000ffffffff
   header: cb0100000000ffffffff

   kid: 0x0000000100000000
   ctr: 0x0000000100000000
   header: cc01000000000100000000

   kid: 0x0000000100000000
   ctr: 0x000000ffffffffff
   header: cc0100000000ffffffffff

   kid: 0x0000000100000000
   ctr: 0x0000010000000000
   header: cd0100000000010000000000

   kid: 0x0000000100000000
   ctr: 0x0000ffffffffffff
   header: cd0100000000ffffffffffff

   kid: 0x0000000100000000
   ctr: 0x0001000000000000
   header: ce010000000001000000000000

   kid: 0x0000000100000000
   ctr: 0x00ffffffffffffff
   header: ce0100000000ffffffffffffff

   kid: 0x0000000100000000
   ctr: 0x0100000000000000
   header: cf01000000000100000000000000

   kid: 0x0000000100000000
   ctr: 0xffffffffffffffff
   header: cf0100000000ffffffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0000000000000000
   header: c0ffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0000000000000001
   header: c1ffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x00000000000000ff
   header: c8ffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0000000000000100
   header: c9ffffffffff0100

   kid: 0x000000ffffffffff
   ctr: 0x000000000000ffff
   header: c9ffffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0000000000010000
   header: caffffffffff010000

   kid: 0x000000ffffffffff
   ctr: 0x0000000000ffffff
   header: caffffffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0000000001000000
   header: cbffffffffff01000000

   kid: 0x000000ffffffffff
   ctr: 0x00000000ffffffff
   header: cbffffffffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0000000100000000
   header: ccffffffffff0100000000

   kid: 0x000000ffffffffff
   ctr: 0x000000ffffffffff
   header: ccffffffffffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0000010000000000
   header: cdffffffffff010000000000

   kid: 0x000000ffffffffff
   ctr: 0x0000ffffffffffff
   header: cdffffffffffffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0001000000000000
   header: ceffffffffff01000000000000

   kid: 0x000000ffffffffff
   ctr: 0x00ffffffffffffff
   header: ceffffffffffffffffffffffff

   kid: 0x000000ffffffffff
   ctr: 0x0100000000000000
   header: cfffffffffff0100000000000000

   kid: 0x000000ffffffffff
   ctr: 0xffffffffffffffff
   header: cfffffffffffffffffffffffffff

   kid: 0x0000010000000000
   ctr: 0x0000000000000000
   header: d0010000000000

   kid: 0x0000010000000000
   ctr: 0x0000000000000001
   header: d1010000000000

   kid: 0x0000010000000000
   ctr: 0x00000000000000ff
   header: d8010000000000ff

   kid: 0x0000010000000000
   ctr: 0x0000000000000100
   header: d90100000000000100

   kid: 0x0000010000000000
   ctr: 0x000000000000ffff
   header: d9010000000000ffff

   kid: 0x0000010000000000
   ctr: 0x0000000000010000
   header: da010000000000010000

   kid: 0x0000010000000000
   ctr: 0x0000000000ffffff
   header: da010000000000ffffff

   kid: 0x0000010000000000
   ctr: 0x0000000001000000
   header: db01000000000001000000

   kid: 0x0000010000000000
   ctr: 0x00000000ffffffff
   header: db010000000000ffffffff

   kid: 0x0000010000000000
   ctr: 0x0000000100000000
   header: dc0100000000000100000000

   kid: 0x0000010000000000
   ctr: 0x000000ffffffffff
   header: dc010000000000ffffffffff

   kid: 0x0000010000000000
   ctr: 0x0000010000000000
   header: dd010000000000010000000000

   kid: 0x0000010000000000
   ctr: 0x0000ffffffffffff
   header: dd010000000000ffffffffffff

   kid: 0x0000010000000000
   ctr: 0x0001000000000000
   header: de01000000000001000000000000

   kid: 0x0000010000000000
   ctr: 0x00ffffffffffffff
   header: de010000000000ffffffffffffff

   kid: 0x0000010000000000
   ctr: 0x0100000000000000
   header: df0100000000000100000000000000

   kid: 0x0000010000000000
   ctr: 0xffffffffffffffff
   header: df010000000000ffffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0000000000000000
   header: d0ffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0000000000000001
   header: d1ffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x00000000000000ff
   header: d8ffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0000000000000100
   header: d9ffffffffffff0100

   kid: 0x0000ffffffffffff
   ctr: 0x000000000000ffff
   header: d9ffffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0000000000010000
   header: daffffffffffff010000

   kid: 0x0000ffffffffffff
   ctr: 0x0000000000ffffff
   header: daffffffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0000000001000000
   header: dbffffffffffff01000000

   kid: 0x0000ffffffffffff
   ctr: 0x00000000ffffffff
   header: dbffffffffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0000000100000000
   header: dcffffffffffff0100000000

   kid: 0x0000ffffffffffff
   ctr: 0x000000ffffffffff
   header: dcffffffffffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0000010000000000
   header: ddffffffffffff010000000000

   kid: 0x0000ffffffffffff
   ctr: 0x0000ffffffffffff
   header: ddffffffffffffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0001000000000000
   header: deffffffffffff01000000000000

   kid: 0x0000ffffffffffff
   ctr: 0x00ffffffffffffff
   header: deffffffffffffffffffffffffff

   kid: 0x0000ffffffffffff
   ctr: 0x0100000000000000
   header: dfffffffffffff0100000000000000

   kid: 0x0000ffffffffffff
   ctr: 0xffffffffffffffff
   header: dfffffffffffffffffffffffffffff

   kid: 0x0001000000000000
   ctr: 0x0000000000000000
   header: e001000000000000

   kid: 0x0001000000000000
   ctr: 0x0000000000000001
   header: e101000000000000

   kid: 0x0001000000000000
   ctr: 0x00000000000000ff
   header: e801000000000000ff

   kid: 0x0001000000000000
   ctr: 0x0000000000000100
   header: e9010000000000000100

   kid: 0x0001000000000000
   ctr: 0x000000000000ffff
   header: e901000000000000ffff

   kid: 0x0001000000000000
   ctr: 0x0000000000010000
   header: ea01000000000000010000

   kid: 0x0001000000000000
   ctr: 0x0000000000ffffff
   header: ea01000000000000ffffff

   kid: 0x0001000000000000
   ctr: 0x0000000001000000
   header: eb0100000000000001000000

   kid: 0x0001000000000000
   ctr: 0x00000000ffffffff
   header: eb01000000000000ffffffff

   kid: 0x0001000000000000
   ctr: 0x0000000100000000
   header: ec010000000000000100000000

   kid: 0x0001000000000000
   ctr: 0x000000ffffffffff
   header: ec01000000000000ffffffffff

   kid: 0x0001000000000000
   ctr: 0x0000010000000000
   header: ed01000000000000010000000000

   kid: 0x0001000000000000
   ctr: 0x0000ffffffffffff
   header: ed01000000000000ffffffffffff

   kid: 0x0001000000000000
   ctr: 0x0001000000000000
   header: ee0100000000000001000000000000

   kid: 0x0001000000000000
   ctr: 0x00ffffffffffffff
   header: ee01000000000000ffffffffffffff

   kid: 0x0001000000000000
   ctr: 0x0100000000000000
   header: ef010000000000000100000000000000

   kid: 0x0001000000000000
   ctr: 0xffffffffffffffff
   header: ef01000000000000ffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0000000000000000
   header: e0ffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0000000000000001
   header: e1ffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x00000000000000ff
   header: e8ffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0000000000000100
   header: e9ffffffffffffff0100

   kid: 0x00ffffffffffffff
   ctr: 0x000000000000ffff
   header: e9ffffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0000000000010000
   header: eaffffffffffffff010000

   kid: 0x00ffffffffffffff
   ctr: 0x0000000000ffffff
   header: eaffffffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0000000001000000
   header: ebffffffffffffff01000000

   kid: 0x00ffffffffffffff
   ctr: 0x00000000ffffffff
   header: ebffffffffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0000000100000000
   header: ecffffffffffffff0100000000

   kid: 0x00ffffffffffffff
   ctr: 0x000000ffffffffff
   header: ecffffffffffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0000010000000000
   header: edffffffffffffff010000000000

   kid: 0x00ffffffffffffff
   ctr: 0x0000ffffffffffff
   header: edffffffffffffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0001000000000000
   header: eeffffffffffffff01000000000000

   kid: 0x00ffffffffffffff
   ctr: 0x00ffffffffffffff
   header: eeffffffffffffffffffffffffffff

   kid: 0x00ffffffffffffff
   ctr: 0x0100000000000000
   header: efffffffffffffff0100000000000000

   kid: 0x00ffffffffffffff
   ctr: 0xffffffffffffffff
   header: efffffffffffffffffffffffffffffff

   kid: 0x0100000000000000
   ctr: 0x0000000000000000
   header: f00100000000000000

   kid: 0x0100000000000000
   ctr: 0x0000000000000001
   header: f10100000000000000

   kid: 0x0100000000000000
   ctr: 0x00000000000000ff
   header: f80100000000000000ff

   kid: 0x0100000000000000
   ctr: 0x0000000000000100
   header: f901000000000000000100

   kid: 0x0100000000000000
   ctr: 0x000000000000ffff
   header: f90100000000000000ffff

   kid: 0x0100000000000000
   ctr: 0x0000000000010000
   header: fa0100000000000000010000

   kid: 0x0100000000000000
   ctr: 0x0000000000ffffff
   header: fa0100000000000000ffffff

   kid: 0x0100000000000000
   ctr: 0x0000000001000000
   header: fb010000000000000001000000

   kid: 0x0100000000000000
   ctr: 0x00000000ffffffff
   header: fb0100000000000000ffffffff

   kid: 0x0100000000000000
   ctr: 0x0000000100000000
   header: fc01000000000000000100000000

   kid: 0x0100000000000000
   ctr: 0x000000ffffffffff
   header: fc0100000000000000ffffffffff

   kid: 0x0100000000000000
   ctr: 0x0000010000000000
   header: fd0100000000000000010000000000

   kid: 0x0100000000000000
   ctr: 0x0000ffffffffffff
   header: fd0100000000000000ffffffffffff

   kid: 0x0100000000000000
   ctr: 0x0001000000000000
   header: fe010000000000000001000000000000

   kid: 0x0100000000000000
   ctr: 0x00ffffffffffffff
   header: fe0100000000000000ffffffffffffff

   kid: 0x0100000000000000
   ctr: 0x0100000000000000
   header: ff010000000000000001000000000000
           00

   kid: 0x0100000000000000
   ctr: 0xffffffffffffffff
   header: ff0100000000000000ffffffffffffff
           ff

   kid: 0xffffffffffffffff
   ctr: 0x0000000000000000
   header: f0ffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0000000000000001
   header: f1ffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x00000000000000ff
   header: f8ffffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0000000000000100
   header: f9ffffffffffffffff0100

   kid: 0xffffffffffffffff
   ctr: 0x000000000000ffff
   header: f9ffffffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0000000000010000
   header: faffffffffffffffff010000

   kid: 0xffffffffffffffff
   ctr: 0x0000000000ffffff
   header: faffffffffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0000000001000000
   header: fbffffffffffffffff01000000

   kid: 0xffffffffffffffff
   ctr: 0x00000000ffffffff
   header: fbffffffffffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0000000100000000
   header: fcffffffffffffffff0100000000

   kid: 0xffffffffffffffff
   ctr: 0x000000ffffffffff
   header: fcffffffffffffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0000010000000000
   header: fdffffffffffffffff010000000000

   kid: 0xffffffffffffffff
   ctr: 0x0000ffffffffffff
   header: fdffffffffffffffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0001000000000000
   header: feffffffffffffffff01000000000000

   kid: 0xffffffffffffffff
   ctr: 0x00ffffffffffffff
   header: feffffffffffffffffffffffffffffff

   kid: 0xffffffffffffffff
   ctr: 0x0100000000000000
   header: ffffffffffffffffff01000000000000
           00

   kid: 0xffffffffffffffff
   ctr: 0xffffffffffffffff
   header: ffffffffffffffffffffffffffffffff
           ff

C.2.  AEAD Encryption/Decryption Using AES-CTR and HMAC

   For each case, we provide:

   *  cipher_suite: The index of the cipher suite in use (see
      Section 8.1)

   *  key: The key input to encryption/decryption

   *  enc_key: The encryption subkey produced by the derive_subkeys()
      algorithm

   *  auth_key: The encryption subkey produced by the derive_subkeys()
      algorithm

   *  nonce: The nonce input to encryption/decryption

   *  aad: The aad input to encryption/decryption

   *  pt: The plaintext

   *  ct: The ciphertext

   An implementation should verify that the following are true, where
   AEAD.Encrypt and AEAD.Decrypt are as defined in Section 4.5.1:

   *  AEAD.Encrypt(key, nonce, aad, pt) == ct

   *  AEAD.Decrypt(key, nonce, aad, ct) == pt

   The other values in the test vector are intermediate values provided
   to facilitate debugging of test failures.

   cipher_suite: 0x0001
   key: 000102030405060708090a0b0c0d0e0f
        101112131415161718191a1b1c1d1e1f
        202122232425262728292a2b2c2d2e2f
   enc_key: 000102030405060708090a0b0c0d0e0f
   auth_key: 101112131415161718191a1b1c1d1e1f
             202122232425262728292a2b2c2d2e2f
   nonce: 101112131415161718191a1b
   aad: 4945544620534672616d65205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 6339af04ada1d064688a442b8dc69d5b
       6bfa40f4bef0583e8081069cc60705

   cipher_suite: 0x0002
   key: 000102030405060708090a0b0c0d0e0f
        101112131415161718191a1b1c1d1e1f
        202122232425262728292a2b2c2d2e2f
   enc_key: 000102030405060708090a0b0c0d0e0f
   auth_key: 101112131415161718191a1b1c1d1e1f
             202122232425262728292a2b2c2d2e2f
   nonce: 101112131415161718191a1b
   aad: 4945544620534672616d65205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 6339af04ada1d064688a442b8dc69d5b
       6bfa40f4be6e93b7da076927bb

   cipher_suite: 0x0003
   key: 000102030405060708090a0b0c0d0e0f
        101112131415161718191a1b1c1d1e1f
        202122232425262728292a2b2c2d2e2f
   enc_key: 000102030405060708090a0b0c0d0e0f
   auth_key: 101112131415161718191a1b1c1d1e1f
             202122232425262728292a2b2c2d2e2f
   nonce: 101112131415161718191a1b
   aad: 4945544620534672616d65205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 6339af04ada1d064688a442b8dc69d5b
       6bfa40f4be09480509

C.3.  SFrame Encryption/Decryption

   For each case, we provide:

   *  cipher_suite: The index of the cipher suite in use (see
      Section 8.1)

   *  kid: A KID value

   *  ctr: A CTR value

   *  base_key: The base_key input to the derive_key_salt algorithm

   *  sframe_key_label: The label used to derive sframe_key in the
      derive_key_salt algorithm

   *  sframe_salt_label: The label used to derive sframe_salt in the
      derive_key_salt algorithm

   *  sframe_secret: The sframe_secret variable in the derive_key_salt
      algorithm

   *  sframe_key: The sframe_key value produced by the derive_key_salt
      algorithm

   *  sframe_salt: The sframe_salt value produced by the derive_key_salt
      algorithm

   *  metadata: The metadata input to the SFrame encrypt algorithm

   *  pt: The plaintext

   *  ct: The SFrame ciphertext

   An implementation should verify that the following are true, where
   encrypt and decrypt are as defined in Section 4.4, using an SFrame
   context initialized with base_key assigned to kid:

   *  encrypt(ctr, kid, metadata, plaintext) == ct

   *  decrypt(metadata, ct) == pt

   The other values in the test vector are intermediate values provided
   to facilitate debugging of test failures.

   cipher_suite: 0x0001
   kid: 0x0000000000000123
   ctr: 0x0000000000004567
   base_key: 000102030405060708090a0b0c0d0e0f
   sframe_key_label: 534672616d6520312e30205365637265
                     74206b65792000000000000001230001
   sframe_salt_label: 534672616d6520312e30205365637265
                      742073616c7420000000000000012300
                      01
   sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
                  ceff8cceee34f574d23909eb314c40c0
   sframe_key: 3f7d9a7c83ae8e1c8a11ae695ab59314
               b367e359fadac7b9c46b2bc6f81f46e1
               6b96f0811868d59402b7e870102720b3
   sframe_salt: 50b29329a04dc0f184ac3168
   metadata: 4945544620534672616d65205747
   nonce: 50b29329a04dc0f184ac740f
   aad: 99012345674945544620534672616d65
        205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 9901234567449408b6f490086165b9d6
       f62b24ae1a59a56486b4ae8ed036b889
       12e24f11

   cipher_suite: 0x0002
   kid: 0x0000000000000123
   ctr: 0x0000000000004567
   base_key: 000102030405060708090a0b0c0d0e0f
   sframe_key_label: 534672616d6520312e30205365637265
                     74206b65792000000000000001230002
   sframe_salt_label: 534672616d6520312e30205365637265
                      742073616c7420000000000000012300
                      02
   sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
                  ceff8cceee34f574d23909eb314c40c0
   sframe_key: e2ec5c797540310483b16bf6e7a570d2
               a27d192fe869c7ccd8584a8d9dab9154
               9fbe553f5113461ec6aa83bf3865553e
   sframe_salt: e68ac8dd3d02fbcd368c5577
   metadata: 4945544620534672616d65205747
   nonce: e68ac8dd3d02fbcd368c1010
   aad: 99012345674945544620534672616d65
        205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 99012345673f31438db4d09434e43afa
       0f8a2f00867a2be085046a9f5cb4f101
       d607

   cipher_suite: 0x0003
   kid: 0x0000000000000123
   ctr: 0x0000000000004567
   base_key: 000102030405060708090a0b0c0d0e0f
   sframe_key_label: 534672616d6520312e30205365637265
                     74206b65792000000000000001230003
   sframe_salt_label: 534672616d6520312e30205365637265
                      742073616c7420000000000000012300
                      03
   sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
                  ceff8cceee34f574d23909eb314c40c0
   sframe_key: 2c5703089cbb8c583475e4fc461d97d1
               8809df79b6d550f78eb6d50ffa80d892
               11d57909934f46f5405e38cd583c69fe
   sframe_salt: 38c16e4f5159700c00c7f350
   metadata: 4945544620534672616d65205747
   nonce: 38c16e4f5159700c00c7b637
   aad: 99012345674945544620534672616d65
        205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 990123456717fc8af28a5a695afcfc6c
       8df6358a17e26b2fcb3bae32e443

   cipher_suite: 0x0004
   kid: 0x0000000000000123
   ctr: 0x0000000000004567
   base_key: 000102030405060708090a0b0c0d0e0f
   sframe_key_label: 534672616d6520312e30205365637265
                     74206b65792000000000000001230004
   sframe_salt_label: 534672616d6520312e30205365637265
                      742073616c7420000000000000012300
                      04
   sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
                  ceff8cceee34f574d23909eb314c40c0
   sframe_key: d34f547f4ca4f9a7447006fe7fcbf768
   sframe_salt: 75234edefe07819026751816
   metadata: 4945544620534672616d65205747
   nonce: 75234edefe07819026755d71
   aad: 99012345674945544620534672616d65
        205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 9901234567b7412c2513a1b66dbb4884
       1bbaf17f598751176ad847681a69c6d0
       b091c07018ce4adb34eb

   cipher_suite: 0x0005
   kid: 0x0000000000000123
   ctr: 0x0000000000004567
   base_key: 000102030405060708090a0b0c0d0e0f
   sframe_key_label: 534672616d6520312e30205365637265
                     74206b65792000000000000001230005
   sframe_salt_label: 534672616d6520312e30205365637265
                      742073616c7420000000000000012300
                      05
   sframe_secret: 0fc3ea6de6aac97a35f194cf9bed94d4
                  b5230f1cb45a785c9fe5dce9c188938a
                  b6ba005bc4c0a19181599e9d1bcf7b74
                  aca48b60bf5e254e546d809313e083a3
   sframe_key: d3e27b0d4a5ae9e55df01a70e6d4d28d
               969b246e2936f4b7a5d9b494da6b9633
   sframe_salt: 84991c167b8cd23c93708ec7
   metadata: 4945544620534672616d65205747
   nonce: 84991c167b8cd23c9370cba0
   aad: 99012345674945544620534672616d65
        205747
   pt: 64726166742d696574662d736672616d
       652d656e63
   ct: 990123456794f509d36e9beacb0e261d
       99c7d1e972f1fed787d4049f17ca2135
       3c1cc24d56ceabced279

Acknowledgements

   The authors wish to specially thank Dr. Alex Gouaillard as one of the
   early contributors to the document.  His passion and energy were key
   to the design and development of SFrame.

Contributors

   Frédéric Jacobs
   Apple
   Email: frederic.jacobs@apple.com


   Marta Mularczyk
   Amazon
   Email: mulmarta@amazon.com


   Suhas Nandakumar
   Cisco
   Email: snandaku@cisco.com


   Tomas Rigaux
   Cisco
   Email: trigaux@cisco.com


   Raphael Robert
   Phoenix R&D
   Email: ietf@raphaelrobert.com


Authors' Addresses

   Emad Omara
   Apple
   Email: eomara@apple.com


   Justin Uberti
   Fixie.ai
   Email: justin@fixie.ai


   Sergio Garcia Murillo
   CoSMo Software
   Email: sergio.garcia.murillo@cosmosoftware.io


   Richard Barnes (editor)
   Cisco
   Email: rlb@ipv.sx


   Youenn Fablet
   Apple
   Email: youenn@apple.com
