Internet Engineering Task Force Baugher (Cisco)
MSEC Working Group Carrara (Ericsson)
INTERNET-DRAFT
EXPIRES: March 2006 September 2005
The Use of TESLA in SRTP
<draft-ietf-msec-srtp-tesla-04.txt>
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
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 September 2005
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..................7
4.4. SRTP Processing..............................................8
4.4.1 Sender Processing...........................................9
4.4.2 Receiver Processing.........................................9
4.5. SRTCP Packet Format.........................................10
4.6. TESLA MAC...................................................12
4.7. PRFs........................................................13
5. TESLA Bootstrapping and Cleanup...............................13
6. SRTP TESLA Default parameters.................................13
6.2 Transform-dependent Parameters for TESLA MAC.................14
7. Security Considerations.......................................15
8. IANA Considerations...........................................16
9. Acknowledgements..............................................16
10. Author's Addresses...........................................16
11. References...................................................16
1. Introduction
Multicast and broadcast communications 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 had 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
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MAC in this case only guarantees that the packet was not manipulated
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 [TESLA].
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. Few of their details are explained in this document, and the
reader can find them in their respective specifications, [RFC3711]
and [TESLA]. This specification uses the same definitions as TESLA
for common terms and assumes that the reader is familiar with the
TESLA algorithms and protocols [TESLA].
2. SRTP
The Secure Real-time Transport Protocol (SRTP) [RFC3711] 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). Note, the term "SRTP" may often be used to indicate SRTCP
as well.
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
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associations. Fortunately, it is straightforward to add TESLA to
the SRTP cryptographic framework.
The TESLA extension to SRTP is defined in this specification, which
assumes that the reader is familiar with the SRTP specification
[RFC3711], its packet structure, and processing rules.
3. TESLA
TESLA provides delayed per-packet data authentication and is
specified in [TESLA].
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
[TESLA]. 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,
RFC3830]. 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
[RFC3711] and describes the use of TESLA with only a single key
chain and delayed-authentication [TESLA].
4.1. The TESLA extension
TESLA is an OPTIONAL authentication transform for SRTP. When used,
TESLA adds the fields shown 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 SRTP integrity protection tag, when the
SRTP master key is shared by more than two endpoints [RFC3711].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| i |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Disclosed Key ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TESLA MAC ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: The "TESLA authentication extension".
i: 32 bit, MANDATORY
Identifier of the time interval i, corresponding to the key K_i
that is used to calculate the TESLA MAC of the current packet
(and other packets sent in the current time interval i).
Disclosed Key: variable length, MANDATORY
The disclosed key (K_(i-d)), that can be used to authenticate
previous packets from earlier time intervals [TESLA].
TESLA MAC (Message Authentication Code): variable length, MANDATORY
The MAC computed using the key K'_i (derived from K_i) [TESLA],
which is disclosed in a subsequent packet (in the Disclosed Key
field). 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 (SRTP MAC).
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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 | | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| | i | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ 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.
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
[RFC3711] by the inclusion of the TESLA authentication extension.
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The "TESLA Authenticated Portion" of an SRTP packet consists of the
RTP header and the Encrypted Portion of the SRTP packet. As shown in
Figure 2, SRTP HMAC-SHA1 covers up to the MKI field but does not
include MKI. SRTP does not cover the MKI field (because it does not
need to be covered for SRTP packet integrity). In order to make the
two tags (SRTP-TESLA and SRTP-HMAC_SHA1) contiguous, we would need
to redefine the SRTP specification to include the MKI in HMAC-SHA1
coverage. This change is impossible and so the MKI field separates
the TESLA MAC from the SRTP MAC in the packet layout of Figure 2.
This change to the packet format presents no problem to an
implementation that supports the new SRTP-TESLA authentication
transform.
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.
Transform-dependent Parameters
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. to
indicate HMAC-SHA1, see Section 6.2 for the default value.
2. a non-negative integer n_p, 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), see Section 6.2 for the
default value.
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. to indicate HMAC-SHA1, see
Section 6.2 for the default value.
4. a non-negative integer n_f, determining the length of the
output of F', i.e. of the key for the TESLA MAC, see Section
6.2 for the default value.
5. an identifier for the TESLA MAC, that accepts the output of
F'(x) as its key, e.g. to indicate HMAC-SHA1, see Section 6.2
for the default value.
6. a non-negative integer n_m, determining the length of the
output of the TESLA MAC, see Section 6.2 for the default value.
7. the beginning of the session T_0,
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8. the interval duration T_int (in msec),
9. the key disclosure delay d (in number of intervals)
10. the upper bound D_t (in sec) on the lag of the receiver clock
relative to the sender clock (this quantity has to be
calculated by the peers out-of-band)
11. non-negative integer n_c, determining the length of the key
chain, which is determined based upon the expected duration of
the stream.
12. the initial key of the chain to which the sender has
committed himself.
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.
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 [RFC3711]. The use of TESLA slightly changes the
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
3.5-3.7 of [TESLA]. The words "TESLA computation" and "TESLA
verification" hereby imply all those steps, which are not all
spelled out in the following. In particular, notice that the TESLA
verification implies checking the safety condition (Section 3.5 of
[TESLA]).
As pointed out in [TESLA], if the packet is deemed "unsafe", then
the receiver considers the packet unauthenticated. It should discard
unsafe packets but, at its own risk, it may choose to use them
unverified. Hence, if the safe condition does not hold, it is
RECOMMENDED to discard the packet and log the event.
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4.4.1 Sender Processing
The sender processing is as described in Section 3.3 of [RFC3711],
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
current interval identifier (as i field), the disclosed key of the
chain for an earlier interval, 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 in the SRTP crypto context is set to one,
append the MKI to the packet.
8. When TESLA is applied, and if the SRTP authentication (external
tag) is required (for DoS), 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 8 in Section 3.3 of
[RFC3711]).
4.4.2 Receiver Processing
The receiver processing is as described in Section 3.3 of [RFC3711],
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
[RFC3711]). Note however, the SRTP replay list contains SRTP
indices of recently received packets that have been authenticated
by TESLA (i.e. replay list updates MUST NOT be based on SRTP MAC).
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
the verification is unsuccessful, the packet MUST be discarded
from further processing and the event SHOULD be logged.
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If the verification is successful, remove and store 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.
(Note that the order of decryption and TESLA verification is not
mandated. It is RECOMMENDED to perform the TESLA verification
before decryption. TESLA application designers might choose to
implement optimistic processing techniques such as notification of
TESLA verification results after decryption or even after plaintext
processing. Optimistic verification is beyond the scope of this
document.)
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 [RFC3711] that
in SRTCP the MKI is OPTIONAL, while the E-bit, the SRTCP index, and
the authentication tag are MANDATORY. This means that the SRTP
(external) MAC is MANDATORY also when TESLA is used.
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
[RFC3711] 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) and the Encrypted
Portion of the SRTCP packet.
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 [RFC3711] and in Section 4.6
of this memo.
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 | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| | i | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ Disclosed Key ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ TESLA MAC ~ | |
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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<|-+
| ~ 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.
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 and SRTP Encrypted Portion, see Figure 2) 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 and SRTCP Encrypted
Portion).
The normal authentication tag (OPTIONAL for SRTP, MANDATORY for
SRTCP) SHALL be applied with the same coverage as specified in
[RFC3711], i.e.:
- for SRTP: Authenticated Portion || ROC (with the extended
definition of SRTP Authentication Portion as for Section 4.2)
- for SRTCP: Authenticated Portion (with the extended definition of
SRTCP Authentication Portion as for Section 4.2).
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.
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4.7. PRFs
TESLA requires two pseudo-random functions (PRFs), f and f', to
implement
* 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 [RFC3711] when adding new security parameters.
5. TESLA Bootstrapping and Cleanup
The extensions to the SRTP cryptographic context include a set of
TESLA parameters that are listed in section 4.3 of this document.
Furthermore, TESLA MUST be bootstrapped at session set-up (for the
parameter exchange and the initial key commitment) through a regular
data authentication system (a digital signature algorithm is
RECOMMENDED). Key management procedures can take care of this
bootstrapping prior to the commencement of an SRTP session where
TESLA authentication is used. The bootstrapping mechanism is out of
scope for this document (it could for example be part of the key
management protocol).
A critical factor for the security of TESLA is that the sender and
receiver need to be loosely synchronized. TESLA requires a bound on
clock drift to be known (D_t). Use of TESLA in SRTP assumes that
the time synchronization is guaranteed by out-of-band schemes (e.g.
key management), i.e. it is not in the scope of SRTP.
It also should be noted that TESLA has some reliability requirements
in that a key is disclosed for a packet in a subsequent packet,
which can get lost. Since a key in a lost packet can be derived from
a future packet, TESLA is robust to packet loss. This repetition
might abruptly stop, however, if the key-bearing packets stop
abruptly at the end of the stream. To avoid this nasty boundary
condition, send null packets with TESLA keys for one entire interval
following the interval in which the stream ceases.
6. SRTP TESLA Default parameters
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Key management procedures establish SRTP TESLA operating parameters,
which are listed in section 4.3 of this document. The operating
parameters appear in the SRTP cryptographic context and have the
default values that are described in this section. 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.
It is RECOMMENDED that the SRTP MAC be truncated to 32 bits since the
SRTP MAC provides only group authentication and serves only as
protection against external DoS.
6.1 Transform-independent Parameters
The value of the flag indicating the use of TESLA in SRTP is by
default zero (TESLA not used).
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
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 [TESLA].
K_i=HMAC_SHA1(K_{i+1},0), i=0..N-1
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where N-1=n_c from the cryptographic context.
The TESLA MAC key generator is defined as follows [TESLA].
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.
7. Security Considerations
Denial of Service (DoS) attacks on delayed authentication are
discussed in [PCST]. TESLA requires receiver 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 external SRTP MAC, based on the group key, MAY be used
in addition to the TESLA MAC. The short size of the SRTP MAC
(default 32 bits) is here motivated by the fact that that MAC serves
purely for DoS prevention from attackers external to the group.
[TESLA] describes other mechanisms that can be used to prevent DoS,
in place of the external group-key MAC. If used, they need to be
added as processing steps (following the guidelines of [TESLA]).
The use of TESLA in SRTP defined in this specification is subject to
the security considerations discussed in the SRTP specification
[RFC3711] and in the TESLA specification [TESLA]. In particular, the
TESLA security is dependent on the computation of the "safety
condition" as defined in Section 3.5 of [TESLA].
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.
The length of the TESLA MAC is by default 80 bits. RFC 2104 requires
the MAC length to be at least 80 bits and at least half the output
size of the underlying hash function. The SHA-1 output size is 160
bits, so both of these requirements are met with the 80 bit MAC
specified in this document. Note that IPsec implementations tend to
use 96 bits for their MAC values to align the header with a 64 bit
boundary. Both MAC sizes are well beyond the reach of current
cryptanalytic techniques.
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8. IANA Considerations
No IANA registration is required.
Note that it is the task of each particular key management protocol
to register the cryptographic transforms (here, TESLA, as value in
the identifier for the message authentication algorithm in the SRTP
crypto context) and related parameters.
9. Acknowledgements
The authors would like to thanks Ran Canetti, Karl Norrman, Mats
N„slund, Fredrik Lindholm, David McGrew, and Bob Briscoe for their
valuable help.
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
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.
[RFC1305] Mills D., Network Time Protocol (Version 3) Specification,
Implementation and Analysis, RFC 1305, March, 1992.
[RFC3711] Baugher, McGrew, Naslund, Carrara, Norrman, "The Secure
Real-time Transport Protocol", RFC 3711, March 2004.
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[TESLA] Perrig, Song, Canetti, Tygar, Briscoe, "TESLA: Multicast
Source Authentication Transform Introduction", RFC 4082, June 2005.
Informative
[gkmarch] Baugher, Canetti, Dondeti, Lindholm, "MSEC Group Key
Management Architecture", May 2005, work in progress.
[GDOI] Baugher, Weis, Hardjono, Harney, "The Group Domain of
Interpretation", RFC 3547, July 2003.
[RFC3830] Arkko et al., "MIKEY: Multimedia Internet KEYing",
December 2003, RFC 3830, August 2004.
Copyright Notice
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Disclaimer of Validity
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
This draft expires in March 2006.
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