RFC 9605 | SFrame | August 2024
Omara, et al. | Standards Track | [Page]

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

# RFC 9605

# 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.¶

## 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.¶

¶
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.¶

¶
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.¶

¶
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.¶

¶
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).¶

¶
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.¶

¶
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:¶

Table 1: SFrame Cipher Suite Constants
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

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))

¶
¶
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))

¶
¶
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.¶

¶
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 220 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 212 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:¶

Table 2: SFrame Cipher Suites
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 - 0xFFFF | Reserved for Private Use | - | RFC 9605 | IETF

## 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.¶

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

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"):¶

Table 4: SFrame Overhead for Audio Streams
Scenario | Frame length | fps | Base kbps | Overhead kbps | Overhead %
--- | --- | --- | --- | --- | ---
Narrow-band speech | 120 ms | 8.3 | 8 | 1.4 | 17.9%
Full-band speech | 20 ms | 50 | 32 | 8.6 | 26.9%
Full-band stereo music | 10 ms | 100 | 128 | 17.2 | 13.4%

### 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.¶

Table 5: SFrame Overhead for a Video Stream Encrypted per Frame
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 6: SFrame Overhead for a Video Stream Encrypted per Packet
Scenario | fps | Packets per Second (pps) | Base kbps | Overhead kbps | Overhead %
--- | --- | --- | --- | --- | ---
426 x 240 | 7.5 | 7.5 | 45 | 1.3 | 2.9%
640 x 360 | 15 | 30 | 200 | 5.2 | 2.6%
640 x 360 | 30 | 60 | 400 | 10.3 | 2.6%
1280 x 720 | 30 | 180 | 1500 | 30.9 | 2.1%
1920 x 1080 | 60 | 780 | 7200 | 134.1 | 1.9%

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%.¶

Table 7: SFrame Overhead for a Two-Person Conference
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%

### 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].¶

¶
Figure 11: SRTP Packet with SFrame-Protected Payload
¶
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
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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x00000000000000ff
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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x000000000000ffff
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¶

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¶

    kid: 0x000000000000ffff
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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x000000000000ffff
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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x0000000000010000
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¶

    kid: 0x0000000000010000
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¶

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¶

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¶

    kid: 0x0000000000010000
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¶

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¶

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¶

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¶

    kid: 0x0000000000010000
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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x0000000000ffffff
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¶

    kid: 0x0000000000ffffff
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¶

    kid: 0x0000000000ffffff
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¶

    kid: 0x0000000000ffffff
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¶

    kid: 0x0000000000ffffff
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¶

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¶

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¶

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¶

    kid: 0x0000000000ffffff
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¶

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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

    kid: 0x0000000001000000
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¶

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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x0000000001000000
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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x00000000ffffffff
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x0000000100000000
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¶

    kid: 0x000000ffffffffff
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¶

    kid: 0x000000ffffffffff
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¶

    kid: 0x000000ffffffffff
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¶

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¶

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¶

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¶

    kid: 0x000000ffffffffff
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¶

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¶

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¶

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¶

    kid: 0x000000ffffffffff
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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x000000ffffffffff
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¶

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¶

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¶

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¶

    kid: 0x0000010000000000
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¶

    kid: 0x0000010000000000
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¶

    kid: 0x0000010000000000
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¶

    kid: 0x0000010000000000
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¶

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¶

    kid: 0x0000010000000000
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¶

    kid: 0x0000010000000000
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¶

    kid: 0x0000010000000000
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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x0000010000000000
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¶

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¶

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    header: d1ffffffffffff

¶

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    header: d8ffffffffffffff

¶

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¶

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    header: d9ffffffffffffffff

¶

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¶

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    header: daffffffffffffffffff

¶

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    header: dbffffffffffff01000000

¶

    kid: 0x0000ffffffffffff
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    header: dbffffffffffffffffffff

¶

    kid: 0x0000ffffffffffff
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    header: dcffffffffffff0100000000

¶

    kid: 0x0000ffffffffffff
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¶

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    header: ddffffffffffff010000000000

¶

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    header: ddffffffffffffffffffffffff

¶

    kid: 0x0000ffffffffffff
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    header: deffffffffffff01000000000000

¶

    kid: 0x0000ffffffffffff
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    header: deffffffffffffffffffffffffff

¶

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¶

    kid: 0x0000ffffffffffff
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

    kid: 0x0001000000000000
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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

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¶

    kid: 0x00ffffffffffffff
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¶

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¶

    kid: 0x0100000000000000
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¶

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¶

    kid: 0x0100000000000000
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¶

    kid: 0x0100000000000000
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¶

    kid: 0x0100000000000000
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¶

    kid: 0x0100000000000000
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¶

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¶

    kid: 0x0100000000000000
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¶

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¶

    kid: 0x0100000000000000
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¶

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¶

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¶

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¶

    kid: 0x0100000000000000
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¶

    kid: 0x0100000000000000
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            00

¶

    kid: 0x0100000000000000
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            ff

¶

    kid: 0xffffffffffffffff
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¶

    kid: 0xffffffffffffffff
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¶

    kid: 0xffffffffffffffff
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¶

    kid: 0xffffffffffffffff
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¶

    kid: 0xffffffffffffffff
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¶

    kid: 0xffffffffffffffff
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¶

    kid: 0xffffffffffffffff
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¶

    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

## Authors' Addresses