Network Working Group D. McGrew
Internet-Draft F. Andreasen
Intended status: Standards Track Cisco Systems, Inc.
Expires: September 5, 2007 L. Dondeti
QUALCOMM
March 4, 2007
Encrypted Key Transport for Secure RTP
draft-mcgrew-srtp-ekt-02.txt
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Abstract
SRTP Encrypted Key Transport (EKT) is an extension to SRTP that
provides for the secure transport of SRTP master keys, Rollover
Counters, and other information, within SRTCP. This facility enables
SRTP to work for decentralized conferences with minimal control, and
to handle situations caused by SIP forking and early media.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions Used In This Document . . . . . . . . . . . . 5
2. Encrypted Key Transport . . . . . . . . . . . . . . . . . . . 6
2.1. Authentication Tag Format . . . . . . . . . . . . . . . . 6
2.2. Packet Processing and State Machine . . . . . . . . . . 8
2.2.1. Outbound (Tag Generation) . . . . . . . . . . . . . . 8
2.2.2. Inbound (Tag Verification) . . . . . . . . . . . . . . 9
2.3. Ciphers . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.1. The Default Cipher . . . . . . . . . . . . . . . . . . 13
2.3.2. The AES Key Wrap Cipher . . . . . . . . . . . . . . . 13
2.3.3. Other EKT Ciphers . . . . . . . . . . . . . . . . . . 14
2.4. Synchronizing Operation . . . . . . . . . . . . . . . . 14
2.5. Timing and Reliability Consideration . . . . . . . . . . . 15
3. EKT and SDP Security Descriptions . . . . . . . . . . . . . . 16
3.1. SDP Security Descriptions Recap . . . . . . . . . . . . . 16
3.2. Relationship between EKT and SDP Security Descriptions . . 17
3.3. Overview of Combined EKT and SDP Security Description
Operation . . . . . . . . . . . . . . . . . . . . . . . . 19
3.4. EKT Extensions to SDP Security Descriptions . . . . . . . 19
3.4.1. EKT_Cipher . . . . . . . . . . . . . . . . . . . . . . 19
3.4.2. EKT_Key . . . . . . . . . . . . . . . . . . . . . . . 20
3.4.3. EKT_SPI . . . . . . . . . . . . . . . . . . . . . . . 20
3.5. Offer/Answer Procedures . . . . . . . . . . . . . . . . . 20
3.5.1. Generating the Initial Offer - Unicast Streams . . . . 20
3.5.2. Generating the Initial Answer - Unicast Streams . . . 22
3.5.3. Processing of the Initial Answer - Unicast Streams . . 23
3.6. SRTP-Specific Use Outside Offer/Answer . . . . . . . . . . 24
3.7. Modifying the Session . . . . . . . . . . . . . . . . . . 24
3.8. Backwards Compatibility Considerations . . . . . . . . . . 24
3.9. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 25
4. Use of EKT with MIKEY . . . . . . . . . . . . . . . . . . . . 26
4.1. EKT transform attribute mapping in MIKEY . . . . . . . . . 26
5. Design Rationale . . . . . . . . . . . . . . . . . . . . . . . 29
6. RTP Transport . . . . . . . . . . . . . . . . . . . . . . . . 31
7. Security Considerations . . . . . . . . . . . . . . . . . . . 32
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
10.1. Normative References . . . . . . . . . . . . . . . . . . . 35
10.2. Informative References . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
Intellectual Property and Copyright Statements . . . . . . . . . . 38
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1. Introduction
RTP is designed to allow decentralized groups with minimal control to
establish sessions, e.g. 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, the
SRTP rollover counter (ROC) of each SRTP source in the session needs
to be provided to that participant.
The inability of SRTP to work in the absence of central control was
well understood during the design of that protocol; that 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 signaling control that is needed in an SRTP session. EKT
securely distributes the SRTP master key and other information for
each SRTP source, using SRTCP to 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 with new SRTP master keys (see
Section 2.2) within a session without coordinating with other
entities via 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 to replace an old SRTP source that has
reached the packet-count limit. 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 simplify many architectures that
implement SRTP.
EKT provides a way for an SRTP session participant, either 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.
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EKT does not control the manner in which the SSRC and master key are
generated; it is concerned only 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 [SDES] or MIKEY [RFC3830].
Instead, it is used in conjunction with those methods, and it
relieves them of the burden of tightly coordinating every SRTP source
among every SRTP participant.
This document is organized as follows. A complete normative
definition of EKT is provided in Section 2. It consists of packet
processing algorithms (Section 2.2) and cryptographic definitions
(Section 2.3) . In Section 3, the use of EKT with SDP Security
Descriptions is defined. In Section 4 we outline the use of EKT with
MIKEY. Section 5 provides a design rationale. Security
Considerations are provided in Section 7, and IANA considerations are
provided in Section 8.
1.1. 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].
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2. Encrypted Key Transport
In EKT, an SRTP master key is encrypted with a Key Encrypting Key
(KEK), and the resulting ciphertext is transported in every SRTCP
packet. A single KEK suffices for a single SRTP session, regardless
of the number of participants in the session. However, there can be
multiple KEKs used within a particular session.
In order to convey the ciphertext of the SRTP master key, and other
additional information, the SRTCP Authentication Tag field is
subdivided as shown in Figure 1. EKT defines a new SRTCP
authentication function, which uses this format. It incorporates a
conventional SRTCP authentication function, which is called the base
authentication function in this specification. Any SRTCP
authentication function, such as the default one of HMAC-SHA1 with a
160-bit key and an 80-bit authentication tag, can be used as a base
authentication function. EKT also defines a new method of providing
SRTP master keys to an endpoint.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Base Authentication Tag :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Encrypted Master Key :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rollover Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Sequence Number | Security Parameter Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: The EKT Authentication Tag format.
2.1. Authentication Tag Format
The Authentication Tag field contains the following sub-fields:
Base Authentication Tag This field contains the authentication tag
computed by the base authentication function. The value of this
field is used to check the authenticity of the packet.
Encrypted Master Key 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 is inferable
from that field. The Encrypted Master Key field is included
outside of the authenticated portion of the SRTCP packet,
immediately following the Authentication Tag. It contains the
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ciphertext value resulting from the encryption of the SRTP master
key corresponding to the SSRC contained in the packet. The
encryption and decryption of this value is done using a cipher as
defined in Section 2.3.
Rollover Counter The length of this field is fixed at 32 bits. This
field is set to the current value of the SRTP rollover counter in
the SRTP context associated with the SSRC in the SRTCP packet.
This field immediately follows the Encrypted Master Key field.
Initial Sequence Number (ISN) The length of this field is fixed at
16 bits. If this field is nonzero, then 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 Encrypted
Master Key field of this packet. If this field is zero, it
indicates that the initial RTP 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. The ISN
field follows the Rollover Counter field.
Security Parameter Index (SPI) The length of this field is fixed at
16 bits. This field indicates the appropriate Key Encrypting 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 parameters
that are identified by this field are:
The Key Encrypting 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.
Together, these 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. The SPI field follows the Initial Sequence Number.
Since it is the last field in the packet, and has a fixed length,
it is always possible to unambiguously parse this field.
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2.2. Packet Processing and State Machine
At any given time, each SRTP/SRTCP source has associated with it a
single EKT parameter set. This parameter set is used to process all
outbound packets, and is called the outbound parameter set. There
may be other EKT parameter sets that are used by other SRTP/SRTCP
sources in the same session. All of these EKT parameter sets SHOULD
be stored by all of the participants in an SRTP session, for use in
processing inbound SRTCP traffic.
We next review the SRTCP authentication method and show how the EKT
authentication method is built on top of the base method. An SRTCP
authentication method consists of a tag-generation function and a
verification function. The tag-generation function takes as input a
secret key, the data to be authenticated, and the SRTCP packet index.
It provides an authentication tag as its sole output, and is used in
the processing of outbound packets. The verification function takes
as input a secret key, the data to be authenticated, the SRTCP packet
index, and the authentication tag. It returns an indication of
whether or not the data, index, and tag are valid or not. It is used
in the processing of inbound packets. EKT defines a tag-generation
function in terms of the base tag-generation function, and defines a
verification function in terms of the base verification function.
The tag-generation function is used to process outbound packets, and
the verification function is used to process inbound packets.
2.2.1. Outbound (Tag Generation)
When an SRTCP packet needs to be sent, the EKT tag generation
function works as follows. Given an RTCP packet, the Rollover
Counter field in the SRTCP 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
RTP sequence number of the initial RTP packet that was or will be
protected by that key. When the SRTP master key corresponding to a
source is changed, the new key SHOULD be communicated in advance via
EKT. (Note that the ISN field allows the receiver to know when it
should start using the new key to process SRTP packets.) This
enables the rekeying event to be communicated before any RTP packets
are protected with the new key. 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).
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The Security Parameter Index field is set to the value of the
Security Parameter Index that is associated with the outbound
parameter set.
The Encrypted Master Key field is set to the ciphertext created by
encrypting the SRTP master key with the EKT cipher, using the KEK 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.
2.2.1.1. Base Authentication Tag
The Base Authentication Tag field is computed using the base tag-
generation function as follows. It can only be computed after all of
the other fields have been set. The authenticated input consists of
the following elements, in order:
the SRTCP authenticated portion,
a string of zero bits whose length exactly matches that of the
Base Authentication Tag field,
the Encrypted Master Key field,
the Rollover Counter field,
the Initial Sequence Number field, and
the Security Parameter Index field.
Implementation note: the string of zero bits is included in the
authenticated input in order to allow implementations to compute
the base authentication tag using a single pass of the base
authentication function. Implementations MAY write zeros into the
Base Authentication Tag field prior to computing that function, on
the sending side.
2.2.2. Inbound (Tag Verification)
The EKT verification function proceeds as follows (see Figure 2), or
uses an equivalent set of steps. Recall that the verification
function is a component of SRTCP processing. When a packet does not
pass the verification step, the function indicates this fact to the
SRTCP packet processing function when it returns control to that
function.
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1. The Security Parameter Index field is checked to determine which
EKT parameter set should be used when processing the packet. If
multiple parameter sets been defined for the SRTP session, then
the one that is associated with the Security Parameter Index
value that matches the Security Parameter Index 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 authentication
failure, and the steps described below are not performed.
2. The Encrypted Master Key field is decrypted using the EKT
cipher's decryption function. That field is used as the
ciphertext input, and the Key Encrypting Key in the matching
parameter set is used as the decryption key. The decryption
process is detailed in Section 2.3. The plaintext resulting from
this decryption is provisionally accepted as the SRTP master key
corresponding to the SSRC in the packet. If an MKI is present in
the packet, then the provisional key corresponds to the
particular SSRC and MKI combination. A provisional key MUST be
used only to process one single packet. A provisional SRTCP
authentication key is generated from the provisional master key,
and the SRTP master salt from the matching parameter set, using
the SRTP key derivation algorithm (see Section 4.3 of [RFC3711]).
3. An authentication check is performed on the packet, using the
provisional SRTCP authentication key. This key is provided to
the base SRTCP authentication function (see Figure 2), which is
evaluated as described in Section 2.2.1.1. If the Base
Authentication Tag field matches the tag computed by the base
authentication function, then the packet passes the check.
Implementation note: an SRTP receiver MAY copy the Base
Authentication Tag field into temporary storage, then write
zeros into that field, prior to computing the base
authentication tag value. This step allows the base
authentication function to be computed in a single pass.
4. If the base authentication check using the provisional key fails,
then the provisional key MUST be discarded and it MUST NOT affect
any subsequent processing. The verification function MUST return
an indication of authentication failure, and the steps described
below are not performed.
5. Otherwise, if the base authentication check is passed, the
provisional key is also accepted as the SRTP master key
corresponding to the SRTP source that sent the packet. If an MKI
is present in the packet, then the master key corresponds to the
particular SSRC and MKI combination. If there is no SRTP crypto
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context corresponding to the SSRC in the packet, then a new
crypto context is created. The rollover counter in the context
is set to the value of the Rollover Counter field.
6. 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, the SSRC from the packet, and the MKI
from the packet, if one is present, are stored in the crypto
context associated with the SRTP source. 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.
8. The verification function then returns an indication that the
packet passed the verification.
Implementation note: the value of the Encrypted Master Key
field is identical in successive packets protected by the same
KEK and SRTP master key. This value MAY be cached by an SRTP
receiver to minimize computational effort, by allowing it to
recognize when the SRTP master key is unchanged, and thus
avoid repeating Steps 2, 6, and 7.
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+------- Encrypted Master Key
|
v
+------------+
| Decryption |
| Function |<-------------------------- Key Encrypting Key
+------------+
| +----------------+ EKT
+--------+-- provisional ---->| SRTCP |<-- master
| master key | Key Derivation | salt
| +----------------+
| |
| provisional SRTCP authentication key
| |
| v
| +----------------+
| authenticated portion --> | Base SRTCP |
| authentication tag -----> | Verification |
| +----------------+
| |
| +----------------+ +---+
| | return FAIL |<- FAIL -| ? |
| +----------------+ +---+
| |
| +----------------+ |
+------->| set master key,|<- PASS ---+
| ROC, and MKI |
+----------------+
|
v
+----------------+
| return PASS |
+----------------+
Figure 2: EKT inbound processing.
2.3. Ciphers
EKT uses a cipher to encrypt the SRTP master keys. 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. This cipher 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. call signaling).
An EKT cipher consists of an encryption function and a decryption
function. The encryption function E(K, P) takes the following
inputs:
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a secret key K with a length of L bytes, and
a plaintext value P with a length of M bytes.
The encryption function returns a ciphertext value C whose length is
N bytes, where N is at least M. The decryption function D(K, C) takes
the following inputs:
a secret key K with a length of L bytes, and
a ciphertext value C with a length of N bytes.
The decryption function returns a plaintext value P that is M bytes
long. These functions have the property that D(K, E(K, P)) = P for
all values of K and P. Each cipher also has a limit T on the number
of times that it can be used with any fixed key value. 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.
An EKT cipher MUST resist attacks in which both ciphertexts and
plaintexts can be adaptively chosen. For each randomly chosen key,
the encryption and decryption functions cannot be distinguished from
a random permutation and its inverse with non-negligible advantage.
This must be true even for adversaries that can query both the
encryption and decryption functions adaptively. The advantage is
defined as the difference between the probability that the adversary
will identify the cipher as such and the probability that the
adversary will identify the random permutation as the cipher, when
each case is equally likely.
2.3.1. The Default Cipher
The default cipher is the Advanced Encryption Standard (AES)
[FIPS197] with 128-bit keys, in Electronic Codebook (ECB) Mode. Its
parameters are fixed at L=16, M=16, and T=2^48. Note that M matches
the size of the SRTP master keys used by the default SRTP key
derivation function; if an SRTP cipher with a different SRTP master
key length is to be used with EKT, then another EKT cipher must be
used. ECB is the simplest mode of operation of a block cipher, in
which the block cipher is used in its raw form.
2.3.2. The AES Key Wrap Cipher
The AES Key Wrap [RFC3394] defines a cipher that can be used with
plaintexts larger than 16 bytes in length. It requires a plaintext
length M that is a multiple of eight bytes, and it returns a
ciphertext with a length of N = M + 8 bytes. It can be used with key
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sizes of L = 16, 24, and 32. The key size determines the length of
the AES key used by the Key Wrap algorithm. With this cipher,
T=2^48.
2.3.3. Other EKT Ciphers
Other specification MAY extend this one by defining other EKT
ciphers. This section defines how those ciphers interact with this
specification.
An EKT cipher determines how the Encrypted Master Key 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
EKT is 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
out-of-order. This can lead to various race conditions, which we
review below.
When joining an SRTP session, SRTP packets may be received before any
SRTCP (EKT) packets, which implies the crypto context has not been
established, unless other external signaling mechanism has done so.
Rather than automatically discarding such SRTP packets, the receiver
MAY want to provisionally place them in a jitter buffer and delay
discarding them until playout time.
When an SRTP source using EKT performs a rekeying operation, there is
a race between the actual rekeying signaled via SRTCP and the SRTP
packets secured by the new keying material. If the SRTP packets are
received first, they will fail authentication; alternatively, if
authentication is not being used, they will decrypt to unintelligible
random-looking plaintext. (Note, however, that [RFC3711] says that
SRTP "SHOULD NOT be used without message authentication".) In order
to address this problem, the rekeying event can be sent before
packets using the new SRTP master key are sent (by use of the ISN
field). Another solution involves using an MKI at the expense of
added overhead in each SRTP packet. Alternatively, receivers MAY
want to delay discarding packets from known SSRCs that fail
authentication in anticipation of receiving a rekeying event via EKT
(SRTCP) shortly.
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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 [SRTP] Section 3.3.1.
2.5. Timing and Reliability Consideration
SRTCP communicates the master key and ROC for the SRTP session.
Thus, as explained above, if SRTP packets are received prior to the
corresponding SRTCP (EKT) packet, a race condition occurs. From an
EKT point of view, it is therefore desirable for an SRTP sender to
send an SRTCP packet as soon as possible, and in no case any later
than when the initial SRTP packet is sent. SRTCP however MUST obey
the timing rules associated with the profile under which it runs
(e.g. RTP/SAVP or RTP/SAVPF). Subject to that constraint, SRTP
senders SHOULD send an SRTCP packet as soon as possible after joining
a session. Note that there is no need for SRTP receivers to do so.
Also note, that per RFC 3550, Section 6.2, it is permissible to send
a compound RTCP packet immediately after joining a unicast session
(but not a multicast session).
SRTCP is not reliable and hence SRTCP packets may be lost. This is
obviously a problem for endpoints joining an SRTP session and
receiving SRTP traffic (as opposed to SRTCP), or for endpoints
receiving SRTP traffic following a rekeying event. To reduce the
impact of lost packets, SRTP senders SHOULD send SRTCP packets as
often as allowed by the profile under which they operate.
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3. EKT and SDP Security Descriptions
The SDP Security Descriptions (SDES) [SDES] specification defines a
generic framework for negotiating security parameters for media
streams negotiated via the Session Description Protocol by use of a
new SDP "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 SDP Security Descriptions
contains a tag followed by three types of parameters (refer to [SDES]
for details):
Crypto-suite. Identifies the encryption and authentication
transform
Key parameters. SRTP keying material and parameters.
Session parameters. Additional (optional) SRTP parameters such as
Key Derivation Rate, Forward Error Correction Order, use of
unencrypted SRTP, etc.
The crypto attributes in the example SDP in Figure 3 illustrate these
parameters.
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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 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=crypto:2 F8_128_HMAC_SHA1_80
inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4;
inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4
FEC_ORDER=FEC_SRTP
Figure 3: SDP Security Descriptions example. Line breaks are
included for formatting purposes only.
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, the master key lifetime (number of packets), and
the (optional) Master Key Identifier (MKI) whose value is "1" and has
a byte length of "4" in the SRTP 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" and uses the crypto-suite
"F8_128_HMAC_SHA1_80". It includes two SRTP master keys and
associated salts. The first one is used with the MKI value 1,
whereas the second one is used with the MKI value 2. 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 [SDES] 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, SDP
Security Descriptions (SDES) also defines how to use it specifically
to negotiate security parameters for Secure RTP.
EKT and SDES are complementary. SDP Security Descriptions can
negotiate several of the SRTP security parameters (e.g. cipher and
use of Master Key Identifier/MKI) as well as SRTP master keys. SDP
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Security Descriptions however does not negotiate SSRCs and their
associated Rollover Counter (ROC). Instead, SDES 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
established for a while and hence has a non-zero ROC. 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. SDP
Security Descriptions 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). SDP Security Descriptions 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 SDES
provides a better solution to SRTP negotiation for offer/answer than
either of them alone. SDES negotiates the various SRTP crypto
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parameters (which EKT does not), whereas EKT addresses the
shortcomings of SDES.
3.3. Overview of Combined EKT and SDP Security Description Operation
We define three session extension parameters to SDES to communicate
the EKT cipher, EKT key, and Security Parameter Index to the peer.
The original SDES parameters are used as defined in [SDES], however
the procedures associated with the SRTP master key differ slightly,
since both SDES and EKT communicate an SRTP master key. In
particular, the SRTP master key communicated via SDES 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 message from the
peer. Once a valid EKT message is received for the SSRC, the crypto
context is initialized accordingly, and the SRTP master key will then
be derived from the EKT message. 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. Note that with these rules, once a valid EKT message 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 SDES.
3.4. EKT Extensions to SDP Security Descriptions
In order to use EKT and SDES in conjunction, we now define the
following new SDES session parameters, each of which 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, we provide additional detail on each of these attributes.
3.4.1. EKT_Cipher
The (optional) EKT_Cipher parameter parameter defines the EKT cipher
used to encrypt the EKT key with in SRTCP packets. The default value
is "AES_128" in accordance with Section 2.3.1. For the AES Key Wrap
cipher (see Section 2.3.2, 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 in SRTCP packets. The value is base64 encoded (see
[RFC3548], Section 3). When base64 decoding the key, padding
characters (i.e., one or two "=" at the end of the base64 encoded
data) are discarded (see [RFC3548] for details). Base64 encoding
assumes that the base64 encoding input is an integral number of
octets. If a given EKT cipher requires the use of a key with a
length that is not an integral number of octets, said cipher MUST
define a padding scheme that results in the base64 input being an
integral number of octets. For example, if the length defined was
250 bits, then 6 padding bits would be needed, which could be defined
to be the last 6 bits in a 256 bit input. 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 SDES parameters defined in Section
Section 3.4. Since SDES 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 [SDES] Section 7.1.1 as well as the following steps.
For each unicast media line using SDES and where use of EKT is
desired, the offerer MUST include one EKT_Key parameter and one
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EKT_SPI parameter in at least one "crypto" attribute (see [SDES]).
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 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 (incl. EKT_SPI and EKT_KEY) that is already in use.
If an EKT_Cipher other than the default cipher is to be used, then
the EKT_Cipher parameter MUST be included as well.
If a given crypto attribute includes more than one set of SRTP key
parameters (SRTP master key, salt, lifetime, MKI), they MUST all use
the same salt. (EKT requires a single shared salt between all the
participants in the direct SRTP session).
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 an RTP translator, 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.
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3.5.2. Generating the Initial Answer - Unicast Streams
When the initial answer is generated, the answerer MUST follow the
steps defined in [SDES] Section 7.1.2 as well as the following steps.
For each unicast media line using SDES, 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 SDES, the offerer and answerer MUST use the
same SRTP salt. Thus, the SRTP key parameter(s) in the answer crypto
attribute MUST use the same 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.
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3.5.3. Processing of the Initial Answer - Unicast Streams
When the offerer receives the answer, it MUST perform the steps in
[SDES] 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.
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 per
[SDES] Section 5.1.3, 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.
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 [SDES] Section 7.1.3 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 SRTCP packet 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.
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3.6. SRTP-Specific Use Outside Offer/Answer
SDES use for SRTP is not defined outside offer/answer and hence
neither is SDES 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 [SDES] Section 7.1.4 as
well as the following steps. SDES 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 SDES and EKT communicate the SRTP
Master Key with EKT Master Keys taking precedence. Reverting back to
an SDES 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.
Finally, subsequent offer/answer exchanges MUST NOT remap a given SPI
value to a different EKT parameter set until 2^32 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
2^32 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, SDES allows for session parameters to be prefixed with "-" to
indicate that they are optional. If the answerer does not support
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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 SRTCP packets prior
to receipt of such an answer will obviously be problematic (as is
normally the case for SDES without EKT).
Alternatively, SDES 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 Augmented Backus-Naur Form (ABNF) syntax [RFC4234] for the three
new SDES session parameters is as in Figure 4.
EKT_Cipher = "EKT_Cipher=" EKT_Cipher_Name
EKT_Cipher_Name = 1*(ALPHA / DIGIT / "_") ; "AES_128", "AESKW_128"
; "AESKW_192" and "AESKW_256"
; defined in this document.
EKT_Key = 1*(base64) ; See Section 3 in RFC3548
base64 = ALPHA / DIGIT / "+" / "/" / "="
EKT_SPI = 4HEXDIG ; See RFC4234
Figure 4: ABNF for the EKT session parameters.
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4. 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.
A n SRTP sender or a group controller can use MIKEY to establish a
SRTP cryptographic context. This capability includes the
distribution of the TEK or a TEK generation key (TGK) , 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 can be used to generate multiple TEKs from a single
TGK. For group communication, the sender or group controller sends
the same TGK, CSB_ID, and CS_ID to all the members. For interactive
conferencing, each sender distributes the same SRTP crypto context to
the rest of the members.
The MIKEY specification [RFC3830] suggests the use of unicast for
rekeying. This method does not scale well to large groups or
interactive groups. MIKEY also provides a way to provide ROC values
to members when they join the group. It is desirable to not require
the group controller to track the ROC values of each member. For
example, in mobile and wireless environments, members may go in and
out of coverage, and in those cases, key management based ROC
synchronization is not reliable or sufficient. A better alternative
to support ROC synchronization is to send ROC values via SRTP, as EKT
does. A separate SRTP extension is being proposed [RCC] to include
the ROC as part of a modified authentication tag. Unlike EKT, this
extension uses SRTP and not SRTCP as its transport. A new MIKEY
extension [KEYID] specifies the use of MIKEY to update group keys via
multicast or broadcast for 3GPP MBMS networks.
The EKT extension of SRTP/SRTCP provides a combined solution for
rekeying and ROC synchronization. It also offers several advantages.
With EKT, an SRTP session participant can start a new SRTP source
without coordinating with a group controller about the selection of
keys or SSRC values. With EKT, SRTP can handle early media, since
its SPI allows the receiver to identify the cryptographic keys and
parameters used by the source. EKT also allows SRTP to work with SIP
forking.
MIKEY can readily be extended so that it can establish the EKT key,
cipher and SPI values.
4.1. EKT transform attribute mapping in MIKEY
Interactive group communication using MIKEY currently requires each
member to send its own TGK and SSRC information to the other members,
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resulting in O(n^2) MIKEY sessions. That is not desirable. With
EKT, the conference organizer or only one of the members needs to
distribute the EKT parameters to all the members. After that, each
member can distribute its SRTP master key and ROC values using EKT.
Cryptographic policy initially distributed via MIKEY will apply to
all sessions. MIKEY specifies a security policy (SP) payload to
negotiate or distribute SRTP security policy. Policy payload
attributes include Session Encryption key length, Authentication
algorithm, Session Authentication key length, Session Salt key
length, SRTP PRF, Authentication tag length and other fields (see
Section 6.10.1 of RFC 3830).
For the EKT_Cipher parameter, we propose to specify a new SRTP Policy
Type in the Security Policy (SP) payload of MIKEY (see Section 6.10
of RFC 3830). The SP payload contains a number of Policy Param TLVs.
We define Type = TBD: (will be requested from IANA) for EKT_Cipher.
As with other payloads specifying cryptographic algorithms, we only
specify Type and Values only.
EKT_Cipher | Value
-------------------
AES_128 | 0
AESKW_128 | 1
AESKW_192 | 2
AESKW_256 | 3
Figure 5: EKT_Cipher Table
We propose to use the KEMAC payload to transport the two mandatory
EKT parameters: EKT_Key and EKT_SPI MIKEY KEMAC payload, as specified
in RFC 3830 carries the Traffic Encryption Key (TEK) or the TEK
Generation Key (TGK). One or more TEKs or TGKs 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 RFC 3830, 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 len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Key data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Salt len (optional) ! Salt data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! KV data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Key Data Sub-Payload of MIKEY
The Type field, 4 bits in length, indicates the type of key included
in the payload. We define Type = TBD (will be requested from IANA)
to indicate transport of EKT_Key.
KV (4 bits): indicates the type of key validity period specified.
KV=1 is currently specified as an SPI. We propose to use that value
to indicate the KV_data contains the ETK_SPI. KV_data would be 16
octets in length, but it is also possible to interpret the length
from the 'Key data len' field.
KV data MUST NOT be optional when KV = 1.
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5. 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, using SRTCP 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 streams 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. Such a system provides a secure entity
authentication method and a way to revoke group membership, both of
which are out of scope of EKT.
It is natural to use SRTCP to transport encrypted keying material for
SRTP, as it provides a secure control channel for (S)RTP. However,
there are several different places in SRTCP in which the encrypted
SRTP master key and ROC could be conveyed. We briefly review some of
the alternatives in order to motivate the particular choice used in
this specification. One alternative is to have those values carried
as a new SDES item or RTCP packet. This would require that the
normal SRTCP encryption be turned off for the packets containing that
SDES item, since on the receiver's side, SRTCP processing completes
before the RTCP processing starts. This tension between encryption
and the desire for RTCP privacy is highly undesirable. Additionally,
this alternative makes SRTCP dependent upon the parsing of the RTCP
compound packet, which adds complexity. It is simpler to carry the
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encrypted key in a new SRTCP field. One way to do this and to be
backwards compatible with the existing specification is to define a
new crypto function that incorporates the encrypted key. We define a
new authentication transform because EKT relies on the normal SRTCP
authentication to provide implicit authentication of the encrypted
key.
An SRTP packet containing an SSRC that has not been seen will be
discarded. This practice may induce a burst of packet loss at the
outset of an SRTP stream, due to the loss or reorder of the first
SRTCP packet with the EKT containing the key and rollover counter for
that stream. However, this practice matches the conservative RTP
memory-allocation strategy; many existing applications accept this
risk of initial packet loss. Alternatively, implementations may wish
to delay discarding such packets for a short period of time as
described in Section 2.4.
EKT adds eight additional bytes to each SRTCP packet, plus the length
of the Encrypted Master Key field. Using the SRTP and EKT defaults,
the total overhead is 24 bytes. This overhead does not detract from
the total bandwidth used by SRTP, since it is included in the RTCP
bandwidth computation. However, it will cause the control protocol
to issue packets less frequently.
If EKT is used of SRTP, there will be a loss of bandwidth due to the
additional 24 bytes in each RTP packet. For some applications, this
bandwidth loss is significant. It may be desirable to carry the EKT
fields only in some of the SRTP packets, e.g. by adding a flag bit
that indicates the presence or absence of those fields. We leave
this point open for discussion.
The only motivation for defining the ability to run EKT over SRTP
instead of RTCP is the unfortunate fact that RTCP is not always
available, because some RTP stacks are incomplete and some firewalls
and NAT devices can pass RTP but not RTCP.
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6. RTP Transport
EKT MAY be used over SRTP instead of SRTCP if the latter protocol is
not available, though implementations SHOULD otherwise use SRTCP. If
EKT over SRTP is used in an SRTP session in which SRTCP is available,
then EKT MUST be used for both SRTP and SRTCP.
The packet processing, state machine, and Authentication Tag format
for EKT over SRTP is identical to that for EKT over SRTCP.
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7. Security Considerations
With EKT, each SRTP sender and receiver can 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 provides complete security, even in the absence of
further out-of-band coordination of SSRCs, and even when SSRC values
collide.
EKT uses encrypted key transport with implicit authentication. A
strong cipher is used to ensure the confidentiality of the master
keys as they are transported. The authenticity of the master keys is
ensured by the base authentication check, which uses the plaintext
form of that key. If the base authentication function and the cipher
cannot be defeated by a particular attacker, then that attacker will
be unable to defeat the implicit authentication.
In order to avoid potential security issues, the SRTP authentication
tag length used by the base authentication method MUST be at least
ten octets.
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8. IANA Considerations
This section registers with IANA the following SRTP session
parameters for SDP Security Descriptions [SDES]:
EKT_KEY
EKT_CIPHER
EKT_SPI
The definition of these parameters is provided in Section 3.4.
We request the following IANA assignments from existing MIKEY IANA
tables:
From the "Key Data payload name spaces:" a value to indicate the
type as the "EKT_Key."
From the "SRTP" policy table name space, a new value to be
assigned for "EKT_Cipher."
Furthermore, we need a new table to be defined:
EKT_Cipher | Value
-------------------
AES_128 | 0
AESKW_128 | 1
AESKW_192 | 2
AESKW_256 | 3
Figure 7: EKT_Cipher Table
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9. Acknowledgements
Thanks to Dan Wing and Rob Raymond for fruitful discussions.
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10. References
10.1. Normative References
[FIPS197] "The Advanced Encryption Standard (AES)", FIPS-197 Federal
Information Processing Standard.
[KEYID] Carrara, E., Lehtovirta, V., and K. Norrman, "The Key ID
Information Type for the General Extension Payload in
MIKEY", Work In
Progress. <draft-ietf-msec-newtype-keyid-04.txt>.
[RCC] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-over Counter", Work In
Progress. <draft-lehtovirta-srtp-rcc-01.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, September 2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[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.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[SDES] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol Security Descriptions for Media
Streams", Work In
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Progress. <draft-ietf-mmusic-sdescriptions-11.txt>.
10.2. Informative References
[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.
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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
Flemming Andreason
Cisco Systems, Inc.
499 Thornall Street
Edison, NJ 08837
US
Email: fandreas@cisco.com
Lakshminath Dondeti
QUALCOMM
5775 Morehouse Drive
San Diego, CA 92121
US
Email: ldondeti@qualcomm.com
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