AVTCORE Working Group D. McGrew
Internet-Draft D. Wing
Intended status: Standards Track F. Andreasen
Expires: August 18, 2014 Cisco
February 14, 2014
Encrypted Key Transport for Secure RTP
draft-ietf-avtcore-srtp-ekt-02
Abstract
Encrypted Key Transport (EKT) is an extension to Secure Real-time
Transport Protocol (SRTP) that provides for the secure transport of
SRTP master keys, Rollover Counters, and other information. This
facility enables SRTP to work for decentralized conferences with
minimal control.
This note defines EKT, and also describes how to use it with SDP
Security Descriptions, DTLS-SRTP, and MIKEY. With EKT, these other
key management protocols provide an EKT key to everyone in a session,
and EKT coordinates the SRTP keys within the session.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 18, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions Used In This Document . . . . . . . . . . . . 5
2. Encrypted Key Transport . . . . . . . . . . . . . . . . . . . 5
2.1. EKT Field Formats . . . . . . . . . . . . . . . . . . . . 5
2.2. Packet Processing and State Machine . . . . . . . . . . . 8
2.2.1. Outbound Processing . . . . . . . . . . . . . . . . . 8
2.2.2. Inbound Processing . . . . . . . . . . . . . . . . . 9
2.3. Ciphers . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3.1. The Default Cipher . . . . . . . . . . . . . . . . . 12
2.3.2. Other EKT Ciphers . . . . . . . . . . . . . . . . . . 12
2.4. Synchronizing Operation . . . . . . . . . . . . . . . . . 13
2.5. Transport . . . . . . . . . . . . . . . . . . . . . . . . 13
2.6. Timing and Reliability Consideration . . . . . . . . . . 14
3. Use of EKT with SDP Security Descriptions . . . . . . . . . . 15
3.1. SDP Security Descriptions Recap . . . . . . . . . . . . . 15
3.2. Relationship between EKT and SDP Security Descriptions . 16
3.3. Overview of Combined EKT and SDP Security Description
Operation . . . . . . . . . . . . . . . . . . . . . . . . 18
3.4. EKT Extensions to SDP Security Descriptions . . . . . . . 18
3.4.1. EKT_Cipher . . . . . . . . . . . . . . . . . . . . . 18
3.4.2. EKT_Key . . . . . . . . . . . . . . . . . . . . . . . 19
3.4.3. EKT_SPI . . . . . . . . . . . . . . . . . . . . . . . 19
3.5. Offer/Answer Procedures . . . . . . . . . . . . . . . . . 19
3.5.1. Generating the Initial Offer - Unicast Streams . . . 19
3.5.2. Generating the Initial Answer - Unicast Streams . . . 20
3.5.3. Processing of the Initial Answer - Unicast Streams . 21
3.6. SRTP-Specific Use Outside Offer/Answer . . . . . . . . . 22
3.7. Modifying the Session . . . . . . . . . . . . . . . . . . 23
3.8. Backwards Compatibility Considerations . . . . . . . . . 23
3.9. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 24
4. Use of EKT with DTLS-SRTP Key Transport . . . . . . . . . . . 25
4.1. DTLS-SRTP Recap . . . . . . . . . . . . . . . . . . . . . 25
4.2. EKT Extensions to DTLS-SRTP . . . . . . . . . . . . . . . 25
4.3. Offer/Answer Considerations . . . . . . . . . . . . . . . 27
4.3.1. Generating the Initial Offer . . . . . . . . . . . . 27
4.3.2. Generating the Initial Answer . . . . . . . . . . . . 28
4.3.3. Processing the Initial Answer . . . . . . . . . . . . 28
4.3.4. Sending DTLS EKT Key Reliably . . . . . . . . . . . . 29
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4.3.5. Modifying the Session . . . . . . . . . . . . . . . . 29
5. Use of EKT with MIKEY . . . . . . . . . . . . . . . . . . . . 29
5.1. EKT extensions to MIKEY . . . . . . . . . . . . . . . . . 31
5.2. Offer/Answer considerations . . . . . . . . . . . . . . . 32
5.2.1. Generating the Initial Offer . . . . . . . . . . . . 32
5.2.2. Generating the Initial Answer . . . . . . . . . . . . 33
5.2.3. Processing the Initial Answer . . . . . . . . . . . . 33
5.2.4. Modifying the Session . . . . . . . . . . . . . . . . 34
6. Using EKT for interoperability between key management systems 34
7. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 35
7.1. Alternatives . . . . . . . . . . . . . . . . . . . . . . 36
8. Security Considerations . . . . . . . . . . . . . . . . . . . 36
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
11.1. Normative References . . . . . . . . . . . . . . . . . . 39
11.2. Informative References . . . . . . . . . . . . . . . . . 40
Appendix A. Using EKT to Optimize Interworking DTLS-SRTP with
Security Descriptions . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
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 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 (and SSRC,
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 most of 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, an
extension to SRTP that fits within the SRTP framework and reduces the
amount of external signaling control that is needed in an SRTP
session. EKT securely distributes the SRTP master key and other
information for each SRTP source (SSRC), using SRTCP or SRTP to
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transport that information. With this method, SRTP entities are free
to choose SSRC values as they see fit, and to start up new SRTP
sources (SSRC) with new SRTP master keys (see Section 2.2) within a
session without coordinating with other entities via external
signaling or other external means. This fact allows to reinstate the
RTP collision detection and repair mechanism, which is nullified by
the current SRTP specification because of the need to control SSRC
values closely. An SRTP endpoint using EKT can generate new keys
whenever an existing SRTP master key has been overused, or start up a
new SRTP source (SSRC) to replace an old SRTP source that has reached
the packet-count limit. However, EKT does not allow SRTP's ROC to
rollover; that requires re-keying outside of EKT (e.g., using MIKEY
or DTLS-SRTP). EKT also solves the problem in which the burst loss
of the N initial SRTP packets can confuse an SRTP receiver, when the
initial RTP sequence number is greater than or equal to 2^16 - N.
These features can simplify many architectures that implement SRTP.
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, possibly in conjunction with additional data provided by an
external signaling protocol, furnishes the information needed by the
receiver to instantiate an SRTP/SRTCP receiver context.
EKT does not control the manner in which the SSRC is generated; it is
only concerned with their secure transport. Those values may be
generated on demand by the SRTP endpoint, or may be dictated by an
external mechanism such as a signaling agent or a secure group
controller.
EKT is not intended to replace external key establishment mechanisms
such as SDP Security Descriptions [RFC4568], DTLS-SRTP [RFC5764], or
MIKEY [RFC3830][RFC4563]. Instead, it is used in conjunction with
those methods, and it relieves them of the burden of tightly
coordinating every SRTP source (SSRC) among every SRTP participant.
1.1. History
[[RFC Editor Note: please remove this section prior to publication as
an RFC.]]
A substantial change occurred between the EKT documents draft-ietf-
avt-srtp-ekt-03 and draft-ietf-avtcore-srtp-ekt-00. The change makes
it possible for the EKT data to be removed from a packet without
affecting the ability of the receiver to correctly process the data
that is present in that packet. This capability facilitates
interoperability between SRTP implementations with different SRTP key
management methods. The changes also greatly simplify the EKT
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processing rules, and makes the EKT data that must be carried in SRTP
and/or SRTCP packets somewhat larger.
In draft-ietf-avtcore-srtp-ekt-02 (this document), SRTP master keys
have to be always generated randomly and never shared, MKI is no
longer allowed with EKT (as MKI duplicates some of EKT's functions),
and text clarifies that EKT must be negotiated during call setup.
Some text was consolidated and re-written, notably Section 2.6
("Timing and Reliability").
1.2. 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].
2. Encrypted Key Transport
In EKT, an SRTP master key is encrypted with a key encrypting key and
the resulting ciphertext is transported in selected SRTCP packets or
in selected SRTP packets. The key encrypting key is called an EKT
key. A single such key suffices for a single SRTP session,
regardless of the number of participants in that session. However,
there can be multiple EKT keys used within a particular session.
EKT defines a new method of providing SRTP master keys to an
endpoint. In order to convey the ciphertext of the SRTP master key,
and other additional information, an additional EKT field is added to
SRTP or SRTCP packets. When added to SRTCP, the EKT field appears at
the end of the packet, after the authentication tag, if that tag is
present, or after the SRTCP index otherwise. When added to SRTP, The
EKT field appears at the end of the SRTP packet, after the
authentication tag (if that tag is present), or after the ciphertext
of the encrypted portion of the packet otherwise.
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.
2.1. EKT Field Formats
The EKT Field uses one of the two formats defined below. These two
formats can always be unambiguously distinguished on receipt by
examining the final bit of the EKT Field, which is also the final bit
of the SRTP or SRTCP packet. The first format is the Full EKT Field
(or Full_EKT_Field), and the second is the Short EKT Field (or
Short_EKT_Field). The formats are defined as
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EKT_Plaintext = SRTP_Master_Key || SSRC || ROC || ISN
EKT_Ciphertext = EKT_Encrypt(EKT_Key, EKT_Plaintext)
Full_EKT_Field = EKT_Ciphertext || SPI || '1'
Short_EKT_Field = Reserved || '0'
Figure 1: EKT data formats
Here || denotes concatenation, and '1' and '0' denote single one and
zero bits, respectively. These fields and data elements are defined
as follows:
EKT_Plaintext: 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.
EKT_Ciphertext: The data that is output from the EKT encryption
operation, described in Section 2.3. This field is included in
SRTP and SRTCP packets when EKT is in use. The length of this
field is variable, and is equal to the ciphertext size N defined
in Section 2.3. Note that the length of the field is inferable
from the SPI field, since the particular EKT cipher used by the
sender of a packet can be inferred from that field.
SRTP_Master_Key: On the sender side, the SRTP Master Key associated
with the indicated SSRC. The length of this field depends on the
cipher suite negotiated during call setup for SRTP or SRTCP.
SSRC: On the sender side, this field is the SSRC for this SRTP
source. The length of this field is fixed at 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 fixed at 32 bits.
Initial Sequence Number (ISN): If this field is nonzero, it
indicates the RTP sequence number of the initial RTP packet that
is protected using the SRTP master key conveyed (in encrypted
form) by the EKT Ciphertext field of this packet. When this field
is present in an RTCP packet it indicates the RTP sequence number
of the first RTP packet encrypted by this master key. If the ISN
field is zero, it indicates that the initial RTP/RTCP packet
protected using the SRTP master key conveyed in this packet
preceded, or was concurrent with, the last roll-over of the RTP
sequence number, and thus should be used as the current master key
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for processing this packet. The length of this field is fixed at
16 bits.
Security Parameter Index (SPI): This field is included in SRTP and
SRTCP packets when EKT is in use. It indicates the appropriate
EKT key and other parameters for the receiver to use when
processing the packet. It is an "index" into a table of
possibilities (which are established via signaling or some other
out-of-band means), much like the IPsec Security Parameter Index
[RFC4301]. The length of this field is fixed at 15 bits. The
parameters identified by this field are:
* The EKT key used to process the packet.
* The EKT cipher used to process the packet.
* The Secure RTP parameters associated with the SRTP Master Key
carried by the packet and the SSRC value in the packet.
Section 8.2. of [RFC3711] summarizes the parameters defined by
that specification.
* The Master Salt associated with the Master Key. (This value is
part of the parameters mentioned above, but we call it out for
emphasis.) The Master Salt is communicated separately, via
signaling, typically along with the EKT key.
Together, these data elements are called an EKT parameter set.
Within each SRTP session, each distinct EKT parameter set that may
be used MUST be associated with a distinct SPI value, to avoid
ambiguity.
Reserved: The length of this field is 7 bits. MUST be all zeros on
transmission, and MUST be ignored on reception.
The Full_EKT_Field and Short_EKT_Field formats are shown in Figure 2
and Figure 3, respectively. These figures show the on-the-wire data.
The Ciphertext field holds encrypted data, and thus has no apparent
inner structure.
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 |1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 2: Full EKT Field format
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+-+
| Reserved |0|
+-+-+-+-+-+-+-+-+-+
Figure 3: Short EKT Field format
2.2. Packet Processing and State Machine
At any given time, each SRTP/SRTCP source (SSRC) 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.
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 these
EKT parameter sets SHOULD be stored by all of the participants in an
SRTP session, for use in processing inbound SRTP and SRTCP traffic.
All SRTP master keys MUST NOT be re-used, MUST be randomly generated
according to [RFC4086], MUST NOT be equal to or derived from other
SRTP master keys.
2.2.1. Outbound Processing
See Section 2.6 which describes when to send an EKT packet and
describes if a Full EKT Field or Short EKT Field is sent.
When an SRTP or SRTCP packet is to be sent, the EKT field for that
packet is created as follows, or uses an equivalent set of steps.
The creation of the EKT field MUST precede the normal SRTP or SRTCP
packet processing. The ROC used in EKT processing MUST be the same
as the one used in the SRTP processing.
If the Short format is used, an all-zero octet is appended to the
packet. Otherwise, processing continues as follows.
The Rollover Counter field in the packet is set to the current value
of the SRTP rollover counter (represented as an unsigned integer in
network byte order).
The Initial Sequence Number field is set to zero, if the initial RTP
packet protected using the current SRTP master key for this source
preceded, or was concurrent with, the last roll-over of the RTP
sequence number. Otherwise, that field is set to the value of the
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RTP sequence number of the initial RTP packet that was or will be
protected by that key. See "rekey" in Section 2.6. The rekeying
event MUST NOT change the value of ROC (otherwise, the current value
of the ROC would not be known to late joiners of existing sessions).
This means rekeying near the end of sequence number space (e.g., 100
packets before sequence number 65535) is not possible because ROC
needs to roll over.
The Security Parameter Index field is set to the value of the
Security Parameter Index that is associated with the outbound
parameter set.
The EKT_Plaintext field is computed from the SRTP Master Key, SSRC,
ROC, and ISN fields, as shown in Figure 1.
The EKT_Ciphertext field is set to the ciphertext created by
encrypting the EKT_Plaintext with the EKT cipher, using the EKT Key
as the encryption key. The encryption process is detailed in
Section 2.3. Implementations MAY cache the value of this field to
avoid recomputing it for each packet that is sent.
Implementation note: Because of the format of the Full EKT Field, a
packet containing the Full EKT Field MUST be sent when the ROC
changes (i.e., every 2^32 packets).
2.2.2. Inbound Processing
When an SRTP or SRTCP packet containing a Full EKT Field or Short EKT
Field is received, it is processed as follows or using an equivalent
set of steps. Inbound EKT processing MUST take place prior to the
usual SRTP or SRTCP processing. Implementation note: the receiver
may want to have a sliding window to retain old master keys for some
brief period of time, so that out of order packets can be processed.
The following steps show processing as packets are received in order.
1. The final bit is checked to determine which EKT format is in use.
If the packet contains a Short EKT Field then the Short EKT Field
is removed and normal SRTP or SRTCP processing is applied. If
the packet contains a Full EKT Field, then processing continues
as described below.
2. The Security Parameter Index (SPI) field is checked to determine
which EKT parameter set should be used when processing the
packet. If multiple parameter sets have been defined for the
SRTP session, then the one that is associated with the value of
the SPI field in the packet is used. This parameter set is
called the matching parameter set below. If there is no matching
SPI, then the verification function MUST return an indication of
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authentication failure, and the steps described below are not
performed.
3. The EKT_Ciphertext is decrypted using the EKT_Key and EKT_Cipher
in the matching parameter set, as described in Section 2.3. If
the EKT decryption operation returns an authentication failure,
then the packet processing halts with an indication of failure.
Otherwise, the resulting EKT_Plaintext is parsed as described in
Figure 1, to recover the SRTP Master Key, SSRC, ROC, and ISN
fields.
4. The SSRC field output from the decryption operation is compared
to the SSRC field from the SRTP header. If the values of the two
fields do not match, then packet processing halts with an
indication of failure. Otherwise, it continues as follows.
5. If an SRTP context associated with the SSRC in the previous step
already exists and the ROC from the EKT_Plaintext is less than
the ROC in the SRTP context, then EKT processing halts and the
packet is processed as an out-of-order packet (if within the
implementation's sliding window) or dropped (as it is a replay).
Otherwise, the ROC in the SRTP context is set to the value of the
ROC from the EKT_Plaintext, and the SRTP Master Key from the
EKT_Plaintext is accepted as the SRTP master key corresponding to
the SSRC indicated in the EKT_Plaintext, beginning at the
sequence number indicated by the ISN (see next step).
6. If the ISN from the EKT_Plaintext is less than the RTP sequence
number of an authenticated received SRTP packet, then EKT
processing halts (as this is a replay). If the Initial Sequence
Number field is nonzero, then the initial sequence number for the
SRTP master key is set to the packet index created by appending
that field to the current rollover counter and treating the
result as a 48-bit unsigned integer. The initial sequence number
for the master key is equivalent to the "From" value of the
<From, To> pair of indices (Section 8.1.1 of [RFC3711]) that can
be associated with a master key.
7. The newly accepted SRTP master key, the SRTP parameters from the
matching parameter set, and the SSRC from the packet are stored
in the crypto context associated with the SRTP source (SSRC).
The SRTP Key Derivation algorithm is run in order to compute the
SRTP encryption and authentication keys, and those keys are
stored for use in SRTP processing of inbound packets. The Key
Derivation algorithm takes as input the newly accepted SRTP
master key, along with the Master Salt from the matching
parameter set.
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8. At this point, EKT processing has successfully completed, and the
normal SRTP or SRTCP processing takes place.
Implementation note: the value of the EKT Ciphertext field is
identical in successive packets protected by the same EKT
parameter set and the same SRTP master key, ROC, and ISN.
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 Steps 2 through 6.
2.3. Ciphers
EKT uses an authenticated cipher to encrypt the EKT Plaintext, which
is comprised of the SRTP master keys, SSRC, ROC, and ISN. We first
specify the interface to the cipher, in order to abstract the
interface away from the details of that function. We then define the
cipher that is used in EKT by default. The default cipher described
in Section 2.3.1 MUST be implemented, but another cipher that
conforms to this interface MAY be used, in which case its use MUST be
coordinated by external means (e.g., key management).
The master salt length for the offered cipher suites MUST be the
same. In practice the easiest way to achieve this is by offering the
same crypto suite.
An EKT cipher 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 is at least 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
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times that it can be used with any fixed key value. For each key,
the encryption function MUST NOT be invoked on more than T distinct
values of P, and the decryption function MUST NOT be invoked on more
than T distinct values of C.
The length of the EKT Plaintext is ten bytes, plus the length of the
SRTP Master Key.
Security requirements for EKT ciphers are discussed in Section 8.
2.3.1. The Default Cipher
The default EKT Cipher is the Advanced Encryption Standard (AES)
[FIPS197] 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 + 8 octets. It can be used with
key sizes of L = 16, 24, and 32, and its use with those key sizes is
indicated as AESKW_128, AESKW_192, and AESKW_256, respectively. The
key size determines the length of the AES key used by the Key Wrap
algorithm. With this cipher, T=2^48.
length of length of
SRTP EKT EKT EKT length of
transform transform plaintext ciphertext Full EKT Field
--------- ------------ --------- ---------- --------------
AES-128 AESKW_128 (m) 26 40 42
AES-192 AESKW_192 34 48 50
AES-256 AESKW_256 42 56 58
F8-128 AESKW_128 26 40 42
SEED-128 AESKW_128 26 40 42
Figure 4: AESKW Table
The mandatory to implement transform is AESKW_128, indicated by (m).
As AES-128 is the mandatory to implement transform in SRTP [RFC3711],
AESKW_128 MUST be implemented for EKT.
For all the SRTP transforms listed in the table, the corresponding
EKT transform MUST be used, unless a stronger EKT transform is
negotiated by key management.
2.3.2. Other EKT Ciphers
Other specifications may extend this one by defining other EKT
ciphers per Section 9. This section defines how those ciphers
interact with this specification.
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An EKT cipher determines how the EKT Ciphertext 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, M, N, and T MUST be
defined by each EKT cipher, and those values MUST be inferable from
the EKT parameter set.
2.4. Synchronizing Operation
A participant in a session MAY opt to use a particular EKT parameter
set to protect outbound packets after it accepts that EKT parameter
set for protecting inbound traffic. In this case, the fact that one
participant has changed to using a new EKT key for outbound traffic
can trigger other participants to switch to using the same key.
If a source has its EKT key changed by key management, it MUST also
change its SRTP master key, which will cause it to send out a new
Full EKT Field. This ensures that if key management thought the EKT
key needs changing (due to a participant leaving or joining) and
communicated that in key management 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 EKT key.
The use of EKT MUST be negotiated during key management or call setup
(e.g., using DTLS-SRTP, Security Descriptions, MIKEY, or similar).
2.5. Transport
EKT SHOULD be used over SRTP, and MAY be used over SRTCP. SRTP is
preferred because it shares fate with transmitted media and because
SRTP rekeying can occur without concern for RTCP transmission limits.
The packet processing, state machine, and Authentication Tag format
for EKT over SRTP are nearly identical to that for EKT over SRTCP.
Differences are highlighted in Section 2.2.1 and Section 2.2.2.
The Full EKT Field is appended to the SRTP or SRTCP payload and is
42, 50, or 58 octets long for AES-128, AES-192, or AES-192,
respectively. This length impacts the maximum payload size of the
SRTP (or SRTCP) packet itself. To remain below the network path MTU,
senders SHOULD constrain the SRTP (or SRTCP) payload size by this
length of the Full EKT Field.
EKT can be transported over SRTCP, but some of the information that
it conveys is used for SRTP processing; some elements of the EKT
parameter set apply to both SRTP and SRTCP. Furthermore, SRTCP
packets can be lost and both SRTP and SRTCP packets may be delivered
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out of order. This can lead to various race conditions if EKT is
transported over SRTCP but not SRTP, which we review below.
The ROC signaled via EKT over SRTCP may be off by one when it is
received by the other party(ies) in the session. In order to deal
with this, receivers should simply follow the SRTP packet index
estimation procedures defined in Section 3.3.1 [RFC3711].
2.6. Timing and Reliability Consideration
A system using EKT has the SRTP keys distributed with EKT, rather
than with call signaling. This means a receiver can immediately
decrypt an SRTP (or SRTCP packet) using that new key, provided the
SRTP packet (or SRTCP packet) also contains the EKT key. However, a
receiver cannot immediately authenticate or decrypt an SRTCP packets
without the EKT key. Fortunately, the inability to authenticate or
decrypt SRTCP has little immediate impact on the end user.
This section describes how to reliably and expediently deliver EKT
keys to receivers.
There are three cases to consider. The first case is a new sender
joining a session which needs to communicate its key to all the
receivers. The second case is a sender changing its 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 key.
New sender: A new sender SHOULD send a packet containing the Full EKT
Field as soon as possible, always before or coincident with sending
its initial SRTP packet. To accommodate packet loss, it is
RECOMMENDED that three consectutive packets contain the Full EKT
Field be transmitted. Inclusion of that Full EKT Field can be
stopped early if the sender determines all receivers have received
the new EKT key by receipt of an SRTCP receiver report or explicit
ACK for a sequence number with the new key.
Rekey: By sending EKT over SRTP, the rekeying event shares fate with
the SRTP packets protected with that new key. To avoid sending large
SRTP packets (such as video key frames) with the Full EKT Field, it
can be advantageous to send smaller SRTP packets with the Full EKT
Field with the Initial Sequence Number prior to the actual rekey
event, but this does eliminate the benefits of fate-sharing EKT with
the SRTP packets with the new key, which increases the chance a new
receiver won't have seen the new key.
New receiver: When a new receiver joins a session it does not need to
communicate its sending key (because it is a receiver). When a new
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receiver joins a session the sender is generally unaware of the
receiver joining the session. Thus, senders SHOULD periodically
transmit the Full EKT Field. 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 Full EKT Field. The RECOMMENDED frequency is
the same as the key frame frequency if sending video or every 5
seconds. When joining a session it is likely that SRTP or SRTCP
packets might be received before a packet containing the Full EKT
Field is received. Thus, to avoid doubling the authentication
effort, an implementation joining an EKT session SHOULD buffer
received SRTP and SRTCP packets until it receives the Full EKT Field
packet and use the information in that packet to authenticate and
decrypt the received SRTP/SRTCP packets.
3. Use of EKT with SDP Security Descriptions
The SDP Security Descriptions (SDESC) [RFC4568] specification defines
a generic framework for negotiating security parameters for media
streams negotiated via the Session Description Protocol with the
"crypto" attribute and the Offer/Answer procedures defined in
[RFC3264]. In addition to the general framework, SDES also defines
how to use that framework specifically to negotiate security
parameters for Secure RTP. Below, we first provide a brief recap of
the crypto attribute when used for SRTP and we then explain how it is
complementary to EKT. In the rest of this Section, we provide
extensions to the crypto attribute and associated offer/answer
procedures to define its use with EKT.
3.1. SDP Security Descriptions Recap
The SRTP crypto attribute defined for SDESC contains a tag followed
by three types of parameters (refer to [RFC4568] for details):
o Crypto-suite. Identifies the encryption and authentication
transform.
o Key parameters. SRTP keying material and parameters.
o Session parameters. Additional (optional) SRTP parameters such as
Key Derivation Rate, Forward Error Correction Order, use of
unencrypted SRTP, and other parameters defined by SDESC.
The crypto attributes in the example SDP in Figure 5 illustrate these
parameters.
v=0
o=sam 2890844526 2890842807 IN IP4 192.0.2.5
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s=SRTP Discussion
i=A discussion of Secure RTP
u=http://www.example.com/seminars/srtp.pdf
e=marge@example.com (Marge Simpson)
c=IN IP4 192.0.2.12
t=2873397496 2873404696
m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20
FEC_ORDER=FEC_SRTP
a=crypto:2 F8_128_HMAC_SHA1_80
inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjiiKt|2^20
FEC_ORDER=FEC_SRTP
Figure 5: SDP Security Descriptions example
For legibility the SDP shows line breaks that are not present on the
wire.
The first crypto attribute has the tag "1" and uses the crypto-suite
AES_CM_128_HMAC_SHA1_80. The "inline" parameter provides the SRTP
master key and salt and the master key lifetime (number of packets).
Finally, the FEC_ORDER session parameter indicates the order of
Forward Error Correction used (FEC is applied before SRTP processing
by the sender of the SRTP media).
The second crypto attribute has the tag "2", the crypto-suite
F8_128_HMAC_SHA1_80, two SRTP master keys and associated salts.
Finally, the FEC_ORDER session parameter indicates the order of
Forward Error Correction used.
3.2. Relationship between EKT and SDP Security Descriptions
SDP Security Descriptions [RFC4568] define a generic framework for
negotiating security parameters for media streams negotiated via the
Session Description Protocol by use of the Offer/Answer procedures
defined in [RFC3264]. In addition to the general framework, SDESC
also defines how to use it specifically to negotiate security
parameters for Secure RTP.
EKT and SDP Security Descriptions are complementary. SDP Security
Descriptions can negotiate several of the SRTP security parameters
(e.g., cipher and use of Master Key Identifier) as well as SRTP
master keys. SDESC, however, does not negotiate SSRCs and their
associated Rollover Counter (ROC). Instead, SDESC relies on a so-
called "late binding", where a newly observed SSRC will have its
crypto context initialized to a ROC value of zero. Clearly, this
does not work for participants joining an SRTP session that has been
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established for a while and hence has a non-zero ROC. It is
impossible to use SDESC to join an SRTP session that is already in
progress. In this case, EKT on the endpoint running SDESC can
provide the additional signaling necessary to communicate the ROC
(Section 6.4.1 of [RFC4568]). The use of EKT solves this problem by
communicating the ROC associated with the SSRC in the media plane.
SDP Security Descriptions negotiates different SRTP master keys in
the send and receive direction. The offer contains the master key
used by the offerer to send media, and the answer contains the master
key used by the answerer to send media. Consequently, if media is
received by the offerer prior to the answer being received, the
offerer does not know the master key being used. Use of SDP security
preconditions can solve this problem, however it requires an
additional round-trip as well as a more complicated state machine.
EKT solves this problem by simply sending the master key used in the
media plane thereby avoiding the need for security preconditions.
If multiple crypto-suites were offered, the offerer also will not
know which of the crypto-suites offered was selected until the answer
is received. EKT solves this problem by using a correlator, the
Security Parameter Index (SPI), which uniquely identifies each crypto
attribute in the offer.
One of the primary call signaling protocols using offer/answer is the
Session Initiation Protocol (SIP) [RFC3261]. SIP uses the INVITE
message to initiate a media session and typically includes an offer
SDP in the INVITE. An INVITE may be "forked" to multiple recipients
which potentially can lead to multiple answers being received.
SDESC, however, does not properly support this scenario, mainly
because SDP and RTP/RTCP does not contain sufficient information to
allow for correlation of an incoming RTP/RTCP packet with a
particular answer SDP. Note that extensions providing this
correlation do exist (e.g., Interactive Connectivity Establishment
(ICE)). SDESC addresses this point-to-multipoint problem by moving
each answer to a separate RTP transport address thereby turning a
point-to-multipoint scenario into multiple point-to-point scenarios.
There are however significant disadvantages to doing so. As long as
the crypto attribute in the answer does not contain any declarative
parameters that differ from those in the offer, EKT solves this
problem by use of the SPI correlator and communication of the
answerer's SRTP master key in EKT.
As can be seen from the above, the combination of EKT and SDESC
provides a better solution to SRTP negotiation for offer/answer than
either of them alone. SDESC negotiates the various SRTP crypto
parameters (which EKT does not), whereas EKT addresses the
shortcomings of SDESC.
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3.3. Overview of Combined EKT and SDP Security Description Operation
We define three session extension parameters to SDESC to communicate
the EKT cipher, EKT key, and Security Parameter Index to the peer.
The original SDESC parameters are used as defined in [RFC4568],
however the procedures associated with the SRTP master key differ
slightly, since both SDESC and EKT communicate an SRTP master key.
In particular, the SRTP master key communicated via SDESC is used
only if there is currently no crypto context established for the SSRC
in question. This will be the case when an entity has received only
the offer or answer, but has yet to receive a valid EKT packet from
the peer. Once a valid EKT packet is received for the SSRC, the
crypto context is initialized accordingly, and the SRTP master key
will then be derived from the EKT packet. Subsequent offer/answer
exchanges do not change this: The most recent SRTP master key
negotiated via EKT will be used, or, if none is available for the
SSRC in question, the most recent SRTP master key negotiated via
offer/answer will be used. This is done to avoid race conditions
between the offer/answer exchange and EKT, even though this breaks
some offer/answer rules. Note that with the rules described in this
paragraph, once a valid EKT packet has been received for a given
SSRC, rekeying for that SSRC can only be done via EKT. The
associated SRTP crypto parameters however can be changed via SDESC.
3.4. EKT Extensions to SDP Security Descriptions
In order to use EKT and SDESC in conjunction with each other, the
following new SDESC session parameters are defined. These MUST NOT
appear more than once in a given crypto attribute:
EKT_Cipher: The EKT cipher used to encrypt the SRTP Master Key
EKT_Key: The EKT key used to encrypt the SRTP Master Key
EKT_SPI: The EKT Security Parameter Index
Below are details on each of these attributes.
3.4.1. EKT_Cipher
The (optional) EKT_Cipher parameter defines the EKT cipher used to
encrypt the EKT key within SRTP and SRTCP packets. The default value
is "AESKW_128" in accordance with Section 2.3.1. For the AES Key
Wrap cipher, the values "AESKW_128", "AESKW_192", and "AESKW_256" are
defined for values of L=16, 24, and 32 respectively. In the Offer/
Answer model, the EKT_Cipher parameter is a negotiated parameter.
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3.4.2. EKT_Key
The (mandatory) EKT_Key parameter is the key K used to encrypt the
SRTP Master Key within SRTP and SRTCP packets. The value is base64
encoded with "=" padding if padding is necessary (see Section 3.2 and
4 of [RFC4648]). In the Offer/Answer model, the EKT_Key parameter is
a negotiated parameter.
3.4.3. EKT_SPI
The (mandatory) EKT_SPI parameter is the Security Parameter Index.
It is encoded as an ASCII string representing the hexadecimal value
of the Security Parameter Index. The SPI identifies the *offer*
crypto attribute (including the EKT Key and Cipher) being used for
the associated SRTP session. A crypto attribute corresponds to an
EKT Parameter Set and hence the SPI effectively identifies a
particular EKT parameter set. Note that the scope of the SPI is the
SRTP session, which may or may not be limited to the scope of the
associated SIP dialog. In particular, if one of the participants in
an SRTP session is an SRTP translator, the scope of the SRTP session
is not limited to the scope of a single SIP dialog. However, if all
of the participants in the session are endpoints or mixers, the scope
of the SRTP session will correspond to a single SIP dialog. In the
Offer/Answer model, the EKT_SPI parameter is a negotiated parameter.
3.5. Offer/Answer Procedures
In this section, we provide the offer/answer procedures associated
with use of the three new SDESC parameters defined in Section 3.4.
Since SDESC is defined only for unicast streams, we provide only
offer/answer procedures for unicast streams here as well.
3.5.1. Generating the Initial Offer - Unicast Streams
When the initial offer is generated, the offerer MUST follow the
steps defined in [RFC4568] Section 7.1.1 as well as the following
steps.
[[Editor's Note: following paragraph would benefit from rewording.]]
For each unicast media line using Security Descriptions and where use
of EKT is desired, the offerer MUST include one EKT_Key parameter and
one EKT_SPI parameter in at least one "crypto" attribute (see
[RFC4568]). The EKT_SPI parameter serves to identify the EKT
parameter set used for a particular SRTCP packet. Consequently,
within a single media line, a given EKT_SPI value MUST NOT be used
with multiple crypto attributes. Note that the EKT parameter set to
use for the session is not yet established at this point; each
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offered crypto attribute contains a candidate EKT parameter set.
Furthermore, if the media line refers to an existing SRTP session,
then any SPI values used for EKT parameter sets in that session MUST
NOT be remapped to any different EKT parameter sets. When an offer
describes an SRTP session that is already in progress, the offer
SHOULD use an EKT parameter set (including EKT_SPI and EKT_KEY) that
is already in use.
If a given crypto attribute includes more than one set of SRTP key
parameters (SRTP master key, salt, lifetime), they MUST all use the
same salt. (EKT requires a single shared salt between all the
participants in the direct SRTP session).
As EKT is not defined for use with MKI, the offer and the answer MUST
NOT contain MKI.
Important Note: The scope of the offer/answer exchange is the SIP
dialog(s) established as a result of the INVITE, however the scope
of EKT is the direct SRTP session, i.e., all the participants that
are able to receive SRTP and SRTCP packets directly. If an SRTP
session spans multiple SIP dialogs, the EKT parameter sets MUST be
synchronized between all the SIP dialogs where SRTP and SRTCP
packets can be exchanged. In the case where the SIP entity
operates as an RTP mixer (and hence re-originates SRTP and SRTCP
packets with its own SSRC), this is not an issue, unless the mixer
receives traffic from the various participants on the same
destination IP address and port, in which case further
coordination of SPI values and crypto parameters may be needed
between the SIP dialogs (note that SIP forking with multiple early
media senders is an example of this). However, if it operates as
a transport translator (relay) then synchronized negotiation of
the EKT parameter sets on *all* the involved SIP dialogs will be
needed. This is non-trivial in a variety of use cases, and hence
use of the combined SDES/EKT mechanism with RTP translators should
be considered very carefully. It should be noted, that use of
SRTP with RTP translators in general should be considered very
carefully as well.
The EKT session parameters can either be included as optional or
mandatory parameters, however within a given crypto attribute, they
MUST all be either optional or mandatory.
3.5.2. Generating the Initial Answer - Unicast Streams
When the initial answer is generated, the answerer MUST follow the
steps defined in [RFC4568] Section 7.1.2 as well as the following
steps.
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For each unicast media line using SDESC, the answerer examines the
associated crypto attribute(s) for the presence of EKT parameters.
If mandatory EKT parameters are included with a "crypto" attribute,
the answerer MUST support those parameters in order to accept that
offered crypto attribute. If optional EKT parameters are included
instead, the answerer MAY accept the offered crypto attribute without
using EKT. However, doing so will prevent the offerer from
processing any packets received before the answer. If neither
optional nor mandatory EKT parameters are included with a crypto
attribute, and that crypto attribute is accepted in the answer, EKT
MUST NOT be used. If a given a crypto attribute includes a mixture
of optional and mandatory EKT parameters, or an incomplete set of
mandatory EKT parameters, that crypto attribute MUST be considered
invalid.
When EKT is used with SDESC, the offerer and answerer MUST use the
same SRTP master salt. Thus, the SRTP key parameter(s) in the answer
crypto attribute MUST use the same master salt as the one accepted
from the offer.
When the answerer accepts the offered media line and EKT is being
used, the crypto attribute included in the answer MUST include the
same EKT parameter values as found in the accepted crypto attribute
from the offer (however, if the default EKT cipher is being used, it
may be omitted). Furthermore, the EKT parameters included MUST be
mandatory (i.e., no "-" prefix).
Acceptance of a crypto attribute with EKT parameters leads to
establishment of the EKT parameter set for the corresponding SRTP
session. Consequently, the answerer MUST send packets in accordance
with that particular EKT parameter set only. If the answerer wants
to enable the offerer to process SRTP packets received by the offerer
before it receives the answer, the answerer MUST NOT include any
declarative session parameters that either were not present in the
offered crypto attribute, or were present but with a different value.
Otherwise, the offerer's view of the EKT parameter set would differ
from the answerer's until the answer is received. Similarly, unless
the offerer and answerer has other means for correlating an answer
with a particular SRTP session, the answer SHOULD NOT include any
declarative session parameters that either were not present in the
offered crypto attribute, or were present but with a different value.
If this recommendation is not followed and the offerer receives
multiple answers (e.g., due to SIP forking), the offerer may not be
able to process incoming media stream packets correctly.
3.5.3. Processing of the Initial Answer - Unicast Streams
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When the offerer receives the answer, it MUST perform the steps in
[RFC4568] Section 7.1.3 as well as the following steps for each SRTP
media stream it offered with one or more crypto lines containing EKT
parameters in it.
[[Editor's Note: following paragraph would benefit from rewording.]]
If the answer crypto line contains EKT parameters, and the
corresponding crypto line from the offer contained the same EKT
values, use of EKT has been negotiated successfully and MUST be used
for the media stream. When determining whether the values match,
optional and mandatory parameters MUST be considered equal.
Furthermore, if the default EKT cipher is being used, it MAY be
either present or absent in the offer and/or answer.
If the answer crypto line does not contain EKT parameters, then EKT
MUST NOT be used for the corresponding SRTP session. Note that if
the accepted crypto attribute contained mandatory EKT parameters in
the offer, and the crypto attribute in the answer does not contain
EKT parameters, then negotiation has failed (Section 5.1.3 of
[RFC4568]).
If the answer crypto line contains EKT parameters but the
corresponding offered crypto line did not, or if the parameters don't
match or are invalid, then the offerer MUST consider the crypto line
invalid (see Section 7.1.3 of [RFC4568] for further operation).
The EKT parameter set is established when the answer is received,
however there are a couple of special cases to consider here. First
of all, if an SRTP packet containing a Full EKT Field is received
prior to the answer, then the EKT parameter set is established
provisionally based on the SPI included. Once the answer (which may
include declarative session parameters) is received, the EKT
parameter set is fully established. The second case involves receipt
of multiple answers due to SIP forking. In this case, there will be
multiple EKT parameter sets; one for each SRTP session. As mentioned
earlier, reliable correlation of SIP dialogs to SRTP sessions
requires extensions, and hence if one or more of the answers include
declarative session parameters, it may be difficult to fully
establish the EKT parameter set for each SRTP session. In the
absence of a specific correlation mechanism, it is RECOMMENDED, that
such correlation be done based on the signaled receive IP-address in
the SDP and the observed source IP-address in incoming SRTP/SRTCP
packets, and, if necessary, the signaled receive UDP port and the
observed source UDP port.
3.6. SRTP-Specific Use Outside Offer/Answer
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Security Descriptions use for SRTP is not defined outside offer/
answer and hence neither does Security Descriptions with EKT.
3.7. Modifying the Session
When a media stream using the SRTP security descriptions has been
established, and a new offer/answer exchange is performed, the
offerer and answerer MUST follow the steps in Section 7.1.4 of
[RFC4568] as well as the following steps. SDESC allows for all
parameters of the session to be modified, and the EKT session
parameters are no exception to that, however, there are a few
additional rules to be adhered to when using EKT.
It is permissible to start a session without the use of EKT, and then
subsequently start using EKT, however the converse is not. Thus,
once use of EKT has been negotiated on a particular media stream, EKT
MUST continue to be used on that media stream in all subsequent offer
/answer exchanges.
The reason for this is that both SDESC and EKT communicate the SRTP
Master Key with EKT Master Keys taking precedence. Reverting back to
an SDESC-controlled master key in a synchronized manner is difficult.
Once EKT is being used, the salt for the direct SRTP session MUST NOT
be changed. Thus, a new offer/answer which does not create a new
SRTP session (e.g., because it reuses the same IP address and port)
MUST use the same salt for all crypto attributes as is currently used
for the direct SRTP session.
[[Editor's Note: following paragraph would benefit from re-arranging
into earlier-described steps.]]
Finally, subsequent offer/answer exchanges MUST NOT remap a given SPI
value to a different EKT parameter set until 2^15 other mappings have
been used within the SRTP session. In practice, this requirements is
most easily met by using a monotonically increasing SPI value (modulo
2^15 and starting with zero) per direct SRTP session. Note that a
direct SRTP session may span multiple SIP dialogs, and in such cases
coordination of SPI values across those SIP dialogs will be required.
In the simple point-to-point unicast case without translators, the
requirement simply applies within each media line in the SDP. In the
point-to-multipoint case, the requirement applies across all the
associated SIP dialogs.
3.8. Backwards Compatibility Considerations
Backwards compatibility can be achieved in a couple of ways. First
of all, Security Descriptions allows for session parameters to be
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prefixed with "-" to indicate that they are optional. If the
answerer does not support the EKT session parameters, such optional
parameters will simply be ignored. When the answer is received,
absence of the parameters will indicate that EKT is not being used.
Receipt of SRTP or SRTCP packets prior to receipt of such an answer
will obviously be problematic (as is normally the case for Security
Descriptions without EKT).
Alternatively, Security Descriptions allows for multiple crypto lines
to be included for a particular media stream. Thus, two crypto lines
that differ in their use of EKT parameters (presence in one, absence
in the other) can be used as a way to negotiate use of EKT. When the
answer is received, the accepted crypto attribute will indicate
whether EKT is being used or not.
3.9. Grammar
The ABNF [RFC5234] syntax for the one new SDP Security Descriptions
session parameter, EKT, comprising three parts is shown in Figure 6.
ekt = "EKT=" cipher "|" key "|" spi
cipher = cipher-ext / "AESKW_128" / "AESKW_192" / "AESKW_256"
cipher-ext = 1*64(ALPHA / DIGIT / "_")
key = 1*(base64) ; See Section 4 of [RFC4648]
base64 = ALPHA / DIGIT / "+" / "/" / "="
spi = 4HEXDIG ; See [RFC5234]
Figure 6: ABNF for the EKT session parameters
Using the example from Figure 6 with the EKT extensions to SDP
Security Descriptions results in the following example SDP:
v=0
o=sam 2890844526 2890842807 IN IP4 192.0.2.5
s=SRTP Discussion
i=A discussion of Secure RTP
u=http://www.example.com/seminars/srtp.pdf
e=marge@example.com (Marge Simpson)
c=IN IP4 192.0.2.12
t=2873397496 2873404696
m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP EKT=AESKW_128|WWVzQUxvdmVseUVLVGtleQ|AAE0
a=crypto:2 F8_128_HMAC_SHA1_80
inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4;
inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4
FEC_ORDER=FEC_SRTP EKT=AESKW_128|VHdvTG92ZWx5RUtUa2V5cw|AAE0
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For legibility the SDP shows line breaks that are not present on the
wire.
4. Use of EKT with DTLS-SRTP Key Transport
This document defines an extension to DTLS-SRTP called Key Transport.
The EKT with the DTLS-SRTP Key Transport enables secure transport of
EKT keying material from one DTLS-SRTP peer to another. This enables
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
(strong authentication of the endpoint and flexibility) with the
advantages of EKT (allowing secure multiparty RTP with loose
coordination and efficient communication of per-source keys).
4.1. DTLS-SRTP Recap
DTLS-SRTP [RFC5764] uses an extended DTLS exchange between two peers
to exchange keying material, algorithms, and parapeters 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.
4.2. EKT Extensions to DTLS-SRTP
This document adds a new TLS negotiated extension called "ekt". This
adds a new TLS content type, EKT, and a new negotiated extension EKT.
The negotiated extension MUST only be requested in conjunction with
the "use_srtp" extension (Section 3.2 of [RFC5764]). The DTLS server
MUST include "dtls-srtp-ekt" in its SDP (as a session or media level
attribute) and "ekt" in its TLS ServerHello message. If a DTLS
client includes "ekt" in its ClientHello, but does not receive "ekt"
in the ServerHello, the DTLS client MUST NOT send DTLS packets with
the "ekt" content-type.
The formal description of the dtls-srtp-ekt attribute is defined by
the following ABNF [RFC5234] syntax:
attribute = "a=dtls-srtp-ekt"
Using the syntax described in DTLS [RFC6347], the following
structures are used:
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enum {
ekt_key(0),
ekt_key_ack(1),
ekt_key_error(254),
(255)
} SRTPKeyTransportType;
struct {
SRTPKeyTransportType keytrans_type;
uint24 length;
uint16 message_seq;
uint24 fragment_offset;
uint24 fragment_length;
select (SRTPKeyTransportType) {
case ekt_key:
EKTkey;
};
} KeyTransport;
enum {
RESERVED(0),
AESKW_128(1),
AESKW_192(2),
AESKW_256(3),
} ektcipher;
struct {
ektcipher EKT_Cipher;
uint EKT_Key_Value<1..256>;
uint EKT_Master_Salt<1..256>;
uint16 EKT_SPI;
} EKTkey;
Figure 7: Additional TLS Data Structures
The diagram below shows a message flow of DTLS client and DTLS server
using the DTLS-SRTP Key Transport extension. SRTP packets exchanged
prior to the ekt_message are encrypted using the SRTP master key
derived from the normal DTLS-SRTP key derivation function. After the
ekt_key message, they can be encrypted using the SRTP key carried by
EKT.
Client Server
ClientHello + use_srtp + EKT
-------->
ServerHello + use_srtp + EKT
Certificate*
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ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
SRTP packets <-------> SRTP packets
SRTP packets <-------> SRTP packets
ekt_key -------->
SRTP packets <-------> SRTP packets
SRTP packets <-------> SRTP packets
Figure 8: Handshake Message Flow
4.3. Offer/Answer Considerations
This section describes Offer/Answer considerations for the use of EKT
together with DTLS-SRTP for unicast and multicast streams. The
offerer and answerer MUST follow the procedures specified in
[RFC5764] as well as the following ones.
As most DTLS-SRTP processing is performed on the media channel,
rather than in SDP, there is little processing performed in SDP other
than informational and to redirect DTLS-SRTP to an alternate host.
Advertising support for the extension is necessary in SDP because in
some cases it is required to establish an SRTP call. For example, a
mixer may be able to only support SRTP listeners if those listeners
implement DTLS Key Transport (because it lacks the CPU cycles
necessary to encrypt SRTP uniquely for each listener).
4.3.1. Generating the Initial Offer
The initial offer contains a new SDP attribute, "dtls-srtp-ekt",
which contains no value. This attribute MUST only appear at the
media level. This attribute indicates the offerer is capable of
supporting DTLS-SRTP with EKT extensions, and indicates the desire to
use the "ekt" extension during the DTLS-SRTP handshake.
An example of SDP containing the dtls-srtp-ekt attribute::
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v=0
o=sam 2890844526 2890842807 IN IP4 192.0.2.5
s=SRTP Discussion
i=A discussion of Secure RTP
u=http://www.example.com/seminars/srtp.pdf
e=marge@example.com (Marge Simpson)
c=IN IP4 192.0.2.12
t=2873397496 2873404696
m=audio 49170 UDP/TLS/RTP/SAVP 0
a=fingerprint:SHA-1
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=dtls-srtp-ekt
For legibility the SDP shows line breaks that are not present on the
wire.
4.3.2. Generating the Initial Answer
Upon receiving the initial offer, the presence of the dtls-srtp-ekt
attribute indicates a desire to receive the EKT extension in the
DTLS-SRTP handshake. DTLS messages should be constructed according
to those two attributes.
If the answerer does not wish to perform EKT, it MUST NOT include a
=dtls-srtp-ekt in the SDP answer, and it MUST NOT negotiate EKT
during its DTLS-SRTP exchange.
Otherwise, the dtls-srtp-ekt attribute SHOULD be included in the
answer, and EKT SHOULD be negotiated in the DTLS-SRTP handshake.
4.3.3. Processing the Initial Answer
The presence of the dtls-srtp-ekt attribute indicates a desire by the
answerer to perform DTLS-SRTP with EKT extensions. There are two
indications the remote peer does not want to do EKT: the dtls-srtp-
ekt attribute is not present in the answer, or the DTLS-SRTP exchange
fails to negotiate the EKT extension. If the dtls-srtp-ekt attribute
is not present in the answer, the DTLS-SRTP exchange MUST NOT attempt
to negotiate the EKT extension. If the dtls-srtp-ekt attribute is
present in the answer but the DTLS-SRTP exchange fails to negotiate
the EKT extension, EKT MUST NOT be used with that media stream.
After successful DTLS negotiation of the EKT extension, the DTLS
client and server MAY exchange SRTP packets, encrypted using the KDF
described in [RFC5764]. This is normal and expected, even if Key
Transport was negotiated by both sides, as neither side may (yet)
have a need to alter the SRTP key. However, it is also possible that
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one (or both) peers will immediately send an EKT packet before
sending any SRTP, and also possible that SRTP, encrypted with an
unknown key, may be received before the EKT packet is received.
4.3.4. Sending DTLS EKT Key Reliably
In the absence of a round trip time estimate, the DTLS ekt_key
message is sent using an exponential backoff initialized to 250ms, so
that if the first message is sent at time 0, the next transmissions
are at 250ms, 500ms, 1000ms, and so on. If a recent round trip time
estimate is available, exponential backoff is used with the first
transmission at 1.5 times the round trip time estimate. In either
case, re-transmission stops when ekt_key_ack or ekt_key_error message
is received for the matching message_seq.
4.3.5. Modifying the Session
As DTLS-SRTP-EKT processing is done on the DTLS-SRTP channel (media
channel) rather than signaling, no special processing for modifying
the session is necessary.
If the initial offer and initial answer both contained EKT attributes
(indicating the answerer desired to perform EKT), a subsequent offer/
answer exchange MUST also contain those same EKT attributes. If not,
operation is undefined and the sesion MAY be terminated. If the
initial offer and answer failed to negotiate EKT (that is, the answer
did not contain EKT attributes), EKT negotiation failed and a
subsequent offer SHOULD NOT include EKT attributes.
5. Use of EKT with MIKEY
The advantages outlined in Section 1 are useful in some scenarios in
which MIKEY is used to establish SRTP sessions. In this section, we
briefly review MIKEY and related work, and discuss these scenarios.
An SRTP sender or a group controller can use MIKEY to establish a
SRTP cryptographic context. This capability includes the
distribution of a TEK generation key (TGK) or the TEK itself,
security policy payload, crypto session bundle ID (CSB_ID) and a
crypto session ID (CS_ID). The TEK directly maps to an SRTP master
key, whereas the TGK is used along with the CSB_ID and a CS_ID to
generate a TEK. The CS_ID is used to generate multiple TEKs (SRTP
master keys) from a single TGK. For a media stream in SDP, MIKEY
allocates two consecutive numbers for the crypto session IDs, so that
each direction uses a different SRTP master key (see [RFC4567]).
The MIKEY specification [RFC3830] defines three modes to exchange
keys, associated parameters and to protect the MIKEY message: pre-
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shared key, public-key encryption and Diffie-Hellman key exchange.
In the first two modes the MIKEY initiator only chooses and
distributes the TGK or TEK, whereas in the third mode both MIKEY
entities (the initiator and responder) contribute to the keys. All
three MIKEY modes have in common that for establishing a SRTP session
the exchanged key is valid for the send and receive direction.
Especially for group communications it is desirable to update the
SRTP master key individually per direction. EKT provides this
property by distributing the SRTP master key within the SRTP/SRTCP
packet.
MIKEY already supports synchronization of ROC values between the
MIKEY initiator and responder. The SSRC / ROC value pair is part of
the MIKEY Common Header payload. This allows providing the current
ROC value to late joiners of a session. However, in some scenarios a
key management based ROC synchronization is not sufficient. For
example, in mobile and wireless environments, members may go in and
out of coverage and may miss a sequence number overrun. In point-to-
multipoint translator scenarios it is desirable to not require the
group controller to track the ROC values of each member, but to
provide the ROC value by the originator of the SRTP packet. A better
alternative to synchronize the ROC values is to send them directly
via SRTP/SRTCP as EKT does. A separate SRTP extension [RFC4771]
includes the ROC in a modified authentication tag but that extension
does not support updating the SRTP master key.
Besides the ROC, MIKEY synchronizes also the SSRC values of the SRTP
streams. Each sender of a stream sends the associated SSRC within
the MIKEY message to the other party. If an SRTP session participant
starts a new SRTP source (SSRC) or a new participant is added to a
group, subsequent SDP offer/answer and MIKEY exchanges are necessary
to update the SSRC values. EKT improves these scenarios by updating
the keys and SSRC values without coordination on the signaling
channel. With EKT, SRTP can handle early media, since the EKT SPI
allows the receiver to identify the cryptographic keys and parameters
used by the source.
The MIKEY specification [RFC3830] suggests the use of unicast for
rekeying. This method does not scale well to large groups or
interactive groups. The EKT extension of SRTP/SRTCP provides a
solution for rekeying the SRTP master key and for ROC/SSRC
synchronization. EKT is not a substitution for MIKEY, but rather a
complementary addition to address the above described limitations of
MIKEY.
In the next section we provide an extension to MIKEY for support of
EKT. EKT can be used only with the pre-shared key or public-key
encryption MIKEY mode of [RFC3830]. The Diffie-Hellman exchange mode
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is not suitable in conjunction with EKT, because it is not possible
to establish one common EKT key over multiple EKT entities.
Additional MIKEY modes specified in separate documents are not
considered for EKT.
5.1. EKT extensions to MIKEY
In order to use EKT with MIKEY, the EKT cipher, EKT key and EKT SPI
must be negotiated in the MIKEY message exchange.
For EKT we specify a new SRTP Policy Type in the Security Policy (SP)
payload of MIKEY (see Section 6.10 of [RFC3830]). The SP payload
contains a set of policies. Each policy consists of a number of
Policy Param TLVs.
Prot type | Value
-------------------
EKT | TBD (will be assigned by IANA) NOTE TO RFC EDITOR
Figure 9: EKT Security Policy
The EKT Security Policy has one parameter representing the EKT
cipher.
Type | Meaning | Possible values
----------------------------------------------------
0 | EKT cipher | see below
Figure 10: EKT Security Policy Parameters
EKT cipher | Value
-------------------
(reserved) | 0
AESKW_128 | 1
AESKW_192 | 2
AESKW_256 | 3
Figure 11: EKT Cipher Parameters
The two mandatory EKT parameters (EKT_Key and EKT_SPI) are
transported in the MIKEY KEMAC payload within one separate Key Data
sub-payload. As specified in Section 6.2 of [RFC3830], the KEMAC
payload carries the TEK Generation Key (TGK) or the Traffic
Encryption Key (TEK). One or more TGKs or TEKs are carried in
individual Key Data sub-payloads within the KEMAC payload. The KEMAC
payload is encrypted as part of MIKEY. The Key Data sub- payload,
specified in Section 6.13 of [RFC3830], has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload | Type | KV | Key data length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Key data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Salt length (optional) ! Salt data (optional) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: KV data (optional) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Key Data Sub-Payload of MIKEY
These fields are described below:
Type: 4 bits in length, indicates the type of key included in the
payload. We define Type = TBD (will be requested from IANA [NOTE
TO RFC EDITOR]) to indicate transport of the EKT key.
KV: (4 bits): indicates the type of key validity period specified.
KV=1 is currently specified as an SPI. We use that value to
indicate the KV data contains the EKT_SPI for the key type
EKT_Key. KV data would be 16 bits in length, but it is also
possible to interpret the length from the 'Key data len' field.
KV data MUST be present for the key type EKT_Key when KV=1.
Salt length, Salt Data: These optional fields SHOULD be omitted for
the key type EKT_Key, if the SRTP master salt is already present
in the TGK or TEK Key Data sub-payload. The EKT_Key sub-payload
MUST contain a SRTP master salt, if the SRTP master salt is not
already present in the TGK or TEK Key Data sub-payload.
KV Data: length determined by Key Data Length field.
5.2. Offer/Answer considerations
This section describes Offer/Answer considerations for the use of EKT
together with MIKEY for unicast streams. The offerer and answerer
MUST follow the procedures specified in [RFC3830] and [RFC4567] as
well as the following ones.
5.2.1. Generating the Initial Offer
If it is intended to use MIKEY together with EKT, the offerer MUST
include at least one MIKEY key-mgmt attribute with one EKT_Key Key
Data sub-payload and the EKT_Cipher Security Policy payload. MIKEY
can be used on session or media level. On session level, MIKEY
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provides the keys for multiple SRTP sessions in the SDP offer. The
EKT SPI references a EKT parameter set including the Secure RTP
parameters as specified in Section 8.2 in [RFC3711]. If MIKEY is
used on session level, it is only possible to use one EKT SPI value.
Therefore, the session-level MIKEY message MUST contain one SRTP
Security Policy payload only, which is valid for all related SRTP
media lines. If MIKEY is used on media level, different SRTP
Security Policy parameters (and consequently different EKT SPI
values) can be used for each media line. If MIKEY is used on session
and media level, the media level content overrides the session level
content.
EKT requires a single shared SRTP master salt between all
participants in the direct SRTP session. If a MIKEY key-mgmt
attribute contains more than one TGK or TEK Key Data sub-payload, all
the sub-payloads MUST contain the same master salt value.
Consequently, the EKT_Key Key Data sub-payload MAY also contain the
same salt or MAY omit the salt value. If the SRTP master salt is not
present in the TGK and TEK Key Data sub-payloads, the EKT_Key sub-
payload MUST contain a master salt.
5.2.2. Generating the Initial Answer
For each media line in the offer using MIKEY, provided on session and
/or on media level, the answerer examines the related MIKEY key-mgmt
attributes for the presence of EKT parameters. In order to accept
the offered key-mgmt attribute, the MIKEY message MUST contain one
EKT_Key Key Data sub-payload and the EKT_Cipher Security Policy
payload. The answerer examines also the existence of a SRTP master
salt in the TGK/TEK and/or the EKT_Key sub-payloads. If multiple
salts are available, all values MUST be equal. If the salt values
differ or no salt is present, the key-mgmt attribute MUST be
considered as invalid.
The MIKEY responder message in the SDP answer does not contain a
MIKEY KEMAC or Security Policy payload and consequently does not
contain any EKT parameters. If a key-mgmt attribute for a media line
was accepted by the answerer, the EKT parameter set of the offerer is
valid for both directions of the SRTP session.
5.2.3. Processing the Initial Answer
On reception of the answer, the offerer examines if EKT has been
accepted for the offered media lines. If a MIKEY key-mgmt attribute
is received containing a valid MIKEY responder message, EKT has been
successfully negotiated. On receipt of a MIKEY error message, EKT
negotiation has failed. For example, this may happen if an EKT
extended MIKEY initiator message is sent to a MIKEY entity not
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supporting EKT. A MIKEY error code 'Invalid SP' or 'Invalid DT' is
returned to indicate that the EKT_Cipher Security Policy payload or
the EKT_Key sub-payload is not supported. In this case, the offerer
may send a second SDP offer with a MIKEY key-mgmt attribute without
the additional EKT extensions.
This behavior can be improved by defining an additional key-mgmt
prtcl-id value 'mikeyekt' and offering two key-mgmt SDP attributes.
One attribute offers MIKEY with SRTP and EKT and the other attribute
offers MIKEY with SRTP without EKT.
5.2.4. Modifying the Session
Once an SRTP stream has been established, a new offer/answer exchange
can modify the session including the EKT parameters. If the EKT key
or EKT cipher is modified (i.e., a new EKT parameter set is created)
the offerer MUST also provide a new EKT SPI value. The offerer MUST
NOT remap an existing EKT SPI value to a new EKT parameter set.
Similar, a modification of the SRTP Security Policy leads to a new
EKT parameter set and requires a fresh EKT SPI, even if the EKT key
or cipher did not change.
Once EKT is being used, the SRTP master salt for the SRTP session
MUST NOT be changed. The salt in the Key Data sub-payloads within
the subsequent offers MUST be the same as the one already used.
After EKT has been successfully negotiated for a session and an SRTP
master key has been transported by EKT, it is difficult to switch
back to a pure MIKEY based key exchange in a synchronized way.
Therefore, once EKT is being used for a session, EKT MUST be used
also in all subsequent offer/answer exchanges for that session.
6. Using EKT for interoperability between key management systems
A media gateway (MGW) can provide interoperability between an SRTP-
EKT endpoint and a non-EKT SRTP endpoint. When doing this function,
the MGW can perform non-cryptographic transformations on SRTP packets
outlined above. However, there are some uses of cryptography that
will be required for that gateway. If a new SRTP master key is
communicated to the MGW (via EKT from the EKT leg, or via Security
Descriptions without EKT from the Security Descriptions leg), the MGW
needs to convert that information for the other leg, and that process
will incur some cryptographic operations. Specifically, if the new
key arrived via EKT, the key must be decrypted and then sent in
Security Descriptions (e.g., as a SIP re-INVITE); likewise, if a new
key arrives via Security Descriptions that must be encrypted via EKT
and sent in SRTP/SRTCP.
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Additional non-normative information can be found in Appendix A.
7. Design Rationale
From [RFC3550], a primary function of RTCP is to carry the CNAME, a
"persistent transport-level identifier for an RTP source" since
"receivers require the CNAME to keep track of each participant." EKT
works in much the same way but uses SRTP to carry information needed
for the proper processing of the SRTP traffic.
With EKT, SRTP gains the ability to synchronize the creation of
cryptographic contexts across all of the participants in a single
session. This feature provides some, but not all, of the
functionality that is present in IKE phase two (but not phase one).
Importantly, EKT does not provide a way to indicate SRTP options.
With EKT, external signaling mechanisms provide the SRTP options and
the EKT Key, but need not provide the key(s) for each individual SRTP
source. EKT provides a separation between the signaling mechanisms
and the details of SRTP. The signaling system need not coordinate
all SRTP streams, nor predict in advance how many sources will be
present, nor communicate SRTP-level information (e.g., rollover
counters) of current sessions.
EKT is especially useful for multi-party sessions, and for the case
where multiple RTP sessions are sent to the same destination
transport address (see the example in the definition of "RTP session"
in [RFC3550]). A SIP offer that is forked in parallel (sent to
multiple endpoints at the same time) can cause multiple RTP sessions
to be sent to the same transport address, making EKT useful for use
with SIP.
EKT can also be used in conjunction with a scalable group-key
management system like GDOI [RFC6407]. In such a combination GDOI
would provide a secure entity authentication method for group
members, and a scalable way to revoke group membership; by itself,
EKT does not attempt to provide either capability.
EKT carries the encrypted key in a new SRTP field (at the end of the
SRTP packet). This maintains compatibility with the existing SRTP
specification by defining a new crypto function that incorporates the
encrypted key, and a new authentication transform to provide implicit
authentication of the encrypted key.
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The main motivation for the use of the variable-length EKT format is
bandwidth conservation. When EKT is sent over SRTP, there will be a
loss of (usable) bandwidth due to the additional EKT bytes in each
RTP packet. For some applications, this bandwidth loss is
significant.
7.1. Alternatives
In its current design, EKT requires that the Master Salt be
established out of band. That requirement is undesirable. In an
offer/answer environment, it forces the answerer to re-use the same
Master Salt value used by the offerer. The Master Salt value could
be carried in EKT packets though that would consume yet more
bandwidth.
In some scenarios, two SRTP sessions may be combined into a single
session. When using EKT in such sessions, it is desirable to have an
SPI value that is larger than 15 bits, so that collisions between SPI
values in use in the two different sessions are unlikely (since each
collision would confuse the members of one of the sessions).
An alternative that addresses both of these needs is as follows: the
SPI value can be lengthed from 15 bits to 63 bits, and the Master
Salt can be identical to, or constructed from, the SPI value. SRTP
conventionally uses a 14-byte Master Salt, but shorter values are
acceptable. This alternative would add six bytes to each EKT packet;
that overhead may be a reasonable tradeoff for addressing the
problems outlined above. This is considered too high a bandwidth
penalty.
8. Security Considerations
EKT inherits the security properties of the SRTP keying it uses:
Security Descriptions, DTLS-SRTP, or MIKEY.
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 in the absence of out-of-band
coordination of SSRCs, and even when SSRC values collide.
The EKT Cipher includes its own authentication/integrity check. For
an attacker to successfully forge a full EKT packet, it would need to
defeat the authentication mechanisms of both the EKT Cipher and the
SRTP authentication mechanism.
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The presence of the SSRC in the EKT_Plaintext ensures that an
attacker cannot substitute an EKT_Ciphertext from one SRTP stream
into another SRTP stream.
An attacker who strips a Full_EKT_Field from an SRTP packet may
prevent the intended receiver of that packet from being able to
decrypt it. This is a minor denial of service vulnerability.
Similarly, an attacker who adds a Full_EKT_Field can disrupt service.
An attacker could send packets containing either Short EKT Field or
Full EKT Field, in an attempt to consume additional CPU resources of
the receiving system. In the case of the Short EKT Field, this field
is stripped and normal SRTP or SRTCP processing is performed. In the
case of the Full EKT Field, the attacker would have to have guessed
or otherwise determined the SPI being used by the receiving system.
If an invalid SPI is provided by the attacker, processing stops. If
a valid SPI is provided by the attacker, the receiving system will
decrypt the EKT ciphertext and return an authentication failure (Step
3 of Section 2.2.2).
EKT can rekey an SRTP stream until the SRTP rollover counter (ROC)
needs to roll over. EKT does not extend SRTP's rollover counter
(ROC), and like SRTP itself EKT cannot properly handle a ROC
rollover. Thus even if using EKT, new (master or session) keys need
to be established after 2^48 packets are transmitted in a single SRTP
stream as described in Section 3.3.1 of [RFC3711]. Due to the
relatively low packet rates of typical RTP sessions, this is not
expected to be a burden.
The confidentiality, integrity, and authentication of the EKT cipher
MUST be at least as strong as the SRTP cipher.
Part of the EKT_Plaintext 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. 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.
9. IANA Considerations
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IANA is requested to register EKT from Section 3.9 into the Session
Description Protocol (SDP) Security Descriptions [iana-sdp-sdesc]
registry for "SRTP Session Parameters".
IANA is requested to register the following new attributes into the
SDP Attributes registry [iana-sdp-attr].
Attribute name: dtls-srtp-ekt
Long form name: DTLS-SRTP with EKT
Type of attribute: Media-level
Subject to charset: No
Purpose: Indicates support for DTLS-SRTP with EKT
Appropriate values: No values
Contact name: Dan Wing, dwing@cisco.com
We request the following IANA assignments from the existing
[iana-mikey] name spaces in the IETF consensus range (0-240)
[RFC3830]:
o From the Key Data payload name spaces, a value to indicate the
type as the 'EKT_Key'.
o From the Security Policy table name space, a new value to be
assigned for 'EKT' (see Figure 9).
Furthermore, we need the following two new IANA registries created,
populated with the initial values in this document. New values for
both of these registries can be defined via Specification Required
[RFC5226].
o EKT parameter type, initially populated with the list from Figure
10
o EKT cipher, initially populated with the list from Figure 11
10. Acknowledgements
Thanks to Lakshminath Dondeti for assistance with earlier versions of
this document. Thanks to Kai Fischer for writing the MIKEY section.
Thanks to Nermeen Ismail, Eddy Lem, and Rob Raymond for fruitful
discussions and comments. Thanks to Felix Wyss for his review and
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comments regarding ciphers. Thanks to Michael Peck for his review.
Thanks to John Mattsson for his detailed security review highlighting
the duplicative interaction between SRTP MKI with EKT ISN and
encourged the EKT specification to prohibit sharing SRTP master keys.
Thanks to Magnus Westerlund for his review.
11. References
11.1. Normative References
[FIPS197] National Institute of Standards and Technology (NIST),
"The Advanced Encryption Standard (AES)", FIPS-197 Federal
Information Processing Standard, November 2001.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4563] Carrara, E., Lehtovirta, V., and K. Norrman, "The Key ID
Information Type for the General Extension Payload in
Multimedia Internet KEYing (MIKEY)", RFC 4563, June 2006.
[RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session
Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", RFC 4567, July 2006.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[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, May 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
11.2. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure Real-
time Transport Protocol (SRTP)", RFC 4771, January 2007.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
September 2009.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, October 2011.
[iana-mikey]
IANA, ., "Multimedia Internet KEYing (Mikey) Payload Name
Spaces", 2011, <http://www.iana.org/assignments/mikey-
payloads/mikey-payloads.xhtml>.
[iana-sdp-attr]
IANA, ., "SDP Parameters", 2011, <http://www.iana.org/
assignments/sdp-parameters/sdp-parameters.xml>.
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[iana-sdp-sdesc]
IANA, ., "Session Description Protocol (SDP) Security
Descriptions: SRTP Session Parameters", 2011, <http://
www.iana.org/assignments/sdp-security-descriptions/sdp-
security-descriptions.xml#sdp-security-descriptions-4>.
Appendix A. Using EKT to Optimize Interworking DTLS-SRTP with Security
Descriptions
Today, SDP Security Descriptions [RFC4568] is used for distributing
SRTP keys in several different IP PBX systems. The IP PBX systems
are typically used within a single enterprise. A Session Border
Controller is a reasonable solution to interwork between Security
Descriptions in one network and DTLS-SRTP in another network. For
example, a mobile operator (or an Enterprise) could operate Security
Descriptions within their network and DTLS-SRTP towards the Internet.
However, due to the way Security Descriptions and DTLS-SRTP manage
their SRTP keys, such an SBC has to authenticate, decrypt, re-
encrypt, and re-authenticate the SRTP (and SRTCP) packets in one
direction, as shown in Figure 13, below. This is computationally
expensive.
RFC4568 endpoint SBC DTLS-SRTP endpoint
| | |
1. |---key=A------------->| |
2. | |<-DTLS-SRTP handshake->|
3. |<--key=B--------------| |
4. | |<--SRTP, encrypted w/B-|
5. |<-SRTP, encrypted w/B-| |
6. |-SRTP, encrypted w/A->| |
7. | (decrypt, re-encrypt) |
8. | |-SRTP, encrypted w/C-->|
| | |
Figure 13: Interworking Security Descriptions and DTLS-SRTP
The message flow is as follows (similar steps occur with SRTCP):
1. The Security Descriptions [RFC4568] endpoint discloses its SRTP
key to the SBC, using a=crypto in its SDP.
2. SBC completes DTLS-SRTP handshake. From this handshake, the SBC
derives the SRTP key for traffic from the DTLS-SRTP endpoint (key
B) and to the DTLS-SRTP endpoint (key C).
3. The SBC communicates the SRTP encryption key (key B) to the
Security Descriptions endpoint (using a=crypto). (There is no
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way, with DTLS-SRTP, to communicate the Security Descriptions key
to the DTLS-SRTP key endpoint.)
4. The DTLS-SRTP endpoint sends an SRTP key, encrypted with its key
B. This is received by the SBC.
5. The received SRTP packet is simply forwarded; the SBC does not
need to do anything with this packet as its key (key B) was
already communicated in step 3.
6. The Security Descriptions endpoint sends an SRTP packet,
encrypted with its key A.
7. The SBC has to authenticate and decrypt the SRTP packet (using
key A), and re-encrypt it and generate an HMAC (using key C).
8. The SBC sends the new SRTP packet.
If EKT is deployed on the DTLS-SRTP endpoints, EKT helps to avoid the
computationally expensive operation so the SBC does not need to
perform any per-packet operations on the SRTP (or SRTCP) packets in
either direction. With EKT the SBC can simply forward the SRTP (and
SRTCP) packets in both directions without per-packet HMAC or
cryptographic operations.
To accomplish this interworking, DTLS-SRTP EKT must be supported on
the DTLS-SRTP endpoint, which allows the SBC to transport the
Security Description key to the EKT endpoint and send the DTLS-SRTP
key to the Security Descriptions endpoint. This works equally well
for both incoming and outgoing calls. An abbreviated message flow is
shown in Figure 14, below.
RFC4568 endpoint SBC DTLS-SRTP endpoint
| | |
1. |---key=A------------->| |
2. | |<-DTLS-SRTP handshake->|
3. |<--key=B--------------| |
4. | |--ekt:A--------------->|
5. | |<--SRTP, encrypted w/B-|
5. |<-SRTP, encrypted w/B-| |
6. |-SRTP, encrypted w/A->| |
7. | |-SRTP, encrypted w/A-->|
| | |
Figure 14: Interworking Security Descriptions and EKT
The message flow is as follows (similar steps occur with SRTCP):
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1. Security Descriptions endpoint discloses its SRTP key to the SBC
(a=crypto).
2. SBC completes DTLS-SRTP handshake. From this handshake, the SBC
derives the SRTP key for traffic from the DTLS-SRTP endpoint (key
B) and to the DTLS-SRTP endpoint (key C).
3. The SBC communicates the SRTP encryption key (key B) to the
Security Descriptions endpoint.
4. The SBC sends an EKT packet indicating that SRTP will be
encrypted with 'key A' towards the DTLS-SRTP endpoint.
5. The DTLS-SRTP endpoint sends an SRTP key, encrypted with its key
B. This is received by the SBC.
6. The received SRTP packet is simply forwarded; the SBC does not
need to do anything with this packet as its key (key B) was
communicated in step 3.
7. The Security Descriptions endpoint sends an SRTP packet,
encrypted with its key A.
8. The received SRTP packet is simply forwarded; the SBC does not
need to do anything with this packet as its key (key A) was
communicated in step 4.
Authors' Addresses
David A. McGrew
Cisco Systems, Inc.
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, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
US
Phone: (408) 853 4197
Email: dwing@cisco.com
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Flemming Andreason
Cisco Systems, Inc.
499 Thornall Street
Edison, NJ 08837
US
Email: fandreas@cisco.com
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