| Internet-Draft | SFrame | August 2023 |
| Omara, et al. | Expires 5 February 2024 | [Page] |
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.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://sframe-wg.github.io/sframe/draft-ietf-sframe-enc.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-sframe-enc/.¶
Discussion of this document takes place on the Secure Media Frames Working Group mailing list (mailto:[email protected]), which is archived at https://mailarchive.ietf.org/arch/browse/sframe/. Subscribe at https://www.ietf.org/mailman/listinfo/sframe/.¶
Source for this draft and an issue tracker can be found at https://github.com/sframe-wg/sframe.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 5 February 2024.¶
Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
Modern multi-party video call systems use Selective Forwarding Unit (SFU) servers to efficiently route media streams to call endpoints based on factors such as available bandwidth, desired video size, codec support, and other factors. An SFU typically does not need access to the media content of the conference, allowing for the media to be "end-to-end" encrypted so that it cannot be decrypted by the SFU. In order for the SFU to work properly, though, it usually needs 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:¶
The Secure Real-Time Protocol (SRTP) is already widely used for HBH encryption [RFC3711]. The SRTP "double encryption" scheme defines a way to do E2E encryption in SRTP [RFC8723]. Unfortunately, this scheme has poor efficiency and high complexity, and its entanglement with RTP makes it unworkable in several realistic SFU scenarios.¶
This document proposes a new end-to-end encryption mechanism known as SFrame, specifically designed to work in group conference calls with SFUs. SFrame is a general encryption framing that can be used to protect media payloads, agnostic of transport.¶
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.¶
Initialization Vector¶
Message Authentication Code¶
End to End Encryption¶
Hop By Hop¶
We use "Selective Forwarding Unit (SFU)" and "media stream" in a less formal sense than in [RFC7656]. An SFU is a selective switching function for media payloads, and a media stream a sequence of media payloads, in both cases regardless of whether those media payloads are transported over RTP or some other protocol.¶
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:¶
This document defines an encryption mechanism that provides effective end-to-end encryption, is simple to implement, has no dependencies on RTP, and minimizes encryption bandwidth overhead. Because SFrame can encrypt a full frame, rather than individual packets, bandwidth overhead can be reduced by adding encryption overhead only once per media frame, instead of once per packet.¶
SFrame is a general encryption framing, intended to be used as an E2E encryption layer over an underlying HBH-encrypted transport such as SRTP or QUIC [RFC3711][I-D.ietf-moq-transport].¶
The scale at which SFrame encryption is applied to media determines the overall amount of overhead that SFrame adds to the media stream, as well as the engineering complexity involved in integrating SFrame into a particular environment. Two patterns are common: Either using SFrame to encrypt whole media frames (per-frame) or individual transport-level media payloads (per-packet).¶
For example, Figure 1 shows a typical media sender stack that takes media in from some source, encodes it into frames, divides those frames into media packets, and then sends those payloads in SRTP packets. The receiver stack performs the reverse operations, reassembling frames from SRTP packets and decoding. Arrows indicate two different ways that SFrame protection could be integrated into this media stack, to encrypt whole frames or individual media packets.¶
Applying SFrame per-frame in this system offers higher efficiency, but may require a more complex integration in environments where depacketization relies on the content of media packets. Applying SFrame per-packet avoids this complexity, at the cost of higher bandwidth consumption. Some quantitative discussion of these trade-offs is provided in Appendix C.¶
As noted above, however, SFrame is a general media encapsulation, and can be applied in other scenarios. The important thing is that the sender and receivers of an SFrame-encrypted object agree on that object's semantics. SFrame does not provide this agreement; it must be arranged by the application.¶
Like SRTP, SFrame does not define how the keys used for SFrame are exchanged by the parties in the conference. Keys for SFrame might be distributed over an existing E2E-secure channel (see Section 5.1), or derived from an E2E-secure shared secret (see Section 5.2). The key management system MUST ensure that each key used for encrypting media is used by exactly one media sender, in order to avoid reuse of IVs.¶
An SFrame ciphertext comprises an SFrame header followed by the output of an AEAD encryption of the plaintext [RFC5116], with the header provided as additional authenticated data (AAD).¶
The SFrame header is a variable-length structure described in detail in Section 4.3. The structure of the encrypted data and authentication tag are determined by the AEAD algorithm in use.¶
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:¶
Applications MUST ensure that each (KID, CTR) combination is used for exactly one encryption operation. A typical approach to achieving this gaurantee is outlined in Section 9.1.¶
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:¶
This field MUST be set to zero on sending, and MUST be ignored by receivers.¶
This field indicates the length of the CTR field in bytes, minus one (the range of possible values is thus 1-8).¶
Indicates if the key field contains the key id or the key length.¶
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:¶
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:¶
SFrame encryption uses an AEAD encryption algorithm and hash function defined by the cipher suite in use (see Section 4.5). We will refer to the following aspects of the AEAD algorithm below:¶
AEAD.Encrypt and AEAD.Decrypt - The encryption and decryption functions
for the AEAD. We follow the convention of RFC 5116 [RFC5116] and consider
the authentication tag part of the ciphertext produced by AEAD.Encrypt (as
opposed to a separate field as in SRTP [RFC3711]).¶
AEAD.Nk - The size in bytes of a key for the encryption algorithm¶
AEAD.Nn - The size in bytes of a nonce for the encryption algorithm¶
AEAD.Nt - The overhead in bytes of the encryption algorithm (typically the
size of a "tag" that is added to the plaintext)¶
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:¶
def derive_key_salt(KID, base_key):
sframe_secret = HKDF-Extract("", base_key)
sframe_key = HKDF-Expand(sframe_secret, "SFrame 1.0 Secret key " + KID, AEAD.Nk)
sframe_salt = HKDF-Expand(sframe_secret, "SFrame 1.0 Secret salt " + KID, AEAD.Nn)
return sframe_key, sframe_salt
¶
In the derivation of sframe_secret, the + operator represents concatenation
of byte strings and the KID value is encoded as an 8-byte big-endian integer
(not the compressed form used in the SFrame header).¶
The hash function used for HKDF is determined by the cipher suite in use.¶
SFrame encryption uses the AEAD encryption algorithm for the cipher suite in use.
The key for the encryption is the sframe_key and the nonce is formed by XORing
the sframe_salt with the current counter, encoded as a big-endian integer of
length AEAD.Nn.¶
The encryptor forms an SFrame header using the CTR, and KID values provided. The encoded header is provided as AAD to the AEAD encryption operation, together with application-provided metadata about the encrypted media (see Section 9.4).¶
def encrypt(CTR, KID, metadata, plaintext): sframe_key, sframe_salt = key_store[KID] ctr = encode_big_endian(CTR, AEAD.Nn) nonce = xor(sframe_salt, CTR) header = encode_sframe_header(CTR, KID) aad = header + metadata ciphertext = AEAD.Encrypt(sframe_key, nonce, aad, plaintext) return header + ciphertext¶
For example, the metadata input to encryption allows for frame metadata to be authenticated when SFrame is applied per-frame. After encoding the frame and before packetizing it, the necessary media metadata will be moved out of the encoded frame buffer, to be sent in some channel visible to the SFU (e.g., an RTP header extension).¶
Before decrypting, a client needs to assemble a full SFrame ciphertext. When an SFrame ciphertext may be fragmented into multiple parts for transport (e.g., a whole encrypted frame sent in multiple SRTP packets), the receiving client collects all the fragments of the ciphertext, using an appropriate sequencing and start/end markers in the transport. Once all of the required fragments are available, the client reassembles them into the SFrame ciphertext, then passes the ciphertext to SFrame for decryption.¶
The KID field in the SFrame header is used to find the right key and salt for the encrypted frame, and the CTR field is used to construct the nonce.¶
def decrypt(metadata, sframe_ciphertext): KID, CTR, ciphertext = parse_ciphertext(sframe_ciphertext) sframe_key, sframe_salt = key_store[KID] ctr = encode_big_endian(CTR, AEAD.Nn) nonce = xor(sframe_salt, ctr) aad = header + metadata return AEAD.Decrypt(sframe_key, nonce, aad, ciphertext)¶
If a ciphertext fails to decrypt because there is no key available for the KID in the SFrame header, the client MAY buffer the ciphertext and retry decryption once a key with that KID is received.¶
Each SFrame session uses a single cipher suite that specifies the following primitives:¶
This document defines the following cipher suites, with the constants defined in Section 4.4:¶
| Name | Nh | Nk | Nn | Nt |
|---|---|---|---|---|
AES_128_CTR_HMAC_SHA256_80
|
32 | 16 | 12 | 10 |
AES_128_CTR_HMAC_SHA256_64
|
32 | 16 | 12 | 8 |
AES_128_CTR_HMAC_SHA256_32
|
32 | 16 | 12 | 4 |
AES_128_GCM_SHA256_128
|
32 | 16 | 12 | 16 |
AES_256_GCM_SHA512_128
|
64 | 32 | 12 | 16 |
Numeric identifiers for these cipher suites are defined in the IANA registry created in Section 8.1.¶
In the suite names, the length of the authentication tag is indicated by the last value: "_128" indicates a hundred-twenty-eight-bit tag, "_80" indicates a eighty-bit tag, "_64" indicates a sixty-four-bit tag and "_32" indicates a thirty-two-bit tag.¶
In a session that uses multiple media streams, different cipher suites might be configured for different media streams. For example, in order to conserve bandwidth, a session might use a cipher suite with eighty-bit tags for video frames and another cipher suite with thirty-two-bit tags for audio frames.¶
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,
Nh represents the output size of the hash function, and Nt represents the
size of a tag for the cipher in bytes (as in Table 2):¶
def derive_subkeys(sframe_key): tag_len = encode_big_endian(Nt, 8) aead_label = "SFrame 1.0 AES CTR AEAD " + tag_len aead_secret = HKDF-Extract(aead_label, sframe_key) 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 Nt fixed by the cipher suite.¶
def compute_tag(auth_key, nonce, aad, ct):
aad_len = encode_big_endian(len(aad), 8)
ct_len = encode_big_endian(len(ct), 8)
tag_len = encode_big_endian(Nt, 8)
auth_data = aad_len + ct_len + tag_len + nonce + aad + ct
tag = HMAC(auth_key, auth_data)
return truncate(tag, Nt)
def AEAD.Encrypt(key, nonce, aad, pt):
enc_key, auth_key = derive_subkeys(key)
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:¶
base_key mappings to participating clients¶
It is the responsibility of the application to provide the key management framework, as described in Section 9.2.¶
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 (e.g., an RTP SSRC). SFrame KID values are then used to distinguish between versions of the sender's key.¶
Key IDs in this scheme have two parts, a "key generation" and a "ratchet step". Both are unsigned integers that begin at zero. The key generation increments each time the sender distributes a new key to receivers. The "ratchet step" is incremented each time the sender ratchets their key forward for forward secrecy:¶
sender_base_key[i+1] = HKDF-Expand(
HKDF-Extract("", sender_base_key[i]),
"SFrame 1.0 Ratchet", CipherSuite.Nh)
¶
For compactness, we do not send the whole ratchet step. Instead, we send only
its low-order R bits, where R is a value set by the application. Different
senders may use different values of R, but each receiver of a given sender
needs to know what value of R is used by the sender so that they can recognize
when they need to ratchet (vs. expecting a new key). R effectively defines a
re-ordering window, since no more than 2R ratchet steps can be
active at a given time. The key generation is sent in the remaining 64 - R
bits of the key ID.¶
KID = (key_generation << R) + (ratchet_step % (1 << R))¶
The sender signals such a ratchet step update by sending with a KID value in which the ratchet step has been incremented. A receiver who receives from a sender with a new KID computes the new key as above. The old key may be kept for some time to allow for out-of-order delivery, but should be deleted promptly.¶
If a new participant joins mid-call, they will need to receive from each sender (a) the current sender key for that sender and (b) the current KID value for the sender. Evicting a participant requires each sender to send a fresh sender key to all receivers.¶
The Messaging Layer Security (MLS) protocol provides group authenticated key exchange [I-D.ietf-mls-architecture] [I-D.ietf-mls-protocol]. In principle, it could be used to instantiate the sender key scheme above, but it can also be used more efficiently directly.¶
MLS creates a linear sequence of keys, each of which is shared among the members of a group at a given point in time. When a member joins or leaves the group, a new key is produced that is known only to the augmented or reduced group. Each step in the lifetime of the group is know as an "epoch", and each member of the group is assigned an "index" that is constant for the time they are in the group.¶
To generate keys and nonces for SFrame, we use the MLS exporter function to
generate a base_key value for each MLS epoch. Each member of the group is
assigned a set of KID values, so that each member has a unique sframe_key and
sframe_salt that it uses to encrypt with. Senders may choose any KID value
within their assigned set of KID values, e.g., to allow a single sender to send
multiple uncoordinated outbound media streams.¶
base_key = MLS-Exporter("SFrame 1.0 Base Key", "", AEAD.Nk)
¶
For compactness, we do not send the whole epoch number. Instead, we send only
its low-order E bits, where E is a value set by the application. E
effectively defines a re-ordering window, since no more than 2E
epochs can be active at a given time. Receivers MUST be prepared for the epoch
counter to roll over, removing an old epoch when a new epoch with the same E
lower bits is introduced.¶
Let S be the number of bits required to encode a member index in the group,
i.e., the smallest value such that group_size < (1 << S). The sender index
is encoded in the S bits above the epoch. The remaining 64 - S - E bits of
the KID value are a context value chosen by the sender (context value 0` will
produce the shortest encoded KID).¶
KID = (context << (S + E)) + (sender_index << E) + (epoch % (1 << E))¶
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.¶
Selective Forwarding Units (SFUs) (e.g., those described in Section 3.7 of [RFC7667]) receive the media streams from each participant and select which ones should be forwarded to each of the other participants. There are several approaches about how to do this stream selection but in general, in order to do so, the SFU needs to access metadata associated to each frame and modify the RTP information of the incoming packets when they are transmitted to the received participants.¶
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 avoid the overhead involved in establishing new transport streams, the SFU may decide to reuse previously existing streams or even pre-allocate a predefined number of streams and choose in each moment in time which participant media will be sent through it.¶
This means that in the same transport-level stream (e.g., an RTP stream defined by either SSRC or MID) may carry media from different streams of different participants. As different keys are used by each participant for encoding their media, the receiver will be able to verify which is the sender of the media coming within the RTP stream at any given point in time, preventing the SFU trying to impersonate any of the participants with another participant's media.¶
Note that in order to prevent impersonation by a malicious participant (not the SFU), a mechanism based on digital signature would be required. SFrame does not protect against such attacks.¶
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 asynchronously and on a different path than the SFU signaling and media. So it may happen that when a new participant joins the call and the SFU side requests a key frame, the sender generates the e2ee 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 has arrived and was 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 SHOULD 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.¶
The handling of replay is out of the scope of this document. However, senders MUST reject requests to encrypt multiple times with the same key and nonce, since several AEAD algorithms fail badly in such cases (see, e.g., Section 5.1.1 of [RFC5116]).¶
This document requests the creation of the following new IANA registries:¶
This registries should be under a heading of "SFrame", and assignments are made via the Specification Required policy [RFC8126].¶
RFC EDITOR: Please replace XXXX throughout with the RFC number assigned to this document¶
This registry lists identifiers for SFrame cipher suites, as defined in Section 4.5. The cipher suite field is two bytes wide, so the valid cipher suites are in the range 0x0000 to 0xFFFF.¶
Template:¶
Initial contents:¶
| Value | Name | Reference |
|---|---|---|
| 0x0001 |
AES_128_CTR_HMAC_SHA256_80
|
RFC XXXX |
| 0x0002 |
AES_128_CTR_HMAC_SHA256_64
|
RFC XXXX |
| 0x0003 |
AES_128_CTR_HMAC_SHA256_32
|
RFC XXXX |
| 0x0004 |
AES_128_GCM_SHA256_128
|
RFC XXXX |
| 0x0005 |
AES_256_GCM_SHA512_128
|
RFC XXXX |
To use SFrame, an application needs to define the inputs to the SFrame encryption and decryption operations, and how SFrame ciphertexts are delivered from sender to receiver (including any fragmentation and reassembly). In this section, we lay out additional requirements that an integration must meet in order for SFrame to operate securely.¶
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.¶
It is up to the application to provision SFrame with a mapping of KID values to
base_key values and the resulting keys and salts. More importantly, the
application specifies which KID values are used for which purposes (e.g., by
which senders). An applications KID assignment strategy MUST be structured to
assure the non-reuse properties discussed above.¶
It is also up to the application to define a rotation schedule for keys. For example, one application might have an ephemeral group for every call and keep rotating keys when end points join or leave the call, while another application could have a persistent group that can be used for multiple calls and simply derives ephemeral symmetric keys for a specific call.¶
It should be noted that KID values are not encrypted by SFrame, and are thus visible to any application-layer intermediaries that might handle an SFrame ciphertext. If there are application semantics included in KID values, then this information would be exposed to intermediaries. For example, in the scheme of Section 5.1, the number of ratchet steps per sender is exposed, and in the scheme of Section 5.2, the number of epochs and the MLS sender ID of the SFrame sender are exposed.¶
It is the responsibility of the application to handle anti-replay. Replay by network attackers is assumed to be prevented by network-layer facilities (e.g., TLS, SRTP). As mentioned in Section 7.4, senders MUST reject requests to encrypt multiple times with the same key and nonce.¶
It is not mandatory to implement anti-replay on the receiver side. Receivers MAY apply time or counter based anti-replay mitigations.¶
The metadata input to SFrame operations is pure application-specified data. As
such, it is up to the application to define what information should go in the
metadata input and ensure that it is provided to the encryption and decryption
functions at the appropriate points. A receiver SHOULD NOT use SFrame-authenticated
metadata until after the SFrame decrypt function has authenticated it.¶
Note: The metadata input is a feature at risk, and needs more confirmation that it
is useful and/or needed.¶
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.¶
This section is not normative.¶
This section describes a notional API that an SFrame implementation might expose. The core concept is an "SFrame context", within which KID values are meaningful. In the key management scheme described in Section 5.1, each sender has a different context; in the scheme described in Section 5.2, all senders share the same context.¶
An SFrame context stores mappings from KID values to "key contexts", which are different depending on whether the KID is to be used for sending or receiving (an SFrame key should never be used for both operations). A key context tracks the key and salt associated to the KID, and the current CTR value. A key context to be used for sending also tracks the next CTR value to be used.¶
The primary operations on an SFrame context are as follows:¶
Figure 8 shows an example of the types of structures and methods that could be used to create an SFrame API in Rust.¶
type KeyId = u64;
type Counter = u64;
type CipherSuite = u16;
struct SendKeyContext {
key: Vec<u8>,
salt: Vec<u8>,
next_counter: Counter,
}
struct RecvKeyContext {
key: Vec<u8>,
salt: Vec<u8>,
}
struct SFrameContext {
cipher_suite: CipherSuite,
send_keys: HashMap<KeyId, SendKeyContext>,
recv_keys: HashMap<KeyId, RecvKeyContext>,
}
trait SFrameContextMethods {
fn create(cipher_suite: CipherSuite) -> Self;
fn add_send_key(&self, kid: KeyId, base_key: &[u8]);
fn add_recv_key(&self, kid: KeyId, base_key: &[u8]);
fn encrypt(&mut self, kid: KeyId, metadata: &[u8], plaintext: &[u8]) -> Vec<u8>;
fn decrypt(&self, metadata: &[u8], ciphertext: &[u8]) -> Vec<u8>;
}
Any use of SFrame will impose overhead in terms of the amount of bandwidth necessary to transmit a given media stream. Exactly how much overhead will be added depends on several factors:¶
Overall, the overhead rate in kilobits per second can be estimated as:¶
OverheadKbps = (1 + |CTR| + |KID| + |TAG|) * 8 * CTPerSecond / 1024
¶
Here the constant value 1 reflects the fixed SFrame header; |CTR| and
|KID| reflect the lengths of those fields; |TAG| reflects the cipher
overhead; and CTPerSecond reflects the number of SFrame ciphertexts
sent per second (e.g., packets or frames per second).¶
In the remainder of this secton, we compute overhead estimates for a collection of common scenarios.¶
In the below calculations, we make conservative assumptions about SFrame overhead, so that the overhead amounts we compute here are likely to be an upper bound on those seen in practice.¶
| Field | Bytes | Explanataion |
|---|---|---|
| Fixed header | 1 | Fixed |
| Key ID (KID) | 2 | >255 senders; or MLS epoch (E=4) and >16 senders |
| Counter (CTR) | 3 | More than 24 hours of media in common cases |
| Cipher overhead | 16 | Full GCM tag (longest defined here) |
In total, then, we assume that each SFrame encryption will add 22 bytes of overhead.¶
We consider two scenarios, applying SFrame per-frame and per-packet. In each scenario, we compute the SFrame overhead in absolute terms (Kbps) and as a percentage of the base bandwidth.¶
In audio streams, there is typically a one-to-one relationship between frames and packets, so the overhead is the same whether one uses SFrame at a per-packet or per-frame level.¶
The below table considers three scenarios, based on recommended configurations of the Opus codec [RFC6716]:¶
| Scenario | fps | Base Kbps | Overhead Kbps | Overhead % |
|---|---|---|---|---|
| NB speech, 120ms packets | 8.3 | 8 | 1.4 | 17.9% |
| FB speech, 20ms packets | 50 | 32 | 8.6 | 26.9% |
| FB stereo, 10ms packets | 100 | 128 | 17.2 | 13.4% |
Video frames can be larger than an MTU and thus are commonly split across multiple frames. Table 5 and Table 6 show the estimated overhead of encrypting a video stream, where SFrame is applied per-frame and per-packet, respectively. The choices of resolution, frames per second, and bandwidth are chosen to roughly reflect the capabilities of modern video codecs across a range from very low to very high quality.¶
| Scenario | fps | Base Kbps | Overhead Kbps | Overhead % |
|---|---|---|---|---|
| 426 x 240 | 7.5 | 45 | 1.3 | 2.9% |
| 640 x 360 | 15 | 200 | 2.6 | 1.3% |
| 640 x 360 | 30 | 400 | 5.2 | 1.3% |
| 1280 x 720 | 30 | 1500 | 5.2 | 0.3% |
| 1920 x 1080 | 60 | 7200 | 10.3 | 0.1% |
| Scenario | fps | pps | Base Kbps | Overhead Kbps | Overhead % |
|---|---|---|---|---|---|
| 426 x 240 | 7.5 | 7.5 | 45 | 1.3 | 2.9% |
| 640 x 360 | 15 | 30 | 200 | 5.2 | 2.6% |
| 640 x 360 | 30 | 60 | 400 | 10.3 | 2.6% |
| 1280 x 720 | 30 | 180 | 1500 | 30.9 | 2.1% |
| 1920 x 1080 | 60 | 780 | 7200 | 134.1 | 1.9% |
In the per-frame case, the SFrame percentage overhead approaches zero as the quality of the video goes up, since bandwidth is driven more by picture size than frame rate. In the per-packet case, the SFrame percentage overhead approaches the ratio between the SFrame overhead per packet and the MTU (here 22 bytes of SFrame overhead divided by an assumed 1200-byte MTU, or about 1.8%).¶
Real conferences usually involve several audio and video streams. The overhead of SFrame in such a conference is the aggregate of the overhead over all the individual streams. Thus, while SFrame incurs a large percentage overhead on an audio stream, if the conference also involves a video stream, then the audio overhead is likely negligible relative to the overall bandwidth of the conference.¶
For example, Table 7 shows the overhead estimates for a two person conference where one person is sending low-quality media and the other sending high-quality. (And we assume that SFrame is applied per-frame.) The video streams dominate the bandwidth at the SFU, so the total bandwidth overhead is only around 1%.¶
| Stream | Base Kbps | Overhead Kbps | Overhead % |
|---|---|---|---|
| Participant 1 audio | 8 | 1.4 | 17.9% |
| Participant 1 video | 45 | 1.3 | 2.9% |
| Participant 2 audio | 32 | 9 | 26.9% |
| Participant 2 video | 1500 | 5 | 0.3% |
| Total at SFU | 1585 | 16.5 | 1.0% |
SFrame is a generic encapsulation format, but many of the applications in which it is likely to be integrated are based on RTP. This section discusses how an integration between SFrame and RTP could be done, and some of the challenges that would need to be overcome.¶
As discussed in Section 4.1, there are two natural patterns for integrating SFrame into an application: applying SFrame per-frame or per-packet. In RTP-based applications, applying SFrame per-packet means that the payload of each RTP packet will be an SFrame ciphertext, starting with an SFrame Header, as shown in Figure 9. Applying SFrame per-frame means that different RTP payloads will have different formats: The first payload of a frame will contain the SFrame headers, and subsequent payloads will contain further chunks of the ciphertext, as shown in Figure 10.¶
In order for these media payloads to be properly interpreted by receivers, receivers will need to be configured to know which of the above schemes the sender has applied to a given sequence of RTP packets. SFrame does not provide a mechanism for distributing this configuration information. In applications that use SDP for negotiating RTP media streams [RFC4566], an appropriate extension to SDP could provide this function.¶
Applying SFrame per-frame also requires that packetization and depacketization be done in a generic manner that does not depend on the media content of the packets, since the content being packetized / depacketized will be opaque ciphertext (except for the SFrame header). In order for such a generic packetization scheme to work interoperably one would have to be defined, e.g., as proposed in [I-D.codec-agnostic-rtp-payload-format].¶
This section provides a set of test vectors that implementations can use to verify that they correctly implement SFrame encryption and decryption. In addition to test vectors for the overall process of SFrame encryption/decryption, we also provide test vectors for header encoding/decoding, and for AEAD encryption/decryption using the AES-CTR construction defined in Section 4.5.1.¶
All values are either numeric or byte strings. Numeric values are represented
as hex values, prefixed with 0x. Byte strings are represented in hex
encoding.¶
Line breaks and whitespace within values are inserted to conform to the width requirements of the RFC format. They should be removed before use.¶
These test vectors are also available in JSON format at [TestVectors]. In the JSON test vectors, numeric values are JSON numbers and byte string values are JSON strings containing the hex encoding of the byte strings.¶
For each case, we provide:¶
An implementation should verify that:¶
kid: 0x0000000000000000 ctr: 0x0000000000000000 header: 0000¶
kid: 0x0000000000000000 ctr: 0x0000000000000001 header: 0001¶
kid: 0x0000000000000000 ctr: 0x00000000000000ff header: 00ff¶
kid: 0x0000000000000000 ctr: 0x0000000000000100 header: 100100¶
kid: 0x0000000000000000 ctr: 0x000000000000ffff header: 10ffff¶
kid: 0x0000000000000000 ctr: 0x0000000000010000 header: 20010000¶
kid: 0x0000000000000000 ctr: 0x0000000000ffffff header: 20ffffff¶
kid: 0x0000000000000000 ctr: 0x0000000001000000 header: 3001000000¶
kid: 0x0000000000000000 ctr: 0x00000000ffffffff header: 30ffffffff¶
kid: 0x0000000000000000 ctr: 0x0000000100000000 header: 400100000000¶
kid: 0x0000000000000000 ctr: 0x000000ffffffffff header: 40ffffffffff¶
kid: 0x0000000000000000 ctr: 0x0000010000000000 header: 50010000000000¶
kid: 0x0000000000000000 ctr: 0x0000ffffffffffff header: 50ffffffffffff¶
kid: 0x0000000000000000 ctr: 0x0001000000000000 header: 6001000000000000¶
kid: 0x0000000000000000 ctr: 0x00ffffffffffffff header: 60ffffffffffffff¶
kid: 0x0000000000000000 ctr: 0x0100000000000000 header: 700100000000000000¶
kid: 0x0000000000000000 ctr: 0xffffffffffffffff header: 70ffffffffffffffff¶
kid: 0x0000000000000001 ctr: 0x0000000000000000 header: 0100¶
kid: 0x0000000000000001 ctr: 0x0000000000000001 header: 0101¶
kid: 0x0000000000000001 ctr: 0x00000000000000ff header: 01ff¶
kid: 0x0000000000000001 ctr: 0x0000000000000100 header: 110100¶
kid: 0x0000000000000001 ctr: 0x000000000000ffff header: 11ffff¶
kid: 0x0000000000000001 ctr: 0x0000000000010000 header: 21010000¶
kid: 0x0000000000000001 ctr: 0x0000000000ffffff header: 21ffffff¶
kid: 0x0000000000000001 ctr: 0x0000000001000000 header: 3101000000¶
kid: 0x0000000000000001 ctr: 0x00000000ffffffff header: 31ffffffff¶
kid: 0x0000000000000001 ctr: 0x0000000100000000 header: 410100000000¶
kid: 0x0000000000000001 ctr: 0x000000ffffffffff header: 41ffffffffff¶
kid: 0x0000000000000001 ctr: 0x0000010000000000 header: 51010000000000¶
kid: 0x0000000000000001 ctr: 0x0000ffffffffffff header: 51ffffffffffff¶
kid: 0x0000000000000001 ctr: 0x0001000000000000 header: 6101000000000000¶
kid: 0x0000000000000001 ctr: 0x00ffffffffffffff header: 61ffffffffffffff¶
kid: 0x0000000000000001 ctr: 0x0100000000000000 header: 710100000000000000¶
kid: 0x0000000000000001 ctr: 0xffffffffffffffff header: 71ffffffffffffffff¶
kid: 0x00000000000000ff ctr: 0x0000000000000000 header: 08ff00¶
kid: 0x00000000000000ff ctr: 0x0000000000000001 header: 08ff01¶
kid: 0x00000000000000ff ctr: 0x00000000000000ff header: 08ffff¶
kid: 0x00000000000000ff ctr: 0x0000000000000100 header: 18ff0100¶
kid: 0x00000000000000ff ctr: 0x000000000000ffff header: 18ffffff¶
kid: 0x00000000000000ff ctr: 0x0000000000010000 header: 28ff010000¶
kid: 0x00000000000000ff ctr: 0x0000000000ffffff header: 28ffffffff¶
kid: 0x00000000000000ff ctr: 0x0000000001000000 header: 38ff01000000¶
kid: 0x00000000000000ff ctr: 0x00000000ffffffff header: 38ffffffffff¶
kid: 0x00000000000000ff ctr: 0x0000000100000000 header: 48ff0100000000¶
kid: 0x00000000000000ff ctr: 0x000000ffffffffff header: 48ffffffffffff¶
kid: 0x00000000000000ff ctr: 0x0000010000000000 header: 58ff010000000000¶
kid: 0x00000000000000ff ctr: 0x0000ffffffffffff header: 58ffffffffffffff¶
kid: 0x00000000000000ff ctr: 0x0001000000000000 header: 68ff01000000000000¶
kid: 0x00000000000000ff ctr: 0x00ffffffffffffff header: 68ffffffffffffffff¶
kid: 0x00000000000000ff ctr: 0x0100000000000000 header: 78ff0100000000000000¶
kid: 0x00000000000000ff ctr: 0xffffffffffffffff header: 78ffffffffffffffffff¶
kid: 0x0000000000000100 ctr: 0x0000000000000000 header: 09010000¶
kid: 0x0000000000000100 ctr: 0x0000000000000001 header: 09010001¶
kid: 0x0000000000000100 ctr: 0x00000000000000ff header: 090100ff¶
kid: 0x0000000000000100 ctr: 0x0000000000000100 header: 1901000100¶
kid: 0x0000000000000100 ctr: 0x000000000000ffff header: 190100ffff¶
kid: 0x0000000000000100 ctr: 0x0000000000010000 header: 290100010000¶
kid: 0x0000000000000100 ctr: 0x0000000000ffffff header: 290100ffffff¶
kid: 0x0000000000000100 ctr: 0x0000000001000000 header: 39010001000000¶
kid: 0x0000000000000100 ctr: 0x00000000ffffffff header: 390100ffffffff¶
kid: 0x0000000000000100 ctr: 0x0000000100000000 header: 4901000100000000¶
kid: 0x0000000000000100 ctr: 0x000000ffffffffff header: 490100ffffffffff¶
kid: 0x0000000000000100 ctr: 0x0000010000000000 header: 590100010000000000¶
kid: 0x0000000000000100 ctr: 0x0000ffffffffffff header: 590100ffffffffffff¶
kid: 0x0000000000000100 ctr: 0x0001000000000000 header: 69010001000000000000¶
kid: 0x0000000000000100 ctr: 0x00ffffffffffffff header: 690100ffffffffffffff¶
kid: 0x0000000000000100 ctr: 0x0100000000000000 header: 7901000100000000000000¶
kid: 0x0000000000000100 ctr: 0xffffffffffffffff header: 790100ffffffffffffffff¶
kid: 0x000000000000ffff ctr: 0x0000000000000000 header: 09ffff00¶
kid: 0x000000000000ffff ctr: 0x0000000000000001 header: 09ffff01¶
kid: 0x000000000000ffff ctr: 0x00000000000000ff header: 09ffffff¶
kid: 0x000000000000ffff ctr: 0x0000000000000100 header: 19ffff0100¶
kid: 0x000000000000ffff ctr: 0x000000000000ffff header: 19ffffffff¶
kid: 0x000000000000ffff ctr: 0x0000000000010000 header: 29ffff010000¶
kid: 0x000000000000ffff ctr: 0x0000000000ffffff header: 29ffffffffff¶
kid: 0x000000000000ffff ctr: 0x0000000001000000 header: 39ffff01000000¶
kid: 0x000000000000ffff ctr: 0x00000000ffffffff header: 39ffffffffffff¶
kid: 0x000000000000ffff ctr: 0x0000000100000000 header: 49ffff0100000000¶
kid: 0x000000000000ffff ctr: 0x000000ffffffffff header: 49ffffffffffffff¶
kid: 0x000000000000ffff ctr: 0x0000010000000000 header: 59ffff010000000000¶
kid: 0x000000000000ffff ctr: 0x0000ffffffffffff header: 59ffffffffffffffff¶
kid: 0x000000000000ffff ctr: 0x0001000000000000 header: 69ffff01000000000000¶
kid: 0x000000000000ffff ctr: 0x00ffffffffffffff header: 69ffffffffffffffffff¶
kid: 0x000000000000ffff ctr: 0x0100000000000000 header: 79ffff0100000000000000¶
kid: 0x000000000000ffff ctr: 0xffffffffffffffff header: 79ffffffffffffffffffff¶
kid: 0x0000000000010000 ctr: 0x0000000000000000 header: 0a01000000¶
kid: 0x0000000000010000 ctr: 0x0000000000000001 header: 0a01000001¶
kid: 0x0000000000010000 ctr: 0x00000000000000ff header: 0a010000ff¶
kid: 0x0000000000010000 ctr: 0x0000000000000100 header: 1a0100000100¶
kid: 0x0000000000010000 ctr: 0x000000000000ffff header: 1a010000ffff¶
kid: 0x0000000000010000 ctr: 0x0000000000010000 header: 2a010000010000¶
kid: 0x0000000000010000 ctr: 0x0000000000ffffff header: 2a010000ffffff¶
kid: 0x0000000000010000 ctr: 0x0000000001000000 header: 3a01000001000000¶
kid: 0x0000000000010000 ctr: 0x00000000ffffffff header: 3a010000ffffffff¶
kid: 0x0000000000010000 ctr: 0x0000000100000000 header: 4a0100000100000000¶
kid: 0x0000000000010000 ctr: 0x000000ffffffffff header: 4a010000ffffffffff¶
kid: 0x0000000000010000 ctr: 0x0000010000000000 header: 5a010000010000000000¶
kid: 0x0000000000010000 ctr: 0x0000ffffffffffff header: 5a010000ffffffffffff¶
kid: 0x0000000000010000 ctr: 0x0001000000000000 header: 6a01000001000000000000¶
kid: 0x0000000000010000 ctr: 0x00ffffffffffffff header: 6a010000ffffffffffffff¶
kid: 0x0000000000010000 ctr: 0x0100000000000000 header: 7a0100000100000000000000¶
kid: 0x0000000000010000 ctr: 0xffffffffffffffff header: 7a010000ffffffffffffffff¶
kid: 0x0000000000ffffff ctr: 0x0000000000000000 header: 0affffff00¶
kid: 0x0000000000ffffff ctr: 0x0000000000000001 header: 0affffff01¶
kid: 0x0000000000ffffff ctr: 0x00000000000000ff header: 0affffffff¶
kid: 0x0000000000ffffff ctr: 0x0000000000000100 header: 1affffff0100¶
kid: 0x0000000000ffffff ctr: 0x000000000000ffff header: 1affffffffff¶
kid: 0x0000000000ffffff ctr: 0x0000000000010000 header: 2affffff010000¶
kid: 0x0000000000ffffff ctr: 0x0000000000ffffff header: 2affffffffffff¶
kid: 0x0000000000ffffff ctr: 0x0000000001000000 header: 3affffff01000000¶
kid: 0x0000000000ffffff ctr: 0x00000000ffffffff header: 3affffffffffffff¶
kid: 0x0000000000ffffff ctr: 0x0000000100000000 header: 4affffff0100000000¶
kid: 0x0000000000ffffff ctr: 0x000000ffffffffff header: 4affffffffffffffff¶
kid: 0x0000000000ffffff ctr: 0x0000010000000000 header: 5affffff010000000000¶
kid: 0x0000000000ffffff ctr: 0x0000ffffffffffff header: 5affffffffffffffffff¶
kid: 0x0000000000ffffff ctr: 0x0001000000000000 header: 6affffff01000000000000¶
kid: 0x0000000000ffffff ctr: 0x00ffffffffffffff header: 6affffffffffffffffffff¶
kid: 0x0000000000ffffff ctr: 0x0100000000000000 header: 7affffff0100000000000000¶
kid: 0x0000000000ffffff ctr: 0xffffffffffffffff header: 7affffffffffffffffffffff¶
kid: 0x0000000001000000 ctr: 0x0000000000000000 header: 0b0100000000¶
kid: 0x0000000001000000 ctr: 0x0000000000000001 header: 0b0100000001¶
kid: 0x0000000001000000 ctr: 0x00000000000000ff header: 0b01000000ff¶
kid: 0x0000000001000000 ctr: 0x0000000000000100 header: 1b010000000100¶
kid: 0x0000000001000000 ctr: 0x000000000000ffff header: 1b01000000ffff¶
kid: 0x0000000001000000 ctr: 0x0000000000010000 header: 2b01000000010000¶
kid: 0x0000000001000000 ctr: 0x0000000000ffffff header: 2b01000000ffffff¶
kid: 0x0000000001000000 ctr: 0x0000000001000000 header: 3b0100000001000000¶
kid: 0x0000000001000000 ctr: 0x00000000ffffffff header: 3b01000000ffffffff¶
kid: 0x0000000001000000 ctr: 0x0000000100000000 header: 4b010000000100000000¶
kid: 0x0000000001000000 ctr: 0x000000ffffffffff header: 4b01000000ffffffffff¶
kid: 0x0000000001000000 ctr: 0x0000010000000000 header: 5b01000000010000000000¶
kid: 0x0000000001000000 ctr: 0x0000ffffffffffff header: 5b01000000ffffffffffff¶
kid: 0x0000000001000000 ctr: 0x0001000000000000 header: 6b0100000001000000000000¶
kid: 0x0000000001000000 ctr: 0x00ffffffffffffff header: 6b01000000ffffffffffffff¶
kid: 0x0000000001000000 ctr: 0x0100000000000000 header: 7b010000000100000000000000¶
kid: 0x0000000001000000 ctr: 0xffffffffffffffff header: 7b01000000ffffffffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0000000000000000 header: 0bffffffff00¶
kid: 0x00000000ffffffff ctr: 0x0000000000000001 header: 0bffffffff01¶
kid: 0x00000000ffffffff ctr: 0x00000000000000ff header: 0bffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0000000000000100 header: 1bffffffff0100¶
kid: 0x00000000ffffffff ctr: 0x000000000000ffff header: 1bffffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0000000000010000 header: 2bffffffff010000¶
kid: 0x00000000ffffffff ctr: 0x0000000000ffffff header: 2bffffffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0000000001000000 header: 3bffffffff01000000¶
kid: 0x00000000ffffffff ctr: 0x00000000ffffffff header: 3bffffffffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0000000100000000 header: 4bffffffff0100000000¶
kid: 0x00000000ffffffff ctr: 0x000000ffffffffff header: 4bffffffffffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0000010000000000 header: 5bffffffff010000000000¶
kid: 0x00000000ffffffff ctr: 0x0000ffffffffffff header: 5bffffffffffffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0001000000000000 header: 6bffffffff01000000000000¶
kid: 0x00000000ffffffff ctr: 0x00ffffffffffffff header: 6bffffffffffffffffffffff¶
kid: 0x00000000ffffffff ctr: 0x0100000000000000 header: 7bffffffff0100000000000000¶
kid: 0x00000000ffffffff ctr: 0xffffffffffffffff header: 7bffffffffffffffffffffffff¶
kid: 0x0000000100000000 ctr: 0x0000000000000000 header: 0c010000000000¶
kid: 0x0000000100000000 ctr: 0x0000000000000001 header: 0c010000000001¶
kid: 0x0000000100000000 ctr: 0x00000000000000ff header: 0c0100000000ff¶
kid: 0x0000000100000000 ctr: 0x0000000000000100 header: 1c01000000000100¶
kid: 0x0000000100000000 ctr: 0x000000000000ffff header: 1c0100000000ffff¶
kid: 0x0000000100000000 ctr: 0x0000000000010000 header: 2c0100000000010000¶
kid: 0x0000000100000000 ctr: 0x0000000000ffffff header: 2c0100000000ffffff¶
kid: 0x0000000100000000 ctr: 0x0000000001000000 header: 3c010000000001000000¶
kid: 0x0000000100000000 ctr: 0x00000000ffffffff header: 3c0100000000ffffffff¶
kid: 0x0000000100000000 ctr: 0x0000000100000000 header: 4c01000000000100000000¶
kid: 0x0000000100000000 ctr: 0x000000ffffffffff header: 4c0100000000ffffffffff¶
kid: 0x0000000100000000 ctr: 0x0000010000000000 header: 5c0100000000010000000000¶
kid: 0x0000000100000000 ctr: 0x0000ffffffffffff header: 5c0100000000ffffffffffff¶
kid: 0x0000000100000000 ctr: 0x0001000000000000 header: 6c010000000001000000000000¶
kid: 0x0000000100000000 ctr: 0x00ffffffffffffff header: 6c0100000000ffffffffffffff¶
kid: 0x0000000100000000 ctr: 0x0100000000000000 header: 7c01000000000100000000000000¶
kid: 0x0000000100000000 ctr: 0xffffffffffffffff header: 7c0100000000ffffffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0000000000000000 header: 0cffffffffff00¶
kid: 0x000000ffffffffff ctr: 0x0000000000000001 header: 0cffffffffff01¶
kid: 0x000000ffffffffff ctr: 0x00000000000000ff header: 0cffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0000000000000100 header: 1cffffffffff0100¶
kid: 0x000000ffffffffff ctr: 0x000000000000ffff header: 1cffffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0000000000010000 header: 2cffffffffff010000¶
kid: 0x000000ffffffffff ctr: 0x0000000000ffffff header: 2cffffffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0000000001000000 header: 3cffffffffff01000000¶
kid: 0x000000ffffffffff ctr: 0x00000000ffffffff header: 3cffffffffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0000000100000000 header: 4cffffffffff0100000000¶
kid: 0x000000ffffffffff ctr: 0x000000ffffffffff header: 4cffffffffffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0000010000000000 header: 5cffffffffff010000000000¶
kid: 0x000000ffffffffff ctr: 0x0000ffffffffffff header: 5cffffffffffffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0001000000000000 header: 6cffffffffff01000000000000¶
kid: 0x000000ffffffffff ctr: 0x00ffffffffffffff header: 6cffffffffffffffffffffffff¶
kid: 0x000000ffffffffff ctr: 0x0100000000000000 header: 7cffffffffff0100000000000000¶
kid: 0x000000ffffffffff ctr: 0xffffffffffffffff header: 7cffffffffffffffffffffffffff¶
kid: 0x0000010000000000 ctr: 0x0000000000000000 header: 0d01000000000000¶
kid: 0x0000010000000000 ctr: 0x0000000000000001 header: 0d01000000000001¶
kid: 0x0000010000000000 ctr: 0x00000000000000ff header: 0d010000000000ff¶
kid: 0x0000010000000000 ctr: 0x0000000000000100 header: 1d0100000000000100¶
kid: 0x0000010000000000 ctr: 0x000000000000ffff header: 1d010000000000ffff¶
kid: 0x0000010000000000 ctr: 0x0000000000010000 header: 2d010000000000010000¶
kid: 0x0000010000000000 ctr: 0x0000000000ffffff header: 2d010000000000ffffff¶
kid: 0x0000010000000000 ctr: 0x0000000001000000 header: 3d01000000000001000000¶
kid: 0x0000010000000000 ctr: 0x00000000ffffffff header: 3d010000000000ffffffff¶
kid: 0x0000010000000000 ctr: 0x0000000100000000 header: 4d0100000000000100000000¶
kid: 0x0000010000000000 ctr: 0x000000ffffffffff header: 4d010000000000ffffffffff¶
kid: 0x0000010000000000 ctr: 0x0000010000000000 header: 5d010000000000010000000000¶
kid: 0x0000010000000000 ctr: 0x0000ffffffffffff header: 5d010000000000ffffffffffff¶
kid: 0x0000010000000000 ctr: 0x0001000000000000 header: 6d01000000000001000000000000¶
kid: 0x0000010000000000 ctr: 0x00ffffffffffffff header: 6d010000000000ffffffffffffff¶
kid: 0x0000010000000000 ctr: 0x0100000000000000 header: 7d0100000000000100000000000000¶
kid: 0x0000010000000000 ctr: 0xffffffffffffffff header: 7d010000000000ffffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0000000000000000 header: 0dffffffffffff00¶
kid: 0x0000ffffffffffff ctr: 0x0000000000000001 header: 0dffffffffffff01¶
kid: 0x0000ffffffffffff ctr: 0x00000000000000ff header: 0dffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0000000000000100 header: 1dffffffffffff0100¶
kid: 0x0000ffffffffffff ctr: 0x000000000000ffff header: 1dffffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0000000000010000 header: 2dffffffffffff010000¶
kid: 0x0000ffffffffffff ctr: 0x0000000000ffffff header: 2dffffffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0000000001000000 header: 3dffffffffffff01000000¶
kid: 0x0000ffffffffffff ctr: 0x00000000ffffffff header: 3dffffffffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0000000100000000 header: 4dffffffffffff0100000000¶
kid: 0x0000ffffffffffff ctr: 0x000000ffffffffff header: 4dffffffffffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0000010000000000 header: 5dffffffffffff010000000000¶
kid: 0x0000ffffffffffff ctr: 0x0000ffffffffffff header: 5dffffffffffffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0001000000000000 header: 6dffffffffffff01000000000000¶
kid: 0x0000ffffffffffff ctr: 0x00ffffffffffffff header: 6dffffffffffffffffffffffffff¶
kid: 0x0000ffffffffffff ctr: 0x0100000000000000 header: 7dffffffffffff0100000000000000¶
kid: 0x0000ffffffffffff ctr: 0xffffffffffffffff header: 7dffffffffffffffffffffffffffff¶
kid: 0x0001000000000000 ctr: 0x0000000000000000 header: 0e0100000000000000¶
kid: 0x0001000000000000 ctr: 0x0000000000000001 header: 0e0100000000000001¶
kid: 0x0001000000000000 ctr: 0x00000000000000ff header: 0e01000000000000ff¶
kid: 0x0001000000000000 ctr: 0x0000000000000100 header: 1e010000000000000100¶
kid: 0x0001000000000000 ctr: 0x000000000000ffff header: 1e01000000000000ffff¶
kid: 0x0001000000000000 ctr: 0x0000000000010000 header: 2e01000000000000010000¶
kid: 0x0001000000000000 ctr: 0x0000000000ffffff header: 2e01000000000000ffffff¶
kid: 0x0001000000000000 ctr: 0x0000000001000000 header: 3e0100000000000001000000¶
kid: 0x0001000000000000 ctr: 0x00000000ffffffff header: 3e01000000000000ffffffff¶
kid: 0x0001000000000000 ctr: 0x0000000100000000 header: 4e010000000000000100000000¶
kid: 0x0001000000000000 ctr: 0x000000ffffffffff header: 4e01000000000000ffffffffff¶
kid: 0x0001000000000000 ctr: 0x0000010000000000 header: 5e01000000000000010000000000¶
kid: 0x0001000000000000 ctr: 0x0000ffffffffffff header: 5e01000000000000ffffffffffff¶
kid: 0x0001000000000000 ctr: 0x0001000000000000 header: 6e0100000000000001000000000000¶
kid: 0x0001000000000000 ctr: 0x00ffffffffffffff header: 6e01000000000000ffffffffffffff¶
kid: 0x0001000000000000 ctr: 0x0100000000000000 header: 7e010000000000000100000000000000¶
kid: 0x0001000000000000 ctr: 0xffffffffffffffff header: 7e01000000000000ffffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0000000000000000 header: 0effffffffffffff00¶
kid: 0x00ffffffffffffff ctr: 0x0000000000000001 header: 0effffffffffffff01¶
kid: 0x00ffffffffffffff ctr: 0x00000000000000ff header: 0effffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0000000000000100 header: 1effffffffffffff0100¶
kid: 0x00ffffffffffffff ctr: 0x000000000000ffff header: 1effffffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0000000000010000 header: 2effffffffffffff010000¶
kid: 0x00ffffffffffffff ctr: 0x0000000000ffffff header: 2effffffffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0000000001000000 header: 3effffffffffffff01000000¶
kid: 0x00ffffffffffffff ctr: 0x00000000ffffffff header: 3effffffffffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0000000100000000 header: 4effffffffffffff0100000000¶
kid: 0x00ffffffffffffff ctr: 0x000000ffffffffff header: 4effffffffffffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0000010000000000 header: 5effffffffffffff010000000000¶
kid: 0x00ffffffffffffff ctr: 0x0000ffffffffffff header: 5effffffffffffffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0001000000000000 header: 6effffffffffffff01000000000000¶
kid: 0x00ffffffffffffff ctr: 0x00ffffffffffffff header: 6effffffffffffffffffffffffffff¶
kid: 0x00ffffffffffffff ctr: 0x0100000000000000 header: 7effffffffffffff0100000000000000¶
kid: 0x00ffffffffffffff ctr: 0xffffffffffffffff header: 7effffffffffffffffffffffffffffff¶
kid: 0x0100000000000000 ctr: 0x0000000000000000 header: 0f010000000000000000¶
kid: 0x0100000000000000 ctr: 0x0000000000000001 header: 0f010000000000000001¶
kid: 0x0100000000000000 ctr: 0x00000000000000ff header: 0f0100000000000000ff¶
kid: 0x0100000000000000 ctr: 0x0000000000000100 header: 1f01000000000000000100¶
kid: 0x0100000000000000 ctr: 0x000000000000ffff header: 1f0100000000000000ffff¶
kid: 0x0100000000000000 ctr: 0x0000000000010000 header: 2f0100000000000000010000¶
kid: 0x0100000000000000 ctr: 0x0000000000ffffff header: 2f0100000000000000ffffff¶
kid: 0x0100000000000000 ctr: 0x0000000001000000 header: 3f010000000000000001000000¶
kid: 0x0100000000000000 ctr: 0x00000000ffffffff header: 3f0100000000000000ffffffff¶
kid: 0x0100000000000000 ctr: 0x0000000100000000 header: 4f01000000000000000100000000¶
kid: 0x0100000000000000 ctr: 0x000000ffffffffff header: 4f0100000000000000ffffffffff¶
kid: 0x0100000000000000 ctr: 0x0000010000000000 header: 5f0100000000000000010000000000¶
kid: 0x0100000000000000 ctr: 0x0000ffffffffffff header: 5f0100000000000000ffffffffffff¶
kid: 0x0100000000000000 ctr: 0x0001000000000000 header: 6f010000000000000001000000000000¶
kid: 0x0100000000000000 ctr: 0x00ffffffffffffff header: 6f0100000000000000ffffffffffffff¶
kid: 0x0100000000000000 ctr: 0x0100000000000000 header: 7f01000000000000000100000000000000¶
kid: 0x0100000000000000 ctr: 0xffffffffffffffff header: 7f0100000000000000ffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0000000000000000 header: 0fffffffffffffffff00¶
kid: 0xffffffffffffffff ctr: 0x0000000000000001 header: 0fffffffffffffffff01¶
kid: 0xffffffffffffffff ctr: 0x00000000000000ff header: 0fffffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0000000000000100 header: 1fffffffffffffffff0100¶
kid: 0xffffffffffffffff ctr: 0x000000000000ffff header: 1fffffffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0000000000010000 header: 2fffffffffffffffff010000¶
kid: 0xffffffffffffffff ctr: 0x0000000000ffffff header: 2fffffffffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0000000001000000 header: 3fffffffffffffffff01000000¶
kid: 0xffffffffffffffff ctr: 0x00000000ffffffff header: 3fffffffffffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0000000100000000 header: 4fffffffffffffffff0100000000¶
kid: 0xffffffffffffffff ctr: 0x000000ffffffffff header: 4fffffffffffffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0000010000000000 header: 5fffffffffffffffff010000000000¶
kid: 0xffffffffffffffff ctr: 0x0000ffffffffffff header: 5fffffffffffffffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0001000000000000 header: 6fffffffffffffffff01000000000000¶
kid: 0xffffffffffffffff ctr: 0x00ffffffffffffff header: 6fffffffffffffffffffffffffffffff¶
kid: 0xffffffffffffffff ctr: 0x0100000000000000 header: 7fffffffffffffffff0100000000000000¶
kid: 0xffffffffffffffff ctr: 0xffffffffffffffff header: 7fffffffffffffffffffffffffffffffff¶
For each case, we provide:¶
cipher_suite: The index of the cipher suite in use (see
Section 8.1)¶
key: The key input to encryption/decryption¶
aead_label: The aead_label variable in the derive_subkeys() algorithm¶
aead_secret: The aead_secret variable in the derive_subkeys() algorithm¶
enc_key: The encryption subkey produced by the derive_subkeys() algorithm¶
auth_key: The encryption subkey produced by the derive_subkeys() algorithm¶
nonce: The nonce input to encryption/decryption¶
aad: The aad input to encryption/decryption¶
pt: The plaintext¶
ct: The ciphertext¶
An implementation should verify that the following are true, where
AEAD.Encrypt and AEAD.Decrypt are as defined in Section 4.5.1:¶
The other values in the test vector are intermediate values provided to facilitate debugging of test failures.¶
cipher_suite: 0x0001 key: 000102030405060708090a0b0c0d0e0f aead_label: 534672616d6520312e302041455320435452204145414420000000000000000a aead_secret: fda0fef7af62639ae1c6440f430395f54623f9a49db659201312ed6d9999a580 enc_key: d6a61ca11fe8397b24954cda8b9543cf auth_key: 0a43277c91120b7c7b6584bede06fcdfe0d07f9d1c9f15fcf0cad50aaecdd585 nonce: 101112131415161718191a1b aad: 4945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: 1075c7114e10c12f20a709450ef8a891e9f070d4fae7b01f558599c929fdfd¶
cipher_suite: 0x0002 key: 000102030405060708090a0b0c0d0e0f aead_label: 534672616d6520312e3020414553204354522041454144200000000000000008 aead_secret: a0d71a69b2033a5a246eefbed19d95aee712a7639a752e5ad3a2b44c9f331caa enc_key: 0ef75d1dd74b81e4d2252e6daa7226da auth_key: 5584d32db18ede79fe8071a334ff31eb2ca0249a7845a61965d2ec620a50c59e nonce: 101112131415161718191a1b aad: 4945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: f8551395579efc8dfdda575ed1a048f8b6cbf0e85653f0a514dea191e4¶
cipher_suite: 0x0003 key: 000102030405060708090a0b0c0d0e0f aead_label: 534672616d6520312e3020414553204354522041454144200000000000000004 aead_secret: ff69640f46d50930ce38bcf5aa5f6417a5bff98a991c79da06a0be460211dd36 enc_key: 96a673a94981bd85e71fcf05c79f2a01 auth_key: bbf3b39da1eb8ed31fc5e0b26896a070f1a43e5ad3009b4c9d6c32e77ac68fce nonce: 101112131415161718191a1b aad: 4945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: d6455bdbe7b5e8cdda861a8e90835637c0f7990349ce9052e6¶
For each case, we provide:¶
cipher_suite: The index of the cipher suite in use (see
Section 8.1)¶
kid: A KID value¶
ctr: A CTR value¶
base_key: The base_key input to the derive_key_salt algorithm¶
sframe_label: The sframe_label variable in the derive_key_salt algorithm¶
sframe_secret: The sframe_secret variable in the derive_key_salt algorithm¶
sframe_key: The sframe_key value produced by the derive_key_salt algorithm¶
sframe_salt: The sframe_salt value produced by the derive_key_salt algorithm¶
metadata: The metadata input to the SFrame encrypt algorithm¶
pt: The plaintext¶
ct: The SFrame ciphertext¶
An implementation should verify that the following are true, where
encrypt and decrypt are as defined in Section 4.4, using an SFrame
context initialized with base_key assigned to kid:¶
The other values in the test vector are intermediate values provided to facilitate debugging of test failures.¶
cipher_suite: 0x0001 kid: 0x0000000000000123 ctr: 0x0000000000004567 base_key: 000102030405060708090a0b0c0d0e0f sframe_label: 534672616d6520312e3020 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0 sframe_key: 52cef96e29191912a6be442a9651c43a sframe_salt: 9655c98fdad276683deb279c metadata: 4945544620534672616d65205747 nonce: 9655c98fdad276683deb62fb aad: 19012345674945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: 1901234567b6a27b2d24f1f06d49cffe6c82af5a96e0d89443a7a93a8700f96fdda3e43c¶
cipher_suite: 0x0002 kid: 0x0000000000000123 ctr: 0x0000000000004567 base_key: 000102030405060708090a0b0c0d0e0f sframe_label: 534672616d6520312e3020 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0 sframe_key: 52cef96e29191912a6be442a9651c43a sframe_salt: 9655c98fdad276683deb279c metadata: 4945544620534672616d65205747 nonce: 9655c98fdad276683deb62fb aad: 19012345674945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: 190123456728b1faac3515d5ca29f3db9c52f27789c5ec8386ff0b570853ebcf721c¶
cipher_suite: 0x0003 kid: 0x0000000000000123 ctr: 0x0000000000004567 base_key: 000102030405060708090a0b0c0d0e0f sframe_label: 534672616d6520312e3020 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0 sframe_key: 52cef96e29191912a6be442a9651c43a sframe_salt: 9655c98fdad276683deb279c metadata: 4945544620534672616d65205747 nonce: 9655c98fdad276683deb62fb aad: 19012345674945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: 190123456754719dcfbe065e1606068cb6b6b5f1a9a371e633ff088485e7¶
cipher_suite: 0x0004 kid: 0x0000000000000123 ctr: 0x0000000000004567 base_key: 000102030405060708090a0b0c0d0e0f sframe_label: 534672616d6520312e3020 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203ceff8cceee34f574d23909eb314c40c0 sframe_key: 52cef96e29191912a6be442a9651c43a sframe_salt: 9655c98fdad276683deb279c metadata: 4945544620534672616d65205747 nonce: 9655c98fdad276683deb62fb aad: 19012345674945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: 1901234567d4dfcd537dbd054dcf4bdab53bf451826843325838178391f63dc15b9475d6081b59c776a5¶
cipher_suite: 0x0005 kid: 0x0000000000000123 ctr: 0x0000000000004567 base_key: 000102030405060708090a0b0c0d0e0f sframe_label: 534672616d6520312e3020 sframe_secret: 0fc3ea6de6aac97a35f194cf9bed94d4b5230f1cb45a785c9fe5dce9c188938ab6ba005bc4c0a19181599e9d1bcf7b74aca48b60bf5e254e546d809313e083a3 sframe_key: 5f3f7c1b277d9cad86b906da39702c3fcdf720902817977ae99bd10f2e5ad56a sframe_salt: a653f558a8018877314fb8d9 metadata: 4945544620534672616d65205747 nonce: a653f558a8018877314ffdbe aad: 19012345674945544620534672616d65205747 pt: 64726166742d696574662d736672616d652d656e63 ct: 19012345672f55e5feb46d118576dc715566003f4becf5252149c839aea7dd5434bf8eceb8b4d59bbfb2¶