Secure Frame (SFrame)
draft-omara-sframe-00
The information below is for an old version of the document.
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| Authors | Emad Omara , Justin Uberti , Dr. Alex Gouaillard , Sergio Garcia Murillo | ||
| Last updated | 2020-05-19 | ||
| Replaced by | draft-ietf-sframe-enc | ||
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draft-omara-sframe-00
Network Working Group E. Omara
Internet-Draft J. Uberti
Intended status: Informational Google
Expires: November 20, 2020 A. GOUAILLARD
S. Murillo
CoSMo Software
May 19, 2020
Secure Frame (SFrame)
draft-omara-sframe-00
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 (SFUs) can
access the media metadata needed to make forwarding decisions without
having access to the actual media. The proposed mechanism differs
from other approaches through its use of media frames as the
encryptable unit, instead of individual RTP packets, which makes it
more bandwidth efficient and also allows use with non-RTP transports.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 20, 2020.
Copyright Notice
Copyright (c) 2020 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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. SFrame . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. SFrame Format . . . . . . . . . . . . . . . . . . . . . . 7
4.2. SFrame Header . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Encryption Schema . . . . . . . . . . . . . . . . . . . . 8
4.3.1. Key Derivation . . . . . . . . . . . . . . . . . . . 8
4.3.2. Encryption . . . . . . . . . . . . . . . . . . . . . 9
4.3.3. Decryption . . . . . . . . . . . . . . . . . . . . . 10
4.3.4. Duplicate Frames . . . . . . . . . . . . . . . . . . 11
4.3.5. Key Rotation . . . . . . . . . . . . . . . . . . . . 11
4.4. Authentication . . . . . . . . . . . . . . . . . . . . . 12
4.5. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . 14
4.5.1. SFrame . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . 15
5. Key Management . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. MLS-SFrame . . . . . . . . . . . . . . . . . . . . . . . 15
6. Media Considerations . . . . . . . . . . . . . . . . . . . . 16
6.1. SFU . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.1. LastN and RTP stream reuse . . . . . . . . . . . . . 16
6.1.2. Simulcast . . . . . . . . . . . . . . . . . . . . . . 16
6.1.3. SVC . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2. Video Key Frames . . . . . . . . . . . . . . . . . . . . 17
6.3. Partial Decoding . . . . . . . . . . . . . . . . . . . . 17
7. Overhead . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Audio . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2. Video . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3. SFrame vs PERC-lite . . . . . . . . . . . . . . . . . . . 18
7.3.1. Audio . . . . . . . . . . . . . . . . . . . . . . . . 19
7.3.2. Video . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8.1. Key Management . . . . . . . . . . . . . . . . . . . . . 19
8.2. Authentication tag length . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
Modern multi-party video call systems use Selective Forwarding Unit
(SFU) servers to efficiently route RTP streams to call endpoints
based on factors such as available bandwidth, desired video size,
codec support, and other factors. In order for the SFU to work
properly though, it needs to be able to access RTP metadata and RTCP
feedback messages, which is not possible if all RTP/RTCP traffic is
end-to-end encrypted.
As such, two layers of 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
While DTLS-SRTP can be used as an efficient HBH mechanism, it is
inherently point-to-point and therefore not suitable for a SFU
context. In addition, given the various scenarios in which video
calling occurs, minimizing the bandwidth overhead of end-to-end
encryption is also an important goal.
This document proposes a new end-to-end encryption mechanism known as
SFrame, specifically designed to work in group conference calls with
SFUs.
+-------------------------------+-------------------------------+^+
|V=2|P|X| CC |M| PT | sequence number | |
+-------------------------------+-------------------------------+ |
| timestamp | |
+---------------------------------------------------------------+ |
| synchronization source (SSRC) identifier | |
|=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=| |
| contributing source (CSRC) identifiers | |
| .... | |
+---------------------------------------------------------------+ |
| RTP extension(s) (OPTIONAL) | |
+^---------------------+------------------------------------------+ |
| | payload header | | |
| +--------------------+ payload ... | |
| | | |
+^+---------------------------------------------------------------+^+
| : authentication tag : |
| +---------------------------------------------------------------+ |
| |
++ Encrypted Portion* Authenticated Portion +--+
SRTP packet format
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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.
SFU: Selective Forwarding Unit (AKA RTP Switch)
IV: Initialization Vector
MAC: Message Authentication Code
E2EE: End to End Encryption
HBH: Hop By Hop
KMS: Key Management System
3. Goals
SFrame is designed to be a suitable E2EE protection scheme for
conference call media in a broad range of scenarios, as outlined by
the following goals:
1. Provide 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 KMS.
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.
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.
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4. SFrame
We propose a frame level 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
encrypts the full frame, rather than individual packets, bandwidth
overhead is reduced by having a single IV and authentication tag for
each media frame.
Also, because media is encrypted prior to packetization, the
encrypted frame is packetized using a generic RTP packetizer instead
of codec-dependent packetization mechanisms. With this move to a
generic packetizer, media metadata is moved from codec-specific
mechanisms to a generic frame RTP header extension which, while
visible to the SFU, is authenticated end-to-end. This extension
includes metadata needed for SFU routing such as resolution, frame
beginning and end markers, etc.
The generic packetizer splits the E2E encrypted media frame into one
or more RTP packets and adds the SFrame header to the beginning of
the first packet and an auth tag to the end of the last packet.
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+-------------------------------------------------------+
| |
| +----------+ +------------+ +-----------+ |
| | | | SFrame | |Packetizer | | DTLS+SRTP
| | Encoder +----->+ Enc +----->+ +-------------------------+
,+. | | | | | | | | +--+ +--+ +--+ |
`|' | +----------+ +-----+------+ +-----------+ | | | | | | | |
/|\ | ^ | | | | | | | |
+ | | | | | | | | | |
/ \ | | | +--+ +--+ +--+ |
Alice | +-----+------+ | Encrypted Packets |
| |Key Manager | | |
| +------------+ | |
| || | |
| || | |
| || | |
+-------------------------------------------------------+ |
|| |
|| v
+------------+ +-----+------+
E2EE channel | Messaging | | Media |
via the | Server | | Server |
Messaging Server | | | |
+------------+ +-----+------+
|| |
|| |
+-------------------------------------------------------+ |
| || | |
| || | |
| || | |
| +------------+ | |
| |Key Manager | | |
,+. | +-----+------+ | Encrypted Packets |
`|' | | | +--+ +--+ +--+ |
/|\ | | | | | | | | | |
+ | v | | | | | | | |
/ \ | +----------+ +-----+------+ +-----------+ | | | | | | | |
Bob | | | | SFrame | | De+ | | +--+ +--+ +--+ |
| | Decoder +<-----+ Dec +<-----+Packetizer +<------------------------+
| | | | | | | | DTLS+SRTP
| +----------+ +------------+ +-----------+ |
| |
+-------------------------------------------------------+
The E2EE keys used to encrypt the frame are exchanged out of band
using a secure E2EE channel.
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4.1. SFrame Format
+------------+------------------------------------------+^+
|S|LEN|X|KID | Frame Counter | |
+^+------------+------------------------------------------+ |
| | | |
| | | |
| | | |
| | | |
| | Encrypted Frame | |
| | | |
| | | |
| | | |
| | | |
+^+-------------------------------------------------------+^+
| | Authentication Tag | |
| +-------------------------------------------------------+ |
| |
| |
+----+Encrypted Portion Authenticated Portion+---+
4.2. SFrame Header
Since each endpoint can send multiple media layers, each frame will
have a unique frame counter that will be used to derive the
encryption IV. The frame counter must be unique and monotonically
increasing to avoid IV reuse.
As each sender will use their own key for encryption, so the SFrame
header will include the key id to allow the receiver to identify the
key that needs to be used for decrypting.
Both the frame counter and the key id are encoded in a variable
length format to decrease the overhead, so the first byte in the
Sframe header is fixed and contains the header metadata with the
following format:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|LEN |X| K |
+-+-+-+-+-+-+-+-+
SFrame header metadata
Signature flag (S): 1 bit This field indicates the payload contains a
signature if set. Counter Length (LEN): 3 bits This field indicates
the length of the CTR fields in bytes. Extended Key Id Flag (X): 1
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bit Indicates if the key field contains the key id or the key length.
Key or Key Length: 3 bits This field contains the key id (KID) if the
X flag is set to 0, or the key length (KLEN) if set to 1.
If X flag is 0 then the KID is in the range of 0-7 and the frame
counter (CTR) is found in the next LEN bytes:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+---------------------------------+
|S|LEN |0| KID | CTR... (length=LEN) |
+-+-+-+-+-+-+-+-+---------------------------------+
Key id (KID): 3 bits The key id (0-7). Frame counter (CTR):
(Variable length) Frame counter value up to 8 bytes long.
if X flag is 1 then KLEN is the length of the key (KID), that is
found after the SFrame header metadata byte. After the key id (KID),
the frame counter (CTR) will be found in the next LEN bytes:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+---------------------------+---------------------------+
|S|LEN |1|KLEN | KID... (length=KLEN) | CTR... (length=LEN) |
+-+-+-+-+-+-+-+-+---------------------------+---------------------------+
Key length (KLEN): 3 bits The key length in bytes. Key id (KID):
(Variable length) The key id value up to 8 bytes long. Frame counter
(CTR): (Variable length) Frame counter value up to 8 bytes long.
4.3. Encryption Schema
4.3.1. Key Derivation
Each client creates a 32 bytes secret key K and share it with with
other participants via an E2EE channel. From K, we derive 3 secrets:
1- Salt key used to calculate the IV
Key = HKDF(K, 'SFrameSaltKey', 16)
2- Encryption key to encrypt the media frame
Key = HKDF(K, 'SFrameEncryptionKey', 16)
3- Authentication key to authenticate the encrypted frame and the
media metadata
Key = HKDF(K, 'SFrameAuthenticationKey', 32)
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The IV is 128 bits long and calculated from the CTR field of the
Frame header:
IV = CTR XOR Salt key
4.3.2. Encryption
After encoding the frame and before packetizing it, the necessary
media metadata will be moved out of the encoded frame buffer, to be
used later in the RTP generic frame header extension. The encoded
frame, the metadata buffer and the frame counter are passed to SFrame
encryptor. The encryptor constructs SFrame header using frame
counter and key id and derive the encryption IV. The frame is
encrypted using the encryption key and the header, encrypted frame,
the media metadata and the header are authenticated using the
authentication key. The authentication tag is then truncated (If
supported by the cipher suite) and prepended at the end of the
ciphertext.
The encrypted payload is then passed to a generic RTP packetized to
construct the RTP packets and encrypts it using SRTP keys for the HBH
encryption to the media server.
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+---------------+ +---------------+
| | | frame metadata+----+
| | +---------------+ |
| frame | |
| | |
| | |
+-------+-------+ |
| |
CTR +---------------> IV |Enc Key <----Master Key |
derive IV | | |
+ | | |
| + v |
| encrypt Auth Key |
| | + |
| | | |
| v | |
| +-------+-------+ | |
| | | | |
| | encrypted | v |
| | frame +---->Authenticate<-----+
+ | | +
encode CTR | | |
+ +-------+-------+ |
| | |
| | |
| | |
| generic RTP packetize |
| + |
| | |
| | +--------------+
+----------+ v |
| |
| +---------------+ +---------------+ +---------------+ |
+-> | SFrame header | | | | | |
+---------------+ | | | payload N/N | |
| | | payload 2/N | | | |
| payload 1/N | | | +---------------+ |
| | | | | auth tag | <-+
+---------------+ +---------------+ +---------------+
Encryption flow
4.3.3. Decryption
The receiving clients buffer all packets that belongs to the same
frame using the frame beginning and ending marks in the generic RTP
frame header extension, and once all packets are available, it passes
it to Frame for decryption. SFrame maintains multiple decryptor
objects, one for each client in the call. Initially the client might
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not have the mapping between the incoming streams the user's keys, in
this case SFrame tries all unmapped keys until it finds one that
passes the authentication verification and use it to decrypt the
frame. If the client has the mapping ready, it can push it down to
SFrame later.
The KeyId field in the SFrame header is used to find the right key
for that user, which is incremented by the sender when they switch to
a new key.
For frames that are failed to decrypt because there is not key
available yet, SFrame will buffer them and retries to decrypt them
once a key is received.
4.3.4. Duplicate Frames
Unlike messaging application, in video calls, receiving a duplicate
frame doesn't necessary mean the client is under a replay attack,
there are other reasons that might cause this, for example the sender
might just be sending them in case of packet loss. SFrame decryptors
use the highest received frame counter to protect against this. It
allows only older frame pithing a short interval to support out of
order delivery.
4.3.5. Key Rotation
Because the E2EE keys could be rotated during the call when people
join and leave, these new keys are exchanged using the same E2EE
secure channel used in the initial key negotiation. Sending new
fresh keys is an expensive operation, so the key management component
might chose to send new keys only when other clients leave the call
and use hash ratcheting for the join case, so no need to send a new
key to the clients who are already on the call. SFrame supports both
modes
4.3.5.1. Key Ratcheting
When SFrame decryptor fails to decrypt one of the frames, it
automatically ratchets the key forward and retries again until one
ratchet succeed or it reaches the maximum allowed ratcheting window.
If a new ratchet passed the decryption, all previous ratchets are
deleted.
K(i) = HKDF(K(i-1), 'SFrameRatchetKey', 32)
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4.3.5.2. New Key
SFrame will set the key immediately on the decrypts when it is
received and destroys the old key material, so if the key manager
sends a new key during the call, it is recommended not to start using
it immediately and wait for a short time to make sure it is delivered
to all other clients before using it to decrease the number of
decryption failure. It is up to the application and the key manager
to define how long this period is.
4.4. Authentication
Every client in the call knows the secret key for all other clients
so it can decrypt their traffic, it also means a malicious client can
impersonate any other client in the call by using the victim key to
encrypt their traffic. This might not be a problem for consumer
application where the number of clients in the call is small and
users know each others, however for enterprise use case where large
conference calls are common, an authentication mechanism is needed to
protect against malicious users. This authentication will come with
extra cost.
Adding a digital signature to each encrypted frame will be an
overkill, instead we propose adding signature over multiple frames.
The signature is calculated by concatenating the authentication tags
of the frames that the sender wants to authenticate (in reverse sent
order) and signing it with the signature key. Signature keys are
exchanged out of band along the encryption keys.
Signature = Sign(Key, AuthTag(Frame N) || AuthTag(Frame N-1) || ...|| AuthTag(Frame N-M))
The authentication tags for the previous frames covered by the
signature and the signature itself will be appended at end of the
frame, after the current frame authentication tag, in the same order
that the signature was calculated, and the SFrame header metadata
signature bit (S) will be set to 1.
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+^ +------------------+
| | SFrame header S=1|
| +------------------+
| | Encrypted |
| | payload |
| | |
|^ +------------------+ ^+
| | Auth Tag N | |
| +------------------+ |
| | Auth Tag N-1 | |
| +------------------+ |
| | ........ | |
| +------------------+ |
| | Auth Tag N-M | |
| +------------------+ ^|
| | NUM | Signature : |
| +-----+ + |
| : | |
| +------------------+ |
| |
+-> Authenticated with +-> Signed with
Auth Tag N Signature
Encrypted Frame with Signature
Note that the authentication tag for the current frame will only
authenticate the SFrame header and the encrypted payload, ant not the
signature nor the previous frames's authentication tags (N-1 to N-M)
used to calculate the signature.
The last byte (NUM) after the authentication tag list and before the
signature indicates the number of the authentication tags from
previous frames present in the current frame. All the
authentications tags MUST have the same size, which MUST be equal to
the authentication tag size of the current frame. The signature is
fixed size depending on the signature algorithm used (for example, 64
bytes for Ed25519).
The receiver has to keep track of all the frames received but yet not
verified, by storing the authentication tags of each received frame.
When a signature is received, the receiver will verify it with the
signature key associated to the key id of the frame the signature was
sent in. If the verification is successful, the received will mark
the frames as authenticated and remove them from the list of the not
verified frames. It is up to the application to decide what to do
when signature verification fails.
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When using SVC, the hash will be calculated over all the frames of
the different spatial layers within the same superframe/picture.
However the SFU will be able to drop frames within the same stream
(either spatial or temporal) to match target bitrate.
If the signature is sent on a frame which layer that is dropped by
the SFU, the receiver will not receive it and will not be able to
perform the signature of the other received layers.
An easy way of solving the issue would be to perform signature only
on the base layer or take into consideration the frame dependency
graph and send multiple signatures in parallel (each for a branch of
the dependency graph).
In case of simulcast or K-SVC, each spatial layer should be
authenticated with different signatures to prevent the SFU to discard
frames with the signature info.
In any case, it is possible that the frame with the signature is lost
or the SFU drops it, so the receiver MUST be prepared to not receive
a signature for a frame and remove it from the pending to be verified
list after a timeout.
4.5. Ciphersuites
4.5.1. SFrame
Each SFrame session uses a single ciphersuite that specifies the
following primitives:
o A hash function This is used for the Key derivation and frame
hashes for signature. We recommend using SHA256 hash function.
o An AEAD encryption algorithm [RFC5116] While any AEAD algorithm can
be used to encrypt the frame, we recommend using algorithms with safe
MAC truncation like AES-CTR and HMAC to reduce the per-frame
overhead. In this case we can use 80 bits MAC for video frames and
32 bits for audio frames similar to DTLS-SRTP cipher suites:
1- AES_CM_128_HMAC_SHA256_80
2- AES_CM_128_HMAC_SHA256_32
o [Optional] A signature algorithm If signature is supported, we
recommend using ed25519
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4.5.2. DTLS-SRTP
SRTP is used as an HBH encryption, since the media payload is already
encrypted, and SRTP only protects the RTP headers, one implementation
could use 4 bytes outer auth tag to decrease the overhead, however it
is up to the application to use other ciphers like AES-128-GCM with
full authentication tag.
5. Key Management
SFrame must be integrated with an E2EE key management framework to
exchange and rotate the encryption keys. This framework will
maintain a group of participant endpoints who are in the call. At
call setup time, each endpoint will create a fresh key material and
optionally signing key pair for that call and encrypt the key
material and the public signing key to every other endpoints. They
encrypted keys are delivered by the messaging delivery server using a
reliable channel.
The KMS will monitor the group changes, and exchange new keys when
necessary. It is up to the application to define this group, for
example one application could have ephemeral group for every call and
keep rotating key when end points joins or leave the call, while
another application could have a persisted group that can be used for
multiple calls and exchange keys with all group endpoints for every
call.
When a new key material is created during the call, we recommend not
to start using it immediately in SFrame to give time for the new keys
to be delivered. If the application supports delivery receipts, it
can be used to track if the key is delivered to all other endpoints
on the call before using it.
Keys must have a sequential id starting from 0 and incremented eery
time a new key is generated for this endpoint. The key id will be
added in the SFrame header during encryption, so the recipient know
which key to use for the decryption.
5.1. MLS-SFrame
While any other E2EE KMS can be used with SFrame, there is a big
advantage if it is used with [MLSARCH] which natively supports very
large groups efficiently. When [MLSPROTO] is used, the endpoints
keys (AKA Application secret) can be used directly for SFrame without
the need to exchange separate key material. The application secret
is rotated automatically by [MLSPROTO] when group membership changes.
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6. Media Considerations
6.1. SFU
Selective Forwarding Units (SFUs) as described in
https://tools.ietf.org/html/rfc7667#section-3.7 receives the RTP
streams from each participant and selects 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.
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 reduce the number of SDP
O/A required to establish a new RTP stream, the SFU may decide to
reuse previously existing RTP sessions or even pre-allocate a
predefined number of RTP streams and choose in each moment in time
which participant media will be sending through it. This means that
in the same 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 if 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) usage of the signature is
required. In case of video, the a new signature should be started
each time a key frame is sent to allow the receiver to identify the
source faster after a switch.
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
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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 async and on a different path than the SFU
signaling and media. So it may happen that when a new participant
joins the call and the SFU side requests a key frame, the sender
generates the e2ee 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 is arrived and decrypted.
7. Overhead
The encryption overhead will vary between audio and video streams,
because in audio each packet is considered a separate frame, so it
will always have extra MAC and IV, however a video frame usually
consists of multiple RTP packets. The number of bytes overhead per
frame is calculated as the following 1 + FrameCounter length + 4 The
constant 1 is the SFrame header byte and 4 bytes for the HBH
authentication tag for both audio and video packets.
7.1. Audio
Using three different audio frame durations 20ms (50 packets/s) 40ms
(25 packets/s) 100ms (10 packets/s) Up to 3 bytes frame counter (3.8
days of data for 20ms frame duration) and 4 bytes fixed MAC length.
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+------------+-----------+-----------+----------+-----------+
| Counter len| Packets | Overhead | Overhead | Overhead |
| | | bps@20ms | bps@40ms | bps@100ms |
+------------+-----------+-----------+----------+-----------+
| 1 | 0-255 | 2400 | 1200 | 480 |
| 2 | 255 - 65K | 2800 | 1400 | 560 |
| 3 | 65K - 16M | 3200 | 1600 | 640 |
+------------+--------- -+-----------+----------+-----------+
7.2. Video
The per-stream overhead bits per second as calculated for the
following video encodings: 30fps@1000Kbps (4 packets per frame)
30fps@512Kbps (2 packets per frame) 15fps@200Kbps (2 packets per
frame) 7.5fps@30Kbps (1 packet per frame) Overhead bps = (Counter
length + 1 + 4 ) * 8 * fps
+------------+-----------+------------+------------+------------+
| Counter len| Frames | Overhead | Overhead | Overhead |
| | | bps@30fps | bps@15fps | bps@7.5fps |
+------------+-----------+------------+------------+------------+
| 1 | 0-255 | 1440 | 1440 | 720 |
| 2 | 256 - 65K | 1680 | 1680 | 840 |
| 3 | 56K - 16M | 1920 | 1920 | 960 |
| 4 | 16M - 4B | 2160 | 2160 | 1080 |
+------------+-----------+------------+------------+------------+
7.3. SFrame vs PERC-lite
[PERC] has significant overhead over SFrame because the overhead is
per packet, not per frame, and OHB (Original Header Block) which
duplicates any RTP header/extension field modified by the SFU.
[PERCLITE] <https://mailarchive.ietf.org/arch/msg/perc/
SB0qMHWz6EsDtz3yIEX0HWp5IEY/> is slightly better because it doesn't
use the OHB anymore, however it still does per packet encryption
using SRTP. Below the the overheard in [PERCLITE] implemented by
Cosmos Software which uses extra 11 bytes per packet to preserve the
PT, SEQ_NUM, TIME_STAMP and SSRC fields in addition to the extra MAC
tag per packet.
OverheadPerPacket = 11 + MAC length Overhead bps = PacketPerSecond *
OverHeadPerPacket * 8
Similar to SFrame, we will assume the HBH authentication tag length
will always be 4 bytes for audio and video even though it is not the
case in this [PERCLITE] implementation
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7.3.1. Audio
+-------------------+--------------------+--------------------+
| Overhead bps@20ms | Overhead bps@40ms | Overhead bps@100ms |
+-------------------+--------------------+--------------------+
| 6000 | 3000 | 1200 |
+-------------------+--------------------+--------------------+
7.3.2. Video
+---------------------+----------------------+-----------------------+
| Overhead bps@30fps | Overhead bps@15fps | Overhead bps@7.5fps |
|(4 packets per frame)| (2 packets per frame)| (1 packet per frame) |
+---------------------+----------------------+-----------------------+
| 14400 | 7200 | 3600 |
+---------------------+----------------------+-----------------------+
For a conference with a single incoming audio stream (@ 50 pps) and 4
incoming video streams (@200 Kbps), the savings in overhead is 34800
- 9600 = ~25 Kbps, or ~3%.
8. Security Considerations
8.1. Key Management
Key exchange mechanism is out of scope of this document, however
every client MUST change their keys when new clients joins or leaves
the call for "Forward Secrecy" and "Post Compromise Security".
8.2. 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.
9. IANA Considerations
This document makes no requests of IANA.
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/info/rfc2119>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References
[MLSARCH] Omara, E., Barnes, R., Rescorla, E., Inguva, S., Kwon, A.,
and A. Duric, "Messaging Layer Security Architecture",
2020.
[MLSPROTO]
Barnes, R., Millican, J., Omara, E., Cohn-Gordon, K., and
R. Robert, "Messaging Layer Security Protocol", 2020.
[PERC] Jennings, C., Jones, P., Barnes, R., and A. Roach, "PERC",
2020, <https://datatracker.ietf.org/doc/rfc8723/>.
[PERCLITE]
GOUAILLARD, A. and S. Murillo, "PERC-Lite", 2020,
<https://tools.ietf.org/html/draft-murillo-perc-lite-01>.
Authors' Addresses
Emad Omara
Google
Email: emadomara@google.com
Justin Uberti
Google
Email: juberti@google.com
Alexandre GOUAILLARD
CoSMo Software
Email: Alex.GOUAILLARD@cosmosoftware.io
Sergio Garcia Murillo
CoSMo Software
Email: sergio.garcia.murillo@cosmosoftware.io
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