Internet Engineering Task Force Baugher (Cisco)
MSEC Working Group Carrara (Ericsson)
INTERNET-DRAFT
EXPIRES: August 2004 February 2004
The Use of TESLA in SRTP
<draft-ietf-msec-srtp-tesla-00.txt>
Status of this memo
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Abstract
This memo describes the use of the Timed Efficient Stream loss-
tolerant Authentication (TESLA) transform within the Secure Real-
time Transport Protocol (SRTP), to provide data origin
authentication for multicast and broadcast data streams.
INTERNET-DRAFT TESLA-SRTP February, 2004
TABLE OF CONTENTS
1. Introduction...................................................2
1.1. Notational Conventions.......................................3
2. SRTP...........................................................3
3. TESLA..........................................................4
4. Usage of TESLA within SRTP.....................................4
4.1. The TESLA extension..........................................4
4.2. SRTP Packet Format...........................................5
4.3. Extension of the SRTP Cryptographic Context..................6
4.4. SRTP Processing..............................................7
4.4.1 Sender Processing...........................................8
4.4.2 Receiver Processing.........................................8
4.5. SRTCP Packet Format..........................................9
4.6. TESLA MAC...................................................11
4.7. PRFs........................................................11
5. TESLA Bootstrapping...........................................12
6. SRTP TESLA Default parameters.................................12
6.1 Transform-independent Parameter: SRTP MAC with TESLA MAC.....13
6.2 Transform-dependent Parameters for TESLA MAC.................13
7. Security Considerations.......................................14
8. IANA Considerations...........................................14
9. Acknowledgements..............................................14
10. Author's Addresses...........................................15
11. References...................................................15
Intellectual Property Right Considerations.......................16
Full Copyright Statement.........................................16
1. Introduction
Multicast and broadcast communication introduce some new security
challenges compared to unicast communication. Many multicast and
broadcast applications need "data origin authentication" (DOA), or
"source authentication", in order to guarantee that a received
message originated from a given source, and was not manipulated
during the transmission. In unicast communication, a pairwise
security association between one sender and one receiver can provide
data origin authentication using symmetric-key cryptography (such as
a message authentication code, MAC). When the communication is
strictly pairwise, the sender and receiver agree upon a key that is
known only to them.
In groups, however, a key is shared among more than two members, and
this symmetric-key approach does not guarantee data origin
authentication. When there is a group security association
[gkmarch] instead of a pairwise security association, any of the
members can alter the packet and impersonate any other member. The
MAC in this case only guarantees that the packet was not manipulated
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by an attacker outside the group (and hence not in possession of the
group key), and that the packet was sent by a source within the
group.
Some applications cannot tolerate source ambiguity and must discern
the true sender from any other group member. A common way to solve
the problem is by use of asymmetric cryptography, such as digital
signatures. This method, unfortunately, suffers from high overhead,
in terms of time (to sign and verify) and bandwidth (to convey the
signature in the packet).
Several schemes have been proposed to provide efficient data origin
authentication in multicast and broadcast scenarios. The Timed
Efficient Stream loss-tolerant Authentication (TESLA), is one such
scheme.
This memo specifies TESLA authentication for SRTP. SRTP TESLA can
provide data origin authentication to RTP applications that use
group security associations (such as multicast RTP applications) so
long as receivers abide by the TESLA security invariants [TESLA1,
TESLA2].
1.1. Notational Conventions
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification assumes the reader familiar with both SRTP and
TESLA. Almost none of their details will be explained, and the
reader can find them in their respective specifications [SRTP,
TESLA1, TESLA2]. Also, this specification uses the same definitions
as TESLA for common terms.
2. SRTP
The Secure Real-time Transport Protocol (SRTP) [SRTP] is a profile
of RTP, which can provide confidentiality, message authentication,
and replay protection to the RTP traffic and to the RTP control
protocol, the Real-time Transport Control Protocol (RTCP).
SRTP is a framework that allows new security functions and new
transforms to be added. SRTP currently does not define any
mechanism to provide data origin authentication for group security
associations. Fortunately, it is straightforward to add TESLA to
the SRTP cryptographic framework.
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The TESLA extension to SRTP is defined in this specification, which
assumes that the reader is familiar with the SRTP specification
[SRTP], its packet structure, and processing rules.
3. TESLA
TESLA provides delayed per-packet data authentication and is
specified in two documents, an introductory overview [TESLA1] and a
second specification that defines signaling and data packet
parameters [TESLA2]. This specification assumes that the reader is
familiar with these two documents.
In addition to its SRTP data-packet definition given here, TESLA
needs an initial synchronization protocol and initial bootstrapping
procedure. The synchronization protocol allows the sender and the
receiver to compare their clocks and determine an upper bound of the
difference. The synchronization protocol is outside the scope of
this document.
TESLA also requires an initial bootstrapping procedure to exchange
needed parameters and the initial commitment to the key chain
[TESLA2]. For SRTP, it is assumed that the bootstrapping is
performed out-of-band, possibly using the key management protocol
that is exchanging the security parameters for SRTP, e.g. [GDOI,
MIKEY]. Initial bootstrapping of TESLA is outside the scope of this
document.
4. Usage of TESLA within SRTP
The present specification is an extension to the SRTP specification
[SRTP] and describes the use of TESLA with only a single key chain,
and the delayed-authentication TESLA elements of procedure [TESLA1,
TESLA2].
4.1. The TESLA extension
TESLA is an OPTIONAL authentication algorithm for SRTP. When used,
TESLA adds the fields showed in Figure 1 per-packet. The fields
added by TESLA are called "TESLA authentication extensions"
altogether, whereas "authentication tag" or "integrity protection
tag" indicate the normal integrity protection tag when the SRTP
master key is shared by more than two endpoints [SRTP].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Disclosed Key ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TESLA MAC ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: The "TESLA authentication extension".
Id: identifier of K_i, MANDATORY
The identifier of the key that was used to calculate the MAC
present in the packet during interval i.
Disclosed Key: variable length, MANDATORY
The disclosed key, that can be used to authenticate previous
packets from earlier time intervals, i.e. K_{i-d}.
TESLA MAC (Message Authentication Code): variable length, MANDATORY
The MAC computed using K'_i, which is disclosed in a subsequent
packet. The MAC coverage is defined in Section 4.6.
4.2. SRTP Packet Format
Figure 2 illustrates the format of the SRTP packet when TESLA is
applied. When applied to RTP, the TESLA authentication extension
SHALL be inserted before the (optional) SRTP MKI and (recommended)
authentication tag.
As in SRTP, the "Encrypted Portion" of an SRTP packet consists of
the encryption of the RTP payload (including RTP padding when
present) of the equivalent RTP packet.
The "Authenticated Portion" of an SRTP packet consists of the RTP
header, the Encrypted Portion of the SRTP packet, and the TESLA
authentication extension. Note that the definition is extended from
[SRTP] by the inclusion of the TESLA authentication extension.
The "TESLA Authenticated Portion" of an SRTP packet consists of the
RTP header, the Encrypted Portion of the SRTP packet, the TESLA Id
field, and the TESLA disclosed key.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+<+
|V=2|P|X| CC |M| PT | sequence number | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| timestamp | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| synchronization source (SSRC) identifier | | |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| contributing source (CSRC) identifiers | | |
| .... | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| RTP extension (OPTIONAL) | | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | payload ... | | |
| | +-------------------------------+ | |
| | | RTP padding | RTP pad count | | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | Id | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ Disclosed Key ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| ~ TESLA MAC ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<|-+
| ~ MKI ~ | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ MAC ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | |
+- Encrypted Portion TESLA Authenticated Portion ---+ |
|
Authenticated Portion ---+
Figure 2. The format of the SRTP packet when TESLA is applied. Note
that it is OPTIONAL to apply TESLA, i.e. the TESLA fields are
OPTIONAL.
4.3. Extension of the SRTP Cryptographic Context
When TESLA is used, the definition of cryptographic context in
Section 3.2 of SRTP SHALL include the following extensions:
1. an identifier for the PRF, f, implementing the one-way function
F(x) in TESLA (to derive the keys in the chain), e.g. HMAC-
SHA1,
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2. a non-negative integer n_c, determining the length of the F
output, i.e. the length of the keys in the chain (that is also
the key disclosed in an SRTP packet),
3. an identifier for the PRF, f', implementing the one-way
function F'(x) in TESLA (to derive the keys for the TESLA MAC,
from the keys in the chain), e.g. HMAC-SHA1,
4. a non-negative integer n_f, determining the length of the
output of F', i.e. of the key for the TESLA MAC,
5. an identifier for the TESLA MAC, that accepts the output of
F'(x) as its key,
6. a non-negative integer n_m, determining the length of the
output of the TESLA MAC,
7. the identifier id_j of a specific time interval I_j,
8. an NTP timestamp TI_j describing the beginning of I_j,
9. an NTP timestamp T_int describing the interval duration,
10. the key-disclosure interval, d,
11. the id_n of the final key in the keychain, K_n,
12. the interval d_n of the last key chain element.
F(x) is used to compute a keychain of keys in SRTP TESLA, as defined
in Section 6. Also according to TESLA, F'(x) computes a TESLA MAC
key with inputs as defined in Section 6.
Note that the replay list is now containing indices of recently
received packets that have been authenticated by TESLA. I.e. replay
list updates MUST NOT be based on SRTP MAC.
These parameters are all "transform-specific" parameters. There is
one transform-independent parameter that declares that SRTP message
authentication is extended with TESLA DOA authentication. Section 6
of this document defines the default values for the transform-
independent and transform-specific TESLA parameters.
4.4. SRTP Processing
The SRTP packet processing is described in Section 3.3 of the SRTP
specification [SRTP]. The use of TESLA slightly changes the
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processing, as the SRTP MAC is checked upon packet arrival for DoS
prevention, but the current packet is not TESLA-authenticated. Each
packet is buffered until a subsequent packet discloses its TESLA
key. The TESLA verification itself consists of some steps, such as
tests of TESLA security invariants, that are described in Section 4
of [TESLA1]. The words "TESLA computation" and "TESLA verification"
hereby imply all those steps, which are not all spelled out in the
following.
4.4.1 Sender Processing
The sender processing is as described in Section 3.3 of [SRTP], up
to step 5 included. After that the following process is followed:
6. When TESLA is applied, identify the key in the TESLA chain to be
used in the current time interval, and the TESLA MAC key derived
from it. Execute the TESLA computation to obtain the TESLA
authentication extension for the current packet, by appending the
key Id, the disclosed key of the chain for an earlier packet, and
the TESLA MAC under the current key from the chain. This step uses
the related TESLA parameters from the crypto context as for Step 4.
7. If the MKI indicator is set to one, append the MKI to the packet.
8. When TESLA is applied, compute the authentication tag as
described in step 7 of Section 3.3 of the SRTP specification, but
with coverage as defined in this specification (see Section 4.6).
9. If necessary, update the ROC (step 9 in Section 3.3 of [SRTP]).
4.4.2 Receiver Processing
The receiver processing is as described in Section 3.3 of [SRTP], up
to step 4 included.
To authenticate and replay-protect the current packet, the
processing is the following:
First check if the packet has been replayed (as for Section 3.3 of
[SRTP]). If the packet is judged to be replayed, then the packet
MUST be discarded, and the event SHOULD be logged.
Next, perform verification of the SRTP integrity protection tag
(note, not the TESLA MAC), if present, using the rollover counter
from the current packet, the authentication algorithm indicated in
the cryptographic context, and the session authentication key. If
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the verification is unsuccessful, the packet MUST be discarded
from further processing and the event SHOULD be logged.
If the verification is successful, remove the MKI (if present) and
authentication tag fields from the packet. The packet is buffered,
awaiting disclosure of the TESLA key in a subsequent packet.
TESLA authentication is performed on a packet when the key is
disclosed in a subsequent packet. When such key is disclosed,
perform the TESLA verification of the packet using the rollover
counter from the packet, the TESLA security parameters from the
cryptographic context, and the disclosed key. If the verification
is unsuccessful, the packet MUST be discarded from further
processing and the event SHOULD be logged. If the TESLA
verification is successful, remove the TESLA authentication
extension from the packet.
To decrypt the current packet, the processing is the following:
Decrypt the Encrypted Portion of the packet, using the decryption
algorithm indicated in the cryptographic context, the session
encryption key and salt (if used) found in Step 4 with the index
from Step 2.
Update the rollover counter and highest sequence number, s_l, in the
cryptographic context, using the packet index estimated in Step 2.
If replay protection is provided, also update the Replay List (i.e.,
the Replay List is updated after the TESLA authentication is
successfully verified).
4.5. SRTCP Packet Format
Figure 3 illustrates the format of the SRTCP packet when TESLA is
applied. The TESLA authentication extension SHALL be inserted
before the MKI and authentication tag. Recall from [SRTP] that in
SRTCP the MKI is OPTIONAL, while the E-bit, the SRTCP index, and the
authentication tag are MANDATORY.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+<+
|V=2|P| RC | PT=SR or RR | length | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| SSRC of sender | | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| ~ sender info ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ report block 1 ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ report block 2 ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ ... ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| |V=2|P| SC | PT=SDES=202 | length | | |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| | SSRC/CSRC_1 | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ SDES items ~ | |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| ~ ... ~ | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| |E| SRTCP index | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | Id (OPTIONAL) | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ Disclosed Key ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| ~ TESLA MAC ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<|-+
| ~ SRTCP MKI ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| : authentication tag : | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | |
+-- Encrypted Portion TESLA Authenticated Portion -----+ |
|
Authenticated Portion -------+
Figure 3. The format of the SRTCP packet when TESLA is applied.
Note that it is OPTIONAL to apply TESLA, i.e. the TESLA fields are
OPTIONAL.
As in SRTP, the "Encrypted Portion" of an SRTCP packet consists of
the encryption of the RTCP payload of the equivalent compound RTCP
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packet, from the first RTCP packet, i.e., from the ninth (9) octet
to the end of the compound packet.
The "Authenticated Portion" of an SRTCP packet consists of the
entire equivalent (eventually compound) RTCP packet, the E flag, the
SRTCP index (after any encryption has been applied to the payload),
and the TESLA extension. Note that the definition is extended from
[SRTP] by the inclusion of the TESLA authentication extension.
We define the "TESLA Authenticated Portion" of an SRTCP packet as
consisting of the RTCP header (first 8 bytes), the Encrypted Portion
of the SRTCP packet, the Id field, and the TESLA disclosed key.
Processing of an SRTCP packets is similar to the SRTP processing
(Section 4.3), but there are SRTCP-specific changes described in
Section 3.4 of the SRTP specification [SRTP] and in Section 4.6 of
this memo.
4.6. TESLA MAC
Let M' denote packet data to be TESLA-authenticated. In the case of
SRTP, M' SHALL consist of the SRTP TESLA Authenticated Portion (RTP
header, SRTP Encrypted Portion, TESLA Id, and disclosed key) of the
packet concatenated with the ROC of the same packet:
M' = ROC || TESLA Authenticated Portion.
In the case of SRTCP, M' SHALL consist of the SRTCP TESLA
Authenticated Portion only (RTCP header, SRTCP Encrypted Portion,
TESLA Id, and disclosed key).
The normal authentication tag SHALL be applied with the same
coverage as specified in [SRTP], i.e. Authenticated Portion || ROC
for SRTP, and Authenticated Portion for SRTCP.
The pre-defined authentication transform in SRTP, HMAC-SHA1
[RFC2104], is also used to generate the TESLA MAC. For SRTP
(respectively SRTCP), the HMAC SHALL be applied to the key in the
TESLA chain corresponding to a particular time interval, and M' as
specified above. The HMAC output SHALL then be truncated to the n_m
left-most bits. Default values are in Section 6.2.
4.7. PRFs
TESLA requires two pseudo-random functions (PRFs), f and f', to
implement
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* one one-way function F(x) to derive the key chain, and
* one one-way function F'(x) to derive (from each key of the chain)
the key that is actually used to calculate the TESLA MAC.
When TESLA is used within SRTP, the default choice of the two PRFs
SHALL be HMAC-SHA1. Default values are in Section 6.2.
Other PRFs can be chosen, and their use SHALL follow the common
guidelines in [SRTP] when adding new security parameter.
5. TESLA Bootstrapping
The extensions to the SRTP cryptographic context include a set of
TESLA parameters that are listed in section 4.3 of this document.
Key management procedures establish these parameters prior to the
commencement of an SRTP session where TESLA authentication is used.
A critical factor for the security of TESLA is that the sender and
receiver need to be loosely synchronized. TESLA assumes that the
local internal clocks do not drift too much during the session. Use
of TESLA in SRTP assumes that the time synchronization is guaranteed
by out-of-band schemes, i.e. it is not in the scope of SRTP. The
TESLA overview specification [TESLA2] describes some methods, which
might be accomplished as part of SRTP key management. At least one
SRTP key management protocol, MIKEY, requires time synchronization
[MIKEY].
6. SRTP TESLA Default parameters
Key management procedures establish SRTP TESLA operating parameters
listed in section 4.3 of this document. The operating parameters
appear in the SRTP cryptographic context and have the following
default values. In the future, an Internet RFC MAY define
alternative settings for SRTP TESLA that are different than those
specified here. In particular, it should be noted that the settings
defined in this memo can have a large impact on bandwidth, as it
adds 38 bytes to each packet (when the field length values are the
default ones) . For certain applications, this overhead may
represent more than a 50% increase in packet size. Alternative
settings might seek to reduce the number and length of various TESLA
fields and outputs. No such optimizations are considered in this
memo.
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6.1 Transform-independent Parameter: SRTP MAC with TESLA MAC
Section 3.2.1 of the SRTP specification identifies "message
authentication" as one of the transform-independent parameters. By
default, this is HMAC-SHA1 for SRTP. With the addition of TESLA,
SRTP message authentication becomes a compound parameter since it is
necessary to identify two message authentication algorithms, one for
the SRTP MAC and one for the TESLA MAC. Thus, the use or non-use of
TESLA SHALL be indicated by the presence of a TESLA bit in the SRTP
cryptographic context. When this bit is set, the SRTP
implementation MUST inspect the TESLA transform-dependent parameters
to determine the particular TESLA configuration.
It is RECOMMENDED that the SRTP MAC be truncated to four bytes since
the SRTP MAC provides only group authentication and serves only as
protection against DoS.
6.2 Transform-dependent Parameters for TESLA MAC
The default values for the security parameters are listed in the
following. "OWF" denotes a one-way function.
Parameter Mandatory-to-support Default
--------- -------------------- -------
TESLA KEYCHAIN OWF (F(x)) HMAC-SHA1 HMAC-SHA1
OUTPUT LENGTH 160 160
TESLA MAC KEY OWF (F'(F(x))) HMAC-SHA1 HMAC-SHA1
OUTPUT LENGTH n_f 160 160
TESLA MAC HMAC-SHA1 HMAC-SHA1
(TRUNCATED) OUTPUT LENGTH n_m 80 80
id_j
TI_j
T_int
id_n
d_n
As shown above, TESLA implementations MUST support HMAC-SHA1 for the
TESLA MAC, the MAC key generator, and the TESLA keychain generator
one-way function. The TESLA keychain generator is recursively
defined as follows.
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K_i=HMAC_SHA1(K_{i+1},0), i=0..N-1
The TESLA MAC key generator is defined as follows.
K'_i=HMAC_SHA1(K_i,1)
The TESLA MAC uses a truncated output of ten bytes [RFC2104] and is
defined as follows.
HMAC_SHA1(K'_i, M')
where M' is as specified in Section 4.6.
The TESLA interval parameters are id_j and id_n, both are 32 bits in
length. The times associated with the intervals are TI_j, T_int,
and d_n, which are 64-bit values in Network Time Protocol (NTP)
format.
7. Security Considerations
Denial of Service (DoS) attacks when delayed authentication is used
are discussed in [PCST]. TESLA requires receiver's buffering before
authentication, therefore the receiver can suffer a denial of
service attack due to a flood of bogus packets. To address this
problem, the current specification REQUIRES the use of a four-byte
SRTP MAC in addition to TESLA MAC. The shorter size of the SRTP MAC
is here motivated by the fact that that MAC served purely for DoS
prevention from attackers external to the group.
SRTP TESLA depends on the effective security of the systems that
perform bootstrapping (time synchronization) and key management.
These systems are external to SRTP and are not considered in this
specification.
8. IANA Considerations
No IANA registration is required.
9. Acknowledgements
The authors would like to thanks Karl Norrman, Mats Näslund, and Ran
Canetti, for their valuable help.
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10. Author's Addresses
Questions and comments should be directed to the authors and
msec@ietf.org:
Mark Baugher
Cisco Systems, Inc.
5510 SW Orchid Street Phone: +1 408-853-4418
Portland, OR 97219 USA Email: mbaugher@cisco.com
Elisabetta Carrara
Ericsson Research
SE-16480 Stockholm Phone: +46 8 50877040
Sweden EMail: elisabetta.carrara@ericsson.com
11. References
Normative
[PCST] Perrig, A., Canetti, R., Song, D., Tygar, D., "Efficient and
Secure Source Authentication for Multicast", in Proc. of Network and
Distributed System Security Symposium NDSS 2001, pp. 35-46, 2001.
[SRTP] Baugher, McGrew, Carrara, Naslund, Norrman, "The Secure Real-
time Transport Protocol", July 2003, <draft-ietf-avt-srtp-09.txt>.
[TESLA1] Perrig, Canetti, Song, Tygar, Briscoe, "TESLA: Multicast
Source Authentication Transform Introduction", October 2002, draft-
ietf-msec-tesla-intro-01.txt.
[TESLA2] Perrig, Canetti, Whillock, "TESLA: Multicast Source
Authentication Transform Specification", October 2002, draft-ietf-
msec-tesla-spec-00.txt
Informative
[gkmarch] Baugher, Canetti, Dondeti, Lindholm, "MSEC Group Key
Management Architecture", January 2003, <draft-ietf-msec-gkmarch-
07.txt>.
[GDOI] Baugher, Weis, Hardjono, Harney, "The Group Domain of
Interpretation", RFC 3547, July 2003.
[MESP] Baugher, Canetti, Cheng, Rohatgi, "MESP: A Multicast
Framework for the IPsec ESP", March 2003, <draft-ietf-msec-mesp-
01.txt>.
Baugher, Carrara [Page 15]
INTERNET-DRAFT TESLA-SRTP February, 2004
[MIKEY] Arkko, Carrara, Lindholm, Naslund, Norrman, "MIKEY:
Multimedia Internet KEYing", December 2003, <draft-ietf-msec-mikey-
08.txt>
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Baugher, Carrara [Page 16]
INTERNET-DRAFT TESLA-SRTP February, 2004
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Baugher, Carrara [Page 17]