Network Working Group E. Omara
Internet-Draft Apple
Intended status: Standards Track J. Uberti
Expires: 24 April 2024 Google
S. Murillo
CoSMo Software
R. L. Barnes, Ed.
Cisco
Y. Fablet
Apple
22 October 2023
Secure Frame (SFrame)
draft-ietf-sframe-enc-04
Abstract
This document describes the Secure Frame (SFrame) end-to-end
encryption and authentication mechanism for media frames in a
multiparty conference call, in which central media servers (selective
forwarding units or SFUs) can access the media metadata needed to
make forwarding decisions without having access to the actual media.
The proposed 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.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://sframe-
wg.github.io/sframe/draft-ietf-sframe-enc.html. Status information
for this document may be found at https://datatracker.ietf.org/doc/
draft-ietf-sframe-enc/.
Discussion of this document takes place on the Secure Media Frames
Working Group mailing list (mailto:sframe@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/sframe/.
Subscribe at https://www.ietf.org/mailman/listinfo/sframe/.
Source for this draft and an issue tracker can be found at
https://github.com/sframe-wg/sframe.
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Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 24 April 2024.
Copyright Notice
Copyright (c) 2023 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. SFrame . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Application Context . . . . . . . . . . . . . . . . . . . 5
4.2. SFrame Ciphertext . . . . . . . . . . . . . . . . . . . . 8
4.3. SFrame Header . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Encryption Schema . . . . . . . . . . . . . . . . . . . . 10
4.4.1. Key Selection . . . . . . . . . . . . . . . . . . . . 10
4.4.2. Key Derivation . . . . . . . . . . . . . . . . . . . 11
4.4.3. Encryption . . . . . . . . . . . . . . . . . . . . . 12
4.4.4. Decryption . . . . . . . . . . . . . . . . . . . . . 13
4.5. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 14
4.5.1. AES-CTR with SHA2 . . . . . . . . . . . . . . . . . . 15
5. Key Management . . . . . . . . . . . . . . . . . . . . . . . 16
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5.1. Sender Keys . . . . . . . . . . . . . . . . . . . . . . . 16
5.2. MLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Media Considerations . . . . . . . . . . . . . . . . . . . . 19
6.1. Selective Forwarding Units . . . . . . . . . . . . . . . 19
6.1.1. LastN and RTP stream reuse . . . . . . . . . . . . . 20
6.1.2. Simulcast . . . . . . . . . . . . . . . . . . . . . . 20
6.1.3. SVC . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.2. Video Key Frames . . . . . . . . . . . . . . . . . . . . 20
6.3. Partial Decoding . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7.1. No Header Confidentiality . . . . . . . . . . . . . . . . 21
7.2. No Per-Sender Authentication . . . . . . . . . . . . . . 21
7.3. Key Management . . . . . . . . . . . . . . . . . . . . . 21
7.4. Authentication tag length . . . . . . . . . . . . . . . . 22
7.5. Replay . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8.1. SFrame Cipher Suites . . . . . . . . . . . . . . . . . . 22
9. Application Responsibilities . . . . . . . . . . . . . . . . 23
9.1. Header Value Uniqueness . . . . . . . . . . . . . . . . . 23
9.2. Key Management Framework . . . . . . . . . . . . . . . . 23
9.3. Anti-Replay . . . . . . . . . . . . . . . . . . . . . . . 24
9.4. Metadata . . . . . . . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 27
Appendix B. Example API . . . . . . . . . . . . . . . . . . . . 27
Appendix C. Overhead Analysis . . . . . . . . . . . . . . . . . 28
C.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 29
C.2. Audio . . . . . . . . . . . . . . . . . . . . . . . . . . 29
C.3. Video . . . . . . . . . . . . . . . . . . . . . . . . . . 30
C.4. Conferences . . . . . . . . . . . . . . . . . . . . . . . 31
C.5. SFrame over RTP . . . . . . . . . . . . . . . . . . . . . 32
Appendix D. Test Vectors . . . . . . . . . . . . . . . . . . . . 34
D.1. Header encoding/decoding . . . . . . . . . . . . . . . . 35
D.2. AEAD encryption/decryption using AES-CTR and HMAC . . . . 59
D.3. SFrame encryption/decryption . . . . . . . . . . . . . . 61
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 63
1. Introduction
Modern multi-party 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, allowing for the media
to be "end-to-end" encrypted so that it cannot be decrypted by the
SFU. In order for the SFU to work properly, though, it usually needs
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to be able to access RTP metadata and RTCP feedback messages, which
is not possible if all RTP/RTCP traffic is end-to-end encrypted.
As such, two layers of encryptions and authentication are required:
1. Hop-by-hop (HBH) encryption of media, metadata, and feedback
messages between the the endpoints and SFU
2. End-to-end (E2E) encryption of media between the endpoints
The Secure Real-Time Protocol (SRTP) is already widely used for HBH
encryption [RFC3711]. The SRTP "double encryption" scheme defines a
way to do E2E encryption in SRTP [RFC8723]. Unfortunately, this
scheme has poor efficiency and high complexity, and its entanglement
with RTP makes it unworkable in several realistic SFU scenarios.
This document proposes a new end-to-end encryption mechanism known as
SFrame, specifically designed to work in group conference calls with
SFUs. SFrame is a general encryption framing that can be used to
protect media payloads, agnostic of transport.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
IV: Initialization Vector
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 a sequence of media
payloads, in both cases regardless of whether those media payloads
are transported over RTP or some other protocol.
3. Goals
SFrame is designed to be a suitable E2EE protection scheme for
conference call media in a broad range of scenarios, as outlined by
the following goals:
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1. Provide an secure E2EE mechanism for audio and video in
conference calls that can be used with arbitrary SFU servers.
2. Decouple media encryption from key management to allow SFrame to
be used with an arbitrary key management system.
3. Minimize packet expansion to allow successful conferencing in as
many network conditions as possible.
4. Independence from the underlying transport, including use in non-
RTP transports, e.g., WebTransport [I-D.ietf-webtrans-overview].
5. When used with RTP and its associated error resilience
mechanisms, i.e., RTX and FEC, require no special handling for
RTX and FEC packets.
6. Minimize the changes needed in SFU servers.
7. Minimize the changes needed in endpoints.
8. Work with the most popular audio and video codecs used in
conferencing scenarios.
4. SFrame
This document defines an encryption mechanism that provides effective
end-to-end encryption, is simple to implement, has no dependencies on
RTP, and minimizes encryption bandwidth overhead. Because SFrame can
encrypt a full frame, rather than individual packets, bandwidth
overhead can be reduced by adding encryption overhead only once per
media frame, instead of once per packet.
4.1. Application Context
SFrame is a general encryption framing, intended to be used as an E2E
encryption layer over an underlying HBH-encrypted transport such as
SRTP or QUIC [RFC3711][I-D.ietf-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: Either using
SFrame to encrypt 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 in from some source, encodes it into frames, divides those
frames into media packets, and then sends those payloads in SRTP
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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 C.
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.
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+--------------------------------------------------------+
| |
| +----------+ +-------------+ +-----------+ |
.-. | | | | | | 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
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 IVs.
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4.2. SFrame Ciphertext
An SFrame ciphertext comprises an SFrame header followed by the
output of an 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 ---+
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.
4.3. SFrame Header
The SFrame header specifies two values from which encryption
parameters are derived:
* A Key ID (KID) that determines which encryption key should be used
* A counter (CTR) that is used to construct the IV for the
encryption
Applications MUST ensure that each (KID, CTR) combination is used for
exactly one encryption operation. A typical approach to achieving
this gaurantee is outlined in Section 9.1.
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Config Byte
|
.-----' '-----.
| |
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+------------+------------+
|X| K |Y| C | KID... | CTR... |
+-+-+-+-+-+-+-+-+------------+------------+
Figure 2: SFrame header
The SFrame Header has the overall structure shown in Figure 2. The
first byte is a "config byte", with the following fields:
Extended Key Id Flag (X, 1 bit): Indicates if the K field contains
the key id or the key id length.
Key or Key Length (K, 3 bits): This field contains the key id (KID)
if the X flag is set to 0, or the key id length if set to 1.
Extended Counter Flag (Y, 1 bit): Indicates if the C field contains
the counter or the counter length.
Counter or Counter Length (C, 3 bits): This field contains the
counter (CTR) if the Y flag is set to 0, or the counter length if
set to 1.
The Key ID and Counter 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
configuration byte, with the Key ID first and Counter 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 3, depending on which of the X and Y flags are set.
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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 3: Forms of Encoded SFrame Header
4.4. Encryption Schema
SFrame encryption uses an AEAD encryption algorithm and hash function
defined by the cipher suite in use (see Section 4.5). We will refer
to the following aspects of the AEAD 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)
4.4.1. Key Selection
Each SFrame encryption or decryption operation is premised on a
single secret base_key, which is labeled with an integer KID value
signaled in the SFrame header.
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The sender and receivers need to agree on which key should be used
for a given KID. The process for provisioning keys 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 counter value CTR to be used when
encrypting (initially 0).
When encrypting a plaintext, the application specifies which KID is
to be used, and the counter 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 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 SHOULD mark each key as
usable for encryption or decryption, never both.
Note that the set of available keys might change over the lifetime of
a real-time session. In such cases, the client will need to manage
key usage to avoid media loss due to a key being used to encrypt
before all receivers are able to use it to decrypt. For example, an
application may make decryption-only keys available immediately, but
delay the use of keys for encryption until (a) all receivers have
acknowledged receipt of the new key or (b) a timeout expires.
4.4.2. Key Derivation
SFrame encrytion and decryption use a key and salt derived from the
base_key associated to a KID. Given a base_key value, the key and
salt are derived using HKDF [RFC5869] as follows:
def derive_key_salt(KID, base_key):
sframe_secret = HKDF-Extract("", base_key)
info = "SFrame 1.0 Secret key " + KID + cipher_suite
sframe_key = HKDF-Expand(sframe_secret, info, AEAD.Nk)
sframe_salt = HKDF-Expand(sframe_secret, info, AEAD.Nn)
return sframe_key, sframe_salt
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In the derivation of sframe_secret: * The + operator represents
concatenation of byte strings. * The KID value is encoded as an
8-byte big-endian integer, not the compressed form used in the SFrame
header. * The cipher_suite value is a 2-byte big-endian integer
representing the cipher suite in use (see Section 8.1).
The hash function used for HKDF is determined by the cipher suite in
use.
4.4.3. Encryption
SFrame encryption uses the AEAD encryption algorithm for the cipher
suite in use. The key for the encryption is the sframe_key and the
nonce is formed by XORing the sframe_salt with the current counter,
encoded as a big-endian integer of length AEAD.Nn.
The encryptor forms an SFrame header using the CTR, and KID values
provided. The encoded header is provided as AAD to the AEAD
encryption operation, together with application-provided metadata
about the encrypted media (see Section 9.4).
def encrypt(CTR, KID, metadata, plaintext):
sframe_key, sframe_salt = key_store[KID]
ctr = encode_big_endian(CTR, AEAD.Nn)
nonce = xor(sframe_salt, CTR)
header = encode_sframe_header(CTR, KID)
aad = header + metadata
ciphertext = AEAD.Encrypt(sframe_key, nonce, aad, plaintext)
return header + ciphertext
For example, the metadata input to encryption allows for frame
metadata to be authenticated when SFrame is applied per-frame. After
encoding the frame and before packetizing it, the necessary media
metadata will be moved out of the encoded frame buffer, to be sent in
some channel visible to the SFU (e.g., an RTP header extension).
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+----------------+ +---------------+
| metadata | | |
+-------+--------+ | |
| | plaintext |
| | |
| | |
| +-------+-------+
| |
header ----+------------------>| AAD
+-----+ |
| S | |
+-----+ |
| KID +--+--> sframe_key ----->| Key
| | | |
| | +--> sframe_salt --+ |
+-----+ | |
| CTR +---------------------+->| Nonce
| | |
| | |
+-----+ |
| AEAD.Encrypt
| |
| +---------------+ |
+---->| SFrame Header | |
+---------------+ |
| | |
| |<----+
| ciphertext |
| |
| |
+---------------+
Figure 4: Encryption flow
4.4.4. Decryption
Before decrypting, a client needs to assemble a full SFrame
ciphertext. When an SFrame ciphertext may be 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 an 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, then 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.
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def decrypt(metadata, sframe_ciphertext):
KID, CTR, ciphertext = parse_ciphertext(sframe_ciphertext)
sframe_key, sframe_salt = key_store[KID]
ctr = encode_big_endian(CTR, AEAD.Nn)
nonce = xor(sframe_salt, ctr)
aad = header + metadata
return AEAD.Decrypt(sframe_key, nonce, aad, ciphertext)
If a ciphertext fails to decrypt because there is no key available
for the KID in the SFrame header, the client MAY buffer the
ciphertext and retry decryption once a key with that KID is received.
4.5. Cipher Suites
Each SFrame session uses a single cipher suite that specifies the
following primitives:
* A hash function used for key derivation
* An AEAD encryption algorithm [RFC5116] used for frame encryption,
optionally with a truncated authentication tag
This document defines the following cipher suites, with the constants
defined in Section 4.4:
+============================+====+====+====+====+
| Name | Nh | Nk | Nn | Nt |
+============================+====+====+====+====+
| AES_128_CTR_HMAC_SHA256_80 | 32 | 48 | 12 | 10 |
+----------------------------+----+----+----+----+
| AES_128_CTR_HMAC_SHA256_64 | 32 | 48 | 12 | 8 |
+----------------------------+----+----+----+----+
| AES_128_CTR_HMAC_SHA256_32 | 32 | 48 | 12 | 4 |
+----------------------------+----+----+----+----+
| AES_128_GCM_SHA256_128 | 32 | 16 | 12 | 16 |
+----------------------------+----+----+----+----+
| AES_256_GCM_SHA512_128 | 64 | 32 | 12 | 16 |
+----------------------------+----+----+----+----+
Table 1: SFrame cipher suite constants
Numeric identifiers for these cipher suites are defined in the IANA
registry created in Section 8.1.
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In the suite names, the length of the authentication tag is indicated
by the last value: "_128" indicates a hundred-twenty-eight-bit tag,
"_80" indicates a eighty-bit tag, "_64" indicates a sixty-four-bit
tag and "_32" indicates a thirty-two-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 eighty-bit tags for video frames and another cipher suite with
thirty-two-bit tags for audio frames.
4.5.1. AES-CTR with SHA2
In order to allow very short tag sizes, we define a synthetic AEAD
function using the authenticated counter mode of AES together with
HMAC for authentication. We use an encrypt-then-MAC approach, as in
SRTP [RFC3711].
Before encryption or decryption, encryption and authentication
subkeys are derived from the single AEAD key. The overall length of
the AEAD key is Nka + Nh, where Nka represents the key size for the
AES block cipher in use and Nh represents the output size of the hash
function (as in Table 2). The encryption subkey comprises the first
Nka bytes and the authentication subkey comprises the remaining Nh
bytes.
def derive_subkeys(sframe_key):
enc_key = sframe_key[..Nka]
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.
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def compute_tag(auth_key, nonce, aad, ct):
aad_len = encode_big_endian(len(aad), 8)
ct_len = encode_big_endian(len(ct), 8)
tag_len = encode_big_endian(Nt, 8)
auth_data = aad_len + ct_len + tag_len + nonce + aad + ct
tag = HMAC(auth_key, auth_data)
return truncate(tag, Nt)
def AEAD.Encrypt(key, nonce, aad, pt):
enc_key, auth_key = derive_subkeys(key)
iv = nonce + 0x00000000 # append four zero bytes
ct = AES-CTR.Encrypt(enc_key, iv, 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")
iv = nonce + 0x00000000 # append four zero bytes
return AES-CTR.Decrypt(enc_key, iv, inner_ct)
5. Key Management
SFrame must be integrated with an E2E key management framework to
exchange and rotate the keys used for SFrame encryption. The key
management framework provides the following functions:
* Provisioning KID / base_key mappings to participating clients
* Updating the above data as clients join or leave
It is the responsibility of the application to provide the key
management framework, as described in Section 9.2.
5.1. Sender Keys
If the participants in a call have a pre-existing 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.
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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 SSRC).
SFrame KID values are then used to distinguish between versions of
the sender's base_key.
Key IDs 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 re-ordering
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
key ID.
KID = (key_generation << R) + (ratchet_step % (1 << R))
64-R bits R bits
<---------------> <------------>
+-----------------+--------------+
| Key Generation | Ratchet Step |
+-----------------+--------------+
Figure 5: Structure of a KID in the Sender Keys scheme
The sender signals such a ratchet step update by sending with a KID
value in which the ratchet step has been incremented. A receiver who
receives from a sender with a new KID computes the new key as above.
The old key may be kept for some time to allow for out-of-order
delivery, but should be deleted promptly.
If a new participant joins mid-call, 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.
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5.2. MLS
The Messaging Layer Security (MLS) protocol provides group
authenticated key exchange [I-D.ietf-mls-architecture]
[I-D.ietf-mls-protocol]. 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 know as an "epoch", and each member of the group is assigned an
"index" that is constant for the time they are in the group.
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 re-ordering window, since no
more than 2^E epochs can be active at a given time. Receivers MUST
be prepared for the epoch counter to roll over, removing an old epoch
when a new epoch with the same E lower bits is 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 theSbits above the epoch. The remaining64
- S - Ebits of the KID value are acontextvalue chosen by the sender
(context value0` 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 6: Structure of a KID for an MLS Sender
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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 7: An example sequence of KIDs for an MLS-based SFrame
session. We assume that the group has 64 members, S=6.
6. Media Considerations
6.1. Selective Forwarding Units
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 about how to do
this stream selection but in general, in order to do so, the SFU
needs to access metadata associated to each frame and modify the RTP
information of the incoming packets when they are transmitted to the
received participants.
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This section describes how this normal SFU modes of operation
interacts with the E2EE provided by SFrame
6.1.1. LastN and RTP stream reuse
The SFU may choose to send only a certain number of streams based on
the voice activity of the participants. To 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 in the same transport-level stream (e.g., an RTP
stream defined by either SSRC or MID) may carry media from different
streams of different participants. As different keys are used by
each participant for encoding their media, the receiver will be able
to verify which is the sender of the media coming within the RTP
stream at any given point in time, preventing the SFU trying to
impersonate any of the participants with another participant's media.
Note that in order to prevent impersonation by a malicious
participant (not the SFU), 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 counter
for each.
6.1.3. SVC
In both temporal and spatial scalability, the SFU may choose to drop
layers in order to match a certain bitrate or forward specific media
sizes or frames per second. In order to support it, the sender MUST
encode each spatial layer of a given picture in a different frame.
That is, an RTP frame may contain more than one SFrame encrypted
frame with an incrementing frame counter.
6.2. Video Key Frames
Forward and Post-Compromise Security requires that the e2ee keys are
updated anytime a participant joins/leave the call.
The key exchange happens 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
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sender generates the e2ee encrypted frame with a key not known by the
receiver, so it will be discarded. When the sender updates his
sending key with the new key, it will send it in a non-key frame, so
the receiver will be able to decrypt it, but not decode it.
Receiver will re-request an key frame then, but due to sender and SFU
policies, that new key frame could take some time to be generated.
If the sender sends a key frame when the new e2ee key is in use, the
time required for the new participant to display the video is
minimized.
6.3. Partial Decoding
Some codes support partial decoding, where it can decrypt individual
packets without waiting for the full frame to arrive, with SFrame
this won't be possible because the decoder will not access the
packets until the entire frame has arrived and was decrypted.
7. Security Considerations
7.1. No Header Confidentiality
SFrame provides integrity protection to the SFrame Header (the key ID
and counter values), but 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 C.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
Key exchange mechanism is out of scope of this document, however
every client SHOULD change their keys when new clients joins or
leaves the call for "Forward Secrecy" and "Post Compromise Security".
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7.4. Authentication tag length
The cipher suites defined in this draft use short authentication tags
for encryption, however it can easily support other ciphers with full
authentication tag if the short ones are proved insecure.
7.5. 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]).
8. IANA Considerations
This document requests the creation of the following new IANA
registries:
* SFrame Cipher Suites (Section 8.1)
This registries should be under a heading of "SFrame", and
assignments are made via the Specification Required policy [RFC8126].
RFC EDITOR: Please replace XXXX throughout with the RFC number
assigned to this document
8.1. SFrame Cipher Suites
This 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.
Template:
* Value: The numeric value of the cipher suite
* Name: The name of the cipher suite
* Reference: The document where this wire format is defined
Initial contents:
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+=================+============================+===========+
| Value | Name | Reference |
+=================+============================+===========+
| 0x0001 | AES_128_CTR_HMAC_SHA256_80 | RFC XXXX |
+-----------------+----------------------------+-----------+
| 0x0002 | AES_128_CTR_HMAC_SHA256_64 | RFC XXXX |
+-----------------+----------------------------+-----------+
| 0x0003 | AES_128_CTR_HMAC_SHA256_32 | RFC XXXX |
+-----------------+----------------------------+-----------+
| 0x0004 | AES_128_GCM_SHA256_128 | RFC XXXX |
+-----------------+----------------------------+-----------+
| 0x0005 | AES_256_GCM_SHA512_128 | RFC XXXX |
+-----------------+----------------------------+-----------+
| 0xF000 - 0xFFFF | Reserved for private use | RFC XXXX |
+-----------------+----------------------------+-----------+
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 integration must meet in order for
SFrame to operate securely.
9.1. Header Value Uniqueness
Applications MUST ensure that each (KID, CTR) combination is used for
exactly one encryption operation. 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. Note that in addition to its
simplicity, this scheme minimizes overhead by keeping CTR values as
small as possible.
9.2. Key Management Framework
It is up to the application to provision 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 applications KID
assignment strategy MUST be structured to assure the non-reuse
properties discussed above.
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It is also up to the application to define a rotation schedule for
keys. For example, one application might have an ephemeral group for
every call and keep rotating keys when end points 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 7.5,
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.
9.4. Metadata
The metadata input to SFrame operations is pure application-specified
data. As such, it is up to the application 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 SHOULD NOT use SFrame-authenticated
metadata until after the SFrame decrypt function has authenticated
it.
Note: The metadata input is a feature at risk, and needs more
confirmation that it is useful and/or needed.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
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[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/rfc8174>.
10.2. Informative References
[I-D.codec-agnostic-rtp-payload-format]
Murillo, S. G. and A. Gouaillard, "Codec agnostic RTP
payload format for video", Work in Progress, Internet-
Draft, draft-codec-agnostic-rtp-payload-format-00, 19
February 2021, <https://datatracker.ietf.org/doc/html/
draft-codec-agnostic-rtp-payload-format-00>.
[I-D.ietf-mls-architecture]
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-11, 26 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
architecture-11>.
[I-D.ietf-mls-protocol]
Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", Work in Progress, Internet-
Draft, draft-ietf-mls-protocol-20, 27 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
protocol-20>.
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[I-D.ietf-moq-transport]
Curley, L., Pugin, K., Nandakumar, S., and V. Vasiliev,
"Media over QUIC Transport", Work in Progress, Internet-
Draft, draft-ietf-moq-transport-00, 5 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-moq-
transport-00>.
[I-D.ietf-webtrans-overview]
Vasiliev, V., "The WebTransport Protocol Framework", Work
in Progress, Internet-Draft, draft-ietf-webtrans-overview-
06, 6 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
webtrans-overview-06>.
[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/rfc/rfc3711>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/rfc/rfc4566>.
[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/rfc/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/rfc/rfc7656>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/rfc/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/rfc/rfc8723>.
[TestVectors]
"SFrame Test Vectors", 2021,
<https://github.com/eomara/sframe/blob/master/test-
vectors.json>.
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Appendix A. Acknowledgements
The authors wish to specially thank Dr. Alex Gouaillard as one of the
early contributors to the document. His passion and energy were key
to the design and development of SFrame.
Appendix B. 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
ciphersuite and no KID mappings.
* *Adding 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 counter value.
* *Adding 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 8 shows an example of the types of structures and methods that
could be used to create an SFrame API in Rust.
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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 8: An example SFrame API
Appendix C. 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:
* How many senders are involved in a conference (length of KID)
* How long the conference has been going on (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:
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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 secton, we compute overhead estimates for a
collection of common scenarios.
C.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 on those seen in practice.
+==============+=======+============================+
| Field | Bytes | Explanation |
+==============+=======+============================+
| Fixed header | 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 GCM tag (longest |
| overhead | | defined here) |
+--------------+-------+----------------------------+
Table 3
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.
C.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.
The below table considers three scenarios, based on recommended
configurations of the Opus codec [RFC6716]:
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* Narrow-band speech: 120ms packets, 8Kbps
* Full-band speech: 20ms packets, 32Kbps
* Full-band stereo music: 10ms packets, 128Kbps
+===============+=====+===========+===============+============+
| Scenario | fps | Base Kbps | Overhead Kbps | Overhead % |
+===============+=====+===========+===============+============+
| NB speech, | 8.3 | 8 | 1.4 | 17.9% |
| 120ms packets | | | | |
+---------------+-----+-----------+---------------+------------+
| FB speech, | 50 | 32 | 8.6 | 26.9% |
| 20ms packets | | | | |
+---------------+-----+-----------+---------------+------------+
| FB stereo, | 100 | 128 | 17.2 | 13.4% |
| 10ms packets | | | | |
+---------------+-----+-----------+---------------+------------+
Table 4: SFrame overhead for audio streams
C.3. Video
Video frames can be larger than an MTU and thus are commonly split
across multiple frames. Table 5 and Table 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 are chosen to 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
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+=============+=====+=====+===========+===============+============+
| Scenario | fps | pps | Base Kbps | Overhead Kbps | Overhead % |
+=============+=====+=====+===========+===============+============+
| 426 x 240 | 7.5 | 7.5 | 45 | 1.3 | 2.9% |
+-------------+-----+-----+-----------+---------------+------------+
| 640 x 360 | 15 | 30 | 200 | 5.2 | 2.6% |
+-------------+-----+-----+-----------+---------------+------------+
| 640 x 360 | 30 | 60 | 400 | 10.3 | 2.6% |
+-------------+-----+-----+-----------+---------------+------------+
| 1280 x 720 | 30 | 180 | 1500 | 30.9 | 2.1% |
+-------------+-----+-----+-----------+---------------+------------+
| 1920 x 1080 | 60 | 780 | 7200 | 134.1 | 1.9% |
+-------------+-----+-----+-----------+---------------+------------+
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 goes up, 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%).
C.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 over 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 sending high-quality. (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%.
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+=====================+===========+===============+============+
| 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
C.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 9.
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 10.
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 [RFC4566], an appropriate extension to
SDP could provide this function.
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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
/ 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
[I-D.codec-agnostic-rtp-payload-format].
+---+-+-+-------+-+-------------+-------------------------------+<-+
|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 9: SRTP packet with SFrame-protected payload
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+----------------+ +---------------+
| 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 10: Encryption flow with per-frame encryption for RTP
Appendix D. 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.
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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.
D.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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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kid: 0x0100000000000000
ctr: 0x0001000000000000
header: fe010000000000000001000000000000
kid: 0x0100000000000000
ctr: 0x00ffffffffffffff
header: fe0100000000000000ffffffffffffff
kid: 0x0100000000000000
ctr: 0x0100000000000000
header: ff01000000000000000100000000000000
kid: 0x0100000000000000
ctr: 0xffffffffffffffff
header: ff0100000000000000ffffffffffffffff
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
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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: ffffffffffffffffff0100000000000000
kid: 0xffffffffffffffff
ctr: 0xffffffffffffffff
header: ffffffffffffffffffffffffffffffffff
D.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
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* 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: 000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f
enc_key: 000102030405060708090a0b0c0d0e0f
auth_key: 101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f
nonce: 101112131415161718191a1b
aad: 4945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 6339af04ada1d064688a442b8dc69d5b6bfa40f4bef0583e8081069cc60705
cipher_suite: 0x0002
key: 000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f
enc_key: 000102030405060708090a0b0c0d0e0f
auth_key: 101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f
nonce: 101112131415161718191a1b
aad: 4945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 6339af04ada1d064688a442b8dc69d5b6bfa40f4be6e93b7da076927bb
cipher_suite: 0x0003
key: 000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f
enc_key: 000102030405060708090a0b0c0d0e0f
auth_key: 101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f
nonce: 101112131415161718191a1b
aad: 4945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 6339af04ada1d064688a442b8dc69d5b6bfa40f4be09480509
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D.3. SFrame encryption/decryption
For each case, we provide:
* cipher_suite: The index of the cipher suite in use (see
Section 8.1)
* kid: A KID value
* ctr: A CTR value
* base_key: The base_key input to the derive_key_salt algorithm
* sframe_key_label: The label used to derive sframe_key in the
derive_key_salt algorithm
* sframe_salt_label: The label used to derive sframe_salt in the
derive_key_salt algorithm
* sframe_secret: The sframe_secret variable in the derive_key_salt
algorithm
* sframe_key: The sframe_key value produced by the derive_key_salt
algorithm
* sframe_salt: The sframe_salt value produced by the derive_key_salt
algorithm
* metadata: The metadata input to the SFrame encrypt algorithm
* pt: The plaintext
* ct: The SFrame ciphertext
An implementation should verify that the following are true, where
encrypt and decrypt are as defined in Section 4.4, using an SFrame
context initialized with base_key assigned to kid:
* encrypt(ctr, kid, metadata, plaintext) == ct
* decrypt(metadata, ct) == pt
The other values in the test vector are intermediate values provided
to facilitate debugging of test failures.
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cipher_suite: 0x0001
kid: 0x0000000000000123
ctr: 0x0000000000004567
base_key: 000102030405060708090a0b0c0d0e0f
sframe_key_label: 534672616d6520312e3020536563726574206b65792000000000000001230001
sframe_salt_label: 534672616d6520312e30205365637265742073616c742000000000000001230001
sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0
sframe_key: 3f7d9a7c83ae8e1c8a11ae695ab59314b367e359fadac7b9c46b2bc6f81f46e16b96f0811868d59402b7e870102720b3
sframe_salt: 50b29329a04dc0f184ac3168
metadata: 4945544620534672616d65205747
nonce: 50b29329a04dc0f184ac740f
aad: 99012345674945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 9901234567449408b6f490086165b9d6f62b24ae1a59a56486b4ae8ed036b88912e24f11
cipher_suite: 0x0002
kid: 0x0000000000000123
ctr: 0x0000000000004567
base_key: 000102030405060708090a0b0c0d0e0f
sframe_key_label: 534672616d6520312e3020536563726574206b65792000000000000001230002
sframe_salt_label: 534672616d6520312e30205365637265742073616c742000000000000001230002
sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0
sframe_key: e2ec5c797540310483b16bf6e7a570d2a27d192fe869c7ccd8584a8d9dab91549fbe553f5113461ec6aa83bf3865553e
sframe_salt: e68ac8dd3d02fbcd368c5577
metadata: 4945544620534672616d65205747
nonce: e68ac8dd3d02fbcd368c1010
aad: 99012345674945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 99012345673f31438db4d09434e43afa0f8a2f00867a2be085046a9f5cb4f101d607
cipher_suite: 0x0003
kid: 0x0000000000000123
ctr: 0x0000000000004567
base_key: 000102030405060708090a0b0c0d0e0f
sframe_key_label: 534672616d6520312e3020536563726574206b65792000000000000001230003
sframe_salt_label: 534672616d6520312e30205365637265742073616c742000000000000001230003
sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0
sframe_key: 2c5703089cbb8c583475e4fc461d97d18809df79b6d550f78eb6d50ffa80d89211d57909934f46f5405e38cd583c69fe
sframe_salt: 38c16e4f5159700c00c7f350
metadata: 4945544620534672616d65205747
nonce: 38c16e4f5159700c00c7b637
aad: 99012345674945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 990123456717fc8af28a5a695afcfc6c8df6358a17e26b2fcb3bae32e443
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cipher_suite: 0x0004
kid: 0x0000000000000123
ctr: 0x0000000000004567
base_key: 000102030405060708090a0b0c0d0e0f
sframe_key_label: 534672616d6520312e3020536563726574206b65792000000000000001230004
sframe_salt_label: 534672616d6520312e30205365637265742073616c742000000000000001230004
sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0
sframe_key: d34f547f4ca4f9a7447006fe7fcbf768
sframe_salt: 75234edefe07819026751816
metadata: 4945544620534672616d65205747
nonce: 75234edefe07819026755d71
aad: 99012345674945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 9901234567b7412c2513a1b66dbb48841bbaf17f598751176ad847681a69c6d0b091c07018ce4adb34eb
cipher_suite: 0x0005
kid: 0x0000000000000123
ctr: 0x0000000000004567
base_key: 000102030405060708090a0b0c0d0e0f
sframe_key_label: 534672616d6520312e3020536563726574206b65792000000000000001230005
sframe_salt_label: 534672616d6520312e30205365637265742073616c742000000000000001230005
sframe_secret: 0fc3ea6de6aac97a35f194cf9bed94d4b5230f1cb45a785c9fe5dce9c188938ab6ba005bc4c0a19181599e9d1bcf7b74aca48b60bf5e254e546d809313e083a3
sframe_key: d3e27b0d4a5ae9e55df01a70e6d4d28d969b246e2936f4b7a5d9b494da6b9633
sframe_salt: 84991c167b8cd23c93708ec7
metadata: 4945544620534672616d65205747
nonce: 84991c167b8cd23c9370cba0
aad: 99012345674945544620534672616d65205747
pt: 64726166742d696574662d736672616d652d656e63
ct: 990123456794f509d36e9beacb0e261d99c7d1e972f1fed787d4049f17ca21353c1cc24d56ceabced279
Authors' Addresses
Emad Omara
Apple
Email: eomara@apple.com
Justin Uberti
Google
Email: juberti@google.com
Sergio Garcia Murillo
CoSMo Software
Email: sergio.garcia.murillo@cosmosoftware.io
Omara, et al. Expires 24 April 2024 [Page 63]
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Richard L. Barnes (editor)
Cisco
Email: rlb@ipv.sx
Youenn Fablet
Apple
Email: youenn@apple.com
Omara, et al. Expires 24 April 2024 [Page 64]