| Network Working Group | C. Jennings |
| Internet-Draft | Cisco Systems |
| Intended status: Standards Track | J. Mattsson |
| Expires: December 20, 2020 | Ericsson AB |
| D. McGrew | |
| Cisco Systems | |
| D. Wing | |
| Citrix Systems, Inc. | |
| F. Andreason | |
| Cisco Systems | |
| June 18, 2020 |
Encrypted Key Transport for DTLS and Secure RTP
draft-ietf-perc-srtp-ekt-diet-12
Encrypted Key Transport (EKT) is an extension to DTLS (Datagram Transport Layer Security) and Secure Real-time Transport Protocol (SRTP) that provides for the secure transport of SRTP master keys, rollover counters, and other information within SRTP. This facility enables SRTP for decentralized conferences by distributing a common key to all of the conference endpoints.
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/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 20, 2020.
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
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Real-time Transport Protocol (RTP) is designed to allow decentralized groups with minimal control to establish sessions, such as for multimedia conferences. Unfortunately, Secure RTP (SRTP [RFC3711]) cannot be used in many minimal-control scenarios, because it requires that synchronization source (SSRC) values and other data be coordinated among all of the participants in a session. For example, if a participant joins a session that is already in progress, that participant needs to be told the SRTP keys along with the SSRC, rollover counter (ROC) and other details of the other SRTP sources.
The inability of SRTP to work in the absence of central control was well understood during the design of the protocol; the omission was considered less important than optimizations such as bandwidth conservation. Additionally, in many situations SRTP is used in conjunction with a signaling system that can provide the central control needed by SRTP. However, there are several cases in which conventional signaling systems cannot easily provide all of the coordination required.
This document defines Encrypted Key Transport (EKT) for SRTP and reduces the amount of external signaling control that is needed in a SRTP session with multiple receivers. EKT securely distributes the SRTP master key and other information for each SRTP source. With this method, SRTP entities are free to choose SSRC values as they see fit, and to start up new SRTP sources with new SRTP master keys within a session without coordinating with other entities via external signaling or other external means.
EKT extends DTLS and SRTP to enable a common key encryption key (called an EKTKey) to be distributed to all endpoints, so that each endpoint can securely send its SRTP master key and current SRTP rollover counter to the other participants in the session. This data furnishes the information needed by the receiver to instantiate an SRTP/SRTCP receiver context.
EKT can be used in conferences where the central media distributor or conference bridge cannot decrypt the media, such as the type defined for [I-D.ietf-perc-private-media-framework]. It can also be used for large scale conferences where the conference bridge or media distributor can decrypt all the media but wishes to encrypt the media it is sending just once and then send the same encrypted media to a large number of participants. This reduces the amount of CPU time needed for encryption and can be used for some optimization to media sending that use source specific multicast.
EKT does not control the manner in which the SSRC is generated. It is only concerned with distributing the security parameters that an endpoint needs to associate with a given SSRC in order to decrypt SRTP packets from that sender.
EKT is not intended to replace external key establishment mechanisms. Instead, it is used in conjunction with those methods, and it relieves those methods of the burden to deliver the context for each SRTP source to every SRTP participant. This document defines how EKT works with the DTLS-SRTP approach to key establishment, by using keys derived from the DTLS-SRTP handshake to encipher the EKTKey in addition to the SRTP media.
This specification defines a way for the server in a DTLS-SRTP negotiation, see Section 5, to provide an EKTKey to the client during the DTLS handshake. The EKTKey thus obtained can be used to encrypt the SRTP master key that is used to encrypt the media sent by the endpoint. This specification also defines a way to send the encrypted SRTP master key (with the EKTKey) along with the SRTP packet, see Section 4. Endpoints that receive this and know the EKTKey can use the EKTKey to decrypt the SRTP master key which can then be used to decrypt the SRTP packet.
One way to use this is described in the architecture defined by [I-D.ietf-perc-private-media-framework]. Each participant in the conference forms a DTLS-SRTP connection to a common key distributor that distributes the same EKTKey to all the endpoints. Then each endpoint picks its own SRTP master key for the media they send. When sending media, the endpoint also includes the SRTP master key encrypted with the EKTKey in the SRTP packet. This allows all the endpoints to decrypt the media.
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.
EKT defines a new method of providing SRTP master keys to an endpoint. In order to convey the ciphertext corresponding to the SRTP master key, and other additional information, an additional field, called EKTField, is added to the SRTP packets. The EKTField appears at the end of the SRTP packet. It appears after the optional authentication tag if one is present, otherwise the EKTField appears after the ciphertext portion of the packet.
EKT MUST NOT be used in conjunction with SRTP's MKI (Master Key Identifier) or with SRTP's <From, To> [RFC3711], as those SRTP features duplicate some of the functions of EKT. Senders MUST NOT include MKI when using EKT. Receivers SHOULD simply ignore any MKI field received if EKT is in use.
This document defines the use of EKT with SRTP. Its use with SRTCP would be similar, but is reserved for a future specification. SRTP is preferred for transmitting key material because it shares fate with the transmitted media, because SRTP rekeying can occur without concern for RTCP transmission limits, and because it avoids the need for SRTCP compound packets with RTP translators and mixers.
The EKTField uses the format defined in Figure 1 for the FullEKTField and ShortEKTField.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : EKT Ciphertext : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security Parameter Index | Epoch | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length |0 0 0 0 0 0 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: FullEKTField format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+
Figure 2: ShortEKTField format
The following shows the syntax of the EKTField expressed in ABNF [RFC5234]. The EKTField is added to the end of an SRTP or SRTCP packet. The EKTPlaintext is the concatenation of SRTPMasterKeyLength, SRTPMasterKey, SSRC, and ROC in that order. The EKTCiphertext is computed by encrypting the EKTPlaintext using the EKTKey. Future extensions to the EKTField MUST conform to the syntax of ExtensionEKTField.
BYTE = %x00-FF EKTMsgTypeFull = %x02 EKTMsgTypeShort = %x00 EKTMsgTypeExtension = %x03-FF EKTMsgLength = 2BYTE; SRTPMasterKeyLength = BYTE SRTPMasterKey = 1*256BYTE SSRC = 4BYTE; SSRC from RTP ROC = 4BYTE ; ROC from SRTP FOR THE GIVEN SSRC EKTPlaintext = SRTPMasterKeyLength SRTPMasterKey SSRC ROC EKTCiphertext = 1*256BYTE ; EKTEncrypt(EKTKey, EKTPlaintext) Epoch = 2BYTE SPI = 2BYTE FullEKTField = EKTCiphertext SPI Epoch EKTMsgLength EKTMsgTypeFull ShortEKTField = EKTMsgTypeShort ExtensionData = 1*1024BYTE ExtensionEKTField = ExtensionData EKTMsgLength EKTMsgTypeExtension EKTField = FullEKTField / ShortEKTField / ExtensionEKTField
Figure 3: EKTField Syntax
These fields and data elements are defined as follows:
EKTPlaintext: The data that is input to the EKT encryption operation. This data never appears on the wire, and is used only in computations internal to EKT. This is the concatenation of the SRTP Master Key and its length, the SSRC, and the ROC.
EKTCiphertext: The data that is output from the EKT encryption operation, described in Section 4.4. This field is included in SRTP packets when EKT is in use. The length of EKTCiphertext can be larger than the length of the EKTPlaintext that was encrypted.
SRTPMasterKey: On the sender side, the SRTP Master Key associated with the indicated SSRC.
SRTPMasterKeyLength: The length of the SRTPMasterKey in bytes. This depends on the cipher suite negotiated for SRTP using SDP Offer/Answer [RFC3264] for the SRTP.
SSRC: On the sender side, this is the SSRC for this SRTP source. The length of this field is 32 bits. The SSRC value in the EKT tag MUST be the same as the one in the header of the SRTP packet to which the tag is appended.
Rollover Counter (ROC): On the sender side, this is set to the current value of the SRTP rollover counter in the SRTP/SRTCP context associated with the SSRC in the SRTP or SRTCP packet. The length of this field is 32 bits.
Security Parameter Index (SPI): This field indicates the appropriate EKTKey and other parameters for the receiver to use when processing the packet, within a given conference. The length of this field is 16 bits, representing a two-byte integer in network byte order. The parameters identified by this field are:
Epoch: This field indicates how many SRTP keys have been sent for this SSRC under the current EKTKey, prior to the current key, as a two-byte integer in network byte order. It starts at zero at the beginning of a session and resets to zero whenever the EKTKey is changed (i.e., when a new SPI appears). The epoch for an SSRC increments by one every time the sender transmits a new key. The recipient of a FullEKTField MUST reject any future FullEKTField for this SPI and SSRC that has an equal or lower epoch value to an epoch already seen.
Together, these data elements are called an EKT parameter set. Each distinct EKT parameter set that is used MUST be associated with a distinct SPI value to avoid ambiguity.
EKTMsgLength: All EKT messages types other than the ShortEKTField have a length as second from the last element. This is the length in octets (in network byte order) of either the FullEKTField/ExtensionEKTField including this length field and the following EKT Message Type.
Message Type: The last byte is used to indicate the type of the EKTField. This MUST be 2 for the FullEKTField format and 0 in ShortEKTField format. If a received EKT tag has an unknown message type, then the receiver MUST discard the whole EKT tag.
The SPI field identifies the parameters for how the EKT tag should be processed:
Together, these data elements are called an "EKT parameter set". Each distinct EKT parameter set that is used MUST be associated with a distinct SPI value to avoid ambiguity. The association of a given parameter set with a given SPI value is configured by some other protocol, e.g., the DTLS-SRTP extension defined in Section 5.
At any given time, each SRTP/SRTCP source has associated with it a single EKT parameter set. This parameter set is used to process all outbound packets, and is called the outbound parameter set for that SSRC. There may be other EKT parameter sets that are used by other SRTP/SRTCP sources in the same session, including other SRTP/SRTCP sources on the same endpoint (e.g., one endpoint with voice and video might have two EKT parameter sets, or there might be multiple video sources on an endpoint each with their own EKT parameter set). All of the received EKT parameter sets SHOULD be stored by all of the participants in an SRTP session, for use in processing inbound SRTP and SRTCP traffic. If a participant deletes an EKT parameter set (e.g., because of space limitations, then it will be unable to process Full EKT Tags containing updated media keys, and thus unable to receive media from a particpant that has changed its media key.
Either the FullEKTField or ShortEKTField is appended at the tail end of all SRTP packets. The decision on which to send when is specified in Section 4.6.
See Section 4.6 which describes when to send an SRTP packet with a FullEKTField. If a FullEKTField is not being sent, then a ShortEKTField is sent so the receiver can correctly determine how to process the packet.
When an SRTP packet is sent with a FullEKTField, the EKTField for that packet is created as follows, or uses an equivalent set of steps. The creation of the EKTField MUST precede the normal SRTP packet processing.
The computed value of the FullEKTField is appended to the end of the SRTP packet, after the encrypted payload.
When a packet is sent with the ShortEKTField, the ShortEKFField is simply appended to the packet.
Outbound packets SHOULD continue to use the old SRTP Master Key for 250 ms after sending any new key in a FullEKTField value. This gives all the receivers in the system time to get the new key before they start receiving media encrypted with the new key. (The specific value of 250ms is chosen to represent a reasonable upper bound on the amount of latency and jitter that is tolerable in a real-time context.)
When receiving a packet on a RTP stream, the following steps are applied for each SRTP received packet.
The value of the EKTCiphertext field is identical in successive packets protected by the same EKT parameter set and the same SRTP master key, and ROC. SRTP senders and receivers MAY cache an EKTCiphertext value to optimize processing in cases where the master key hasn't changed. Instead of encrypting and decrypting, senders can simply copy the pre-computed value and receivers can compare a received EKTCiphertext to the known value.
Section 4.3.1 recommends that SRTP senders continue using an old key for some time after sending a new key in an EKT tag. Receivers that wish to avoid packet loss due to decryption failures MAY perform trial decryption with both the old key and the new key, keeping the result of whichever decryption succeeds. Note that this approach is only compatible with SRTP transforms that include integrity protection.
When receiving a new EKTKey, implementations need to use the ekt_ttl field (see Section 5.2.2) to create a time after which this key cannot be used and they also need to create a counter that keeps track of how many times the key has been used to encrypt data to ensure it does not exceed the T value for that cipher (see Section 4.4). If either of these limits are exceeded, the key can no longer be used for encryption. At this point implementation need to either use the call signaling to renegotiate a new session or need to terminate the existing session. Terminating the session is a reasonable implementation choice because these limits should not be exceeded except under an attack or error condition.
EKT uses an authenticated cipher to encrypt and authenticate the EKTPlaintext. This specification defines the interface to the cipher, in order to abstract the interface away from the details of that function. This specification also defines the default cipher that is used in EKT. The default cipher described in Section 4.4.1 MUST be implemented, but another cipher that conforms to this interface MAY be used. The cipher used for a given EKTCiphertext value is negotiated using the supported_ekt_ciphers and indicated with the SPI value in the FullEKTField.
An EKTCipher consists of an encryption function and a decryption function. The encryption function E(K, P) takes the following inputs:
The encryption function returns a ciphertext value C whose length is N bytes, where N may be larger than M. The decryption function D(K, C) takes the following inputs:
The decryption function returns a plaintext value P that is M bytes long, or returns an indication that the decryption operation failed because the ciphertext was invalid (i.e. it was not generated by the encryption of plaintext with the key K).
These functions have the property that D(K, E(K, P)) = P for all values of K and P. Each cipher also has a limit T on the number of times that it can be used with any fixed key value. The EKTKey MUST NOT be used for encryption more that T times. Note that if the same FullEKTField is retransmitted 3 times, that only counts as 1 encryption.
Security requirements for EKT ciphers are discussed in Section 6.
The default EKT Cipher is the Advanced Encryption Standard (AES) Key Wrap with Padding [RFC5649] algorithm. It requires a plaintext length M that is at least one octet, and it returns a ciphertext with a length of N = M + (M mod 8) + 8 octets.
It can be used with key sizes of L = 16, and L = 32 octets, and its use with those key sizes is indicated as AESKW128, or AESKW256, respectively. The key size determines the length of the AES key used by the Key Wrap algorithm. With this cipher, T=2^48.
| Cipher | L | T |
|---|---|---|
| AESKW128 | 16 | 2^48 |
| AESKW256 | 32 | 2^48 |
As AES-128 is the mandatory to implement transform in SRTP, AESKW128 MUST be implemented for EKT and AESKW256 MAY be implemented.
Other specifications may extend this document by defining other EKTCiphers as described in Section 7. This section defines how those ciphers interact with this specification.
An EKTCipher determines how the EKTCiphertext field is written, and how it is processed when it is read. This field is opaque to the other aspects of EKT processing. EKT ciphers are free to use this field in any way, but they SHOULD NOT use other EKT or SRTP fields as an input. The values of the parameters L, and T MUST be defined by each EKTCipher. The cipher MUST provide integrity protection.
If a source has its EKTKey changed by the key management, it MUST also change its SRTP master key, which will cause it to send out a new FullEKTField. This ensures that if key management thought the EKTKey needs changing (due to a participant leaving or joining) and communicated that to a source, the source will also change its SRTP master key, so that traffic can be decrypted only by those who know the current EKTKey.
A system using EKT learns the SRTP master keys distributed with the FullEKTField sent with the SRTP, rather than with call signaling. A receiver can immediately decrypt an SRTP packet, provided the SRTP packet contains a FullEKTField.
This section describes how to reliably and expediently deliver new SRTP master keys to receivers.
There are three cases to consider. The first case is a new sender joining a session, which needs to communicate its SRTP master key to all the receivers. The second case is a sender changing its SRTP master key which needs to be communicated to all the receivers. The third case is a new receiver joining a session already in progress which needs to know the sender's SRTP master key.
The three cases are:
This document defines an extension to DTLS-SRTP called SRTP EKTKey Transport which enables secure transport of EKT keying material from the DTLS-SRTP peer in the server role to the client. This allows those peers to process EKT keying material in SRTP (or SRTCP) and retrieve the embedded SRTP keying material. This combination of protocols is valuable because it combines the advantages of DTLS, which has strong authentication of the endpoint and flexibility, along with allowing secure multiparty RTP with loose coordination and efficient communication of per-source keys.
In cases where the DTLS termination point is more trusted than the media relay, the protection that DTLS affords to EKT key material can allow EKT keys to be tunneled through an untrusted relay such as a centralized conference bridge. For more details, see [I-D.ietf-perc-private-media-framework].
DTLS-SRTP [RFC5764] uses an extended DTLS exchange between two peers to exchange keying material, algorithms, and parameters for SRTP. The SRTP flow operates over the same transport as the DTLS-SRTP exchange (i.e., the same 5-tuple). DTLS-SRTP combines the performance and encryption flexibility benefits of SRTP with the flexibility and convenience of DTLS-integrated key and association management. DTLS-SRTP can be viewed in two equivalent ways: as a new key management method for SRTP, and a new RTP-specific data format for DTLS.
This document defines a new TLS negotiated extension supported_ekt_ciphers and a new TLS handshake message type ekt_key. The extension negotiates the cipher to be used in encrypting and decrypting EKTCiphertext values, and the handshake message carries the corresponding key.
Figure 4 shows a message flow of DTLS 1.3 client and server using EKT configured using the DTLS extensions described in this section. (The initial cookie exchange and other normal DTLS messages are omitted.) To be clear, EKT can be used with versions of DTLS prior to 1.3. The only difference is that in a pre-1.3 TLS stacks will not have built-in support for generating and processing ACK messages.
Client Server
ClientHello
+ use_srtp
+ supported_ekt_ciphers
-------->
ServerHello
{EncryptedExtensions}
+ use_srtp
+ supported_ekt_ciphers
{... Finished}
<--------
{... Finished} -------->
[ACK]
<-------- [EKTKey]
[ACK] -------->
|SRTP packets| <-------> |SRTP packets|
+ <EKT tags> + <EKT tags>
{} Messages protected using DTLS handshake keys
[] Messages protected using DTLS application traffic keys
<> Messages protected using the EKTKey and EKT cipher
|| Messages protected using the SRTP Master Key sent in
a Full EKT Tag
Figure 4
In the context of a multi-party SRTP session in which each endpoint performs a DTLS handshake as a client with a central DTLS server, the extensions defined in this document allow the DTLS server to set a common EKTKey for all participants. Each endpoint can then use EKT tags encrypted with that common key to inform other endpoint of the keys it uses to protect SRTP packets. This avoids the need for many individual DTLS handshakes among the endpoints, at the cost of preventing endpoints from directly authenticating one another.
Client A Server Client B
<----DTLS Handshake---->
<--------EKTKey---------
<----DTLS Handshake---->
---------EKTKey-------->
-------------SRTP Packet + EKT Tag------------->
<------------SRTP Packet + EKT Tag--------------
To indicate its support for EKT, a DTLS-SRTP client includes in its ClientHello an extension of type supported_ekt_ciphers listing the ciphers used for EKT by the client supports in preference order, with the most preferred version first. If the server agrees to use EKT, then it includes a supported_ekt_ciphers extension in its ServerHello containing a cipher selected from among those advertised by the client.
The extension_data field of this extension contains an "EKTCipher" value, encoded using the syntax defined in [RFC8446]:
enum {
reserved(0),
aeskw_128(1),
aeskw_256(2),
} EKTCipherType;
struct {
select (Handshake.msg_type) {
case client_hello:
EKTCipherType supported_ciphers<1..255>;
case server_hello:
EKTCipherType selected_cipher;
};
} EKTCipher;
Once a client and server have concluded a handshake that negotiated an EKTCipher, the server MUST provide to the client a key to be used when encrypting and decrypting EKTCiphertext values. EKTKeys are sent in encrypted handshake records, using handshake type ekt_key(TBD). The body of the handshake message contains an EKTKey structure:
[[ NOTE: RFC Editor, please replace "TBD" above with the code point assigned by IANA ]]
struct {
opaque ekt_key_value<1..256>;
opaque srtp_master_salt<1..256>;
uint16 ekt_spi;
uint24 ekt_ttl;
} EKTKey;
The contents of the fields in this message are as follows:
If the server did not provide a supported_ekt_ciphers extension in its ServerHello, then EKTKey messages MUST NOT be sent by the client or the server.
When an EKTKey is received and processed successfully, the recipient MUST respond with an ACK message as described in Section 7 of [I-D.ietf-tls-dtls13]. The EKTKey message and ACK MUST be retransmitted following the rules of the negotiated version of DTLS.
EKT MAY be used with versions of DTLS prior to 1.3. In such cases, the ACK message is still used to provide reliability. Thus, DTLS implementations supporting EKT with DTLS pre-1.3 will need to have explicit affordances for sending the ACK message in response to an EKTKey message, and for verifying that an ACK message was received. The retransmission rules for both sides are otherwise defined by the negotiated version of DTLS.
If an EKTKey message is received that cannot be processed, then the recipient MUST respond with an appropriate DTLS alert.
When using EKT with DTLS-SRTP, the negotiation to use EKT is done at the DTLS handshake level and does not change the [RFC3264] Offer / Answer messaging.
The DTLS EKTKey message is sent using the retransmissions specified in Section 4.2.4. of DTLS [RFC6347]. Retransmission is finished with an ACK message or an alert is received.
EKT inherits the security properties of the the key management protocol that is used to establish the EKTKey, e.g., the DTLS-SRTP extension defined in this document.
With EKT, each SRTP sender and receiver MUST generate distinct SRTP master keys. This property avoids any security concern over the re-use of keys, by empowering the SRTP layer to create keys on demand. Note that the inputs of EKT are the same as for SRTP with key-sharing: a single key is provided to protect an entire SRTP session. However, EKT remains secure even when SSRC values collide.
SRTP master keys MUST be randomly generated, and [RFC4086] offers some guidance about random number generation. SRTP master keys MUST NOT be re-used for any other purpose, and SRTP master keys MUST NOT be derived from other SRTP master keys.
The EKT Cipher includes its own authentication/integrity check. For an attacker to successfully forge a FullEKTField, it would need to defeat the authentication mechanisms of the EKT Cipher authentication mechanism.
The presence of the SSRC in the EKTPlaintext ensures that an attacker cannot substitute an EKTCiphertext from one SRTP stream into another SRTP stream. This mitigates the impact of the cut-and-paste attacks that arise due to the lack of a cryptographic binding between the EKT tag and the rest of the SRTP packet. SRTP tags can only be cut-and-pasted within the stream of packets sent by a given RTP endpoint; an attacker cannot "cross the streams" and use an EKT tag from one SSRC to reset the key for another SSRC. The epoch field in the FullEKTField also prevents an attacker from rolling back to a previous key.
An attacker could send packets containing a FullEKTField, in an attempt to consume additional CPU resources of the receiving system by causing the receiving system to decrypt the EKT ciphertext and detect an authentication failure. In some cases, caching the previous values of the Ciphertext as described in Section 4.3.2 helps mitigate this issue.
In a similar vein, EKT has no replay protection, so an attacker could implant improper keys in receivers by capturing EKTCiphertext values encrypted with a given EKTKey and replaying them in a different context, e.g., from a different sender. When the underlying SRTP transform provides integrity protection, this attack will just result in packet loss. If it does not, then it will result in random data being fed to RTP payload processing. An attacker that is in a position to mount these attacks, however, could achieve the same effects more easily without attacking EKT.
The key encryption keys distributed with EKTKey messages are group shared symmetric keys, which means they do not provide protection within the group. Group members can impersonate each other; for example, any group member can generate an EKT tag for any SSRC. The entity that distributes EKTKeys can decrypt any keys distributed using EKT, and thus any media protected with those keys.
Each EKT cipher specifies a value T that is the maximum number of times a given key can be used. An endpoint MUST NOT encrypt more than T different FullEKTField values using the same EKTKey. In addition, the EKTKey MUST NOT be used beyond the lifetime provided by the TTL described in Section 5.2.
The confidentiality, integrity, and authentication of the EKT cipher MUST be at least as strong as the SRTP cipher and at least as strong as the DTLS-SRTP ciphers.
Part of the EKTPlaintext is known, or easily guessable to an attacker. Thus, the EKT Cipher MUST resist known plaintext attacks. In practice, this requirement does not impose any restrictions on our choices, since the ciphers in use provide high security even when much plaintext is known.
An EKT cipher MUST resist attacks in which both ciphertexts and plaintexts can be adaptively chosen and adversaries that can query both the encryption and decryption functions adaptively.
In some systems, when a member of a conference leaves the conferences, the conferences is rekeyed so that member no longer has the key. When changing to a new EKTKey, it is possible that the attacker could block the EKTKey message getting to a particular endpoint and that endpoint would keep sending media encrypted using the old key. To mitigate that risk, the lifetime of the EKTKey MUST be limited using the ekt_ttl.
IANA is requested to create a new table for "EKT Messages Types" in the "Real-Time Transport Protocol (RTP) Parameters" registry. The initial values in this registry are:
| Message Type | Value | Specification |
|---|---|---|
| Short | 0 | RFCAAAA |
| Full | 2 | RFCAAAA |
| Unallocated | 3-254 | RFCAAAA |
| Reserved | 255 | RFCAAAA |
Note to RFC Editor: Please replace RFCAAAA with the RFC number for this specification.
New entries to this table can be added via "Specification Required" as defined in [RFC8126]. IANA SHOULD prefer allocation of even values over odd ones until the even code points are consumed to avoid conflicts with pre standard versions of EKT that have been deployed. Allocated values MUST be in the range of 0 to 254.
All new EKT messages MUST be defined to have a length as second from the last element, as specified.
IANA is requested to create a new table for "EKT Ciphers" in the "Real-Time Transport Protocol (RTP) Parameters" registry. The initial values in this registry are:
| Name | Value | Specification |
|---|---|---|
| AESKW128 | 0 | RFCAAAA |
| AESKW256 | 1 | RFCAAAA |
| Unallocated | 2-254 | |
| Reserved | 255 | RFCAAAA |
Note to RFC Editor: Please replace RFCAAAA with the RFC number for this specification.
New entries to this table can be added via "Specification Required" as defined in [RFC8126]. The expert SHOULD ensure the specification defines the values for L and T as required in Section 4.4 of RFCAAAA. Allocated values MUST be in the range of 0 to 254.
IANA is requested to add supported_ekt_ciphers as a new extension name to the "TLS ExtensionType Values" table of the "Transport Layer Security (TLS) Extensions" registry:
Value: [TBD-at-Registration] Extension Name: supported_ekt_ciphers TLS 1.3: CH, SH Recommended: Y Reference: RFCAAAA
[[ Note to RFC Editor: TBD will be allocated by IANA. ]]
IANA is requested to add ekt_key as a new entry in the "TLS HandshakeType Registry" table of the "Transport Layer Security (TLS) Parameters" registry:
Value: [TBD-at-Registration] Description: ekt_key DTLS-OK: Y Reference: RFCAAAA Comment:
[[ Note to RFC Editor: TBD will be allocated by IANA. ]]
Thank you to Russ Housley provided detailed review and significant help with crafting text for this document. Thanks to David Benham, Yi Cheng, Lakshminath Dondeti, Kai Fischer, Nermeen Ismail, Paul Jones, Eddy Lem, Jonathan Lennox, Michael Peck, Rob Raymond, Sean Turner, Magnus Westerlund, and Felix Wyss for fruitful discussions, comments, and contributions to this document.
| [I-D.ietf-perc-double] | Jennings, C., Jones, P., Barnes, R. and A. Roach, "SRTP Double Encryption Procedures", Internet-Draft draft-ietf-perc-double-12, August 2019. |
| [I-D.ietf-perc-private-media-framework] | Jones, P., Benham, D. and C. Groves, "A Solution Framework for Private Media in Privacy Enhanced RTP Conferencing (PERC)", Internet-Draft draft-ietf-perc-private-media-framework-12, June 2019. |
| [I-D.ietf-tls-dtls13] | Rescorla, E., Tschofenig, H. and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-dtls13-38, May 2020. |
| [RFC4086] | Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005. |