PERC Working Group C. Jennings
Internet-Draft Cisco
Intended status: Standards Track J. Mattsson, Ed.
Expires: December 31, 2017 Ericsson
D. McGrew
D. Wing
F. Andreasen
Cisco
June 29, 2017
Encrypted Key Transport for DTLS and Secure RTP
draft-ietf-perc-srtp-ekt-diet-05
Abstract
Encrypted Key Transport (EKT) is an extension to DTLS 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.
Status of This Memo
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This Internet-Draft will expire on December 31, 2017.
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Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Conventions Used In This Document . . . . . . . . . . . . . . 4
4. Encrypted Key Transport . . . . . . . . . . . . . . . . . . . 4
4.1. EKT Field Formats . . . . . . . . . . . . . . . . . . . . 4
4.2. Packet Processing and State Machine . . . . . . . . . . . 7
4.2.1. Outbound Processing . . . . . . . . . . . . . . . . . 8
4.2.2. Inbound Processing . . . . . . . . . . . . . . . . . 9
4.3. Implementation Notes . . . . . . . . . . . . . . . . . . 10
4.4. Ciphers . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4.1. Ciphers . . . . . . . . . . . . . . . . . . . . . . . 11
4.4.2. Defining New EKT Ciphers . . . . . . . . . . . . . . 12
4.5. Synchronizing Operation . . . . . . . . . . . . . . . . . 12
4.6. Transport . . . . . . . . . . . . . . . . . . . . . . . . 12
4.7. Timing and Reliability Consideration . . . . . . . . . . 13
5. Use of EKT with DTLS-SRTP . . . . . . . . . . . . . . . . . . 13
5.1. DTLS-SRTP Recap . . . . . . . . . . . . . . . . . . . . . 14
5.2. SRTP EKT Key Transport Extensions to DTLS-SRTP . . . . . 14
5.3. Offer/Answer Considerations . . . . . . . . . . . . . . . 17
5.4. Sending the DTLS EKT_Key Reliably . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7.1. EKT Message Types . . . . . . . . . . . . . . . . . . . . 19
7.2. EKT Ciphers . . . . . . . . . . . . . . . . . . . . . . . 20
7.3. TLS Extensions . . . . . . . . . . . . . . . . . . . . . 20
7.4. TLS Content Type . . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
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
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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. It is also desirable to eliminate the layer
violations that occur when signaling systems coordinate certain SRTP
parameters, such as SSRC values and ROCs.
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 (see
Section 2.2) within a session without coordinating with other
entities via external signaling or other external means.
EKT provides a way for an SRTP session participant, either a sender
or receiver, to securely transport 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 can not 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 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 their secure transport.
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.
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2. Overview
This specification defines a way for the server in a DTLS-SRTP
negotiation to provide an ekt_key to the client during the DTLS
handshake. This ekt_key can be used to encrypt the SRTP master key
used to encrypt the media the endpoint sends. This specification
also defines a way to send the encrypted SRTP master key along with
the SRTP packet. Endpoints that receive this and know the ekt_key
can use the ekt_key to decrypt the SRTP master key then use the SRTP
master key to decrypt the SRTP packet.
One way to use this is used is described in the architecture defined
by [I-D.ietf-perc-private-media-framework]. Each participants in the
conference call forms a DTLS-SRTP connection to a common key
distributor that gives all the endpoints the same ekt_key. Then each
endpoint picks there own SRTP master key for the media they send.
When sending media, the endpoint also includes the SRTP master key
encrypted with the ekt_key. 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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 EKT
field is added to SRTP packets. The EKT field appears at the end of
the SRTP packet. The EKT field appears after the optional
authentication tag if one is present, otherwise the EKT field 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.
4.1. EKT Field Formats
The EKT Field uses the format defined below for the FullEKTField and
ShortEKTField.
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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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 0|
+-+-+-+-+-+-+-+-+
Figure 1: Full EKT Field format
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+
Figure 2: Short EKT Field 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.
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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)
SPI = 2BYTE
FullEKTField = EKTCiphertext SPI 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, 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 [RFC3264]
SDP Offer/Answer for the SRTP.
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SSRC: On the sender side, this field is the SSRC for this SRTP
source. The length of this field is 32 bits.
Rollover Counter (ROC): On the sender side, this field is set to the
current value of the SRTP rollover counter in the SRTP 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
EKT Key and other parameters for the receiver to use when
processing the packet. The length of this field is 16 bits. The
parameters identified by this field are:
* The EKT cipher used to process the packet.
* The EKT Key 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.
Each distinct EKT parameter set that is used MUST be associated
with a distinct SPI value to avoid ambiguity.
EKTMsgLength: All EKT message other that ShortEKTField have a length
as second from the last element. This is the length in octets of
either the FullEKTField/ExtensionEKTField including this length
field and the following message type.
Message Type: The last byte is used to indicate the type of the
EKTField. This MUST be 2 in the FullEKTField format and 0 in
ShortEKTField format. Values less than 64 are mandatory to
understand while other values are optional to understand. A
receiver SHOULD discard the whole EKTField if it contains any
message type value that is less than 64 and that is not
understood. Message type values that are 64 or greater but not
implemented or understood can simply be ignored.
4.2. Packet Processing and State Machine
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
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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.
Either the FullEKTField or ShortEKTField is appended at the tail end
of all SRTP packets. The decision on which to send is specified in
Section 4.7.
4.2.1. Outbound Processing
See Section 4.7 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.
1. The Security Parameter Index (SPI) field is set to the value of
the Security Parameter Index 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 SHOULD be the same as
the one used in the SRTP processing.
3. The EKTCiphertext field is set to the ciphertext created by
encrypting the EKTPlaintext with the EKT cipher, 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 written into the
packet.
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When a packet is sent with the Short EKT Field, the ShortEKFField is
simply appended to the packet.
4.2.2. Inbound Processing
When receiving a packet on a RTP stream, the following steps are
applied for each received packet.
1. The final byte is checked to determine which EKT format is in
use. When an SRTP or SRTCP packet contains a ShortEKTField, the
ShortEKTField is removed from the packet then normal SRTP or
SRTCP 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 or SRTCP part is not known until the
decryption has occurred. At this point in the processing, there
is no easy way to know where the EKT field would start. However,
the whole UDP packet has been received so instead of the starting
at the front of the packet, the parsing works backwards off the
end of the packet and thus the type is put at the very end of the
packet.
2. The Security Parameter Index (SPI) field is used to find which
EKT parameter set to be used when 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, EKTCipher, and SRTP Master Salt.
3. The EKTCiphertext authentication is checked and it is 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 the packet processing stops.
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 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, then all the information from this EKTPlaintext MUST
be discarded and the following steps in this list are not done.
6. The SRTP Master Key, ROC, and SRTP Master Salt from the previous
step are saved in a map indexed by the SSRC found in the
EKTPlaintext and can be used for any future crypto operations on
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the inbound packets with that SSRC. If the SRTP Master Key
recovered from the EKTPlaintext is longer than needed by SRTP
transform in use, the first bytes are used. If the SRTP Master
Key recovered from the EKTPlaintext is shorter than needed by
SRTP transform in use, then the bytes received replace the first
bytes in the existing key but the other bytes after that remain
the same as the old key. This allows for replacing just half the
key for transforms such as [I-D.ietf-perc-double]. Outbound
packets SHOULD continue to use the old SRTP Master Key for 250 ms
after sending any new key. This gives all the receivers in the
system time to get the new key before they start receiving media
encrypted with the new key.
7. At this point, EKT processing has successfully completed, and the
normal SRTP or SRTCP processing takes place including replay
protection.
4.3. Implementation Notes
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. This ciphertext value MAY be cached by an SRTP
receiver to minimize computational effort by noting when the SRTP
master key is unchanged and avoiding repeating the above steps.
The receiver may want to have a sliding window to retain old SRTP
master keys (and related context) for some brief period of time, so
that out of order packets can be processed as well as packets sent
during the time keys are changing.
When receiving a new EKTKey, implementations need to use the ekt_ttl
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 keys
has been used to encrypt data to ensure it does not exceed the T
value for that cipher. 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 renegotiation
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.
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
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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.
An EKTCipher consists of an encryption function and a decryption
function. The encryption function E(K, P) takes the following
inputs:
o a secret key K with a length of L bytes, and
o 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:
o a secret key K with a length of L bytes, and
o a ciphertext value C with a length of N bytes.
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.
4.4.1. Ciphers
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.
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+----------+----+------+
| 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 [RFC3711],
AESKW128 MUST be implemented for EKT and 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 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.
4.6. Transport
EKT SHOULD be used over SRTP, and other specification MAY define how
to use it over SRTCP. SRTP is preferred because it shares fate with
transmitted media, because SRTP rekeying can occur without concern
for RTCP transmission limits, and to avoid SRTCP compound packets
with RTP translators and mixers.
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4.7. Timing and Reliability Consideration
A system using EKT learns the SRTP master keys distributed with
FullEKTFields sent with the SRTP, rather than with call signaling. A
receiver can immediately decrypt an SRTP packet, provided the SRTP
packet contains a Full EKT Field.
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:
New sender: A new sender SHOULD send a packet containing the
FullEKTField as soon as possible, always before or coincident with
sending its initial SRTP packet. To accommodate packet loss, it
is RECOMMENDED that three consecutive packets contain the Full EKT
Field be transmitted.
Rekey: By sending EKT 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 contain 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). 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 FullEKT
Field. 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 100ms.
5. Use of EKT with DTLS-SRTP
This document defines an extension to DTLS-SRTP called SRTP EKT Key
Transport which enables secure transport of EKT keying material from
one DTLS-SRTP peer to another. This allows those peers to process
EKT keying material in SRTP (or SRTCP) and retrieve the embedded SRTP
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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.
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 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
"srtp_ekt_key_transport"and a new TLS content type called EKTMessage.
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Using the syntax described in DTLS [RFC6347], the following
structures are used:
enum {
reserved(0),
aeskw_128(1),
aeskw_256(2),
} EKTCipherType;
struct {
EKTCipherType ekt_ciphers<1..255>;
} SupportedEKTCiphers;
struct {
EKTCipherType ekt_cipher;
uint ekt_key_value<1..256>;
uint srtp_master_salt<1..256>;
uint16 ekt_spi;
uint24 ekt_ttl;
} EKTkey;
enum {
ekt_key(0),
ekt_key_ack(1),
ekt_key_error(254),
(255)
} EKTMessageType;
struct {
EKTMessageType ekt_message_type;
select (EKTMessage.ekt_message_type) {
case ekt_key:
EKTKey;
} message;
} EKTMessage;
Figure 4: Additional TLS Data Structures
If a DTLS client includes "srtp_ekt_key_transport" in its
ClientHello, then a DTLS server that supports this extensions will
includes "srtp_ekt_key_transport" in its ServerHello message. If a
DTLS client includes "srtp_ekt_key_transport" in its ClientHello, but
does not receive "srtp_ekt_key_transport" in the ServerHello, the
DTLS client MUST NOT send DTLS EKTMessage messages. Also, the
"srtp_ekt_key_transport" in the ServerHello MUST select one and only
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one EKTCipherType from the list provided by the client in the
"srtp_ekt_key_transport" in the ClientHello.
When a DTLS client sends the "srtp_ekt_key_transport" in its
ClientHello message, it MUST include the SupportedEKTCiphers as the
extension_data for the extension, listing the EKTCipherTypes the
client is willing to use in preference order, with the most preferred
version first. When the server responds in the
"srtp_ekt_key_transport" in its ServerHello message, it MUST include
a SupportedEKTCiphers list that selects a single EKTCipherType to use
(selected from the list provided by the client) or it returns an
empty list to indicate there is no matching EKTCipherType in the
clients list that the server is also willing to use. The value to be
used in the EKTCipherType for future extensions that define new
ciphers is the value from the "EKT Ciphers Type" IANA registry
defined in Section 7.2.
The figure above defines the contents for a new TLS content type
called EKTMessage which is registered in Section 7.4. The EKTMessage
above is used as the opaque fragment in the TLSPlaintext structure
defined in Section 6.2.1 of [RFC5246] and the "srtp_ekt_message" as
the content type. The "srtp_ekt_message" content type is defined and
registered in Section 7.3.
ekt_ttl: The maximum amount of time, in seconds, that this
ekt_key_value can be used. The ekt_key_value in this message MUST
NOT be used for encrypting or decrypting information after the TTL
expires.
When the Server wishes to provide a new EKT Key, it can send
EKTMessage containing an EKTKey with the new key information. The
client MUST respond with an EKTMessage of type ekt_key_ack, if the
EKTKey was successfully processed and stored or respond with the the
ekt_key_error EKTMessage otherwise.
The diagram below shows a message flow of DTLS client and DTLS server
using the DTLS-SRTP Key Transport extension.
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Client Server
ClientHello + use_srtp + srtp_ekt_key_trans
-------->
ServerHello+use_srtp+srtp_ekt_key_trans
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
ekt_key <--------
ekt_key_ack -------->
SRTP packets <-------> SRTP packets
SRTP packets <-------> SRTP packets
ekt_key (rekey) <-------
ekt_key_ack -------->
SRTP packets <-------> SRTP packets
SRTP packets <-------> SRTP packets
Figure 5: DTLS/SRTP Message Flow
Note that when used in PERC [I-D.ietf-perc-private-media-framework],
the Server is actually split between the Media Distrbutor and Key
Distributor. The messages in the above figure that are "SRTP
packets" will not got to the Key Distributor but the oter packets
will be relayed by the Media Distributor to the Key Distributor.
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 [RFC3264] Offer /
Answer messaging.
5.4. Sending the DTLS EKT_Key Reliably
The DTLS ekt_key is sent using the retransmissions specified in
Section 4.2.4. of DTLS [RFC6347].
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6. Security Considerations
EKT inherits the security properties of the DTLS-SRTP (or other)
keying it uses.
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.
An attacker who tampers with the bits in FullEKTField can prevent the
intended receiver of that packet from being able to decrypt it. This
is a minor denial of service vulnerability. Similarly the attacker
could take an old FullEKTField from the same session and attach it to
the packet. The FullEKTField would correctly decode and pass
integrity but the key extracted from the FullEKTField , when used to
decrypt the SRTP payload, would be wrong and the SRTP integrity check
would fail. Note that the FullEKTField only changes the decryption
key and does not change the encryption key. None of these are
considered significant attacks as any attacker that can modify the
packets in transit and cause the integrity check to fail.
An attacker could send packets containing a Full EKT Field, in an
attempt to consume additional CPU resources of the receiving system
by causing the receiving system will 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 helps
mitigate this issue.
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 Full EKT Field using the same EKTKey. In addition,
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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
SHOULD be limited using the ekt_ttl.
7. IANA Considerations
7.1. EKT Message Types
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 |
| | | |
| Reserved | 63 | RFCAAAA |
| | | |
| Reserved | 255 | RFCAAAA |
+--------------+-------+---------------+
Table 2: EKT Messages Types
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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 [RFC5226]. When requesting a new value, the requestor
needs to indicate if it is mandatory to understand or not. If it is
mandatory to understand, IANA needs to allocate a value less than 64,
if it is not mandatory to understand, a value greater than or equal
to 64 needs to be allocated. 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.
All new EKT messages MUST be defined to have a length as second from
the last element.
7.2. EKT Ciphers
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 | 1 | RFCAAAA |
| | | |
| AESKW256 | 2 | RFCAAAA |
| | | |
| Reserved | 255 | RFCAAAA |
+----------+-------+---------------+
Table 3: EKT Cipher Types
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 [RFC5226]. The expert SHOULD ensure the specification
defines the values for L and T as required in Section 4.4 of RFCAAA.
Allocated values MUST be in the range of 1 to 254.
7.3. TLS Extensions
IANA is requested to add "srtp_ekt_key_transport" as an new extension
name to the "ExtensionType Values" table of the "Transport Layer
Security (TLS) Extensions" registry with a reference to this
specification and allocate a value of TBD to for this. Note to RFC
Editor: TBD will be allocated by IANA.
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Considerations for this type of extension are described in Section 5
of [RFC4366] and requires "IETF Consensus".
7.4. TLS Content Type
IANA is requested to add "srtp_ekt_message" as an new descriptions
name to the "TLS ContentType Registry" table of the "Transport Layer
Security (TLS) Extensions" registry with a reference to this
specification, a DTLS-OK value of "Y", and allocate a value of TBD to
for this content type. Note to RFC Editor: TBD will be allocated by
IANA.
This registry was defined in Section 12 of [RFC5246] and requires
"Standards Action".
8. Acknowledgements
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.
9. References
9.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
DOI 10.17487/RFC5649, September 2009,
<http://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,
<http://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, <http://www.rfc-editor.org/info/rfc6347>.
9.2. Informative References
[I-D.ietf-perc-double]
Jennings, C., Jones, P., and A. Roach, "SRTP Double
Encryption Procedures", draft-ietf-perc-double-02 (work in
progress), October 2016.
[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", draft-ietf-perc-private-media-framework-02
(work in progress), October 2016.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
<http://www.rfc-editor.org/info/rfc4366>.
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Authors' Addresses
Cullen Jennings
Cisco Systems
Calgary, AB
Canada
Email: fluffy@iii.ca
John Mattsson (editor)
Ericsson AB
SE-164 80 Stockholm
Sweden
Phone: +46 10 71 43 501
Email: john.mattsson@ericsson.com
David A. McGrew
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95035
US
Phone: (408) 525 8651
Email: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
Dan Wing
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95035
US
Phone: (408) 853 4197
Email: dwing@cisco.com
Flemming Andreason
Cisco Systems
499 Thornall Street
Edison, NJ 08837
US
Email: fandreas@cisco.com
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