One document matched: draft-ietf-avt-srtp-ekt-03.xml
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<rfc category="std" docName="draft-ietf-avt-srtp-ekt-03" ipr="trust200902">
<front>
<title abbrev="EKT SRTP">Encrypted Key Transport for Secure RTP</title>
<author fullname="David A. McGrew" initials="D.A.M." surname="McGrew">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>510 McCarthy Blvd.</street>
<city>Milpitas</city>
<region>CA</region>
<code>95035</code>
<country>US</country>
</postal>
<phone>(408) 525 8651</phone>
<email>mcgrew@cisco.com</email>
<uri>http://www.mindspring.com/~dmcgrew/dam.htm</uri>
</address>
</author>
<author fullname="Flemming Andreason" initials="F.A." surname="Andreasen">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>499 Thornall Street</street>
<city>Edison</city>
<region>NJ</region>
<code>08837</code>
<country>US</country>
</postal>
<email>fandreas@cisco.com</email>
</address>
</author>
<author fullname="Dan Wing" initials="D." surname="Wing">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>510 McCarthy Blvd.</street>
<city>Milpitas</city>
<region>CA</region>
<code>95035</code>
<country>US</country>
</postal>
<phone>(408) 853 4197</phone>
<email>dwing@cisco.com</email>
</address>
</author>
<author fullname="Kai Fischer" initials="K." surname="Fischer">
<organization abbrev="Siemens Enterprise Communications">Siemens
Enterprise Communications GmbH & Co. KG</organization>
<address>
<postal>
<street>Hofmannstr. 51</street>
<city>Munich</city>
<region>Bavaria</region>
<code>81739</code>
<country>Germany</country>
</postal>
<email>kai.fischer@siemens-enterprise.com</email>
</address>
</author>
<date />
<area>RAI</area>
<workgroup>AVT Working Group</workgroup>
<keyword>RTP</keyword>
<keyword>SRTP</keyword>
<keyword>EKT</keyword>
<abstract>
<t>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 SRTP or SRTCP. This facility
enables SRTP to work for decentralized conferences with minimal control,
and to handle situations caused by early media.</t>
<t>This note defines EKT, and also describes how to use it with SDP
Security Descriptions, DTLS-SRTP, and MIKEY. These other key management
protocols provide an EKT key to everyone in a session, and EKT
coordinates the keys within the session.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>RTP is designed to allow decentralized groups with minimal control to
establish sessions, such as for multimedia conferences. Unfortunately,
Secure RTP (<xref target="RFC3711">SRTP</xref>) 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.</t>
<t>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.</t>
<t>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 or SRTP 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.</t>
<t>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.</t>
<t>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.</t>
<t>EKT is not intended to replace external key establishment mechanisms
such as SDP Security Descriptions <xref target="RFC4568"></xref>,
DTLS-SRTP <xref target="RFC5764"></xref>, or MIKEY <xref
target="RFC3830"></xref><xref target="RFC4563"></xref>. 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.</t>
<t>This document is organized as follows. A complete normative
definition of EKT is provided in <xref target="normative"></xref>. It
consists of packet processing algorithms (<xref
target="processing"></xref>) and cryptographic definitions (<xref
target="cipher"></xref>) . In <xref target="sdes"></xref>, the use of
EKT with SDP Security Descriptions is defined, and in <xref
target="dtls-srtp-kt"></xref> its use with DTLS-SRTP is defined. In
<xref target="mikey"></xref> we outline the use of EKT with MIKEY. <xref
target="rationale"></xref> provides a design rationale. Security
Considerations are provided in <xref target="sec"></xref>, and IANA
considerations are provided in <xref target="iana"></xref>.</t>
<section title="Conventions Used In This Document">
<t>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 <xref
target="RFC2119"></xref>.</t>
</section>
</section>
<section anchor="normative" title="Encrypted Key Transport">
<t>In EKT, an SRTP master key is encrypted with a Key Encrypting
Key (KEK), and the resulting ciphertext is transported (using
the EKT Base Authentication Tag) in selected SRTCP or in
selected SRTP packets. 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. We use terms "KEK" or "EKT key" to mean the
same thing; the latter term is used when describing the relation
of EKT to external key management.<!-- do we need to say
"compound packet" everywhere? --></t>
<t>In order to convey the ciphertext of the SRTP master key, and other
additional information, the Authentication Tag field is subdivided as
defined in <xref target="EKT"></xref>. EKT defines new SRTP and SRTCP
authentication functions, which use this format. It incorporates a
conventional authentication function, which is called the base
authentication function in this specification. Any 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.</t>
<section anchor="EKT" title="Authentication Tag Formats">
<!--
<t>
We use the following definitions. An RTP session consists of the RTP
packets sent to a particular destination transport address or set of
such addresses. An SRTP session is an RTP session protected by SRTP.
A single session participant may have multiple RTP sources.
</t>
-->
<t>The EKT Authentication Tag 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
Authentication Tag, which is also the final bit of the SRTP packet.
The first format is the Full EKT Authentication Tag
(<xref target="tag_format_base"></xref>), and the second is the
Abbreviated EKT Authentication Tag
(<xref target="tag_format_abbreviated"></xref>). </t>
<t>The following figure shows the packet layout for the Full EKT
Authentication Tag:</t>
<figure anchor="tag_format_base"
title="Full EKT Authentication Tag format">
<artwork align="center"><![CDATA[
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 |1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The Full EKT Authentication Tag field contains the following sub-fields:
<list style="hanging">
<t hangText="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.</t>
<t hangText="Encrypted Master Key:">The length of this field is
variable, and is equal to the ciphertext size N defined in <xref
target="cipher"></xref>. 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 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 <xref
target="cipher"></xref>.</t>
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="Security Parameter Index (SPI):">The length of this
field is fixed at 15 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
<xref target="RFC4301"></xref>. The parameters that are identified
by this field are: <list style="symbols">
<t>The Key Encrypting Key used to process the packet.</t>
<t>The EKT cipher used to process the packet.</t>
<t>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 <xref target="RFC3711"></xref> summarizes the
parameters defined by that specification.</t>
<t>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 Encrypting
Key.</t>
</list>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. <!-- Each
value MUST be associated with a Key Encrypting Key, and MAY be
associated with an EKT cipher and an SRTP parameter set.
-->The SPI field follows the Initial Sequence Number. Since it appears
at the end of the packet, and has a fixed length, it is always
possible to unambiguously parse this field.</t>
<t hangText="Final bit:">This MUST be 1. This flag distinguishes
the packet layout between <xref target="tag_format_abbreviated"></xref> or
<xref target="tag_format_base"></xref>.</t>
</list>The following figure shows the packet layout of the
Abbreviated EKT Authentication Tag:</t>
<figure anchor="tag_format_abbreviated"
title="Abbreviated EKT Authentication Tag format">
<artwork align="center"><![CDATA[
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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |0|
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The Abbreviated EKT Authentication Tag field contains the following sub-fields:
<list style="hanging">
<t hangText="Base Authentication Tag:">same as described
above.</t>
<t hangText="Reserved:">7 bits. MUST be 0 on transmission and MUST
be ignored on reception.</t>
<t hangText="Final Bit:">This MUST be 0. This flag distinguishes
the packet layout between<xref target="tag_format_base"></xref> or
<xref target="tag_format_abbreviated"></xref>.s</t>
</list></t>
</section>
<section anchor="processing" title="Packet Processing and State Machine">
<t>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.</t>
<!--
adds some additional steps to the SRTCP packet processing rules.
-->
<t>We next review SRTP authentication and show how the EKT
authentication method is built on top of a base authentication method.
An SRTP or 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
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 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.</t>
<section anchor="outbound" title="Outbound (Tag Generation)">
<t>When an SRTP or SRTCP packet needs to be sent, the EKT tag
generation function works as follows. <!--
(see <xref target="fig2"/> and
<xref target="fig3"/>).
-->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).</t>
<t>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).</t>
<t>The Security Parameter Index field is set to the value of the
Security Parameter Index that is associated with the outbound
parameter set.<!-- If only one Key
Encrypting Key is provided for the session, then an all-null value for
the identifier SHOULD be used,
--></t>
<t>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 <xref
target="cipher"></xref>. Implementations MAY cache the value of this
field to avoid recomputing it for each packet that is sent.<!--
Then the Encrypted Master Key
field is computed using the EKT cipher's encryption function. The
SRTP master key corresponding to the SSRC (and MKI, if present) in
the packet is the plaintext input, and the Key Encrypting Key is used
as its key.
<list style="empty">
<t> 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 sender to
minimize computational effort. (This property follows from
the fact that the EKT cipher is deterministic, and so
identical ciphertexts will decrypt to identical plaintexts.
When the ciphertext form of the master key matches the
Encrypted Master Key field in a packet, we know that the
decryption of that field will match the master key.)
</t>
</list>
The computed value of the Encrypted Master Key field is written into
the packet.
--></t>
<section anchor="base" title="Computing the Base Authentication Tag">
<t>If using the Base Authentication Tag format, the 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:
<list style="numbers">
<t>the SRTP or SRTCP authenticated portion,</t>
<t>a string of zero bits whose length exactly matches that of
the Base Authentication Tag field,</t>
<t>the Encrypted Master Key field,</t>
<t>the Rollover Counter field,</t>
<t>the Initial Sequence Number field, and</t>
<t>the Security Parameter Index field.</t>
</list><!--
Both the sender and receiver compute the Base Authentication Tag in
the same manner.
--></t>
<t><list style="empty">
<t>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.</t>
</list></t>
</section>
<section anchor="abbreviated" title="Computing the Abbreviated Authentication Tag">
<t>
If using the Abbreviated Authentication Tag format, the 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:
<list style="numbers">
<t>the SRTP or SRTCP authenticated portion,</t>
<t>a string of zero bits whose length exactly matches that of the
Base Authentication Tag field. Then for SRTP only, place the ROC (in
network order) into the first 4 bytes of the "base authentication tag"
field.</t>
<t>set reserved bits and final bit to zeros.</t>
</list></t>
</section>
<!--
<t>
The Initial Sequence Number (ISN) field is set to zero unless the SRTP
master key has been changed, and the EKT fields
are being carried in an SRTCP packet. In that case, then
that field is set to the SRTP sequence number of the initial packet
that was sent immediately after the current master key was put into
use.
</t>
<t>
The Security Parameter Index field is set to the value of the SPI
that is currently in use.
</t>
<figure anchor="fig2" title="EKT outbound processing; generating the
Encrypted Master Key">
<artwork>
+<<<<< SRTP master key (plaintext)
|
v
++++++++++++++
| Encryption |
| Function |<, Key Encrypting Key
++++++++++++++
|
v
Encrypted Master Key (ciphertext)
</artwork>
</figure>
<figure anchor="fig3" title="EKT outbound processing; generating the
Authentication Tag">
<artwork>
authenticated portion of SRTCP packet
|
v
++++++++++++++++++ SRTCP
| Base SRTCP | authentication
| Tag Generation |< key
++++++++++++++++++
|
v
Authentication Tag
</artwork>
</figure>
-->
</section>
<section anchor="inbound" title="Inbound (Tag Verification)">
<t>The EKT verification function proceeds as follows (see <xref
target="fig4"></xref>), or uses an equivalent set of steps. Recall
that the verification function is a component of SRTP and 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.<!--
The SSRC field in the packet is checked to see if it corresponds to a
known
SRTP source with a known crypto context. If it does not, then the
following steps are taken.
--></t>
<t><list style="numbers">
<t>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.<!-- If there is
only a single session key, it SHOULD be used only if the field is set
to the all-null value.
--></t>
<t>If there is already an SRTP crypto context associated with
the SSRC in the packet, and replay protection is in use, then
the receiver performs the replay check described in Section
3.3.2 of <xref target="RFC3711"></xref>. If the EKT fields are
conveyed in an RTCP packet, then the packet index used in that
check is formed from the Rollover Counter and the Initial
Sequence Number fields in that packet. If the EKT fields are
conveyed in an SRTP packet, then the packet index used in that
check is formed from the EKT Rollover Counter field and the RTP
Sequence Number in that packet.</t>
<t>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 <xref target="cipher"></xref>. The
plaintext resulting from this decryption is provisionally
accepted as the SRTP master key corresponding to the SSRC in the
packet. If an SRTP master key identifier (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 SRTP or 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 <xref
target="RFC3711"></xref>).</t>
<t>An authentication check is performed on the packet, using the
provisional SRTP or SRTCP authentication key. This key is
provided to the base authentication function (see <xref
target="fig4"></xref>), which is evaluated as described in <xref
target="base"></xref>. If the Base Authentication Tag field
matches the tag computed by the base authentication function,
then the packet passes the check. <list style="empty">
<t>Implementation note: a 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 over
the data in the packet.</t>
</list><!--
An SRTCP packet passes its authentication check when
the base authentication tag computed by the reciever matches the Base
Authentication Tag field carried in the packet.
--></t>
<t>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.</t>
<t>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
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. If the crypto
context is not new, then the rollover counter in the context
MUST NOT be set to a value lower than its current value. (If the
replay protection step described above is performed, it ensures
that this requirement is satisfied.)</t>
<t>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 <xref target="RFC3711"></xref>) that
can be associated with a master key.</t>
<t>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. The Key Derivation algorithm
takes as input the newly accepted SRTP master key, along with
the Master Salt from the matching parameter set. <list
style="empty">
<t>Implementation note: the receiver may want to retain old
master keys for some brief period of time, so that out of
order packets can be processed.</t>
</list></t>
<t>The verification function then returns an indication that the
packet passed the verification. <!--
Original text: If the value of the Rollover Counter field (when
considered as an
unsigned integer in network byte order) is greater than the current
value of the SRTP rollover counter, then the rollover counter is set
to the value of the field.
--> <list style="empty">
<t>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.</t>
</list></t>
</list><!--
If the SSRC field in the packet does correspond to a known SRTP
source, then the packet is processed as follows, or using an
equivalent set of steps.
<list style="numbers">
<t>
The appropriate parameter set is selected using the Security Parameter
Index field, as in Step 1 above, and the Encrypted Master Key
field is decrypted using the Key Encrypting Key as in Step 2 above.
The resulting plaintext is used as a provisional SRTP master key.
</t>
<t>
If the provisional key matches the key associated with the SSRC (and
MKI, if one is present), then the packet is processed as follows. The
base authentication function is used to check the authenticity of the
packet, as described in Step 3 above. If the check passes, the packet
is accepted, and then the SRTP rollover counter is set to the value of
the Rollover Counter field.
</t>
<t>
If the provisional key does not match the SRTP master key associated
with the SRTP source (and MKI, if present), then processing proceeds
as follows. The authenticity of the packet is checked using the
provisional key and base SRTCP authentication function, as in Step 3
above. If that check passes, then the packet is accepted, and the
context associated with the SRTP source is set as described in Step 5
above. SRTP master keys can be associated with an initial packet
index (Section 8.1.1. of <xref target="RFC3711"/>). The initial
packet index associated with the new SRTP master key is formed from
the value of the Initial Sequence Number and Rollover Counter fields
(considered as an unsigned integer in network byte order), by
multiplying the ROC by 2^16 then adding the ISN to the result.
</t>
</list>
--></t>
<figure anchor="fig4" title="EKT inbound processing.">
<artwork align="center"><![CDATA[
+------- 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 |
+----------------+
]]></artwork>
</figure>
</section>
</section>
<section anchor="cipher" title="Ciphers">
<t>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).</t>
<t>An EKT cipher consists of an encryption function and a decryption
function. The encryption function E(K, P) takes the following inputs:
<list style="symbols">
<t>a secret key K with a length of L bytes, and</t>
<t>a plaintext value P with a length of M bytes.</t>
</list> 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: <list style="symbols">
<t>a secret key K with a length of L bytes, and</t>
<t>a ciphertext value C with a length of N bytes.</t>
</list> 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.</t>
<t>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.</t>
<section anchor="DefaultCipher" title="The Default Cipher">
<t>
The default EKT Cipher is the AES Key
Wrap <xref target="RFC3394"></xref> algorithm, which can be used
with plaintexts larger than 16 bytes in length, and is thus
suitable for keys of any size. 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 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.
</t>
<!--
The default cipher is the Advanced Encryption Standard (AES)
<xref target="FIPS197"></xref> 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.</t>
-->
</section>
<section title="AES ECB">
<t>
The simplest EKT cipher is the Advanced Encryption
Standard (AES)
<xref target="FIPS197"></xref> with 128-bit keys, in
Electronic Codebook (ECB) Mode. Its use is indicated as
AES_ECB, and 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.
</t>
</section>
<!--
<section anchor="AESKeyWrap" title="The AES Key Wrap Cipher">
<t>The AES Key Wrap 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 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.</t>
</section>
-->
<section title="Other EKT Ciphers">
<t>Other specification MAY extend this one by defining other EKT
ciphers per <xref target="iana"></xref>. This section defines how
those ciphers interact with this specification.</t>
<t>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.</t>
</section>
</section>
<section anchor="SynchronizingOperation" title="Synchronizing Operation">
<t>A participant in a session MAY opt to use a particular EKT key to
protect outbound packets after it accepts that EKT key 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.</t>
<t>An SRTP/SRTCP source SHOULD change its SRTP master key after its
EKT key has been changed. This will ensure that the set of
participants able to decrypt the traffic will be limited to those who
know the current EKT key.</t>
<t>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 out of
order. This can lead to various race conditions, which we review
below.</t>
<t>When joining an SRTP session, SRTP packets may be received before
any EKT over SRTCP 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.</t>
<t>When an SRTP source using EKT over SRTCP 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 <xref
target="RFC3711"></xref> 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.</t>
<t><!--
<list style="empty">
<t>
Note: It was considered to include <From, To> functionality in EKT to
indicate when a particular set of parameters would apply. However, if
this is to be done without increasing the SRTP packet size, then the
From and To values can only include the 16-bit SRTP Sequence Number,
not the full (32-bit) SRTP Packet Index (which includes the
ROC). Thus, once the sequence number wraps, such an indication will no
longer be correct.
</t>
</list>
--></t>
<t>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 <xref target="RFC3711">Section 3.3.1</xref>.</t>
</section>
<section anchor="srtp" title="Transport">
<t>EKT MUST be used over SRTCP, whenever RTCP is in use. EKT MAY be
used over SRTP. When EKT over SRTP is used in an SRTP session in which
SRTCP is available, then EKT MUST be used for both SRTP and SRTCP.</t>
<t>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 <xref target="outbound"></xref> and
<xref target="inbound"></xref>.</t>
</section>
<section title="Timing and Reliability Consideration">
<t>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 EKT packet containing
the Base Authentication Tag as soon as possible, and in no
case any later than when the initial SRTP packet is sent. It
is RECOMMENDED that the Base Authentication Tag be transmitted
3 times (to accomodate packet loss) and to provide a reliable
indication to the receiver that the sender is now using the
EKT key. If the Base Authentication Tag sent in SRTCP, the
SRTCP timing rules associated with the profile under which it
runs (e.g., RTP/SAVP or RTP/SAVPF) MUST be obeyed. Subject to
that constraint, SRTP senders using EKT over SRTCP 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).</t>
<t>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 using EKT over SRTCP SHOULD send
SRTCP packets as often as allowed by the profile under which they
operate.<!-- Editor's
Note: For unicast sessions, use of a reliable protocol to transport
EKT packets should be considered. STUN (as used by ICE) is one option,
however we would need to include SSRC identification as well as
procedures for SSRC collission detection and resolution then.
--></t>
</section>
</section>
<section anchor="sdes" title="Use of EKT with SDP Security Descriptions">
<t>The SDP Security Descriptions (SDESC) <xref target="RFC4568"></xref>
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 <xref target="RFC3264"></xref>. 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.</t>
<section title="SDP Security Descriptions Recap">
<t>The SRTP crypto attribute defined for SDESC contains a tag followed
by three types of parameters (refer to <xref target="RFC4568"></xref>
for details): <list style="symbols">
<t>Crypto-suite. Identifies the encryption and authentication
transform</t>
<t>Key parameters. SRTP keying material and parameters.</t>
<t>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.</t>
</list> The crypto attributes in the example SDP in <xref
target="figSDP"></xref> illustrate these parameters.</t>
<figure anchor="figSDP" title="SDP Security Descriptions example">
<artwork align="center"><![CDATA[
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
]]></artwork>
<postamble></postamble>
</figure>
<t>For legibility the SDP shows line breaks that are not present on
the wire.</t>
<t>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).</t>
<!-- EDITOR's NOTE: EKT only includes the master key. What
about the salt ? -->
<t>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.</t>
</section>
<section title="Relationship between EKT and SDP Security Descriptions">
<t>SDP Security Descriptions <xref target="RFC4568"></xref> 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 <xref target="RFC3264"></xref>. In
addition to the general framework, SDESC also defines how to use it
specifically to negotiate security parameters for Secure RTP.</t>
<t>EKT and SDESC are complementary. SDESC can negotiate several of the
SRTP security parameters (e.g., cipher and use of Master Key
Identifier/MKI) 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 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 SDP Security can
provide the additional signaling necessary to communicate the ROC
(Section 6.4.1 of <xref target="RFC4568"></xref>). The use of EKT
solves this problem by communicating the ROC associated with the SSRC
in the media plane.</t>
<t>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.</t>
<t>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.</t>
<!--
EDITOR'S NOTE: How does EKT help solve that ? One solution would be to
include the "tag" from the selected crypto attribute, however this is
sdes specific (may need something else for MIKEY). It also does not
solve the issue of declarative parameters included in the answer,
which the offerer needs to know about to correctly process incoming
media. Propose to simply note that as a limitation of the solution (if
you want the whole nine yards, you need security preconditions).
-->
<t>One of the primary call signaling protocols using offer/answer is
the Session Initiation Protocol (SIP) <xref target="RFC3261"></xref>.
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.</t>
<!--
EDITOR's NOTE: How does EKT help solve that ? We would need to include
a correlator that is found in both the SDP and the RTCP packet. It
should be noted that ICE already provides this.
-->
<t>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.</t>
</section>
<section title="Overview of Combined EKT and SDP Security Description Operation">
<t>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 <xref
target="RFC4568"></xref>, 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 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 SDESC.<!-- DAM - have not edited this section, shouldn't use default key --></t>
</section>
<section anchor="ext"
title="EKT Extensions to SDP Security Descriptions">
<t>In order to use EKT and SDESC in conjunction with each other, the
following new SDES session parameters are defined. These MUST NOT
appear more than once in a given crypto attribute: <list
style="hanging">
<t hangText="EKT_Cipher:">The EKT cipher used to encrypt the SRTP
Master Key</t>
<t hangText="EKT_Key:">The EKT key used to encrypt the SRTP Master
Key</t>
<t hangText="EKT_SPI:">The EKT Security Parameter Index</t>
</list></t>
<t>Below are details on each of these attributes.</t>
<section title="EKT_Cipher">
<t>The (optional) EKT_Cipher parameter defines the EKT
cipher used to encrypt the EKT key with in SRTCP
packets. The default value is "AESKW_128" in accordance
with <xref target="DefaultCipher"></xref>. 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. For the AES ECB cipher, "AES_ECB" is defined.
In the Offer/Answer model, the EKT_Cipher parameter is a
negotiated parameter.</t>
</section>
<section title="EKT_Key">
<t>The (mandatory) EKT_Key parameter is the key K used to encrypt
the SRTP Master Key in SRTCP packets. The value is base64 encoded as
described in Section 4 <xref target="RFC4648"></xref>. When base64
decoding the key, padding characters (i.e., one or two "=" at the
end of the base64 encoded data) are discarded (see <xref
target="RFC4648"></xref> 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.</t>
</section>
<section title="EKT_SPI">
<t>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.</t>
</section>
</section>
<section title="Offer/Answer Procedures">
<t>In this section, we provide the offer/answer procedures associated
with use of the three new SDESC parameters defined in Section <xref
target="ext"></xref>. Since SDESC is defined only for unicast streams,
we provide only offer/answer procedures for unicast streams here as
well.</t>
<section title="Generating the Initial Offer - Unicast Streams">
<t>When the initial offer is generated, the offerer MUST follow the
steps defined in <xref target="RFC4568"></xref> Section 7.1.1 as
well as the following steps.</t>
<t>For each unicast media line using SDESC 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 <xref
target="RFC4568"></xref>). 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. <!--
Can we be more precise in describing a "crypto attribute" -it's an SDES
thing, I assume.
FSA: Right.
--> 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.</t>
<t>If an EKT_Cipher other than the default cipher is to be used,
then the EKT_Cipher parameter MUST be included as well.</t>
<t>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).</t>
<!--
EDITOR's NOTE: Do we ever see a need to allow for a remapping
? We really just need to make sure there is no ambiguity, and
simply cycling the value once we hit 2^32 would be more than
enough here.
-->
<!--
EDITOR'S NOTE: Do we need the ability to negotiate different
EKT Ciphers in a single O/A exchange ? If so, we will need a
way for EKT to tell us which one was actually chosen. For now,
no such capability has been included.
-->
<t><list style="hanging">
<t hangText="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.</t>
</list></t>
<!-- Editor's Note: Need to dedicate a section to more fully explain
(and exemplify) the issues here. Need to clarify RTP mixer and RTP
translator operation. Also need to explain SIP forking with early
media from multiple entities.
-->
<!--
EDITOR's NOTE: Can we have SRTP session where some
participants support EKT, and some do not ? For now, I assume
the answer is no.
-->
<t>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.</t>
</section>
<section title="Generating the Initial Answer - Unicast Streams">
<t>When the initial answer is generated, the answerer MUST follow
the steps defined in <xref target="RFC4568"></xref> Section 7.1.2 as
well as the following steps.</t>
<t>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.</t>
<t>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.<!-- DAM - should we say "master salt" instead of just "salt"? --></t>
<t>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).</t>
<t>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.</t>
<!--
EDITOR'S NOTE: It would be sufficient to just include the
EKT_SPI as there is enough context to determine what was
accepted otherwise. However, sdes has a rule saying negotiated
session parameters need to be included in the answer.
-->
</section>
<section title="Processing of the Initial Answer - Unicast Streams">
<t>When the offerer receives the answer, it MUST perform the steps
in <xref target="RFC4568"></xref> 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.</t>
<t>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.</t>
<t>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 (<xref
target="RFC4568">Section 5.1.3 of</xref>).</t>
<t>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 <xref target="RFC4568">Section 7.1.3
of</xref> for further operation).</t>
<t>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.</t>
</section>
</section>
<section title="SRTP-Specific Use Outside Offer/Answer">
<t>Security Descriptions use for SRTP is not defined outside
offer/answer and hence neither does Security Descriptions with
EKT.</t>
</section>
<section title="Modifying the Session">
<t>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 <xref target="RFC4568">Section
7.1.4 of</xref> 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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<!--
EDITOR'S NOTE: I'm not entirely happy and comfortable with
this restriction. Should probably take another look and see if
we could accommodate this.
-->
</section>
<!--
<section title="Offer/Answer Example">
<t>
To be provided.
</t>
</section>
-->
<section title="Backwards Compatibility Considerations">
<t>Backwards compatibility can be achieved in a couple of ways. First
of all, SDESC allows for session parameters to be 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 SRTCP packets prior to
receipt of such an answer will obviously be problematic (as is
normally the case for SDESC without EKT).</t>
<t>Alternatively, SDESC 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.</t>
</section>
<section title="Grammar">
<t>The <xref target="RFC5234">ABNF</xref> syntax for the three new SDP
Security Descriptions session parameters is shown in <xref
target="figABNF"></xref>.</t>
<figure anchor="figABNF" title="ABNF for the EKT session parameters">
<artwork align="center"><![CDATA[
EKT = EKT_Cipher "|" EKT_Key "|" EKT_SPI
EKT_Cipher = "EKT=" 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 4 of [RFC4648]
base64 = ALPHA / DIGIT / "+" / "/" / "="
EKT_SPI = 4HEXDIG ; See [RFC5234]
]]></artwork>
</figure>
<t>Using the example from <xref target="figABNF"></xref> with the EKT
extensions to SDP Security Descriptions results in the following
example SDP:</t>
<figure>
<artwork align="center"><![CDATA[
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=AES_128|FE9C|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=AES_128|FE9C|AAE0
]]></artwork>
<postamble></postamble>
</figure>
<t>For legibility the SDP shows line breaks that are not present on
the wire.</t>
</section>
</section>
<section anchor="dtls-srtp-kt"
title="Use of EKT with DTLS-SRTP Key Transport">
<t>This document defines an extension to DTLS-SRTP called Key Transport.
Using EKT with the DTLS-SRTP Key Transport extensions allows securely
transporting SRTP keying material from one DTLS-SRTP peer to another, so
the same SRTP keying material can be used by those peers and so those
peers can process EKT keys. 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).</t>
<section anchor="dtls-srtp-extensions"
title="EKT Extensions to DTLS-SRTP">
<t>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 <xref
target="RFC5764"></xref>). The DTLS server indicates its support for
EKT by including "dtls-srtp-ekt" in its SDP 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.</t>
<figure anchor="tls_datastructure"
title="Additional TLS Data Structures">
<preamble>Using the syntax described in <xref
target="I-D.ietf-tls-rfc4347-bis">DTLS</xref>, the following
structures are used:</preamble>
<artwork align="center"><![CDATA[
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 {
AES_128(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;
]]></artwork>
</figure>
<t>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 EKT key.<list style="empty">
<t>Editor's note: do we need reliability for the ekt_key
messages?</t>
</list></t>
<figure anchor="tls_handshake_message_flow"
title="Handshake Message Flow">
<preamble></preamble>
<artwork align="center"><![CDATA[
Client Server
ClientHello + use_srtp + EKT
-------->
ServerHello + use_srtp + EKT
Certificate*
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
]]></artwork>
</figure>
<section title="Scaling to Large Groups">
<t>In certain scenarios it is useful to perform DTLS-SRTP
with a device that is not the RTP peer. A common scenario is
multicast, where it is necessary to distribute the DTLS-SRTP
(and EKT distribution) to several devices. To allow for
this, a new SDP attribute, dtls-srtp-host, is defined which
follows the general syntax specified in Section 5.13
of <xref target="RFC4566"></xref>. When signaled, it
indicates this host controls the EKT keying for all group
members. For the dtls-srtp-host
attribute: <list style="symbols">
<t>the name is the ASCII string "dtls-srtp-host" (lowercase)</t>
<t>the value is the IP address and port number used for
DTLS-SRTP</t>
<t>This is a media-level attribute and MUST NOT appear at the
session level</t>
</list>The formal description of the attribute is defined by the
following <xref target="RFC5234">ABNF</xref> syntax:</t>
<figure>
<artwork><![CDATA[
attribute = "a=dtls-srtp-host:"
dtls-srtp-host-info *(SP dtls-srtp-host-info)
host-info = nettype space addrtype space
connection-address space port CRLF]]></artwork>
</figure>
<t>Multiple IP/port pairs are provided for IPv6/IPv4 interworking,
and to allow failover. The receiving host SHOULD attempt to use them
in the order provided.</t>
<figure>
<preamble>An example of SDP containing the dtls-srtp-host
attribute:</preamble>
<artwork align="center"><![CDATA[
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
a=dtls-srtp-host:IN IP4 192.0.2.13 56789
]]></artwork>
<postamble>For legibility the SDP shows line breaks that are not
present on the wire.</postamble>
</figure>
</section>
</section>
<section title="Offer/Answer Considerations">
<t>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 <xref
target="RFC5764"></xref> as well as the following ones.</t>
<t>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).</t>
<section title="Generating the Initial Offer">
<t>The initial offer contains a new SDP attribute, "dtls-srtp-ekt",
which contains no value. This 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. If the
offerer wants another host to perform DTLS-SRTP-EKT processing, it
also includes the dtls-srtp-host attribute in its offer (<xref
target="dtls-srtp-extensions"></xref>).</t>
<figure>
<preamble>An example of SDP containing the dtls-srtp-ekt
attribute::</preamble>
<artwork align="center"><![CDATA[
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
]]></artwork>
<postamble>For legibility the SDP shows line breaks that are not
present on the wire.</postamble>
</figure>
<t></t>
</section>
<section title="Generating the Initial Answer">
<t>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. The presence of the
dtls-srtp-host attribute indicates an alternate host to send the
DTLS-SRTP handshake (instead of the host on the c/m lines). DTLS
messages should be constructed according to those two
attributes.</t>
<t>The SDP answer SHOULD contain the dtls-srtp-ekt attribute to
indicate the answerer understands dtls-srtp. It should only contain
the dtls-srtp-host attribute if the answerer also wishes to offload
its DTLS-SRTP processing to another host.</t>
</section>
<section title="Processing the Initial Answer">
<t>The presence of the dtls-srtp-ekt attribute indicates a desire by
the answerer to perform DTLS-SRTP with EKT extensions, and the
dtls-srtp-host attribute indicates an alternate host for DTLS-SRTP
processing.</t>
<t>After successful negotiation of the key_transport extension, the
DTLS client and server MAY exchange SRTP packets, encrypted using
the KDF described in <xref target="RFC5764"></xref>. 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 one (or both) peers will immediately send
new_srtp_key message before sending any SRTP, and also possible that
SRTP, encrypted with an unknown key, may be received before the
new_srtp_key message is received.</t>
</section>
<section title="Modifying the Session">
<t>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.</t>
</section>
</section>
</section>
<section anchor="mikey" title="Use of EKT with MIKEY">
<t>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.</t>
<t>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 <xref target="RFC4567"></xref>).</t>
<t>The MIKEY specification <xref target="RFC3830"></xref> defines three
modes to exchange keys, associated parameters and to protect the MIKEY
message: pre-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.</t>
<t>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 is being proposed <xref target="RFC4771"></xref>
to include the ROC as part of a modified authentication tag. Unlike EKT,
this extension uses only SRTP and not SRTCP as its transport and does
not allow updating the SRTP master key.</t>
<t>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 a SRTP session participant starts a
new SRTP source 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.</t>
<t>The MIKEY specification <xref target="RFC3830"></xref> 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.</t>
<t>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 <xref target="RFC3830"></xref>. The
Diffie-Hellman exchange mode 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.</t>
<section title="EKT extensions to MIKEY">
<t>In order to use EKT with MIKEY, the EKT cipher, EKT key and EKT SPI
must be negotiated in the MIKEY message exchange.</t>
<t>For EKT we specify a new SRTP Policy Type in the Security Policy
(SP) payload of MIKEY (see Section 6.10 of <xref
target="RFC3830"></xref>). The SP payload contains a set of policies.
Each policy consists of a number Policy Param TLVs.</t>
<figure anchor="EKT_SecPol" title="EKT Security Policy">
<preamble></preamble>
<artwork align="center"><![CDATA[
Prot type | Value
-------------------
EKT | TBD (will be requested from IANA)
]]></artwork>
<postamble>For legibility the SDP shows line breaks that are not
present on the wire.</postamble>
</figure>
<t>The EKT Security Policy has one parameter representing the EKT
cipher.</t>
<figure anchor="EKT_SecPolParam"
title="EKT Security Policy Parameters">
<preamble></preamble>
<artwork align="center"><![CDATA[
Type | Meaning | Possible values
----------------------------------------------------
0 | EKT cipher | see below
]]></artwork>
</figure>
<figure anchor="EKT_CipherParam" title="EKT Cipher Parameters">
<preamble></preamble>
<artwork align="center"><![CDATA[
EKT cipher | Value
-------------------
AES_128 | 0
AESKW_128 | 1
AESKW_192 | 2
AESKW_256 | 3
]]></artwork>
<postamble></postamble>
</figure>
<t>AES_128 is the default value for the EKT cipher.</t>
<t>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 <xref
target="RFC3830"></xref>, 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 <xref
target="RFC3830"></xref>, has the following format:</t>
<figure anchor="Key_Data_subpayload"
title="Key Data Sub-Payload of MIKEY">
<preamble></preamble>
<artwork align="center"><![CDATA[
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) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
<postamble></postamble>
</figure>
<t>These fields are described below:<list style="hanging">
<t hangText="Type:">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 the EKT key.</t>
<t hangText="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 ETK_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 NOT be optional for the key type EKT_Key when
KV = 1.</t>
<t hangText="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.</t>
<t hangText="KV Data:">length determined by Key Data Length
field.</t>
</list></t>
</section>
<section title="Offer/Answer considerations ">
<t>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 <xref target="RFC3830"></xref>
and <xref target="RFC4567"></xref> as well as the following ones.</t>
<section title="Generating the Initial Offer">
<t>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
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 <xref
target="RFC3711"></xref>. 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 medial level
content overrides the session level content.</t>
<t>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.</t>
</section>
<section title="Generating the Initial Answer">
<t>For each media line in the offer using MIKEY, provided on session
or/and 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.</t>
<t>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 the 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.</t>
</section>
<section title="Processing the Initial Answer">
<t>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
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.</t>
<t>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 together with EKT and the other one
offers MIKEY without EKT. This is for further discussion.</t>
</section>
<section title="Modifying the Session">
<t>Once a 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
the EKT key or cipher did not change.</t>
<t>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.</t>
<t>After EKT has been successfully negotiated for a session and a
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.</t>
</section>
</section>
</section>
<section anchor="rationale" title="Design Rationale">
<t>From <xref target="RFC3550"></xref>, 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.</t>
<t>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.</t>
<t>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.</t>
<t>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 <xref
target="RFC3550"></xref>). 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.</t>
<t>EKT can also be used in conjunction with a scalable group-key
management system like <xref target="RFC3547">GDOI</xref>. 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.</t>
<!--
There are several scenarios in which there are more than two
participants in an SRTP sessions, besides the important case of
multi-party conferences. For example, a session may be recorded (by a
voicemail system) or monitored (by a supervisor), or may
unintentionally include a third party (for example, a receptionist)
due to a race condition in call signaling.
A session can start out as conversational voice, then switch codecs
and SSRC values to send a fax transmission.
-->
<t>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
SDESC item or RTCP packet. This would require that the normal SRTCP
encryption be turned off for the packets containing that SDESC 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 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.<!--
Recall that SRTP allows a transform definition to "add steps to
the packet processing, or even add fields to the SRTP/SRTCP packets."
--></t>
<t>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 <xref
target="SynchronizingOperation"></xref>.</t>
<t>When EKT is carried in SRTCP, it 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.</t>
<t>The main motivation for the use of the variable-length format is
bandwidth conservation. 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.<!--
A data format
that uses this approach is defined in <xref target="appendix"/>. We
leave this point open for discussion.
--></t>
<!--
<t>
The main motivation for defining the ability to run EKT over
SRTP instead of RTCP are the unfortunate facts that RTCP is not
always available, and that
some firewalls and NAT devices can pass RTP but not RTCP.
</t>
-->
<section title="Alternatives">
<t>
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.
</t>
<t>
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.)
</t>
<t>
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.
</t>
</section>
</section>
<section anchor="sec" title="Security Considerations">
<t>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.</t>
<t>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.</t>
<t>In order to avoid potential security issues, the SRTP authentication
tag length used by the base authentication method MUST be at least ten
octets.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>This section registers with IANA the following SRTP session
parameters for SDP Security Descriptions <xref target="RFC4568"></xref>:
<list style="symbols">
<t>EKT_KEY</t>
<t>EKT_CIPHER</t>
<t>EKT_SPI</t>
</list> The definition of these parameters is provided in <xref
target="ext"></xref>.</t>
<!-- Editor's Note: Do we need an IANA registry for EKT ciphers (as
well as procedures for who can define new ones) ? Similarly, for SDES,
do we need a new IANA subregistry under the existing SDES one for the
new EKT_cipher parameter to register EKT ciphers (I think so)
-->
<!--MIKEY Registrations-->
<t>We request the following IANA assignments from existing MIKEY IANA
tables: <list style="symbols">
<t>From the Key Data payload name spaces, a value to indicate the
type as the 'EKT_Key'.</t>
<t>From the Security Policy table name space, a new value to be
assigned for 'EKT' (see <xref target="EKT_SecPol"></xref>).</t>
</list></t>
<t>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 <xref
target="RFC5226">Specification Required</xref>. <list style="symbols">
<t>EKT parameter type (initially populated with the list from <xref
target="EKT_SecPolParam"></xref>)</t>
<t>EKT cipher (initially populated with the list from <xref
target="EKT_CipherParam"></xref>)</t>
</list></t>
</section>
<section title="Acknowledgements">
<t>Thanks to Lakshminath Dondeti for assistance with earlier versions of
this document. Thanks to Nermeen Ismail, Eddy Lem, and Rob Raymond for
fruitful discussions and comments. Thanks to Romain Biehlmann for his
encouragement to add support DTLS-SRTP-EKT key servers for multicast.
Thanks to Felix Wyss for his review and comments regarding ciphers.</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="FIPS197">
<front>
<title>The Advanced Encryption Standard (AES)</title>
<author fullname="National Institute of Standards and Technology (NIST)">
<organization />
</author>
</front>
<seriesInfo name="FIPS-197"
value="Federal Information Processing Standard" />
</reference>
<!--
<reference anchor="LRW02">
<front>
<title>Tweakable Block Ciphers</title>
<author initials="M." surname="Liskov" fullname="Moses Liskov">
<organization/>
</author>
<author initials="R." surname="Rivest" fullname="Ron Rivest">
<organization/>
</author>
<author initials="D." surname="Wagner" fullname="David Wagner">
<organization/>
</author>
<date month="August" year="2002"/>
</front>
<seriesInfo name="CRYPTO 2002" value="Springer-Verlag" />
</reference>
<reference anchor="SDES">
<front>
<title> Session Description Protocol Security Descriptions
for Media Streams </title>
<author initials="F." surname="Andreasen"
fullname = "Flemming Andreasen">
<organization/>
</author>
<author initials="M." surname="Baugher"
fullname = "Mark Baugher">
<organization/>
</author>
<author initials="D." surname="Wing"
fullname = "Dan Wing">
<organization/>
</author>
</front>
<seriesInfo name="Work In Progress." value="<draft-ietf-mmusic-sdescriptions-11.txt>"/>
</reference>
-->
<!--
<reference anchor="RCC">
<front>
<title> Integrity Transform Carrying Roll-over Counter </title>
<author initials="V." surname="Lehtovirta"
fullname = "Vesa Lehtovirta">
<organization/>
</author>
<author initials="M." surname="Naslund"
fullname = "Mats Naslund">
<organization/>
</author>
<author initials="K." surname="Norrman"
fullname = "Karl Norrman">
<organization/>
</author>
</front>
<seriesInfo name="Work In Progress." value="<draft-lehtovirta-srtp-rcc-01.txt>"/>
</reference>
<reference anchor="KEYID">
<front>
<title> The Key ID Information Type for the General
Extension Payload in MIKEY
</title>
<author initials="E." surname="Carrara"
fullname = "Elisabetta Carrara"> <organization/>
</author>
<author initials="V." surname="Lehtovirta"
fullname = "Vesa Lehtovirta"> <organization/>
</author>
<author initials="K." surname="Norrman"
fullname = "Karl Norrman">
<organization/>
</author>
</front>
<seriesInfo name="Work In Progress." value="<draft-ietf-msec-newtype-keyid-04.txt>"/>
</reference>
-->
&rfc4563;
&rfc4567;
&rfc4568;
&rfc4771;
&rfc2119;
&rfc3261;
&rfc3264;
&rfc3394;
&rfc3550;
&rfc4648;
&rfc3711;
&rfc5234;
&I-D.ietf-tls-rfc4347-bis;
&rfc5764;
&rfc4566;
&rfc5226;
</references>
<references title="Informative References">
&rfc3830;
&rfc4301;
&rfc3547;
</references>
<!--
<section anchor="appendix" title="Alternate Format">
<t>
In this appendix we describe an alternate Authentication Tag format,
which is intended for the purposes of discussion. It allows a sender
to optionally omit the EKT data from a packet. As discussed in
<xref target="rationale"/>, this feature is desirable because it
allows bandwidth conservation, and it gives the sender the discretion
of when to send the EKT data, without any need for external
coordination.
</t>
<t>
The alternate format is shown in <xref target="altFormat"/>. The top
diagram shows the format in the case that the final bit is set to one,
in which case all of the EKT fields are present. The bottom diagram
shows the format in the case that the final bit is set to zero, in
which case the Encrypted Master Key, Rollover Counter, Initial
Sequence Number, and Security Parameter Index fields are absent. The
Reserved field MUST be set to the all-zero value. These two cases can
always be unambiguously distinguished by the final bit, or by checking
to see if the final byte in the packet has the all-zero value.
</t>
<figure anchor="altFormat" title="Alternate EKT Authentication Tag format.">
<artwork>
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 |1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |0|
+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t> In the alternate format, the SPI field is 15 bits long, instead of
16 bits long. Sender-side implementations using the existing format
can achieve interoperability with the alternate format by selecting
SPI values have a final bit that is equal to one. Receiver
implementations using the existing format can interoperate with the
alternate format if SPI values ending in one are used, and the sender
always sends all of the EKT fields.
</t>
<t>
The main motivation for the alternate format is the case when RTCP is
not available and EKT data is carried by RTP, and bandwidth
conservation is important. However, it may be acceptable to use it
for RTCP as well.
</t>
</section>
-->
<section title="Using EKT to Optimize Interworking DTLS-SRTP with Security Descriptions">
<t>Today, <xref target="RFC4568">SDP Security Descriptions</xref> is
used for distributing SRTP keys in several different IP PBX systems and
is expected to be used by 3GPP's Long Term Evolution (LTE). The IP PBX
systems are typically used within a single enterprise, and LTE is used
within the confines of a mobile operator's network. 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.</t>
<t>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 <xref target="interworking-expensive"></xref>, below. This is
computationally expensive.</t>
<figure anchor="interworking-expensive"
title="Interworking Security Descriptions and DTLS-SRTP">
<preamble></preamble>
<artwork align="center"><![CDATA[
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-->|
| | |]]></artwork>
<postamble></postamble>
</figure>
<t>The message flow is as follows (similar steps occur with SRTCP):<list
style="numbers">
<t>The <xref target="RFC4568">Security Descriptions</xref> endpoint
discloses its SRTP key to the SBC, using a=crypto in its SDP.</t>
<t>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).</t>
<t>The SBC communicates the SRTP encryption key (key B) to the
Security Descriptions endpoint (using a=crypto). (There is no way,
with DTLS-SRTP, to communicate the Security Descriptions key to the
DTLS-SRTP key endpoint.)</t>
<t>The DTLS-SRTP endpoint sends an SRTP key, encrypted with its key
B. This is received by the SBC.</t>
<t>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.</t>
<t>The Security Descriptions endpoint sends an SRTP packet,
encrypted with its key A.</t>
<t>The SBC has to authenticate and decrypt the SRTP packet (using
key A), and re-encrypt it and generate an HMAC (using key C).</t>
<t>The SBC sends the new SRTP packet.</t>
</list></t>
<t>If EKT is deployed on the DTLS-SRTP endpoints, EKT helps to avoid the
computationally expensive operation so the SBC does not need not 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.</t>
<t>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
<xref target="interworking-cheap"></xref>, below.</t>
<figure anchor="interworking-cheap"
title="Interworking Security Descriptions and EKT">
<preamble></preamble>
<artwork align="center"><![CDATA[
RFC4568 endpoint SBC DTLS-SRTP endpoint
| | |
1. |---key=A------------->| |
2. | |<-DTLS-SRTP handshake->|
3. |<--key=B--------------| |
4. | |--new_srtp_key:A------>|
5. | |<--SRTP, encrypted w/B-|
5. |<-SRTP, encrypted w/B-| |
6. |-SRTP, encrypted w/A->| |
7. | |-SRTP, encrypted w/A-->|
| | |]]></artwork>
<postamble></postamble>
</figure>
<t>The message flow is as follows (similar steps occur with SRTCP):<list
style="numbers">
<t>Security Descriptions endpoint discloses its SRTP key to the SBC
(a=crypto).</t>
<t>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).</t>
<t>The SBC communicates the SRTP encryption key (key B) to the
Security Descriptions endpoint.</t>
<t>The SBC uses the EKT to indicate that SRTP packets will be
encrypted with 'key A' towards the DTLS-SRTP endpoint.</t>
<t>The DTLS-SRTP endpoint sends an SRTP key, encrypted with its key
B. This is received by the SBC.</t>
<t>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.</t>
<t>The Security Descriptions endpoint sends an SRTP packet,
encrypted with its key A.</t>
<t>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.</t>
</list></t>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 05:33:34 |