Secure Frame (SFrame)
draft-omara-sframe-01
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Authors | Emad Omara , Justin Uberti , Dr. Alex Gouaillard , Sergio Garcia Murillo | ||
| Last updated | 2020-11-16 | ||
| Replaced by | draft-ietf-sframe-enc | ||
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| Stream | Stream state | (No stream defined) | |
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draft-omara-sframe-01
Network Working Group E. Omara
Internet-Draft J. Uberti
Intended status: Informational Google
Expires: May 20, 2021 A. GOUAILLARD
S. Murillo
CoSMo Software
November 16, 2020
Secure Frame (SFrame)
draft-omara-sframe-01
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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 20, 2021.
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 Selection . . . . . . . . . . . . . . . . . . . . 9
4.3.2. Key Derivation . . . . . . . . . . . . . . . . . . . 9
4.3.3. Encryption . . . . . . . . . . . . . . . . . . . . . 10
4.3.4. Decryption . . . . . . . . . . . . . . . . . . . . . 12
4.3.5. Duplicate Frames . . . . . . . . . . . . . . . . . . 12
4.4. Authentication . . . . . . . . . . . . . . . . . . . . . 12
4.5. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . 14
4.5.1. AES-CM with SHA2 . . . . . . . . . . . . . . . . . . 15
5. Key Management . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Sender Keys . . . . . . . . . . . . . . . . . . . . . . . 16
5.2. MLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Media Considerations . . . . . . . . . . . . . . . . . . . . 18
6.1. SFU . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1.1. LastN and RTP stream reuse . . . . . . . . . . . . . 19
6.1.2. Simulcast . . . . . . . . . . . . . . . . . . . . . . 19
6.1.3. SVC . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2. Video Key Frames . . . . . . . . . . . . . . . . . . . . 20
6.3. Partial Decoding . . . . . . . . . . . . . . . . . . . . 20
7. Overhead . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Audio . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Video . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.3. SFrame vs PERC-lite . . . . . . . . . . . . . . . . . . . 21
7.3.1. Audio . . . . . . . . . . . . . . . . . . . . . . . . 22
7.3.2. Video . . . . . . . . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8.1. Key Management . . . . . . . . . . . . . . . . . . . . . 22
8.2. Authentication tag length . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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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.
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+-------------------------------+-------------------------------+^+
|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
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:
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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 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.
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
bit Indicates if the key field contains the key id or the key length.
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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
SFrame encryption uses an AEAD encryption algorithm and hash function
defined by the ciphersuite in use (see Section 4.5). We will refer
to the following aspects of the AEAD algorithm below:
o "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]).
o "AEAD.Nk" - The size of a key for the encryption algorithm, in
bytes
o "AEAD.Nn" - The size of a nonce for the encryption algorithm, in
bytes
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4.3.1. Key Selection
Each SFrame encryption or decryption operation is premised on a
single secret "base\_key", which is labeled with an integer KID value
signaled in the SFrame header.
The sender and receivers need to agree on which 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 MUST 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 frame, 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 encryption-only keys until (a) all receivers have
acknowledged receipt of the new key or (b) a timeout expires.
4.3.2. 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:
sframe_secret = HKDF-Extract(K, 'SFrame10')
sframe_key = HKDF-Expand(sframe_secret, 'key', AEAD.Nk)
sframe_salt = HKDF-Expand(sframe_secret, 'salt', AEAD.Nn)
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The hash function used for HKDF is determined by the ciphersuite in
use.
4.3.3. 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.
SFrame encryption uses the AEAD encryption algorithm for the
ciphersuite 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 S, CTR, and KID values
provided. The encoded header is provided as AAD to the AEAD
encryption operation, with any frame metadata appended.
def encrypt(S, CTR, KID, frame_metadata, frame):
sframe_key, sframe_salt = key_store[KID]
frame_ctr = encode_big_endian(CTR, AEAD.Nn)
frame_nonce = xor(sframe_salt, frame_ctr)
header = encode_sframe_header(S, CTR, KID)
frame_aad = header + frame_metadata
encrypted_frame = AEAD.Encrypt(sframe_key, frame_nonce, frame_aad, frame)
return header + encrypted_frame
The encrypted payload is then passed to a generic RTP packetized to
construct the RTP packets and encrypt it using SRTP keys for the HBH
encryption to the media server.
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+----------------+ +---------------+
| frame metadata | | |
+-------+--------+ | |
| | frame |
| | |
| | |
| +-------+-------+
| |
header ----+------------------>| AAD
+-----+ |
| S | |
+-----+ |
| KID +--+--> sframe_key ----->| Key
| | | |
| | +--> sframe_salt -+ |
+-----+ | |
| CTR +--------------------+-->| Nonce
| | |
| | |
+-----+ |
| AEAD.Encrypt
| |
| V
| +-------+-------+
| | |
| | |
| | encrypted |
| | frame |
| | |
| | |
| +-------+-------+
| |
| generic RTP packetize
| |
| v
V
+---------------+ +---------------+ +---------------+
| SFrame header | | | | |
+---------------+ | | | |
| | | payload 2/N | | payload N/N |
| payload 1/N | | | | |
| | | | | |
+---------------+ +---------------+ +---------------+
Encryption flow
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4.3.4. 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 SFrame for decryption. The KID field in the SFrame header is
used to find the right key for the encrypted frame.
def decrypt(frame_metadata, sframe):
header, encrypted_frame = split_header(sframe)
S, CTR, KID = parse_header(header)
sframe_key, sframe_salt = key_store[KID]
frame_ctr = encode_big_endian(CTR, AEAD.Nn)
frame_nonce = xor(sframe_salt, frame_ctr)
frame_aad = header + frame_metadata
return AEAD.Decrypt(sframe_key, frame_nonce, frame_aad, encrypted_frame)
For frames that are failed to decrypt because there is key available
for the KID in the SFrame header, the client MAY buffer the frame and
retry decryption once a key with that KID is received.
4.3.5. 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.
4.4. 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.
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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.
+^ +------------------+
| | 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
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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.
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
Each SFrame session uses a single ciphersuite that specifies the
following primitives:
o A hash function used for key derivation and hashing signature
inputs
o An AEAD encryption algorithm [RFC5116] used for frame encryption,
optionally with a truncated authentication tag
o [Optional] A signature algorithm
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This document defines the following ciphersuites:
+--------+--------------------------+----+----+-----------+
| Value | Name | Nk | Nn | Reference |
+--------+--------------------------+----+----+-----------+
| 0x0001 | AES_CM_128_HMAC_SHA256_8 | 16 | 12 | RFC XXXX |
| | | | | |
| 0x0002 | AES_CM_128_HMAC_SHA256_4 | 16 | 12 | RFC XXXX |
| | | | | |
| 0x0003 | AES_GCM_128_SHA256 | 16 | 12 | RFC XXXX |
| | | | | |
| 0x0004 | AES_GCM_256_SHA512 | 32 | 12 | RFC XXXX |
+--------+--------------------------+----+----+-----------+
In the "AES_CM" suites, the length of the authentication tag is
indicated by the last value: "_8" indicates an eight-byte tag and
"_4" indicates a four-byte tag.
In a session that uses multiple media streams, different ciphersuites
might be configured for different media streams. For example, in
order to conserve bandwidth, a session might use a ciphersuite with
80-bit tags for video frames and another ciphersuite with 32-bit tags
for audio frames.
4.5.1. AES-CM 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 using HKDF. The subkeys
are derived as follows, where "Nk" represents the key size for the
AES block cipher in use and "Nh" represents the output size of the
hash function:
def derive_subkeys(key):
aead_secret = HKDF-Extract(K, 'SFrame10 AES CM AEAD')
enc_key = HKDF-Expand(aead_secret, 'enc', Nk)
auth_key = HKDF-Expand(aead_secret, 'auth', Nh)
The AEAD encryption and decryption functions are then composed of
individual calls to the CM encrypt function and HMAC. The resulting
MAC value is truncated to a number of bytes "tag_len" fixed by the
ciphersuite.
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def compute_tag(aad, ct):
aad_len = encode_big_endian(len(aad), 8)
auth_data = aad_len + aad + ct
tag = HMAC(auth_key, auth_data)
return truncate(tag, tag_len)
def AEAD.Encrypt(key, nonce, aad, pt):
ct = AES-CM.Encrypt(key, nonce, pt)
tag = compute_tag(aad, ct)
return ct + tag
def AEAD.Decrypt(key, nonce, aad, ct):
inner_ct, tag = split_ct(ct, tag_len)
candidate_tag = compute_tag(aad, inner_ct)
if !constant_time_equal(tag, candidate_tag):
raise Exception("Authentication Failure")
return AES-CM.Decrypt(key, nonce, 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 and/or
signing. The key management framework provides the following
functions:
o Provisioning KID/"base\_key" mappings to participating clients
o (optional) Provisioning clients with a list of trusted signing
keys
o Updating the above data as clients join or leave
It is 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 key when end points joins 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.
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 encryption key and optionally a signing
key pair. The client then uses the E2E-secure channel to send their
encryption key and signing public 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, for example the RTP
SSRC. SFrame KID values are then used to distinguish generations of
the sender's key. At the beginning of a call, each sender encrypts
with KID=0. Thereafter, the sender can ratchet their key forward for
forward secrecy:
sender_key[i+1] = HKDF-Expand(
HKDF-Extract(sender_key[i], 'SFrame10 ratchet'),
'', AEAD.Nk)
The sender signals such an update by incrementing their KID value. 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, (b) the
signing key for the sender, if used, and (c) the current KID value
for the sender. Evicting a participant requires each sender to send
a fresh sender key to all receivers.
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.
In SFrame, we derive per-sender "base\_key" values from the group
secret for an epoch, and use the KID field to signal the epoch and
sender index. First, we use the MLS exporter to compute a shared
SFrame secret for the epoch.
sframe_epoch_secret = MLS-Exporter("SFrame 10 MLS", "", AEAD.Nk)
sender_base_key[index] = HKDF-Expand(sframe_epoch_secret,
encode_big_endian(index, 4), AEAD.Nk)
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For compactness, do not send the whole epoch number. Instead, we
send only its low-order E bits. Note that E effectively defines a
re-ordering window, since no more than 2^E epoch 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. (Sender indices cannot be similarly compressed.)
KID = (sender_index << E) + (epoch % (1 << E))
Once an SFrame stack has been provisioned with the
"sframe_epoch_secret" for an epoch, it can compute the required KIDs
and "sender_base_key" values on demand, as it needs to encrypt/
decrypt for a given member.
...
|
Epoch 17 +--+-- index=33 -> KID = 0x211
| |
| +-- index=51 -> KID = 0x331
|
|
Epoch 16 +--+-- index=2 --> KID = 0x20
|
|
Epoch 15 +--+-- index=3 --> KID = 0x3f
| |
| +-- index=5 --> KID = 0x5f
|
|
Epoch 14 +--+-- index=3 --> KID = 0x3e
| |
| +-- index=7 --> KID = 0x7e
| |
| +-- index=20 -> KID = 0x14e
|
...
MLS also provides an authenticated signing key pair for each
participant. When SFrame uses signatures, these are the keys used to
generate SFrame signatures.
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
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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
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.
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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
[RFC8723] 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.
[I-D.murillo-perc-lite] <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 [I-D.murillo-perc-lite]
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.
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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 [I-D.murillo-perc-lite] implementation
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 | (2 packets per | (1 packet per frame) |
| frame) | 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.
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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>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[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
[I-D.ietf-mls-architecture]
Omara, E., Beurdouche, B., Rescorla, E., Inguva, S., Kwon,
A., and A. Duric, "The Messaging Layer Security (MLS)
Architecture", draft-ietf-mls-architecture-05 (work in
progress), July 2020.
[I-D.ietf-mls-protocol]
Barnes, R., Beurdouche, B., Millican, J., Omara, E., Cohn-
Gordon, K., and R. Robert, "The Messaging Layer Security
(MLS) Protocol", draft-ietf-mls-protocol-10 (work in
progress), October 2020.
[I-D.murillo-perc-lite]
Murillo, S. and A. Gouaillard, "End to End Media
Encryption Procedures", draft-murillo-perc-lite-01 (work
in progress), May 2020.
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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC8723] Jennings, C., Jones, P., Barnes, R., and A. Roach, "Double
Encryption Procedures for the Secure Real-Time Transport
Protocol (SRTP)", RFC 8723, DOI 10.17487/RFC8723, April
2020, <https://www.rfc-editor.org/info/rfc8723>.
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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