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
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Secure Frame (SFrame): Lightweight Authenticated Encryption for
Real-Time Media
Abstract
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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
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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
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Copyright © 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
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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
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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 endpointsThe 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
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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
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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:
- Provide a secure E2EE mechanism for audio and video in conference calls that can be used with arbitrary SFU servers.
- Decouple media encryption from key management to allow SFrame to be used with an arbitrary key management system.
- Minimize packet expansion to allow successful conferencing in as many network conditions as possible.
- Decouple the media encryption framework from the underlying transport, allowing use in non-RTP scenarios, e.g., WebTransport [WEBTRANSPORT].
- 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.
- Minimize the changes needed in SFU servers.
- Minimize the changes needed in endpoints.
- Work with the most popular audio and video codecs used in conferencing scenarios.
4. SFrame
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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):
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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)
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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_saltIn 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
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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):
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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 + ciphertextFor 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
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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)
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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
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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
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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
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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):
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# 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):
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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
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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
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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
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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
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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
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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
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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
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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
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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