title: Secure Frame (SFrame) abbrev: SFrame docname: draft-ietf-sframe-enc-latest category: std
ipr: trust200902 stream: IETF area: "Applications and Real-Time" keyword: Internet-Draft v: 3 venue: group: "Secure Media Frames" type: "Working Group" mail: "[email protected]" arch: "https://mailarchive.ietf.org/arch/browse/sframe/" github: "sframe-wg/sframe" latest: "https://sframe-wg.github.io/sframe/draft-ietf-sframe-enc.html"
ins: E. Omara
name: Emad Omara
organization: Apple
email: [email protected]
- ins: J. Uberti name: Justin Uberti organization: Google email: [email protected]
- ins: S. Murillo name: Sergio Garcia Murillo organization: CoSMo Software email: [email protected]
- ins: R.L. Barnes name: Richard L. Barnes organization: Cisco email: [email protected] role: editor
- ins: Y. Fablet name: Youenn Fablet organization: Apple email: [email protected]
informative: TestVectors: title: "SFrame Test Vectors" target: https://github.com/eomara/sframe/blob/master/test-vectors.json date: 2021
--- 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.
--- middle
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. 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 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:
- Hop-by-hop (HBH) encryption of media, metadata, and feedback messages between the the endpoints and SFU
- 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 payloads sent over SRTP
+---+-+-+-------+-+-------------+-------------------------------+<-+
|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 ---+
{: title="SRTP packet with SFrame-protected payload"}
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
SFrame is designed to be a suitable E2EE protection scheme for conference call media in a broad range of scenarios, as outlined by the following goals:
-
Provide an secure E2EE mechanism for audio and video in conference calls that can be used with arbitrary SFU servers.
-
Decouple media encryption from key management to allow SFrame to be used with an arbitrary key management system.
-
Minimize packet expansion to allow successful conferencing in as many network conditions as possible.
-
Independence from the underlying transport, including use in non-RTP transports, e.g., WebTransport.
-
When used with RTP and its associated error resilience mechanisms, i.e., RTX and FEC, require no special handling for RTX and FEC packets.
-
Minimize the changes needed in SFU servers.
-
Minimize the changes needed in endpoints.
-
Work with the most popular audio and video codecs used in conferencing scenarios.
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 reduced by adding encryption overhead only once per media frame, instead of once per packet.
SFrame is a general encryption framing, which is typically applied in one of two ways: Either to encrypt whole media frames (per-frame) or individual media payloads (per-packet). 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.
For example, {{media-stack}} shows a typical media stack that takes media in from some source, encodes it into frames, divides those frames into media payloads, and then sends those payloads in SRTP packets. Arrows indicate the points where SFrame protection would be integrated into this media stack, when applied per-frame or per-packet.
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 {{overhead}}.
As noted above, however, SFrame is a general media encapsulation, and can be applied in other scenarios. The precise efficiency and complexity trade-offs will depend on the environment in which SFrame is being integrated.
+--------------------------------------------------------+
| |
| +----------+ +-------------+ +-----------+ |
.-. | | | | | | SRTP | |
| | | | Encode |----->| Packetize |----->| Encrypt |-----------+
'+' | | | ^ | | ^ | | | |
/|\ | +----------+ | +-----+-------+ | +-----------+ | |
/ + \ | | ^ | ^ | |
/ \ | SFrame | SFrame | | |
/ \ | Protect | Protect | | |
Alice | (per-frame) | (per-packet) | | |
+--------------------------|--------------------|--------+ |
| | v
| | +-----+------+
E2E Key | HBH Key | | Media |
Management | Management | | Server |
| | +-----+------+
| | |
+--------------------------|--------------------|--------+ |
.-. | SFrame | SFrame | | |
| | | Unprotect | Unprotect | | |
'+' | (per-frame) | (per-packet) | | |
/|\ | | V | V | |
/ + \ | +----------+ | +-----+-------+ | +-----------+ | |
/ \ | | | V | | V | SRTP | | |
/ \ | | Decode |<-----| Depacketize |<-----| Decrypt |<----------+
Bob | | | | | | | |
| +----------+ +-------------+ +-----------+ |
| |
+--------------------------------------------------------+
{: #media-stack "Integration of SFrame in a typical media stack" }
Like SRTP, SFrame does not define how the keys used for SFrame are exchanged by the parties in the conference. Keys for SFrame might be distributed over an existing E2E-secure channel (see {{sender-keys}}), or derived from an E2E-secure shared secret (see {{mls}}). 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.
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 {{sframe-header}}. The structure of the encrypted data and authentication tag are determined by the AEAD algorithm in use.
+-+---+-+----+------------------------------------------+^+
|S|LEN|X|KID | Frame 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.
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. 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.
Both the counter and the key id are encoded as integers in network (big-endian) byte order, in a variable length format to decrease the overhead. The length of each field is up to 8 bytes and is represented in 3 bits in the SFrame header: 000 represents a length of 1, 001 a length of 2, etc.
The first byte in the SFrame header has a fixed format and contains the header metadata:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R| LEN |X| K |
+-+-+-+-+-+-+-+-+
{: title="SFrame header metadata"}
Reserved (R, 1 bit): : This field MUST be set to zero on sending, and MUST be ignored by receivers.
Counter Length (LEN, 3 bits): : This field indicates the length of the CTR field in bytes, minus one (the range of possible values is thus 1-8).
Extended Key Id Flag (X, 1 bit): : Indicates if the key field contains the key id or the key 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 length (KLEN) if set to 1.
If X flag is 0, then the KID is in the range of 0-7 and the counter (CTR) is found in the next LEN bytes:
0 1 2 3 4 5 6 7
+-+-----+-+-----+---------------------------------+
|R|LEN |0| KID | CTR... (length=LEN) |
+-+-----+-+-----+---------------------------------+
{: title="SFrame header with short KID" }
If X flag is 1 then KLEN is the length of the key (KID) in bytes, minus one (the range of possible lengths is thus 1-8). The KID is encoded in the KLEN bytes following the metadata byte, and the counter (CTR) is encoded in the next LEN bytes:
0 1 2 3 4 5 6 7
+-+-----+-+-----+---------------------------+---------------------------+
|R|LEN |1|KLEN | KID... (length=KLEN) | CTR... (length=LEN) |
+-+-----+-+-----+---------------------------+---------------------------+
SFrame encryption uses an AEAD encryption algorithm and hash function defined by the ciphersuite in use (see {{ciphersuites}}). We will refer to the following aspects of the AEAD algorithm below:
-
AEAD.EncryptandAEAD.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 byAEAD.Encrypt(as opposed to a separate field as in SRTP {{?RFC3711}}). -
AEAD.Nk- The size of a key for the encryption algorithm, in bytes -
AEAD.Nn- The size of a nonce for the encryption algorithm, in bytes -
AEAD.Nt- The overhead of the encryption algorithm, in bytes (typically the size of a "tag" that is added to the plaintext)
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 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.
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(base_key, 'SFrame10')
sframe_key = HKDF-Expand(sframe_secret, 'key', AEAD.Nk)
sframe_salt = HKDF-Expand(sframe_secret, 'salt', AEAD.Nn)
The hash function used for HKDF is determined by the ciphersuite in use.
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 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.
def encrypt(S, 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
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 visibile to the SFU (e.g., an RTP header extension).
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.
+----------------+ +---------------+
| frame metadata | | |
+-------+--------+ | |
| | frame |
| | |
| | |
| +-------+-------+
| |
header ----+------------------>| AAD
+-----+ |
| S | |
+-----+ |
| KID +--+--> sframe_key ----->| Key
| | | |
| | +--> sframe_salt --+ |
+-----+ | |
| CTR +---------------------+->| Nonce
| | |
| | |
+-----+ |
| AEAD.Encrypt
| |
| V
| +-------+-------+
| | |
| | |
| | encrypted |
| | frame |
| | |
| | |
| +-------+-------+
| |
| generic RTP packetize
| |
| |
| +----------------------+--------.....--------+
| | | |
V V V V
+---------------+ +---------------+ +---------------+
| SFrame header | | | | |
+---------------+ | | | |
| | | payload 2/N | ... | payload N/N |
| payload 1/N | | | | |
| | | | | |
+---------------+ +---------------+ +---------------+
{: title="Encryption flow with per-frame encryption" }
Before decrypting, a client needs to assemble a full SFrame ciphertext. When SFrame is applied per-packet, this is done by extracting the payload of a decrypted SRTP packet. When SFrame is applied per-frame, the receiving client buffers all packets that belongs to the same frame using the frame beginning and ending marks in the generic RTP frame header extension. Once all packets are available and in order, the receiver forms an SFrame ciphertext by concatenating their payloads, 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.
def decrypt(metadata, sframe):
CTR, KID, ciphertext = parse_ciphertext(sframe)
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.
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.
Each SFrame session uses a single ciphersuite 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 ciphersuites:
| Value | Name | Nh | Nk | Nn | Nt | Reference |
|---|---|---|---|---|---|---|
| 0x0001 | AES_CTR_128_HMAC_SHA256_80 |
32 | 16 | 12 | 10 | RFC XXXX |
| 0x0002 | AES_CTR_128_HMAC_SHA256_64 |
32 | 16 | 12 | 8 | RFC XXXX |
| 0x0003 | AES_CTR_128_HMAC_SHA256_32 |
32 | 16 | 12 | 4 | RFC XXXX |
| 0x0004 | AES_GCM_128_SHA256_128 |
32 | 16 | 12 | 16 | RFC XXXX |
| 0x0005 | AES_GCM_256_SHA512_128 |
64 | 32 | 12 | 16 | RFC XXXX |
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 ciphersuites might be configured for different media streams. For example, in order to conserve bandwidth, a session might use a ciphersuite with eighty-bit tags for video frames and another ciphersuite with thirty-two-bit tags for audio frames.
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(sframe_key):
aead_secret = HKDF-Extract(sframe_key, 'SFrame10 AES CTR AEAD')
enc_key = HKDF-Expand(aead_secret, 'enc', Nk)
auth_key = HKDF-Expand(aead_secret, 'auth', Nh)
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 tag_len fixed by the ciphersuite.
def compute_tag(auth_key, nonce, aad, ct):
aad_len = encode_big_endian(len(aad), 8)
ct_len = encode_big_endian(len(ct), 8)
auth_data = aad_len + ct_len + nonce + aad + ct
tag = HMAC(auth_key, auth_data)
return truncate(tag, tag_len)
def AEAD.Encrypt(key, nonce, aad, pt):
enc_key, auth_key = derive_subkeys(key)
ct = AES-CTR.Encrypt(enc_key, nonce, 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")
return AES-CTR.Decrypt(enc_key, nonce, inner_ct)
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_keymappings to participating clients - 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.
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. The client then uses the E2E-secure channel to send their encryption key to the other participants.
In this scheme, it is assumed that receivers have a signal outside of SFrame for which client has sent a given frame, 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 and (b) the current KID value for the sender. Evicting a participant requires each sender to send a fresh sender key to all receivers.
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)
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
|
...
Selective Forwarding Units (SFUs) as described in {{Section 3.7 of ?RFC7667}} 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
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), a mechanism based on digital signature would be required. SFrame does not protect against such attacks.
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.
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.
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.
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.
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.
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".
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.
This document makes no requests of IANA.
--- back
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.
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.
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.
| 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 |
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 | [email protected] | ||
|---|---|---|---|---|
| 1 | 0-255 | 1440 | 1440 | 720 |
| 2 | 256 - 65K | 1680 | 1680 | 840 |
| 3 | 56K - 16M | 1920 | 1920 | 960 |
| 4 | 16M - 4B | 2160 | 2160 | 1080 |
{{?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.
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
| Overhead bps@20ms | Overhead bps@40ms | Overhead bps@100ms |
|---|---|---|
| 6000 | 3000 | 1200 |
| Overhead bps@30fps | Overhead bps@15fps | Overhead [email protected] |
| (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%.
This section provides a set of test vectors that implementations can use to verify that they correctly implement SFrame encryption and decryption. For each ciphersuite, we provide:
- [in] The
base_keyvalue (hex encoded) - [out] The
secret,key, andsaltvalues derived from thebase_key(hex encoded) - A plaintext value that is encrypted in the following encryption cases
- A sequence of encryption cases, including:
- [in] The
KIDandCTRvalues to be included in the header - [out] The resulting encoded header (hex encoded)
- [out] The nonce computed from the
saltandCTRvalues - The ciphertext resulting from encrypting the plaintext with these parameters (hex encoded)
- [in] The
An implementation should reproduce the output values given the input values:
-
An implementation should be able to encrypt with the input values and the plaintext to produce the ciphertext.
-
An implementation must be able to decrypt with the input values and the ciphertext to generate the plaintext.
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}}.
{::include test-vectors.md}