Internet Engineering Task Force (IETF) C. Jennings
Request for Comments: 8870 Cisco Systems
Category: Standards Track J. Mattsson
ISSN: 2070-1721 Ericsson AB
D. McGrew
Cisco Systems
D. Wing
Citrix
F. Andreasen
Cisco Systems
January 2021
Encrypted Key Transport for DTLS and Secure RTP
Abstract
Encrypted Key Transport (EKT) is an extension to DTLS (Datagram
Transport Layer Security) and the 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8870.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Overview
3. Conventions Used in This Document
4. Encrypted Key Transport
4.1. EKTField Formats
4.2. SPIs and EKT Parameter Sets
4.3. Packet Processing and State Machine
4.3.1. Outbound Processing
4.3.2. Inbound Processing
4.4. Ciphers
4.4.1. AES Key Wrap
4.4.2. Defining New EKT Ciphers
4.5. Synchronizing Operation
4.6. Timing and Reliability Considerations
5. Use of EKT with DTLS-SRTP
5.1. DTLS-SRTP Recap
5.2. SRTP EKT Key Transport Extensions to DTLS-SRTP
5.2.1. Negotiating an EKTCipher
5.2.2. Establishing an EKT Key
5.3. Offer/Answer Considerations
5.4. Sending the DTLS EKTKey Reliably
6. Security Considerations
7. IANA Considerations
7.1. EKT Message Types
7.2. EKT Ciphers
7.3. TLS Extensions
7.4. TLS Handshake Type
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
The 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 informed of 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 an
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 ROC
to the other participants in the session. This data furnishes the
information needed by the receiver to instantiate an SRTP 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
in [RFC8871]. 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 encryption CPU time in general and is necessary when sending
multicast media.
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 of delivering 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.
2. Overview
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 packet 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 specification is described in the architecture
defined by [RFC8871]. 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 it sends. When sending media, the
endpoint may also include the SRTP master key encrypted with the
EKTKey in the SRTP packet. This allows all the endpoints to decrypt
the media.
3. Conventions Used in This Document
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.
4. Encrypted Key Transport
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 the "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 the 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 the
Secure Real-time Transport Control Protocol (SRTCP) would be similar,
but that topic is left for a future specification. SRTP is preferred
for transmitting keying material because (1) it shares fate with the
transmitted media, (2) SRTP rekeying can occur without concern for
RTCP transmission limits, and (3) it avoids the need for SRTCP
compound packets with RTP translators and mixers.
4.1. EKTField Formats
The EKTField uses the formats defined in Figures 1 and 2 for the
FullEKTField and ShortEKTField. The EKTField appended to an SRTP
packet can be referred to as an "EKT Tag".
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
Figure 3 shows the syntax of the EKTField, expressed in ABNF
[RFC5234]. The EKTField is added to the end of an SRTP 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 the
ExtensionEKTField.
BYTE = %x00-FF
EKTMsgTypeFull = %x02
EKTMsgTypeShort = %x00
EKTMsgTypeExtension = %x03-FF ; Message Type %x01 is not available
; for assignment due to its usage by
; legacy implementations.
EKTMsgLength = 2BYTE
SRTPMasterKeyLength = BYTE
SRTPMasterKey = 1*242BYTE
SSRC = 4BYTE ; SSRC from RTP
ROC = 4BYTE ; ROC from SRTP for the given SSRC
EKTPlaintext = SRTPMasterKeyLength SRTPMasterKey SSRC ROC
EKTCiphertext = 1*251BYTE ; 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:
This is the data that is input to the EKT encryption operation.
This data never appears on the wire; it 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:
This is the data that is output from the EKT encryption operation
(see Section 4.4). This field is included in SRTP packets when
EKT is in use. The length of the EKTCiphertext can be larger than
the length of the EKTPlaintext that was encrypted.
SRTPMasterKey:
On the sender side, this is the SRTP master key associated with
the indicated SSRC.
SRTPMasterKeyLength:
This is the length of the SRTPMasterKey in bytes. This depends on
the cipher suite negotiated for SRTP using Session Description
Protocol (SDP) Offer/Answer [RFC3264].
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
ROC in the SRTP context associated with the SSRC in the SRTP
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 as follows:
* The EKT Cipher used to process the packet.
* The EKTKey used to process the packet.
* The SRTP master salt associated with any master key encrypted
with this EKT Key. The master salt is communicated separately,
via signaling, typically along with the EKTKey. (Recall that
the SRTP master salt is used in the formation of Initialization
Vectors (IVs) / nonces.)
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 epoch value equal to or lower
than an epoch already seen.
Together, these data elements are called an "EKT parameter set". To
avoid ambiguity, each distinct EKT parameter set that is used MUST be
associated with a distinct SPI value.
EKTMsgLength:
All EKT Message Types other than the ShortEKTField include a
length in octets (in network byte order) of either the
FullEKTField or the ExtensionEKTField, including this length field
and the EKT Message Type (as defined in the next paragraph).
Message Type:
The last byte is used to indicate the type of the EKTField. This
MUST be 2 for the FullEKTField format and 0 for the ShortEKTField
format. If a received EKT Tag has an unknown Message Type, then
the receiver MUST discard the whole EKT Tag.
4.2. SPIs and EKT Parameter Sets
The SPI identifies the parameters for how the EKT Tag should be
processed:
* The EKTKey and EKT Cipher used to process the packet.
* The SRTP master salt associated with any master key encrypted with
this EKT Key. The master salt is communicated separately, via
signaling, typically along with the EKTKey.
Together, these data elements are called an "EKT parameter set". To
avoid ambiguity, each distinct EKT parameter set that is used MUST be
associated with a distinct SPI value. 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.
4.3. Packet Processing and State Machine
At any given time, the SSRC for each SRTP 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 sources in the same session, including other SRTP 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
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 will be unable to
receive media from a participant that has changed its media key.
Either the FullEKTField or ShortEKTField is appended at the tail end
of all SRTP packets. The decision regarding which parameter to send
and when is specified in Section 4.6.
4.3.1. Outbound Processing
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 per either the steps below or an equivalent
set of steps.
1. The Security Parameter Index (SPI) field is set to the value of
the SPI that is associated with the outbound parameter set.
2. The EKTPlaintext field is computed from the SRTP master key,
SSRC, and ROC fields, as shown in Section 4.1. The ROC, SRTP
master key, and SSRC used in EKT processing MUST be the same as
the one used in SRTP processing.
3. The EKTCiphertext field is set to the ciphertext created by
encrypting the EKTPlaintext with the EKTCipher using the EKTKey
as the encryption key. The encryption process is detailed in
Section 4.4.
4. Then, the FullEKTField is formed using the EKTCiphertext and the
SPI associated with the EKTKey used above. Also appended are the
length and Message Type using the FullEKTField format.
| Note: The value of the EKTCiphertext field is identical in
| successive packets protected by the same EKTKey and SRTP master
| key. This value MAY be cached by an SRTP sender to minimize
| computational effort.
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 ShortEKTField 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 250 ms is chosen to represent a reasonable upper bound on
the amount of latency and jitter that is tolerable in a real-time
context.)
4.3.2. Inbound Processing
When receiving a packet on an RTP stream, the following steps are
applied for each received SRTP packet.
1. The final byte is checked to determine which EKT format is in
use. When an SRTP packet contains a ShortEKTField, the
ShortEKTField is removed from the packet and then normal SRTP
processing occurs. If the packet contains a FullEKTField, then
processing continues as described below. The reason for using
the last byte of the packet to indicate the type is that the
length of the SRTP part is not known until the decryption has
occurred. At this point in the processing, there is no easy way
to know where the EKTField would start. However, the whole SRTP
packet has been received, so instead of starting at the front of
the packet, the parsing works backwards at the end of the packet,
and thus the type is placed at the very end of the packet.
2. The Security Parameter Index (SPI) field is used to find the
right EKT parameter set to be used for processing the packet. If
there is no matching SPI, then the verification function MUST
return an indication of authentication failure, and the steps
described below are not performed. The EKT parameter set
contains the EKTKey, the EKTCipher, and the SRTP master salt.
3. The EKTCiphertext is authenticated and decrypted, as described in
Section 4.4, using the EKTKey and EKTCipher found in the previous
step. If the EKT decryption operation returns an authentication
failure, then EKT processing MUST be aborted. The receiver
SHOULD discard the whole SRTP packet.
4. The resulting EKTPlaintext is parsed as described in Section 4.1,
to recover the SRTP master key, SSRC, and ROC fields. The SRTP
master salt that is associated with the EKTKey is also retrieved.
If the value of the srtp_master_salt (see Section 5.2.2) sent as
part of the EKTKey is longer than needed by SRTP, then it is
truncated by taking the first N bytes from the srtp_master_salt
field.
5. If the SSRC in the EKTPlaintext does not match the SSRC of the
SRTP packet received, then this FullEKTField MUST be discarded
and the subsequent steps in this list skipped. After stripping
the FullEKTField, the remainder of the SRTP packet MAY be
processed as normal.
6. The SRTP master key, ROC, and SRTP master salt from the previous
steps are saved in a map indexed by the SSRC found in the
EKTPlaintext and can be used for any future crypto operations on
the inbound packets with that SSRC.
* Unless the transform specifies other acceptable key lengths,
the length of the SRTP master key MUST be the same as the
master key length for the SRTP transform in use. If this is
not the case, then the receiver MUST abort EKT processing and
SHOULD discard the whole SRTP packet.
* If the length of the SRTP master key is less than the master
key length for the SRTP transform in use and the transform
specifies that this length is acceptable, then the SRTP master
key value is used to replace the first bytes in the existing
master key. The other bytes remain the same as in the old
key. For example, the double GCM transform [RFC8723] allows
replacement of the first ("end-to-end") half of the master
key.
7. At this point, EKT processing has successfully completed, and the
normal SRTP processing takes place.
The value of the EKTCiphertext field is identical in successive
packets protected by the same EKT parameter set, 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
precomputed 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 that it does not exceed the T value for that cipher (see
Section 4.4). If either of these limits is exceeded, the key can no
longer be used for encryption. At this point, implementations need
to either use call signaling to renegotiate a new session or
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.
4.4. Ciphers
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 extension (see
Section 5.2) 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:
* a secret key K with a length of L bytes, and
* a plaintext value P with a length of M bytes.
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:
* a secret key K with a length of L bytes, and
* a ciphertext value C with a length of N bytes.
The decryption function returns a plaintext value P that is M bytes
long, or it 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 than T times. Note that if the same
FullEKTField is retransmitted three times, that only counts as one
encryption.
Security requirements for EKT Ciphers are discussed in Section 6.
4.4.1. AES Key Wrap
The default EKT Cipher is the Advanced Encryption Standard (AES) Key
Wrap with Padding algorithm [RFC5649]. 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 octets or 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) |
+----------+----+--------+
Table 1: EKT Ciphers
As AES-128 is the mandatory-to-implement transform in SRTP, AESKW128
MUST be implemented for EKT. AESKW256 MAY be implemented.
4.4.2. Defining New EKT Ciphers
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.
4.5. Synchronizing Operation
If a source has its EKTKey changed by key management, it MUST also
change its SRTP master key, which will cause it to send out a new
FullEKTField and eventually begin encrypting with it, as described in
Section 4.3.1. 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.
4.6. Timing and Reliability Considerations
A system using EKT learns the SRTP master keys distributed with the
FullEKTField sent with 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. In the first case, a new sender
joins a session and needs to communicate its SRTP master key to all
the receivers. In the second case, a sender changes its SRTP master
key, which needs to be communicated to all the receivers. In the
third case, a new receiver joins a session already in progress and
needs to know the sender's SRTP master key.
The three cases are as follows:
New sender:
A new sender SHOULD send a packet containing the FullEKTField as
soon as possible, ideally in its initial SRTP packet. To
accommodate packet loss, it is RECOMMENDED that the FullEKTField
be transmitted in three consecutive packets. If the sender does
not send a FullEKTField in its initial packets and receivers have
not otherwise been provisioned with a decryption key, then
decryption will fail and SRTP packets will be dropped until the
receiver receives a FullEKTField from the sender.
Rekey:
By sending an EKT Tag over SRTP, the rekeying event shares fate
with the SRTP packets protected with that new SRTP master key. To
accommodate packet loss, it is RECOMMENDED that three consecutive
packets containing the FullEKTField be transmitted.
New receiver:
When a new receiver joins a session, it does not need to
communicate its sending SRTP master key (because it is a
receiver). Also, when a new receiver joins a session, the sender
is generally unaware of the receiver joining the session; thus,
senders SHOULD periodically transmit the FullEKTField. That
interval depends on how frequently new receivers join the session,
the acceptable delay before those receivers can start processing
SRTP packets, and the acceptable overhead of sending the
FullEKTField. If sending audio and video, the RECOMMENDED
frequency is the same as the rate of intra-coded video frames. If
only sending audio, the RECOMMENDED frequency is every 100 ms.
If none of the above three cases apply, a ShortEKTField SHOULD be
sent.
In general, sending FullEKTField tags less frequently will consume
less bandwidth but will increase the time it takes for a join or
rekey to take effect. Applications should schedule the sending of
FullEKTField tags in a way that makes sense for their bandwidth and
latency requirements.
5. Use of EKT with DTLS-SRTP
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 such a peer to process EKT keying material in SRTP 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 multi-party 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 keying material
can allow EKT Keys to be tunneled through an untrusted relay such as
a centralized conference bridge. For more details, see [RFC8871].
5.1. DTLS-SRTP Recap
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 as a new RTP-specific data format
for DTLS.
5.2. SRTP EKT Key Transport Extensions to DTLS-SRTP
This document defines a new TLS negotiated extension called
"supported_ekt_ciphers" and a new TLS handshake message type called
"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 between a 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 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: DTLS 1.3 Message Flow
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 endpoints 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--------------
5.2.1. Negotiating an EKTCipher
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, 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
EncryptedExtensions (or ServerHello for DTLS 1.2) 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;
case encrypted_extensions:
EKTCipherType selected_cipher;
};
} EKTCipher;
5.2.2. Establishing an EKT Key
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(26). The body of the handshake message contains an EKTKey
structure as follows:
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:
ekt_key_value
The EKTKey that the recipient should use when generating
EKTCiphertext values
srtp_master_salt
The SRTP master salt to be used with any master key encrypted with
this EKT Key
ekt_spi
The SPI value to be used to reference this EKTKey and SRTP master
salt in EKT Tags (along with the EKT Cipher negotiated in the
handshake)
ekt_ttl
The maximum amount of time, in seconds, that this EKTKey can be
used. The ekt_key_value in this message MUST NOT be used for
encrypting or decrypting information after the TTL expires.
If the server did not provide a supported_ekt_ciphers extension in
its EncryptedExtensions (or ServerHello for DTLS 1.2), 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
[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,
to provide reliability, the ACK message is still used. Thus, DTLS
implementations supporting EKT with pre-1.3 versions of DTLS 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.
5.3. Offer/Answer Considerations
When using EKT with DTLS-SRTP, the negotiation to use EKT is done at
the DTLS handshake level and does not change the SDP Offer/Answer
messaging [RFC3264].
5.4. Sending the DTLS EKTKey Reliably
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.
6. Security Considerations
EKT inherits the security properties of 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 concerns over the
reuse 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 reused 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.
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 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 key length of the EKT Cipher MUST be at least as long as the SRTP
cipher and at least as long as the DTLS-SRTP ciphers.
Part of the EKTPlaintext is known or is 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 by an attacker querying both the
encryption and decryption functions.
In some systems, when a member of a conference leaves the conference,
that conference is rekeyed so that the member who left the conference
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 by using the ekt_ttl.
7. IANA Considerations
7.1. EKT Message Types
IANA has created a new table for "EKT Message Types" in the "Real-
Time Transport Protocol (RTP) Parameters" registry. The initial
values in this registry are as follows:
+==============+=======+===============+
| Message Type | Value | Specification |
+==============+=======+===============+
| Short | 0 | RFC 8870 |
+--------------+-------+---------------+
| Unassigned | 1 | |
+--------------+-------+---------------+
| Full | 2 | RFC 8870 |
+--------------+-------+---------------+
| Unassigned | 3-254 | |
+--------------+-------+---------------+
| Reserved | 255 | RFC 8870 |
+--------------+-------+---------------+
Table 2: EKT Message Types
New entries in this table can be added via "Specification Required"
as defined in [RFC8126]. To avoid conflicts with pre-standard
versions of EKT that have been deployed, IANA SHOULD give preference
to the allocation of even values over odd values until the even code
points are consumed. Allocated values MUST be in the range of 0 to
254.
All new EKT messages MUST be defined to include a length parameter,
as specified in Section 4.1.
7.2. EKT Ciphers
IANA has created a new table for "EKT Ciphers" in the "Real-Time
Transport Protocol (RTP) Parameters" registry. The initial values in
this registry are as follows:
+============+=======+===============+
| Name | Value | Specification |
+============+=======+===============+
| AESKW128 | 0 | RFC 8870 |
+------------+-------+---------------+
| AESKW256 | 1 | RFC 8870 |
+------------+-------+---------------+
| Unassigned | 2-254 | |
+------------+-------+---------------+
| Reserved | 255 | RFC 8870 |
+------------+-------+---------------+
Table 3: EKT Cipher Types
New entries in this table can be added via "Specification Required"
as defined in [RFC8126]. The expert SHOULD ensure that the
specification defines the values for L and T as required in
Section 4.4 of this document. Allocated values MUST be in the range
of 0 to 254.
7.3. TLS Extensions
IANA has added supported_ekt_ciphers as a new extension name to the
"TLS ExtensionType Values" table of the "Transport Layer Security
(TLS) Extensions" registry:
Value: 39
Extension Name: supported_ekt_ciphers
TLS 1.3: CH, EE
Recommended: Y
Reference: RFC 8870
7.4. TLS Handshake Type
IANA has added ekt_key as a new entry in the "TLS HandshakeType"
table of the "Transport Layer Security (TLS) Parameters" registry:
Value: 26
Description: ekt_key
DTLS-OK: Y
Reference: RFC 8870
Comment:
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
DOI 10.17487/RFC5649, September 2009,
<https://www.rfc-editor.org/info/rfc5649>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
<https://www.rfc-editor.org/info/rfc5764>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
8.2. Informative References
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC8723] Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
"Double Encryption Procedures for the Secure Real-Time
Transport Protocol (SRTP)", RFC 8723,
DOI 10.17487/RFC8723, April 2020,
<https://www.rfc-editor.org/info/rfc8723>.
[RFC8871] Jones, P., Benham, D., and C. Groves, "A Solution
Framework for Private Media in Privacy-Enhanced RTP
Conferencing (PERC)", RFC 8871, DOI 10.17487/RFC8871,
January 2021, <https://www.rfc-editor.org/info/rfc8871>.
[TLS-DTLS13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-39, 2 November 2020,
<https://tools.ietf.org/html/draft-ietf-tls-dtls13-39>.
Acknowledgments
Thank you to Russ Housley, who provided a 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.
Authors' Addresses
Cullen Jennings
Cisco Systems
Email: fluffy@iii.ca
John Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
David A. McGrew
Cisco Systems
Email: mcgrew@cisco.com
Dan Wing
Citrix Systems, Inc.
Email: dwing-ietf@fuggles.com
Flemming Andreasen
Cisco Systems
Email: fandreas@cisco.com