Network Working Group D. McGrew
Internet-Draft Cisco Systems
Expires: December 12, 2006 E. Rescorla
Network Resonance
June 10, 2006
Datagram Transport Layer Security (DTLS) Extension to Establish Keys for
Secure Real-time Transport Protocol (SRTP)
draft-mcgrew-tls-srtp-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Secure Real-time Transport Protocol (SRTP) is a profile of the
Real-time Transport Protocol that can provide confidentiality,
message authentication, and replay protection to the RTP traffic and
to the control traffic for RTP, the Real-time Transport Control
Protocol (RTCP). This document describes a method of using DTLS key
management for SRTP by using a new extension that indicates that SRTP
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is to be used for data protection, and which establishes SRTP keys.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 3
3.1. Usage Model . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. The use_srtp Extension . . . . . . . . . . . . . . . . . . 4
3.2.1. use_srtp Extension Definition . . . . . . . . . . . . 5
3.2.2. SRTP Protection Profiles . . . . . . . . . . . . . . . 6
3.2.3. srtp_mki value . . . . . . . . . . . . . . . . . . . . 8
3.3. Key Derivation . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Key Scope . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5. Key Usage Limitations . . . . . . . . . . . . . . . . . . 11
3.6. Data Protection . . . . . . . . . . . . . . . . . . . . . 11
3.6.1. Transmission . . . . . . . . . . . . . . . . . . . . . 11
3.6.2. Reception . . . . . . . . . . . . . . . . . . . . . . 11
3.7. Rehandshake and Re-key . . . . . . . . . . . . . . . . . . 12
4. Multi-party RTP Sessions . . . . . . . . . . . . . . . . . . . 13
4.1. SIP Forking . . . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
5.1. Security of Negotiation . . . . . . . . . . . . . . . . . 13
5.2. Framing Confusion . . . . . . . . . . . . . . . . . . . . 13
5.3. Sequence Number Interactions . . . . . . . . . . . . . . . 14
5.3.1. Alerts . . . . . . . . . . . . . . . . . . . . . . . . 14
5.3.2. Renegotiation . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informational References . . . . . . . . . . . . . . . . . 16
Appendix A. Open Issue: Key/Stream Interaction . . . . . . . . . 16
Appendix B. Open Issue: Using a single DTLS session per SRTP
session . . . . . . . . . . . . . . . . . . . . . . . 18
B.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 19
B.1.1. SRTP Parameter Profiles for Single-DTLS . . . . . . . 20
B.2. Pros and Cons of Single-DTLS . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
The Secure Real-time Transport Protocol (SRTP) [6] is a profile of
the Real-time Transport Protocol (RTP) [1] that can provide
confidentiality, message authentication, and replay protection to RTP
traffic and to the control traffic for RTP, the Real-time Transport
Control Protocol (RTCP). SRTP does not provide key management
functionality but instead depends on external key management to
provide secret master keys and the algorithms and parameters for use
with those keys.
Datagram Transport Layer Security (DTLS) [5] is a channel security
protocol that offers integrated key management, parameter
negotiation, and secure data transfer. Because DTLS's data transfer
protocol is generic, it is less highly optimized for use with RTP
than is SRTP, which has been specifically tuned for that purpose.
This document describes DTLS-SRTP, an SRTP extension for DTLS which
combine the performance and encryption flexibility benefits of SRTP
with the flexibility and convenience of DTLS's integrated key and
association management. DTLS-SRTP can be viewed in two equivalent
ways: as a new key management method for SRTP, and a new RTP-
specific data format for DTLS.
This extension MUST only be used when the data being transported is
RTP and RTCP [4].
The key points of DTLS-SRTP are that:
o application data is protected using SRTP,
o the DTLS handshake is used to establish keying material,
algorithms, and parameters for SRTP,
o a DTLS extension used to negotiate SRTP algorithms, and
o other DTLS record layer content types are protected using the
ordinary DTLS record format.
The next section provides details of the new extension.
2. Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [2].
3. Protocol Description
In this section we define the DTLS extension and its use.
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3.1. Usage Model
DTLS-SRTP is defined for point-to-point media sessions, in which
there are exactly two participants. Each DTLS-SRTP session contains
a single DTLS association (called a "connection" in TLS jargon), and
an SRTP context. Each RTCP flow is protected by a separate DTLS-SRTP
session that is independent from the DTLS-SRTP session that protects
the RTP packet flow. Between a single pair of participants, there
may be multiple media sessions; each session MUST have its own DTLS-
SRTP session. One or both of the DTLS-SRTP session participants MAY
be RTP mixers.
A DTLS-SRTP session can be indicated by an external signaling
protocol like SIP. When the signaling exchange is integrity-
protected (e.g when SIP Identity protection via digital signatures is
used), DTLS-SRTP can leverage this integrity guarantee to provide
complete security of the media stream. A description of how to
indicate DTLS-SRTP sessions in SIP and SDP, and how to authenticate
the endpoints using fingerprints can be found in [8] and [7].
Each RTP stream participates in a single DTLS session. In a naive
implementation, this results in new DTLS session establishment
(complete with public key cryptography) for each media channel. For
example, a videophone may be sending both an audio stream and a video
stream, each of which would use a separate DTLS session establishment
exchange, which would proceed in parallel. As an optimization, the
DTLS-SRTP implementation SHOULD use the following strategy: a single
DTLS connection is established, and all other DTLS sessions wait
until that connection is established before proceeding with their
session establishment exchanges. This strategy allows the later
sessions to use the DTLS session re-start, which allows the
amortization of the expensive public key cryptography operations over
multiple DTLS session establishment instances.
The SRTP keys used to protect packets originated by the client are
distinct from the SRTP keys used to protect packets originated by the
server. All of the RTP sources originating on the client use the
same SRTP keys, and similarly, all of the RTP sources originating on
the server over the same channel use the same SRTP keys. The SRTP
implementation MUST ensure that all of the SSRC values for all of the
RTP sources originating from the same device are distinct, in order
to avoid the "two-time pad" problem (as described in Section 9.1 of
RFC 3711).
3.2. The use_srtp Extension
In order to negotiate the use of SRTP data protection, clients MAY
include an extension of type "use_srtp" in the extended client hello.
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The "extension_data" field of this extension contains the list of
acceptable SRTP protection profiles, as indicated below.
Servers that receive an extended hello containing a "use_srtp"
extension MAY agree to use SRTP by including an extension of type
"use_srtp", with the chosen protection profile in the extended server
hello. This process is shown below.
Client Server
ClientHello + use_srtp -------->
ServerHello + use_srtp
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
SRTP packets <-------> SRTP packets
Once the "use_srtp" extension is negotiated, packets of type
"application_data" in the newly negotiated association (i.e., after
the change_cipher_spec) MUST be protected using SRTP and packets of
type "application_data" MUST NOT be sent. Records of type other than
"application_data" MUST use ordinary DTLS framing. When the
"use_srtp" extension is in effect, implementations MUST NOT place
more than one "record" per datagram. (This is only meaningful from
the perspective of DTLS because SRTP is inherently oriented towards
one payload per packet, but is stated purely for clarification.)
3.2.1. use_srtp Extension Definition
The client MUST fill the extension_data field of the "use_srtp"
extension with an UseSRTPData value:
uint8 SRTPProtectionProfile[2];
struct {
SRTPProtectionProfiles SRTPProtectionProfiles;
uint8 srtp_mki<255>;
} UseSRTPData;
SRTPProtectionProfile SRTPProtectionProfiles<2^16-1>;
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The SRTPProtectionProfiles list indicates the SRTP protection
profiles that the client is willing to support, listed in descending
order of preference. The srtp_mki value contains the SRTP
MasterKeyIdentifier (MKI) value (if any) which the client will use
for his SRTP messages.
If the server is willing to accept the use_srtp extension, it MUST
respond with its own "use_srtp" extension in the ExtendedServerHello.
The extension_data field MUST contain a UseSRTPData value with a
single SRTPProtectionProfile value which the server has chosen for
use with this connection. The server MUST NOT select a value which
the client has not offered. If there is no shared profile, the
server should not return the use_srtp extension at which point the
connection falls back to the negotiated DTLS cipher suite. If that
is not acceptable the server should return an appropriate DTLS alert.
3.2.2. SRTP Protection Profiles
A DTLS-SRTP SRTP Protection Profile defines the parameters and
options that are in effect for the SRTP processing. This document
defines the following SRTP protection profiles.
SRTPProtectionProfile SRTP_AES128_CM_SHA1_80 = {0x00, 0x01};
SRTPProtectionProfile SRTP_AES128_CM_SHA1_32 = {0x00, 0x02};
SRTPProtectionProfile SRTP_AES256_CM_SHA1_80 = {0x00, 0x03};
SRTPProtectionProfile SRTP_AES256_CM_SHA1_32 = {0x00, 0x04};
SRTPProtectionProfile SRTP_NULL_SHA1_80 = {0x00, 0x05};
SRTPProtectionProfile SRTP_NULL_SHA1_32 = {0x00, 0x06};
The following list indicates the SRTP transform parameters for each
protection profile. The parameters cipher_key_length,
cipher_salt_length, auth_key_length, and auth_tag_length express the
number of bits in the values to which they refer. The
maximum_lifetime parameter indicates the maximum number of packets
that can be protected with each single set of keys when the parameter
profile is in use.
All of the crypto algorithms in these profiles are from [6], except
for the AES256_CM cipher, which is specified in [13].
SRTP_AES128_CM_HMAC_SHA1_80
cipher: AES_128_CM
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cipher_key_length: 128
cipher_salt_length: 112
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 80
SRTP_AES128_CM_HMAC_SHA1_32 Because of the short authentication tag
provides limited authentication, this profile SHOULD NOT be used
with RTCP.
cipher: AES_128_CM
cipher_key_length: 128
cipher_salt_length: 112
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 32
SRTP_AES256_CM_HMAC_SHA1_80
cipher: AES_128_CM
cipher_key_length: 128
cipher_salt_length: 112
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 80
SRTP_AES256_CM_HMAC_SHA1_32 Because of the short authentication tag
provides limited authentication, this profile SHOULD NOT be used
with RTCP.
cipher: AES_128_CM
cipher_key_length: 128
cipher_salt_length: 112
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 32
SRTP_NULL_HMAC_SHA1_80
cipher: NULL
cipher_key_length: 0
cipher_salt_length: 0
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 80
SRTP_NULL_HMAC_SHA1_32 Because the short authentication tag provides
limited authentication, this profile SHOULD NOT be used with RTCP.
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cipher: NULL
cipher_key_length: 0
cipher_salt_length: 0
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 32
With all of these SRTP Parameter profiles, the following SRTP options
are in effect:
The TLS Key Derivation Function (KDF) is used to generate keys to
feed into the SRTP KDF.
The Key Derivation Rate (KDR) is equal to zero. Thus, keys are
not re-derived based on the SRTP sequence number.
For all other parameters, the default values are used.
All SRTP parameters that are not determined by the SRTP Protection
Profile MAY be established via the signaling system. In particular,
the relative order of Forward Error Correction and SRTP processing,
and a suggested SRTP replay window size SHOULD be established in this
manner. An example of how these parameters can be defined for SDP by
is contained in [9].
Applications using DTLS-SRTP SHOULD coordinate the SRTP Protection
Profiles between the DTLS-SRTP session that protects an RTP flow and
the DTLS-SRTP session that protects the associated RTCP flow. In
particular, identical ciphers SHOULD be used.
New SRTPProtectionProfile values must be defined by RFC 2434
Standards Action. See Section Section 6 for IANA Considerations.
3.2.3. srtp_mki value
The srtp_mki value can be used to indicate the capability and desire
to use the SRTP Master Key Indicator (MKI) field in the SRTP and
SRTCP packets. The MKI field indicates to an SRTP receiver which key
was used to protect the packet that contains that field. The
srtp_mki field contains the value of the SRTP MKI which is associated
with the SRTP master keys derived from this handshake. Each SRTP
session MUST have exactly one master key that is used to protect
packets at any given time. The client MUST choose the MKI value so
that it is distinct from the last MKI value that was used, and it
SHOULD make these values unique.
Upon receipt of a "use_srtp" extension containing a "srtp_mki" field,
the server MUST either (assuming it accepts the extension at all):
1. include a matching "srtp_mki" value in its "use_srtp" extension
to indicate that it will make use of the MKI, or
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2. return an empty "srtp_mki" value to indicate that it cannot make
use of the MKI.
If the client detects a nonzero-length MKI in the server's response
that is different than the one the client offered MUST abort the
handshake and SHOULD send an invalid_parameter alert. If the client
and server agree on an MKI, all SRTP packets protected under the new
security parameters MUST contain that MKI.
3.3. Key Derivation
When SRTP mode is in effect, different keys are used for ordinary
DTLS record protection and SRTP packet protection. These keys are
generated as additional keying material at the end of the DTLS key
block. Thus, the key block becomes:
client_write_MAC_secret[SecurityParameters.hash_size]
server_write_MAC_secret[SecurityParameters.hash_size]
client_write_key[SecurityParameters.key_material_len]
server_write_key[SecurityParameters.key_material_len]
client_write_SRTP_master_key[SRTPSecurityParams.master_key_len]
server_write_SRTP_master_key[SRTPSecurityParams.master_key_len]
client_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len]
server_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len]
The last four values are provided as inputs to the SRTP key
derivation mechanism, as shown in Figure 5 and detailed below. By
default, the mechanism defined in Section 4.3 of [6] is used, unless
another key derivation mechanism is specified as part of an SRTP
Protection Profile.
The client_write_SRTP_master_key and client_write_SRTP_master_salt
are provided to one invocation of the SRTP key derivation function,
to generate the SRTP keys used to encrypt and authenticate packets
sent by the client. The server MUST only use these keys to decrypt
and to check the authenticity of inbound packets.
The server_write_SRTP_master_key and server_write_SRTP_master_salt
are provided to one invocation of the SRTP key derivation function,
to generate the SRTP keys used to encrypt and authenticate packets
sent by the server. The client MUST only use these keys to decrypt
and to check the authenticity of inbound packets.
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+------- TLS master secret
|
v +-> client_write_MAC_secret
+-----+ |
| TLS |--+-> server_write_MAC_secret
| KDF | |
+-----+ +-> client_write_key
|
+-> server_write_key
| +------+ SRTP
+-> client_write_SRTP_master_key ----->| SRTP |-> client
| +--->| KDF | write
+-> server_write_SRTP_master_key --|-+ +------+ keys
| | |
+-> client_write_SRTP_master_salt -+ | +------+ SRTP
| +->| SRTP |-> server
+-> server_write_SRTP_master_salt ----->| KDF | write
+------+ keys
Figure 5: The derivation of the SRTP keys.
Each DTLS-SRTP session protects the part of an SRTP session that
passes over a single source/destinaation transport address pair, as
shown in Figure 6. When SRTP is run over that port, the SRTCP keys
are not needed and are discarded. When SRTCP is run over that port,
the SRTP keys are not needed and are discarded.
Client Server
(Sender) (Receiver)
(1) <----- DTLS ------> src/dst = a/b and b/a
------ SRTP ------> src/dst = a/b, uses client write keys
(2) <----- DTLS ------> src/dst = c/d and d/c
------ SRTCP -----> src/dst = c/d, uses client write keys
<----- SRTCP ------ src/dst = d/c, uses server write keys
Figure 6: A DTLS-SRTP session protecting RTP (1) and another one
protecting RTCP (2), showing the transport addresses and keys used.
3.4. Key Scope
Because of the possibility of packet reorder, DTLS-SRTP
implementations SHOULD store multiple SRTP keys sets during a re-key
in order to avoid the need for receivers to drop packets for which
they lack a key.
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3.5. Key Usage Limitations
The maximum_lifetime parameter in the SRTP protection profile
indicates the maximum number of packets that can be protected with
each single encryption and authentication key. Each profile defines
its own limit. When this limit is reached, a new DTLS session SHOULD
be used to establish replacement keys, and SRTP implementations MUST
NOT use the existing keys for the processing of either outbound or
inbound traffic.
3.6. Data Protection
Once the DTLS handshake has completed the peers can send RTP or RTCP
over the newly created channel. We describe the transmission process
first followed by the reception process.
Within each RTP session, SRTP processing MUST NOT take place before
the DTLS handshake completes.
3.6.1. Transmission
DTLS and TLS define a number of record content types. In ordinary
TLS/DTLS, all data is protected using the same record encoding and
mechanisms. When the mechanism described in this document is in
effect, this is modified so that data of type "application_data"
(used to transport data traffic) is encrypted using SRTP rather than
the standard TLS record encoding.
When a user of DTLS wishes to send an RTP packet in SRTP mode it
delivers it to the DTLS implementation as a single write of type
"application_data". The DTLS implementation then invokes the
processing described in RFC 3711 Sections 3 and 4. The resulting
SRTP packet is then sent directly on the wire as a single datagram
with no DTLS framing. This provides an encapsulation of the data
that conforms to and interoperates with SRTP. Note that the RTP
sequence number rather than the DTLS sequence number is used for
these packets.
3.6.2. Reception
When DTLS-SRTP is used to protect an RTP session, the RTP receiver
needs to demultiplex packets that are arriving on the RTP port.
Arriving packets may be of types RTP, DTLS, or STUN[12]. The type of
a packet can be determined by looking at its first byte.
The process for demultiplexing a packet is as follows. The receiver
looks at the first byte of the packet. If the value of this byte is
0 or 1, then the packet is STUN. If the value is in between 128 and
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191 (inclusive), then the packet is RTP. If the value is between 20
and 63 (inclusive), the packet is DTLS. This processes is summarized
in Figure 7.
+----------------+
| 127 < B < 192 -+--> forward to RTP
| |
packet --> | 19 < B < 64 -+--> forward to DTLS
| |
| B < 2 -+--> forward to STUN
+----------------+
Figure 7: The DTLS-SRTP receiver's packet demultiplexing
algorithm. Here the field B denotes the leading byte of the packet.
3.7. Rehandshake and Re-key
Rekeying in DTLS is accomplished by performing a new handshake over
the existing DTLS channel. This handshake can be performed in
parallel with data transport, so no interruption of the data flow is
required. Once the handshake is finished, the newly derived set of
keys is used to protect all outbound packets, both DTLS and SRTP.
Because of packet reordering, packets protected by the previous set
of keys can appear on the wire after the handshake has completed. To
compensate for this fact, receivers SHOULD maintain both sets of keys
for some time in order to be able to decrypt and verify older
packets. The keys should be maintained for the duration of the
maximum segment lifetime (MSL).
If an MKI is used, then the receiver should use the corresponding set
of keys to process an incoming packet. Otherwise, when a packet
arrives after the handshake completed, a receiver SHOULD use the
newly derived set of keys to process that packet unless there is an
MKI (If the packet was protected with the older set of keys, this
fact will become apparent to the receiver as an authentication
failure will occur.) If the authentication check on the packet fails
and no MKI is being used, then the receiver MAY process the packet
with the older set of keys. If that authentication check indicates
that the packet is valid, the packet should be accepted; otherwise,
the packet MUST be discarded and rejected.
Receivers MAY use the SRTP packet sequence number to aid in the
selection of keys. After a packet has been received and
authenticated with the new key set, any packets with sequence numbers
that are greater will also have been protected with the new key set.
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4. Multi-party RTP Sessions
Since DTLS is a point-to-point protocol, DTLS-SRTP is intended only
to protect RTP sessions in which there are exactly two participants.
This does not preclude its use with RTP mixers. For example, a
conference bridge may use DTLS-SRTP to secure the communication to
and from each of the participants in a conference.
4.1. SIP Forking
When SIP parallel forking occurs while establishing an RTP session, a
situation may arise in which two or more sources are sending RTP
packets to a single RTP destination transport address. When this
situation arises and DTLS-SRTP is in use, the receiver MUST use the
source transport IP address and port of each packet to distinguish
between the senders, and treat the flow of packets from each distinct
source transport address as a distinct DTLS-SRTP session for the
purposes of the DTLS association.
When SIP forking occurs, two methods can be used to correlate each
answer to the corresponding DTLS-SRTP session. If the answers have
different certificates then fingerprints in the answers can be used
to correlate the SIP dialogs with the associated DTLS session. Note
that two forks with the same certificate cannot be distinguished at
the DTLS level, but this problem is a generic problem with SIP
forking and should be solved at a higher level.
5. Security Considerations
The use of multiple data protection framings negotiated in the same
handshake creates some complexities, which are discussed here.
5.1. Security of Negotiation
One concern here is that attackers might be able to implement a bid-
down attack forcing the peers to use ordinary DTLS rather than SRTP.
However, because the negotiation of this extension is performed in
the DTLS handshake, it is protected by the Finished messages.
Therefore, any bid-down attack is automatically detected, which
reduces this to a denial of service attack - which any attacker who
can control the channel can always mount.
5.2. Framing Confusion
Because two different framing formats are used, there is concern that
an attacker could convince the receiver to treat an SRTP-framed RTP
packet as a DTLS record (e.g., a handshake message) or vice versa.
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This attack is prevented by using different keys for MAC verification
for each type of data. Therefore, this type of attack reduces to
being able to forge a packet with a valid MAC, which violates a basic
security invariant of both DTLS and SRTP.
As an additional defense against injection into the DTLS handshake
channel, the DTLS record type is included in the MAC. Therefore, an
SRTP record would be treated as an unknown type and ignored. (See
Section 6 of [10]).
5.3. Sequence Number Interactions
As described in Section Section 3.6.1, the SRTP and DTLS sequence
number spaces are distinct. This means that it is not possible to
unambiguously order a given DTLS control record with respect to an
SRTP packet. In general, this is relevant in two situations: alerts
and rehandshake.
5.3.1. Alerts
Because DTLS handshake and change_cipher_spec messages share the same
sequence number space as alerts, they can be ordered correctly.
Because DTLS alerts are inherently unreliable and SHOULD NOT be
generated as a response to data packets, reliable sequencing between
SRTP packets and DTLS alerts is not an important feature. However,
implementations which wish to use DTLS alerts to signal problems with
the SRTP encoding SHOULD simply act on alerts as soon as they are
received and assume that they refer to the temporally contiguous
stream. Such implementations MUST check for alert retransmission and
discard retransmitted alerts to avoid overreacting to replay attacks.
5.3.2. Renegotiation
Because the rehandshake transition algorithm specified in Section
Section 3.7 requires trying multiple sets of keys if no MKI is used,
it slightly weakens the authentication. For instance, if an n-bit
MAC is used and k different sets of keys are present, then the MAC is
weakened by log_2(k) bits to n - log_2(k). In practice, since the
number of keys used will be very small and the MACs in use are
typically strong (the default for SRTP is 80 bits) the decrease in
security involved here is minimal.
Another concern here is that this algorithm slightly increases the
work factor on the receiver because it needs to attempt multiple
validations. However, again, the number of potential keys will be
very small (and the attacker cannot force it to be larger) and this
technique is already used for rollover counter management, so the
authors do not consider this to be a serious flaw.
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6. IANA Considerations
This document a new extension for DTLS, in accordance with [11]:
enum { use_srtp (??) } ExtensionType;
[[ NOTE: This value needs to be assigned by IANA ]]
This extension MUST only be used with DTLS, and not with TLS.
Section Section 3.2.2 requires that all SRTPProtectionProfile values
be defined by RFC 2434 Standards Action. IANA SHOULD create a DTLS
SRTPProtectionProfile registry initially populated with values from
Section Section 3.2.2 of this document. Future values MUST be
allocated via Standards Action as described in [3]
7. Acknowledgments
Special thanks to Jason Fischl, Flemming Andreasen, Dan Wing, and
Cullen Jennings for input, discussions, and guidance.
8. References
8.1. Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 1889, January 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[4] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[5] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[6] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
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8.2. Informational References
[7] Fischl, J., "Session Initiation Protocol (SIP) for Media Over
Datagram Transport Layer Security (DTLS)",
draft-fischl-sipping-media-dtls-00 (work in progress),
March 2006.
[8] Fischl, J. and H. Tschofenig, "Session Description Protocol
(SDP) Indicators for Datagram Transport Layer Security
(DTLS)", draft-fischl-mmusic-sdp-dtls-00 (work in progress),
March 2006.
[9] Andreasen, F., "Session Description Protocol Security
Descriptions for Media Streams",
draft-ietf-mmusic-sdescriptions-12 (work in progress),
September 2005.
[10] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1",
draft-ietf-tls-rfc2246-bis-13 (work in progress), June 2005.
[11] Blake-Wilson, S., "Transport Layer Security (TLS) Extensions",
draft-ietf-tls-rfc3546bis-02 (work in progress), October 2005.
[12] Rosenberg, J., "Simple Traversal of UDP Through Network Address
Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-03
(work in progress), March 2006.
[13] McGrew, D., "The use of AES-192 and AES-256 in Secure RTP",
draft-mcgrew-srtp-big-aes-00 (work in progress), April 2006.
Appendix A. Open Issue: Key/Stream Interaction
Standard practice for security protocols such as TLS, DTLS, and SSH
which do inline key management is to create a separate security
association for each underlying network channel (TCP connection, UDP
host/port quartet, etc.). This has dual advantages of simplicity and
independence of the security contexts for each channel.
Three concerns have been raised about the overhead of this strategy
in the context of RTP security. The first concern is the additional
performance overhead of doing a separate public key operation for
each channel. The conventional procedure here (used in TLS and DTLS)
is to establish a master context which can then be used to derive
fresh traffic keys for new associations. In TLS/DTLS this is called
"session resumption" and can be transparently negotiated between the
peers. Similar techniques could be applied to other inline RTP
security protocols.
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The second concern is network bandwidth overhead for the
establishment of subsequent connections and for rehandshake (for
rekeying) for existing connections. In particular, there is a
concern that the channels will have very narrow capacity requirements
allocated entirely to media which will be overflowed by the
rehandshake. Measurements of the size of the rehandshake (with
resumption) in TLS indicate that it is about 300-400 bytes if a full
selection of cipher suites is offered. (the size of a full handshake
is approximately 1-2k larger because of the certificate and keying
material exchange).
The third concern is the additional round-trips associated with
establishing the 2nd, 3rd, ... channels. In TLS/DTLS these can all
be done in parallel but in order to take advantage of session
resumption they should be done after the first channel is
established. For two channels this provides a ladder diagram
something like this (parenthetical #s are media channel #s)
Alice Bob
-------------------------------------------
<- ClientHello (1)
ServerHello (1) ->
Certificate (1)
ServerHelloDone (1)
<- ClientKeyExchange (1)
ChangeCipherSpec (1)
Finished (1)
ChangeCipherSpec (1)->
Finished (1)->
<--- Channel 1 ready
<- ClientHello (2)
ServerHello (2) ->
ChangeCipherSpec(2)->
Finished(2) ->
<- ChangeCipherSpec (2)
Finished (2)
<--- Channel 2 ready
So, there is an additional 1 RTT after Channel 1 is ready before
Channel 2 is ready. If the peers are potentially willing to forego
resumption they can interlace the handshakes, like so:
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Alice Bob
-------------------------------------------
<- ClientHello (1)
ServerHello (1) ->
Certificate (1)
ServerHelloDone (1)
<- ClientKeyExchange (1)
ChangeCipherSpec (1)
Finished (1)
<- ClientHello (2)
ChangeCipherSpec (1)->
Finished (1)->
<--- Channel 1 ready
ServerHello (2) ->
ChangeCipherSpec(2)->
Finished(2) ->
<- ChangeCipherSpec (2)
Finished (2)
<--- Channel 2 ready
In this case the channels are ready contemporaneously, but if a
message in handshake (1) is lost then handshake (2) requires either a
full rehandshake or that Alice be clever and queue the resumption
attempt until the first handshake completes. Note that just dropping
the packet works as well since Bob will retransmit.
We don't know if this is a problem yet or whether it is possible to
use some of the capacity allocated to other channels (e.g., RTCP) to
perform the rehandshake. Another alternative that has been proposed
is to use one security association connection on a single channel and
reuse the keying material across multiple channels, but this gives up
the simplicity and independence benefits mentioned above and so is
architecturally undesirable unless absolutely necessary.
Another alternative is to take advantage of the fact that an (S)RTP
channel is intended to be paired with an (S)RTCP channel. The DTLS
handshake could be performed on just one of those channels and the
same keys used for both the RTP and RTCP channels. This alternative
is defined in Appendix B for study and discussion.
Appendix B. Open Issue: Using a single DTLS session per SRTP session
In order to address the performance, bandwidth, and latency concerns
described in Appendix A, it may be desirable to use a single DTLS
session for each SRTP session. This appendix outlines one approach
for achieving that goal in the next subsection, and then describes
its benefits in the following one.
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B.1. Definition
This section defines the "Single DTLS" model by descibing its
differences from the DTLS-SRTP as defined in the body of this
document. A point-to-point SRTP session consists of a unidirectional
SRTP flow and a bidirectional SRTCP flow. In the Single-DTLS model,
a DTLS-SRTP session contains a single SRTP session and a single DTLS
connection. Within each SRTP session, there may be multiple SRTP
sources; all of these sources use a single SRTP master key. See
Figure 11. As before, the DTLS connection uses the same transport
addresses as the RTP flow; receivers demultiplex packets by
inspection of their first byte.
+------------------------------------------------+
| TLS Master Secret |
| | |
| v |
| TLS PRF |
| | |
| +--------------------+---------------------+ |
| | | | |
| | v | |
| | SRTP Master Key | |
| | | | |
| | +------------+------------+ | |
| | | | | | |
| | v v v | |
| | +----------+ +----------+ | |
| | | SRTP | | SRTP | ... | |
| | | Source A | | Source B | | |
| | +----------+ +----------+ | |
| | | |
| | SRTP Session | |
| +------------------------------------------+ |
| |
| DTLS-SRTP Session |
+------------------------------------------------+
Figure 11: Key derivation in the Single-DTLS model.
Until the SRTP keys have been established by the DTLS handshake, the
participants MUST reject all SRTP and SRTCP packets that they receive
in the DTLS-SRTP session.
The SRTP keys are derived as defined in Section 3.3, and are used as
shown in Figure 12. This figure should be contrasted with Figure 6.
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Client Server
(Sender) (Receiver)
<----- DTLS ------> src/dst = a/b and b/a
------ SRTP ------> src/dst = a/b, uses client write keys
------ SRTCP -----> src/dst = c/d, uses client write keys
<----- SRTCP ------ src/dst = d/c, uses server write keys
Figure 12: A DTLS-SRTP session in the Single-DTLS model.
B.1.1. SRTP Parameter Profiles for Single-DTLS
The following list indicates the SRTP transform parameters for each
protection profile in the Single-DTLS model. The main difference is
that, in this model, an SRTP Parameter Profile determines the policy
for the protection of RTP and RTCP packets.
SRTP_AES128_CM_HMAC_SHA1_80
SRTP and SRTCP cipher: AES_128_CM
SRTP and SRTCP cipher_key_length: 128
SRTP and SRTCP cipher_salt_length: 112
SRTP maximum_lifetime: 2^48
SRTCP maximum_lifetime: 2^31
SRTP and SRTCP auth_function: HMAC-SHA1
SRTP and SRTCP auth_key_length: 160
SRTP and SRTCP auth_tag_length: 80
SRTP_AES128_CM_HMAC_SHA1_32
SRTP and SRTCP cipher: AES_128_CM
SRTP and SRTCP cipher_key_length: 128
SRTP and SRTCP cipher_salt_length: 112
SRTP maximum_lifetime: 2^48
SRTCP maximum_lifetime: 2^31
SRTP and SRTCP auth_function: HMAC-SHA1
SRTP and SRTCP auth_key_length: 160
SRTP auth_tag_length: 32
SRTCP auth_tag_length: 80
SRTP_AES256_CM_HMAC_SHA1_80
SRTP and SRTCP cipher: AES_128_CM
SRTP and SRTCP cipher_key_length: 128
SRTP and SRTCP cipher_salt_length: 112
SRTP maximum_lifetime: 2^48
SRTCP maximum_lifetime: 2^31
SRTP and SRTCP auth_function: HMAC-SHA1
SRTP and SRTCP auth_key_length: 160
SRTP and SRTCP auth_tag_length: 80
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SRTP_AES256_CM_HMAC_SHA1_32
SRTP and SRTCP cipher: AES_128_CM
SRTP and SRTCP cipher_key_length: 128
SRTP and SRTCP cipher_salt_length: 112
SRTP maximum_lifetime: 2^48
SRTCP maximum_lifetime: 2^31
SRTP and SRTCP auth_function: HMAC-SHA1
SRTP and SRTCP auth_key_length: 160
SRTP auth_tag_length: 32
SRTCP auth_tag_length: 80
SRTP_NULL_HMAC_SHA1_80
SRTP and SRTCP cipher: NULL
SRTP and SRTCP cipher_key_length: 0
SRTP and SRTCP cipher_salt_length: 0
SRTP maximum_lifetime: 2^48
SRTCP maximum_lifetime: 2^31
SRTP and SRTCP auth_function: HMAC-SHA1
SRTP and SRTCP auth_key_length: 160
SRTP and SRTCP auth_tag_length: 80
SRTP_NULL_HMAC_SHA1_32
SRTP and SRTCP cipher: NULL
SRTP and SRTCP cipher_key_length: 0
SRTP and SRTCP cipher_salt_length: 0
SRTP maximum_lifetime: 2^48
SRTCP maximum_lifetime: 2^31
SRTP and SRTCP auth_function: HMAC-SHA1
SRTP and SRTCP auth_key_length: 160
SRTP auth_tag_length: 32
SRTCP auth_tag_length: 80
B.2. Pros and Cons of Single-DTLS
Using a single DTLS session per SRTP session has potential
performance benefits in terms of reducing latency and compution. The
discussion of the performance of multiple parallel DTLS connections
in Appendix A applies here as well. In addition. It provides a good
match for existing SRTP implementations, since it matches their SRTP
policy definitions for cryptographic algorithm configuration and
makes use of all of the derived keys rather than having to discard
half as described in Section 3.3.
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Authors' Addresses
David McGrew
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95305
USA
Email: mcgrew@cisco.com
Eric Rescorla
Network Resonance
2483 E. Bayshore #212
Palo Alto, CA 94303
USA
Email: ekr@networkresonance.com
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