MSEC V. Roca
Internet-Draft A. Francillon
Intended status: Experimental S. Faurite
Expires: May 22, 2008 INRIA
November 19, 2007
Use of TESLA in the ALC and NORM Protocols
draft-ietf-msec-tesla-for-alc-norm-03.txt
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Abstract
This document explains how to integrate the TESLA source
authentication and packet integrity protocol to the ALC and NORM
content delivery protocols. This document only considers the
authentication/integrity of the packets generated by the session's
sender.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions Used in this Document . . . . . . . . . . . . 5
1.2. Terminology and Notations . . . . . . . . . . . . . . . . 5
2. Using TESLA with ALC and NORM . . . . . . . . . . . . . . . . 8
2.1. ALC and NORM Specificities that Impact TESLA . . . . . . . 8
2.2. The Need for Secure Time Synchronization . . . . . . . . . 9
2.2.1. Direct Time Synchronization . . . . . . . . . . . . . 9
2.2.2. Indirect Time Synchronization . . . . . . . . . . . . 9
2.3. Bootstrapping TESLA . . . . . . . . . . . . . . . . . . . 11
2.3.1. Bootstrapping TESLA with an Out-Of-Band Mechanism . . 11
2.3.2. Bootstrapping TESLA with an In-Band Mechanism . . . . 11
3. Time Synchronization and Delay Bound Calculations . . . . . . 13
3.1. Delay Bound Calculation in Direct Time Synchronization
Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Delay Bound Calculation in Indirect time
Synchronization Mode . . . . . . . . . . . . . . . . . . . 13
3.2.1. Single time reference . . . . . . . . . . . . . . . . 13
3.2.2. Multiple time references . . . . . . . . . . . . . . . 14
4. Sender Operations . . . . . . . . . . . . . . . . . . . . . . 15
4.1. TESLA Parameters . . . . . . . . . . . . . . . . . . . . . 15
4.1.1. Time Intervals . . . . . . . . . . . . . . . . . . . . 15
4.1.2. Key Chains . . . . . . . . . . . . . . . . . . . . . . 15
4.1.3. Time Interval Schedule . . . . . . . . . . . . . . . . 17
4.1.4. Timing Parameters . . . . . . . . . . . . . . . . . . 17
4.2. TESLA Messages and Authentication Tags . . . . . . . . . . 18
4.2.1. Bootstrap Information . . . . . . . . . . . . . . . . 18
4.2.2. Direct Time Synchronization Response . . . . . . . . . 19
4.2.3. Authentication Tag . . . . . . . . . . . . . . . . . . 20
4.2.4. Weak Group MAC Tag . . . . . . . . . . . . . . . . . . 20
4.2.5. Use of Digital Signatures . . . . . . . . . . . . . . 21
4.3. TESLA Messages and Authentication Tag Format . . . . . . . 22
4.3.1. Bootstrap Information Format . . . . . . . . . . . . . 22
4.3.2. Format of a Direct Time Synchronization Response . . . 27
4.3.3. Format of a Standard Authentication Tag . . . . . . . 29
4.3.4. Format of a Standard Authentication Tag Without
Key Disclosure . . . . . . . . . . . . . . . . . . . . 30
4.3.5. Format of an Authentication Tag with a New Key
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Chain Commitment . . . . . . . . . . . . . . . . . . . 31
4.3.6. Format of an Authentication Tag with an Old Chain
Last Key Disclosure . . . . . . . . . . . . . . . . . 32
4.3.7. Format of the Compact Authentication Tags . . . . . . 32
5. Receiver Operations . . . . . . . . . . . . . . . . . . . . . 36
5.1. Initialization of a Receiver . . . . . . . . . . . . . . . 36
5.1.1. Processing the Bootstrap Information Message . . . . . 36
5.1.2. Time Synchronization . . . . . . . . . . . . . . . . . 36
5.2. Authentication of Received Packets . . . . . . . . . . . . 38
6. Integration in the ALC and NORM Protocols . . . . . . . . . . 42
6.1. Authentication Header Extension Format . . . . . . . . . . 42
6.2. Use of Authentication Header Extensions . . . . . . . . . 44
6.2.1. EXT_AUTH Header Extension of Type Bootstrap
Information . . . . . . . . . . . . . . . . . . . . . 44
6.2.2. EXT_AUTH Header Extension of Type Authentication
Tag . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.2.3. EXT_AUTH Header Extension of Type Direct Time
Synchronization Request . . . . . . . . . . . . . . . 47
6.2.4. EXT_AUTH Header Extension of Type Direct Time
Synchronization Response . . . . . . . . . . . . . . . 47
6.3. Managing Silent Periods . . . . . . . . . . . . . . . . . 48
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
8. Security Considerations . . . . . . . . . . . . . . . . . . . 51
8.1. Dealing With DoS Attacks . . . . . . . . . . . . . . . . . 51
8.2. Dealing With Replay Attacks . . . . . . . . . . . . . . . 52
8.2.1. Impacts of Replay Attacks on TESLA . . . . . . . . . . 52
8.2.2. Impacts of Replay Attacks on NORM . . . . . . . . . . 53
8.2.3. Impacts of Replay Attacks on ALC . . . . . . . . . . . 53
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 56
10.1. Normative References . . . . . . . . . . . . . . . . . . . 56
10.2. Informative References . . . . . . . . . . . . . . . . . . 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . . . . 59
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1. Introduction
Many applications using multicast and broadcast communications
require that each receiver be able to authenticate the source of any
packet it receives as well as its integrity. For instance, ALC
[draft-ietf-rmt-pi-alc-revised] and NORM
[draft-ietf-rmt-pi-norm-revised] are two Content Delivery Protocols
(CDP) designed to transfer reliably objects (e.g. files) between a
session's sender and several receivers.
The NORM protocol is based on bidirectional transmissions. Each
receiver acknowledges data received or, in case of packet erasures,
asks for retransmissions. The ALC protocol defines unidirectional
transmissions. Reliability can be achieved by means of cyclic
transmissions of the content within a carousel, or by the use of
proactive Forward Error Correction codes (FEC), or by the joint use
of these mechanisms. Being purely unidirectional, ALC is massively
scalable, while NORM is intrinsically limited in terms of the number
of receivers that can be handled in a session. Both protocols have
in common the fact that they operate at application level, on top of
an erasure channel (e.g. the Internet) where packets can be lost
(erased) during the transmission. With some use case, an attacker
might impersonate the ALC or NORM session's sender and inject forged
packets to the receivers, thereby corrupting the objects
reconstructed by the receivers.
The situation is much more complex in case of group communications
than it is with unicast communications. Indeed, in the latter case a
simple solution exist: the sender and receiver can share a secret key
to compute a Message Authentication Code (MAC) of all messages
exchanged. This is no longer feasible in case of a multicast and
broadcast communications since sharing a group key between the sender
and all receivers and using the same MAC scheme means that any group
member can impersonate the sender and send forged messages to other
receivers.
The usual solution to provide the source authentication and message
integrity services in case of multicast and broadcast communications
consists in relying on asymmetric cryptography and using digital
signatures. Yet this solution is limited by high computational costs
and high transmission overheads. The Timed Efficient Stream Loss-
tolerant Authentication protocol (TESLA) is an alternative solution
that provides the two required services, while being compatible with
high rate transmissions over lossy channels.
This document explains how to integrate the TESLA source
authentication and packet integrity protocol to the ALC and NORM
content delivery protocols. Since the FLUTE application [RFC3926] is
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built on top of ALC, it will directly benefit from the services
offered by TESLA at the transport layer.
This specification only considers the authentication/integrity of the
packets generated by the session's sender. This specification does
not consider the packets that will be generated by receivers, for
instance the feedback packets of NORM. Adding authentication/
integrity to the packets sent by receivers is outside the scope of
this document. Of course, this remark does not apply to ALC where
transmissions are purely unidirectional.
For more information on the TESLA protocol and its principles, please
refer to [RFC4082][Perrig04]. For more information on ALC, NORM and
FLUTE, please refer to [draft-ietf-rmt-pi-alc-revised],
[draft-ietf-rmt-bb-lct-revised], [draft-ietf-rmt-pi-norm-revised] and
[RFC3926].
1.1. Conventions Used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Terminology and Notations
The following notations and definitions are used throughout this
document. Cryptographic functions related notations and definitions
[RFC4082][RFC4383]:
o PRF is the Pseudo Random Function;
o MAC is the Message Authentication Code;
o HMAC is the Keyed-Hash Message Authentication Code;
o F is the one-way function used to create the key chain;
o F' is the one-way function used to derive the HMAC keys;
o n_p is the length, in bits, of the F function's output. This is
therefore the length of the keys in the key chain;
o n_f is the length, in bits, of the F' function's output. This is
therefore the length of the HMAC keys;
o n_m is the length of the truncated output of the MAC [RFC2104].
Only the n_m left-most bits (most significant bits) of the MAC
output are kept;
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o N is the length of a key chain. There are N+1 keys in a key
chain: K_0, K_1, .. K_N. When several chains are used, all the
chains MUST have the same length and keys are numbered
consecutively, following the time interval numbering;
o n_c is the number of keys in a key chain. Therefore: n_c = N+1;
o n_tx_lastkey is the number of intervals during which the last key
of the old key chain SHOULD be sent, after switching to a new key
chain and after waiting for the disclosure delay d. This
transmission takes place after the interval during which the last
key is normally disclosed. The n_tx_lastkey value is either 0 (no
extra disclosure) or larger;
o n_tx_newkcc is the number of intervals during which the commitment
to a new key chain SHOULD be sent, before switching to the new key
chain. The n_tx_newkcc value is either 0 (no commitment sent
within authentication tags) or larger;
o K_g is a shared group key, communicated to all group members,
confidentially, before starting the session. The mechanism by
which this group key is shared by the group members is out of the
scope of this document;
o n_w is the length of the truncated output of the MAC of the
optional weak group authentication scheme: only the n_w most
significant bits of the MAC output are kept. n_w is typically a
small value (e.g. 32 bits), multiple of 32 bits;
Time related notations and definitions:
o i is the time interval index. Interval numbering starts at 0 and
increases consecutively.
o t_s is the sender local time value at some absolute time;
o t_r is the receiver local time value at the same absolute time;
o T_0, the start time corresponding to the beginning of the session
(NTP timestamp);
o T_int, the interval duration (in milliseconds);
o d, the key disclosure delay (in number of intervals);
o D_t, the upper bound of the lag of the receiver's clock with
respect to the clock of the sender;
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o S_sr, an estimated bound of the clock drift between the sender and
a receiver throughout the duration of the session;
o D^O_t, the upper bound of the lag of the sender's clock with
respect to the time reference in indirect time synchronization
mode;
o D^R_t, the upper bound of the lag of the receiver s's clock with
respect to the time reference in indirect time synchronization
mode;
o D_err, an upper bound of the time error between all the time
references, in indirect time synchronization mode;
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2. Using TESLA with ALC and NORM
2.1. ALC and NORM Specificities that Impact TESLA
The ALC and NORM protocols have features and requirements that
largely impact the way TESLA can be used.
In case of ALC:
o ALC is massively scalable: nothing in the protocol specification
limits the number of receivers that join a session. Therefore an
ALC session potentially includes a huge number (e.g. millions or
more) of receivers;
o ALC can work on top of purely unidirectional transport channels:
this is one of the assets of ALC, and examples of unidirectional
channels include satellite (even if a back channel might exist in
some use cases) and DVB-H systems;
o ALC defines an on-demand content delivery model
[draft-ietf-rmt-pi-alc-revised] where receivers can arrive at any
time, at their own discretion, download the content and leave the
session. Other models (e.g. push or streaming) are also defined;
o ALC sessions are potentially very long: with some use cases a
session can last several months during which the content is
continuously transmitted within a carousel. The content can be
either static (e.g. a software update) or dynamic (e.g. a web
site).
Depending on the use case, some of the above features may not apply.
For instance ALC can also be used over a bidirectional channel, or
ALC can be used for small groups.
In case of NORM:
o NORM has been designed for limited or medium size sessions:
Indeed, NORM relies on feedback messages and the sender may
collapse if the feedback message rate is too high;
o NORM requires a bidirectional transport channel: the back channel
is not necessarily a high rate channel since only low to medium
rate control traffic will flow on it. Networks with an asymmetric
connectivity (e.g. a high rate satellite downlink and a low-rate
RTC based return channel) are appropriate;
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2.2. The Need for Secure Time Synchronization
The security offered by TESLA relies heavily on time. Therefore the
session's sender and each receiver need to be time synchronized in a
secure way. To that purpose, two general methods exist:
o direct time synchronization, and
o indirect time synchronization.
2.2.1. Direct Time Synchronization
When direct time synchronization is used, each receiver asks the
sender for a time synchronization. To that purpose, a receiver sends
a "Direct Time Synchronization Request" (Section 5.1.2.1). The
sender then directly answers to each request with a "Direct Time
Synchronization Response" (Section 4.3.2), signing this reply. Upon
receiving this response, a receiver first verifies the signature, and
then calculates an upper bound of the lag of his clock with respect
to the clock of the sender, D_t. The details on how to calculate D_t
are given in Section 3.1.
This synchronization method is both simple and secure. Yet there are
two potential issues:
o a bidirectional channel MUST exist between the sender and each
receiver,
o the sender may collapse if the incoming request rate is too high.
Relying on direct time synchronization is not expected to be an issue
with NORM since (1) bidirectional communications already take place,
and (2) NORM scalability is anyway limited. Yet it can be required
that a mechanism, that is out of the scope of this document, be used
to spread the transmission of "Direct time synchronization request"
messages over the time if there is a risk that the sender may
collapse.
But direct time synchronization is potentially incompatible with ALC
since (1) there might not be a back channel to the session's sender,
and (2) there are potentially a huge number of receivers and
therefore a risk that the sender collapses.
2.2.2. Indirect Time Synchronization
When indirect time synchronization is used, the sender and each
receiver must synchronize securely via an external time reference.
Several possibilities exist:
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o sender and receivers can synchronize through a NTPv3 (Network Time
Protocol version 3) [RFC1305] hierarchy of servers. The
authentication mechanism of NTPv3 MUST be used in order to
authenticate each NTP message individually. It prevents for
instance an attacker to impersonate a NTP server;
o similarly sender and receivers can synchronize through a NTPv4
(Network Time Protocol version 4) [draft-ietf-ntp-ntpv4-proto]
hierarchy of servers. The Autokey security protocol of NTPv4 MUST
be used in order to authenticate each NTP message individually;
o similarly, they can synchronize through a SNTPv4 (Simple Network
Time Protocol version 4) [RFC4330] hierarchy of servers. The
authentication features of SNTPv4 must then be used. Note that
TESLA only needs a loose (but secure) time synchronization, which
is in line with the time synchronization service offered by SNTP;
o they can synchronize through a GPS or Galileo (or similar) device
that also provide a high precision time reference. This time
reference is in general trusted, yet depending on the use case,
this trust will or not be acceptable;
o they can synchronize thanks to a dedicated hardware, embedded on
each sender and receiver, that offers a clock with a time-drift
that is negligible in front of the TESLA time accuracy
requirements. This feature enables a device to synchronize its
embedded clock with the official time reference from time to time,
when feasible (in an extreme case once, at manufacturing time),
and then to remain autonomous for some time, depending on the
known maximum clock drift.
A bidirectional channel is required by the NTP/SNTP schemes. On the
opposite, with the GPS/Galileo and high precision clock schemes, no
such assumption is made. In situations where ALC is used on purely
unidirectional transport channels (Section 2.1), using the NTP/SNTP
schemes is not possible. Another aspect is the scalability
requirement of ALC, and to a lesser extent of NORM. From this point
of view, the above mechanisms usually do not raise any problem,
unlike the direct time synchronization schemes. Therefore, using
indirect time synchronization is a good candidate, in particular with
ALC.
The details on how to calculate an upper bound of the lag of a
receiver's clock with respect to the clock of the sender, D_t, are
given in Section 3.2.
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2.3. Bootstrapping TESLA
In order to initialize the TESLA component at a receiver, the sender
must communicate some key information. This information MUST be
communicated in a secure way so that a receiver can check the source
of the information and its integrity. Two general methods are
possible:
o by using an out-of-band mechanism, or
o by using an in-band mechanism.
The current specification does not recommend any mechanism to
bootstrap TESLA. Choosing between an in-band and out-of-band scheme
is left to the implementer, depending on the target use-case.
2.3.1. Bootstrapping TESLA with an Out-Of-Band Mechanism
For instance [RFC4442] describes the use of the MIKEY (Multimedia
Internet Keying) protocol to bootstrap TESLA. As a side effect,
MIKEY also provides a loose time synchronization feature, that TESLA
can benefit. Other solutions, for instance based on an extended
session description, are possible, on condition these solutions are
secure.
2.3.2. Bootstrapping TESLA with an In-Band Mechanism
This specification describes an in-band mechanism. In some use-
cases, it might be desired that bootstrap take place without
requiring the use of an additional external mechanism. For instance
each device may feature a clock with a known time-drift that is
negligible in front of the time accuracy required by TESLA, and each
device may embed the public key of the sender. It is also possible
that the use-case does not feature a bidirectional channel which
prevents the use of out-of-band protocols like MIKEY. For these two
examples, the bootstrap information described in Section 4.3.1 and
the knowledge of a few additional parameters, listed below, are
sufficient to bootstrap TESLA at a receiver.
Some parameters cannot be communicated in-band. In particular, the
sender or a group controller:
o MUST communicate his public key, for each receiver to be able to
verify the signature of the bootstrap (and direct time
synchronization response messages when applicable). As a side
effect, the receivers also know the key length and the signature
length, the two parameters being equal.
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o MAY communicate a certificate (which also means that a PKI has
been setup), for each receiver to be able to check the sender's
public key.
o when time synchronization is performed with (S)NTP, MUST
communicate the list of valid (S)NTP servers, for all group
members (including the server) to synchronize themselves on the
same (S)NTP servers.
o when the Weak Group MAC feature is used, MUST communicate the K_g
group key to the receivers. This key might be periodically
refreshed.
These parameters are communicated to all receivers before they can
bootstrap their TESLA component. For instance it can be communicated
as part of the session description, or initialized in a static way on
the receivers.
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3. Time Synchronization and Delay Bound Calculations
3.1. Delay Bound Calculation in Direct Time Synchronization Mode
In direct time synchronization mode, synchronization between a
receiver and the sender follows the following protocol [RFC4082]:
o The receiver sends a "Direct Time Synchronization Request" message
to the sender, that includes t_r, the receiver local time at the
moment of sending (Section 5.1.2.1).
o Upon receipt of this message, the sender records its local time,
t_s, and sends to the receiver a "Direct Time Synchronization
Response" that includes t_r (taken from the request) and t_s
(Section 4.3.2), signing this reply.
o Upon receiving this response, the receiver first verifies that he
actually sent a request with t_r and then checks the signature.
Then he calculates D_t = t_s - t_r + S_sr, where S_sr is an
estimated bound of the clock drift between the sender and the
receiver throughout the duration of the session. This document
does not specify how S_sr is estimated.
After this initial synchronization, at any point throughout the
session, the receiver knows that: T_s < T_r + D_t, where T_s is the
current time at the sender and T_r is the current time at the
receiver.
3.2. Delay Bound Calculation in Indirect time Synchronization Mode
In indirect time synchronization, the sender and the receivers must
synchronize indirectly with one or several time references.
3.2.1. Single time reference
Let's assume that there is a single time reference.
1. The sender calculates D^O_t, the upper bound of the lag of the
sender's clock with respect to the time reference. This D^O_t
value is then be communicated to the receivers (Section 4.2.1).
2. Similarly, a receiver R calculates D^R_t, the upper bound of the
lag of the receiver's clock with respect to the time reference.
3. Then, for receiver R, the overall upper bound of the lag of the
receiver's clock with respect to the clock of the sender, D_t, is
the sum: D_t = D^O_t + D^R_t.
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The D^O_t and D^R_t calculation depends on the time synchronization
mechanism used (Section 2.2.2). In some cases, the synchronization
scheme specifications provide these values. In other cases, these
parameters can be calculated by means of a scheme similar to the one
specified in Section 3.1, for instance when synchronization is
achieved via a group controller [RFC4082].
3.2.2. Multiple time references
Let's now assume that there are several time references (e.g. several
(S)NTP servers). The sender and receivers use the direct time
synchronization scheme to synchronize with the various time
references. It results in D^O_t and D^R_t. Let D_err be an upper
bound of the time error between all the time references. Then, the
overall value of D_t within receiver R is set to the sum: D_t = D^O_t
+ D^R_t + D_err.
In some cases, the D_t value is part of the time synchronization
scheme specifications. For instance NTPv3 [RFC1305] defines
algorithms that are "capable of accuracies in the order of a
millisecond, even after extended periods when synchronization to
primary reference sources has been lost". In practice, depending on
the NTP server stratum, the accuracy might be a little bit worse. In
that case, D_t = security_factor * (1ms + 1ms), where the
security_factor is meant to compensate several sources of inaccuracy
in NTP.
----- Editor's note: is this D_t = security_factor * (1ms + 1ms)
rule of thumb acceptable? -----
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4. Sender Operations
4.1. TESLA Parameters
4.1.1. Time Intervals
The sender divides the time into uniform intervals of duration T_int.
Time interval numbering starts at 0 and is incremented consecutively.
The interval index MUST be stored in an unsigned 32 bit integer so
that wrapping to 0 takes place only after 2^^32 intervals. For
instance, if T_int is equal to 0.5 seconds, then wrapping takes place
after approximately 68 years.
4.1.2. Key Chains
The sender computes a one-way key chain of n_c = K+1 keys, and
assigns one key from the chain to each interval in sequence. Key
numbering starts at 0 and is incremented consecutively, following the
time interval numbering: K_0, K_1 .. K_N.
In order to compute this chain, the sender must first select a
Primary Key, K_N, and a PRF function, f. The functions F and F' are
two one-way functions that are defined as: F(k)=f_k(0) and
F'(k)=f_k(1). The sender computes all the keys of key chain,
starting with K_N, using: K_{i-1} = F(K_i). The key for MAC
calculation can then be derived from the corresponding K_i key by
K'_i=F'(K_i). The randomness of the Primary Key, K_N, is vital to
the security since no one should be able to guess it.
The key chain has a finite length, N, so the TESLA session must
finish before the end of the key chain. But the longer the key
chain, the higher the memory and computation required to cope with
it. Another solution consists in switching to a new key chain, of
the same length, when necessary (see Figure 1) [Perrig04].
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< -------- old key chain --------- >||< -------- new key chain --...
+-----+-----+ .. +-----+-----+-----+||+-----+-----+-----+-----+-----+
0 1 .. N-2 N-1 N || N+1 N+2 N+3 N+4 N+5
||
Key disclosures: ||
N/A N/A .. K_N-4 K_N-3 K_N-2 || K_N-1 K_N K_N+1 K_N+2 K_N+3
| || | |
< --------------- >|| < --------------- >
Additional key F(K_N+1) || K_N
disclosures (commitment to || (last key of
(in parallel): the new chain) || the old chain)
Figure 1: Switching to a new key chain (d=2, n_tx_newkcc=3,
n_tx_lastkey=3, switching between the first and second key chains).
To do so, the sender commits the new key chain with the old key
chain. Let's say that the old key chain stops at K_N and the new key
chain starts at K_{N+1} (i.e., F(K_{N+1}) and K_N are two different
keys). Then the sender includes the commitment F(K_{N+1}) to the new
key chain to packets authenticated with the old key chain (see
Section 4.3.5). This commitment SHOULD be sent during n_tx_newkcc
intervals before the end of the old key chain. Since several packets
are usually sent during an interval, the sender SHOULD alternate
between sending a disclosed key of the old key chain and the
commitment to the new key chain. The details of how to alternate
between the disclosure and commitment are out of the scope of this
document.
The receiver will keep the commitment until the key K_{N+1} is
disclosed, at interval N+1+d. Then the receiver will be able to test
the validity of that key by computing F(K_{N+1}) and comparing it to
the commitment.
When the key chain is changed, it becomes impossible to recover a
previous key from the old key chain. This is a problem if the
receiver lost the packets disclosing the last key of the old key
chain. A solution consists in re-sending the last key, K_N, of the
old key chain (see Section 4.3.6). This SHOULD be done during
n_tx_lastkey additional intervals after the end of the time interval
where K_N is disclosed. Since several packets are usually sent
during a interval, the sender SHOULD alternate between sending a
disclosed key of the new key chain, and the last key of the old key
chain. The details of how to alternate between the two disclosures
are out of the scope of this document.
In some cases a receiver having experienced a very long disconnection
might have lost the commitment of the new chain. Therefore this
receiver will not be able to authenticate any packet related to the
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new chain and all the following ones. The solution for this receiver
to catch up consists in receiving the bootstrap information. This
can happen by waiting for the next periodic transmission (in indirect
time synchronization mode), by requesting it (in direct time
synchronization mode), or through an external mechanism
(Section 4.2.1).
Since a key cannot be disclosed before the disclose delay, d, no key
will be disclosed during the first d time intervals (intervals 0 and
1 in Figure 1) of the session. The following key chains, if any, are
not concerned.
4.1.3. Time Interval Schedule
The sender must determine the following parameters:
o T_0, the start time corresponding to the beginning of the session;
o T_int, the interval duration, usually ranging from 100
milliseconds to 1 second;
o d, the key disclosure delay (in number of intervals). It is the
time to wait before disclosing a key;
o N, the length of a key chain;
The correct choice of T_int, d, and N is crucial for the efficiency
of the scheme. For instance, a T_int * d product that is too long
will cause excessive delay in the authentication process. A T_int *
d product that is too short will cause too many packets to be
unverifiable by some receivers. A N * T_int product that is too
small will cause the sender to switch too often to new key chains. A
N that is too long with respect to the expected session duration, if
this latter is known, will require the sender to compute too many
keys without using them all. [RFC4082] sections 3.2 and 3.6 give
general guidelines for initializing these parameters.
The T_0, T_int, d and N parameters MUST NOT be changed during the
lifetime of the session. This restriction is meant to prevent
introducing vulnerabilities: for instance, if a sender is authorized
to change the key disclosure schedule, a receiver that did not
receive the notification of change would still believe in the old key
disclosure schedule [RFC4082].
4.1.4. Timing Parameters
In indirect time synchronization mode, the sender must determine the
following parameter:
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o D^O_t, the upper bound of the lag of the sender's clock with
respect to the time reference.
The D^O_t parameter MUST NOT be changed during the lifetime of the
session.
4.2. TESLA Messages and Authentication Tags
At a sender, TESLA produces four types of signaling information:
o The bootstrap information. This information can be either sent
out-of-band or in-band. In the latter case, a digitally signed
packet contains all the information required to bootstrap TESLA at
a receiver;
o The time synchronization response, which enables a receiver to
finish a direct time synchronization;
o The authentication tag, which is sent in all data packets and
contains the MAC of the packet;
o Additionally, an optional weak group authentication tag can be
added to packets to mitigate attacks coming from outside of the
group.
4.2.1. Bootstrap Information
In order to initialize the TESLA component at a receiver, the sender
must communicate some key information in a secure way. This
information can be sent in-band or out-of-band, as discussed in
Section 2.3. Choosing between an in-band and out-of-band scheme is
left to the implementer, depending on the target use-case. In this
section we only consider the in-band scheme.
The TESLA bootstrap information message MUST be digitally signed
(Section 4.2.5). The goal is to enable a receiver to check the
packet source and packet integrity. Then, the bootstrap information
can be:
o unicast to a receiver during a direct time synchronization
request/response exchange;
o broadcast to all receivers. This is typically the case in
indirect time synchronization mode. It can also be used in direct
time synchronization mode, for instance when a large number of
clients arrive at the same time, in which case it is more
efficient to answer globally.
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Let's consider situations where the bootstrap information is
broadcast. This message should be broadcast at the beginning of the
session, before data packets are actually sent. This is particularly
important with ALC or NORM sessions in ``push'' mode, when all
clients join the session in advance. For improved reliability,
bootstrap information might be sent a certain number of times.
Afterward, a periodic broadcast of the bootstrap information message
could be useful when:
o the ALC session uses an ``on-demand'' mode, clients arriving at
their own discretion;
o some clients experience an intermittent connectivity. This is
particularly important when several key chains are used in an ALC
or NORM session, since there is a risk that a receivers lose all
the commitments to the new key chain.
A balance must be found between the signaling overhead and the
maximum initial waiting time at the receiver before starting the
delayed authentication process. A frequency of a few seconds for the
transmission of this bootstrap information is often a reasonable
value.
4.2.2. Direct Time Synchronization Response
In Direct Time Synchronization, upon receipt of a synchronization
request, the sender records its local time, t_s, and sends a response
message that contains both t_r and t_s (Section 3.1). This message
is unicast to the receiver. This Direct Time Synchronization
Response message MUST be digitally signed in order to enable a
receiver to check the packet source and packet integrity
(Section 4.2.5). The receiver MUST also be able the associate this
response and his request, which is the reason why t_r is included in
the message.
The Direct Time Synchronization Response messages are distinct from
the Bootstrap Information message (assuming in-band bootstrap is
used). Therefore, if a large number of receivers try to initialize
their TESLA component at the same time (a reasonable assumption in
"push" mode), a single Bootstrap Information message can be broadcast
to all of them. In some situations, when there is a limited number
of receivers, a sender can also choose to unicast a Bootstrap
Information message to each client individually before sending the
direct time synchronization response message. The choice is outside
the scope of this document.
Note that a single session might include receivers that use the
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direct time synchronization mode while others use the indirect time
synchronization mode.
4.2.3. Authentication Tag
Every packet MUST have an authentication tag containing:
o the interval index, which is also the index of the key used for
computing the MAC of this packet: i. This interval index is
optional when ;
o either a disclosed key (that belongs to the current key chain or
the previous key chain) or a commitment to a new key chain;
o and the MAC of the message: MAC(K'_i, M), where K'_i=F'(K_i);
The computation of MAC(K'_i, M), includes the ALC or NORM header
(with the various header extensions) and the payload when applicable.
The UDP/IP/MAC headers are not included. During this computation,
the MAC(K'_i, M) field of the authentication tag MUST be set to 0.
4.2.4. Weak Group MAC Tag
An optional Weak Group MAC can be used to mitigate DoS attacks coming
from attackers that are not group member [RFC4082]. This feature
assumes that a group key, K_g, is shared by the sender and all
receivers. When the attacker is not a group member, the benefits of
adding a group MAC to every packet sent are threefold:
o a receiver can immediately drop packets identified as unauthentic,
without having to wait for the disclosure delay, d;
o a sender can immediately drop faked direct time synchronization
requests, and in particular avoid to compute the digital
signature, a computation intensive task;
o a receiver can immediately drop faked direct time synchronization
response message, without having to verify the digital signature,
a computation intensive task;
More specifically, before sending a message, the sender computes the
group MAC MAC(K_g, M), which includes the ALC or NORM header (with
the various header extensions), plus the payload when applicable.
During this computation, the Weak Group MAC field MUST be set to 0.
However the digital signature and MAC fields, when present, MUST have
been calculated and are included in the Weak Group MAC calculation
itself. Then the sender truncates the MAC output to keep the n_w
most significant bits and stores the result in the TESLA
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Authentication header. Upon receiving this packet, the receiver
recomputes the group MAC and compares it to the value carried in the
packet. If the check fails, the packet MUST be immediately dropped.
This scheme features a few limits:
o it is of no help if a group member (who knows K_g) impersonates
the sender and sends forged messages to other receivers;
o it requires an additional MAC computing for each packet, both at
the sender and receiver sides;
o it increases the size of the TESLA authentication headers. In
order to limit this problem, the length of the truncated output of
the MAC, n_w, SHOULD be kept small (e.g. 32 bits) (see [RFC3711]
section 9.5). As a side effect, the authentication service is
significantly weakened (the probability that any packet be
successfully forged is one in 2^32). Since the weak group MAC
check is only a pre-check that will be followed by the standard
TESLA authentication check, this is not considered to be an issue.
For a given use-case, the benefits brought by the group MAC must be
balanced against these limitations.
Note that the Weak Group MAC function can be different from the TESLA
MAC function (e.g. it can use a weaker but faster MAC function).
Note also that the mechanism by which the group key, K_g, is
communicated to all group members, and perhaps periodically updated,
is out of the scope of this document.
4.2.5. Use of Digital Signatures
The Bootstrap Information message (with the in-band bootstrap scheme)
and Direct Time Synchronization Response message (with the indirect
time synchronization scheme, either with in-band or out-of-band
bootstrap) both need to be signed by the sender. Within these two
messages, a "Signature" field is reserved to hold the result of the
digital signature. The bootstrap information message also contains
the "Signature Type" and "Signature Length" fields that enable a
receiver to process the "Signature" field. There is no such
"Signature Type" and "Signature Length" fields in case of a Direct
Time Synchronization Response message since it is assumed that these
parameters are already known (i.e. the receiver either received a
bootstrap information message before, or these values have been
communicated out-of-band).
The computation of the signature includes the ALC or NORM header
(with the various header extensions) and the payload when applicable.
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The UDP/IP/MAC headers are not included. During this computation,
the "Signature" field MUST be set to 0.
It is assumed in this document that the receivers have the
possibility to retrieve the sender's public key required to check
this digital signature and the sender's certificate if needed
(Section 2.3). The details of how to do that are out of the scope of
this document.
With RSASSA-PKCS1-v1_5 (default) and RSASSA-PSS signatures
(Section 7), the size of the signature is equal to the "RSA modulus",
unless the "RSA modulus" is not a multiple of 8 bits. In that case,
the signature MUST be prepended with between 1 and 7 bits set to zero
such that the signature is a multiple of 8 bits [RFC4359]. The key
size, which in practice is also equal to the "RSA modulus", has major
security implications. [RFC4359] explains how to choose this value
depending on the maximum expected lifetime of the session. This
choice is out of the scope of this document.
4.3. TESLA Messages and Authentication Tag Format
This section specifies the format of the various kinds of TESLA
messages and authentication tags sent by the session's sender.
Because of the ALC and NORM integration of these TESLA messages in an
EXT_AUTH header extension (Section 6), the following formats are not
aligned on 32 bit word boundaries.
4.3.1. Bootstrap Information Format
When bootstrap information is sent in-band, the following message is
used:
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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
+-+-+-+-+-+-+-+-+ ^
| Reserved |W|A| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | f
| PRF Type | MAC Func Type |SigType| Signature Length | | i
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | x
|WG MAC Fun Type| d | T_int | | e
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | d
| | |
+ T_0 (NTP timestamp) + | l
| | | e
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | n
| N (Number of Keys in chain) | | g
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | t
| i (Interval Index of K_i) | | h
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v
| |
~ K_i +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
~ Signature ~
+ +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| |
+-+ D^O_t Extension (optional, present if A==1) +
| (NTP timestamp diff, positive if P==1, negative if P==0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Bootstrap information format.
The format of the bootstrap information is depicted in Figure 2. The
fields are:
"Reserved" fields (6 bits):
This is a reserved field that MUST be set to zero in this
specification.
"W" (Weak Group MAC Present) flag (1 bits):
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The "W" flag indicates whether the Weak Group MAC feature is used
(W==1) or not (W==0). When it is used, a "Weak Group MAC" field
is added to all the packets containing a TESLA EXT_AUTH Header
Extension (including this bootstrap message).
"A" flag (1 bit):
A==0 indicates that the P flag and D^O_t field are not present.
A==1 indicates that the P flag and D^O_t field are present (which
is required in. indirect time synchronization mode).
"PRF Type" field (8 bits):
"PRF Type" is the reference number of the f function used to
derive the F (for key chain) and F' (for MAC keys) functions
(Section 7).
"MAC Function Type" field (8 bits):
The "MAC Function Type" is the reference number of the function
used to compute the MAC of the packets (Section 7).
"Signature Type" field (4 bits):
The "Signature Type" is the reference number (Section 7) of the
digital signature used to authenticate this bootstrap information
and included in the "Signature" field.
"Signature Key Length" field (12 bits):
The "Signature Length" is an unsigned integer that indicates the
signature field size in bytes in the "Signature Extension" field.
"Weak Group MAC Function Type" field (8 bits):
When W==1, the "Weak Group MAC Function Type" fields contains the
reference number of the function used to compute the group MAC
(Section 7) of the packets, including this bootstrap message.
When W==0, this field MUST be set to zero (i.e. denote an INVALID
MAC function Section 7).
"d" field (8 bits):
d is an unsigned integer that defines the key disclosure delay (in
number of intervals). d MUST be greater or equal to 2.
"T_int" field (16 bits):
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T_int is an unsigned 16 bit integer that defines the interval
duration (in milliseconds).
"T_0" field (64 bits):
"T_0" is an NTP timestamp that indicates the time when this
session began.
"N" field (32 bits):
"N" is an unsigned integer that indicates the number of keys in
the current key chain.
"i" (Interval Index of K_i) field (32 bits):
"i" is an unsigned integer that indicates the interval index
associated to the key disclosed in this bootstrap information,
K_i. For performance reasons, the sender SHOULD always send a
bootstrap information with the highest possible index i since this
will reduce the required computation needed to validate key K_j
with j > i. But using the first interval index of the current key
chain (e.g. O and K_0 in case of the first key chain, N+1 and
K_N+1 in case of the second key chain, etc.) is valid. In any
case, if j is the current interval index, then it is REQUIRED that
i <= j - d.
"K_i" field (variable size):
"K_i" is the key corresponding to interval i. If j is the current
interval index, then it is REQUIRED that i <= j - d. If need be,
this field is padded (with 0) up to a multiple of 32 bits.
"Signature" field (variable size):
The "Signature" field is mandatory. The signature field contains
a digital signature using the type specified in the "Signature
Type" field. If need be, this field is padded (with 0) up to a
multiple of 32 bits.
"P" flag (optional, 1 bit if present):
The "P" flag is optional. It is only used in indirect time
synchronization mode when the A flag is 1. This flag indicates
whether the D^O_t NTP timestamp difference is positive (P==1) or
negative (P==0).
"D^O_t" field (optional, 63 bits if present):
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The "D^O_t" field is optional (controlled by the A flag). It is
only used in indirect time synchronization mode. It is the upper
bound of the lag of the sender's clock with respect to the time
reference. When several time references are specified (e.g.
several NTP servers), then D^O_t is the maximum upper bound of the
lag with each time reference. D^O_t is composed of two unsigned
integers, as with NTP timestamps: the first 31 bits give the time
difference in seconds and the remaining 32 bits give the sub-
second time difference.
"Weak Group MAC" field (optional, variable length, multiple of 32
bits):
This field contains the weak MAC, calculated with a group key,
K_g, shared by all group members. The field length is given by
n_w, in bits.
Note that the first byte and the following six 32-bit words are
mandatory fixed length fields. The K_i and Signature fields are
mandatory but variable length fields. The remaining D^O_t and Weak
Group MAC fields are optional.
In order to prevent attacks, some parameters MUST NOT be changed
during the lifetime of the session (Section 4.1.3, Section 4.1.4).
The following table summarizes the parameters status:
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+--------------------------+----------------------------------------+
| Parameter | Status |
+--------------------------+----------------------------------------+
| W | static (during whole session) |
| | |
| A | static (during whole session) |
| | |
| T_O | static (during whole session) |
| | |
| T_int | static (during whole session) |
| | |
| d | static (during whole session) |
| | |
| N | static (during whole session) |
| | |
| D^O_t (if present) | static (during whole session) |
| | |
| PRF Type | static (during whole session) |
| | |
| MAC Function Type | static (during whole session) |
| | |
| Signature Type | static (during whole session) |
| | |
| Weak Group MAC Func. | static (during whole session) |
| Type | |
| | |
| i | dynamic (related to current key chain) |
| | |
| K_i | dynamic (related to current key chain) |
| | |
| signature | dynamic, packet dependent |
| | |
| Weak Group MAC (if | dynamic, packet dependent |
| present) | |
+--------------------------+----------------------------------------+
Note that because a key cannot be disclosed before the disclose
delay, d, the sender MUST NOT send any bootstrap information message
during the first d intervals: {0 .. d-1} (inclusive).
4.3.2. Format of a Direct Time Synchronization Response
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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
+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ t_s (NTP timestamp) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ t_r (NTP timestamp) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
~ Signature ~
+ +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of a Direct Time Synchronization Response
The response to a direct time synchronization request contains the
following information:
"Reserved" fields (8 bits):
This is a reserved field that MUST be set to zero in this
specification.
"t_s" (NTP timestamp, 64 bits):
t_s is an NTP timestamp that corresponds to the sender local time
value when receiving the direct time synchronization request
message.
"t_r" (NTP timestamp, 64 bits):
t_r is an NTP timestamp that contains the receiver local time
value received in the direct time synchronization request message.
"Signature" field (variable size):
The "Signature" field is MANDATORY. The "Signature" field
contains a digital signature using the type specified either in
the "Signature Type" field of the bootstrap information message
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(if applicable) or out-of-band. Similarly the "Signature" field
length is either indicated in the "Signature Length" field of the
the bootstrap information message (if applicable) or out-of-band.
If need be, this field is padded (with 0) up to a multiple of 32
bits.
"Weak Group MAC" field (optional, variable length, multiple of 32
bits):
This field contains the weak MAC, calculated with a group key,
K_g, shared by all group members. The field length is given by
n_w, in bits.
4.3.3. Format of a Standard Authentication Tag
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
+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| i (Interval Index of K'_i) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Disclosed Key K_{i-d} +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of the authentication tag
Figure 4 shows the format of the authentication tag:
"Reserved" field (8 bits):
The "Reserved" field is not used in the current specification and
MUST be set to zero by the sender.
"i" (Interval Index) field (32 bits):
i is the interval index associated to the key (K'_i) used to
compute the MAC of this packet.
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"Disclosed Key" (variable size):
The "Disclosed Key" is the key used for interval i-d: K_{i-d};
Note that during the first d time intervals of a session, this
field must be initialized to "0" since no key can be disclosed
yet.
"MAC(K'_i, M)" (variable size):
MAC(K'_i, M) is the message authentication code of the current
packet. There is no padding between the "Disclosed Key" and
"MAC(K'_i, M)" fields, and this latter MAY not be aligned on 32
bit boundaries, depending on the n_p parameter.
"Weak Group MAC" field (optional, variable length, multiple of 32
bits):
This field contains the weak MAC, calculated with a group key,
K_g, shared by all group members. The field length is given by
n_w, in bits.
Note that because a key cannot be disclosed before the disclose
delay, d, the sender MUST either set the Disclosed Key field to 0
during the first d intervals: {0 .. d-1} (inclusive), or use a
Standard Authentication Tag Without Key Disclosure.
4.3.4. Format of a Standard Authentication Tag Without Key Disclosure
The authentication tag without key disclosure is meant to be used in
situations where a high number of packets are sent in a given time
interval. In such a case, it can be advantageous to disclose the
K_{i-d} key only in a subset of the packets sent, using a standard
authentication tag, and use the shortened version that does not
disclose the K_{i-d} key in the remaining packets. It is left to the
implementer to decide how many packets should disclose the K_{i-d}
key or not.
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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
+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| i (Interval Index of K'_i) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Format of the authentication tag without key disclosure
4.3.5. Format of an Authentication Tag with a New Key Chain Commitment
During the last n_tx_newkcc intervals of the current key chain, the
sender SHOULD send a commitment to the next key chain. This is done
by replacing the disclosed key of the authentication tag with the new
key chain commitment, F(K_{N+1}) (or F(K_{2N+2}) in case of a switch
between the second and third key chains, etc.). Figure 6 shows the
corresponding format.
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
+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| i (Interval Index of K'_i) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ New Key Commitment F(K_{N+1}) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Format of the authentication tag with a new key chain
commitment
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4.3.6. Format of an Authentication Tag with an Old Chain Last Key
Disclosure
During the first n_tx_lastkey intervals of the new key chain after
the disclosing interval, d, the sender MUST send a commitment to the
old key chain. This is done by replacing the disclosed key of the
authentication tag with the last key of the old chain, K_N (or
K_{2N+1} in case of a switch between the second and third key chains,
etc.). Figure 7 shows the corresponding format. There is no padding
between the "K_N" and "MAC(K'_i, M)" fields, and this latter MAY not
be aligned on 32 bit boundaries, depending on the n_p parameter.
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
+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| i (Interval Index of K'_i) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Last Key of Old Chain, K_N +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Format of the authentication tag with an old chain last key
disclosure
4.3.7. Format of the Compact Authentication Tags
The four compact flavors of the Authentication tags follow.
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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
+-+-+-+-+-+-+-+-+
| i_LSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Disclosed Key K_{i-d} +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | i_NSB (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Format of the compact authentication tag
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
+-+-+-+-+-+-+-+-+
| i_LSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | i_NSB (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format of the compact authentication tag without key
disclosure
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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
+-+-+-+-+-+-+-+-+
| i_LSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ New Key Commitment F(K_{N+1}) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | i_NSB (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Format of the compact authentication tag with a new key
chain commitment
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
+-+-+-+-+-+-+-+-+
| i_LSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Last Key of Old Chain, K_N +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+
| | i_NSB (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Format of the compact authentication tag with an old chain
last key disclosure
where:
"i_LSB" (Interval Index Least Significant Byte) field (8 bits):
the i_LSB field contains the least significant byte of the
interval index associated to the key (K'_i) used to compute the
MAC of this packet.
"i_NSB" (Interval Index Next Significant Bytes) field (variable
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length, depending on the MAC type):
the i_NSB field contains the next significant bytes of the
interval index associated to the key (K'_i) used to compute the
MAC of this packet. This field is present instead of the
"Padding" field when the MAC(K'_i, M) field length is not a
multiple of 32 bits.
The compact version does not include the "i" interval index but the
"i_LSB" field and sometimes, depending on the MAC type, the "i_NSB"
field. Upon receiving such an authentication tag, a receiver infers
the associated "i" value, by estimating the current interval where
the sender is supposed to be, assuming that this packet has not been
significantly delayed by the network. The remaining of the
processing does not change.
For instance, with HMAC-SHA-1, the MAC(K'_i, M) field is 8 byte long.
In that case the i_NSB field contains the bytes 2 and 3 of the "i"
counter. Together with the i_LSB byte, the three least significant
bytes of "i" are carried in the compact tag authentication header
extensions. If T_int is 0.5s, then the {i_NSB; i_LSB} counter is
sufficient (i.e. contains as much information as the 32 bit "i"
field) for sessions that last at most 2330 hours.
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5. Receiver Operations
5.1. Initialization of a Receiver
A receiver must be initialized before being able to authenticate the
source of incoming packets. This can be done by an out-of-band
mechanism, out of the scope of the present document, or an in-band
mechanism (Section 2.3). Let's focus on the in-band mechanism. Two
actions must be performed:
o receive and process a bootstrap information message, and
o calculate an upper bound of the sender's local time. To that
purpose, the receiver must perform time synchronization.
5.1.1. Processing the Bootstrap Information Message
A receiver must first receive a packet containing the bootstrap
information, digitally signed by the sender, and verify its
signature. Because the packet is signed, the receiver also needs to
know the public key of the sender. This document does not specify
how the public key of the sender is communicated reliably and in a
secure way to all possible receivers. Once the bootstrap information
has been verified, the receiver can initialize its TESLA component.
The receiver MUST then ignore the following bootstrap information
messages, if any. There is an exception though: when a new key chain
is used and if a receiver missed all the commitments for this new key
chain, then this receiver MUST process one of the future Bootstrap
information messages (if any) in order to be able to authenticate the
incoming packets associated to this new key chain.
Before TESLA has been initialized, a receiver MUST ignore all packets
other than the bootstrap information message. Yet, a receiver MAY
chose to buffer incoming packets, recording the reception time of
each packet, and proceed with delayed authentication later, once the
receiver will be fully initialized. In that case, the buffer must be
carefully sized in order to prevent memory starvation (e.g. an
attacker who sends faked packets before the session actually starts
can exhaust the memory of receivers who do not limit the maximum
incoming buffer size).
5.1.2. Time Synchronization
First of all, the receiver must know whether the ALC or NORM session
relies on direct or indirect time synchronization. This information
is communicated by an out-of-band mechanism (for instance when
describing the various parameters of a FLUTE session in case of ALC).
In some cases, both mechanisms might be available.
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5.1.2.1. Direct Time Synchronization
In case of a direct time synchronization, a receiver MUST synchronize
with the sender. To that purpose, the receiver sends a direct time
synchronization request message. This message includes the local
time (NTP timestamp) at the receiver when sending the message. This
timestamp will be copied in the sender's response.
The direct time synchronization request message format is the
following:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ t_r (NTP timestamp) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Weak Group MAC (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Format of a Direct Time Synchronization Request
The direct time synchronization request (Figure 12) contains the
following information:
"t_r" (NTP timestamp, 64 bits):
t_r is an NTP timestamp that contains the receiver local time
value when sending this direct time synchronization request
message;
"Weak Group MAC" field (optional, variable length, multiple of 32
bits):
This field contains the weak MAC, calculated with a group key,
K_g, shared by all group members. The field length is given by
n_w, in bits.
Section 4.3.2 specifies the direct time synchronization response
message format.
Note that in an ALC session, the direct time synchronization request
message is sent to the sender by an out-of-band mechanism that is not
specified by the current document.
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5.1.2.2. Indirect Time Synchronization
With the indirect time synchronization method, the sender MAY provide
out-of-band the URL or IP address of the NTP server(s) he trusts
along with an OPTIONAL certificate for each NTP server. When several
NTP servers are specified, a receiver MUST choose one of them. This
document does not specify how the choice is made, but for the sake of
scalability, the clients SHOULD NOT use the same server if several
possibilities are offered. The NTP synchronization between the NTP
server and the receiver MUST be authenticated, either using the
certificate provided by the content delivery server, or another
certificate the client may obtain for this NTP server.
Then the receiver computes the time offset between itself and the NTP
server chosen. Note that the receiver does not need to update the
local time, since this operation often requires root privileges.
Computing the time offset is sufficient.
Since the offset between the server and the time reference, D^O_t, is
indicated in the bootstrap information message (or communicated out-
of-band), the receiver can now calculate an upper bound of the
sender's local time (Section 3.2).
5.2. Authentication of Received Packets
The receiver can now authenticate incoming packets. To that purpose,
he MUST follow different steps (see [RFC4082] section 3.5):
1. The receiver parses the different packet headers. If none of the
eight TESLA authentication tags is present, the receiver MUST
discard the packet.
2. Safe packet test: When the receiver receives packet P_j, it first
records the local time T at which the packet arrived. The
receiver then computes an upper bound t_j on the sender's clock
at the time when the packet arrived: t_j = T + D_t. The receiver
then computes the highest interval the sender could possibly be
in: highest_i = floor((t_j - T_0) / T_int). Two possibilities
arise then:
* with a non compact authentication tag, the "i" interval index
is available. Get it from the header.
* When a compact authentication tag is used, the receiver must
compute the corresponding "i" interval index from the "i_LSB"
and perhaps "i_NSB" fields. The following algorithm is used:
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if (MAC(K'_i, M) is not padded) {
// with HMAC-SHA-256 and higher, the i_LSB field is the only
// field available to guess i.
i_mask = 0xFFFFFF00;
i_low = i_LSB; // lower bits of "i"
} else {
// with a two byte padding (i.e. HMAC-SHA-1 and HMAC-SHA-224),
// the 2 byte i_NSB field is available in addition to i_LSB.
i_mask = 0xFF000000;
i_low = i_LSB + i_NSB; // lower bits of "i"
}
i_high = highest_i & i_mask; // (guessed) higher bits of "i", using
// the highest interval the sender can
// possibly be in.
i = i_high + i_low; // raw guessed "i"
if (i > highest_i) {
// cycling took place. Since "i" cannot be larger than "highest_i",
// decrement it.
i_cycle = (~i_mask) + 1; // length of a cycle
i = i - i_cycle;
}
The receiver can now proceed with the "safe packet" test. If
highest_i < i + d, then the sender is not yet in the interval
during which it discloses the key K_i. The packet is safe (but
not necessarily authentic). If the test fails, the packet is
unsafe, and the receiver MUST discard the packet.
3. Weak Group MAC test: The receiver checks the optional Weak Group
Tag, if present. To that purpose, the receiver recomputes the
group MAC and compares it to the value stored in the "Weak Group
MAC" field. If the check fails, the packet is immediately
dropped.
4. Disclosed Key processing: When the packet discloses a key (i.e.,
with a standard or compact authentication tag, or with a standard
or compact authentication tag with an old chain last key
disclosure), the following tests are performed:
* New key index test: the receiver checks whether a legitimate
key already exists with the same index (i.e., i-d), or with an
index strictly superior (i.e., with an index > i-d). If such
a legitimate key exists, the receiver ignores the current
disclosed key and skips the "Key verification test".
* Key verification test: If the disclosed key index is new, the
receiver checks the legitimacy of K_{i-d} by verifying, for
some earlier disclosed and legitimate key K_v (with v < i-d),
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that K_v = F^{i-d-v}(K_{i-d}). In other words, the receiver
checks the disclosed key by computing the necessary number of
PRF functions to obtain a previously disclosed and legitimate
(i.e., verified) key. If the key verification fails, the
receiver MUST discard the packet. If the key verification
succeeds, this key is said legitimate.
5. New Key Chain Commitment processing: When the packet includes a
new key chain commitment (i.e., with a standard or compact
authentication tag with a new key chain commitment), the receiver
first checks whether a commitment has already been received or
not for this new key chain. If this is a new commitment, the
receiver stores it. If a commitment is already available, it is
recommended that the receiver stores the new commitment. Indeed,
the previously stored commitment(s) may fail the authentication
test and therefore turn out to be useless. When the commitment
is stored, it is marked as non-verified. This commitment will be
validated later on, when the associated packet is authenticated.
6. When applicable, the receiver performs congestion control, even
if the packet has not yet been authenticated
[draft-ietf-rmt-bb-lct-revised]. If this feature leads to a
potential DoS attack (the attacker can send a high data rate
stream of faked packets), it does not compromise the security
features offered by TESLA and enables a rapid reaction in front
of actual congestion problems.
7. The receiver then buffers the packet for a later authentication,
once the corresponding key will be disclosed (after d time
intervals) or deduced from another key (if all packets disclosing
this key are lost).
8. Authentication test: Let v be the smallest index of the
legitimate keys known by the receiver so far. For all the new
keys K_w, with v < w < = i-d, that have been either disclosed by
this packet (i.e. K_{i-d}) or derived by K_{i-d} (i.e. keys in
interval {v+1,.. i-d-1}), the receiver verifies the authenticity
of the safe packets buffered for the corresponding interval w.
To authenticate one of the buffered packets P_h containing
message M_h protected with a MAC that used key index w, the
receiver will compute K'_w = F'(K_w) from which it can compute
MAC( K'_w, M_h). If this MAC equals the MAC stored in the
packet, the packet is successfully authenticated and the receiver
continues processing it. If the MACs do not agree, the receiver
MUST discard the packet.
9. The receiver continues processing all the packets authenticated
during the authentication test.
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In this specification, a receiver using TESLA MUST immediately drop
unsafe packets. But the receiver MAY also decide, at any time, to
continue an ALC or NORM session in unsafe mode, ignoring TESLA
extensions.
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6. Integration in the ALC and NORM Protocols
6.1. Authentication Header Extension Format
The integration of TESLA in ALC or NORM is similar and relies on the
header extension mechanism defined in both protocols. More precisely
this document details the EXT_AUTH==1 header extension defined in
[draft-ietf-rmt-bb-lct-revised].
----- Editor's note: All authentication schemes using the EXT_AUTH
header extension MUST reserve the same 4 bit "ASID" field after
the HET/HEL fields. This way, several authentication schemes can
be used in the same ALC or NORM session, even on the same
communication path. -----
Several fields are added in addition to the HET (Header Extension
Type) and HEL (Header Extension Length) fields (Figure 14).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET (=1) | HEL | ASID | Type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
~ ~
| Content |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Format of the TESLA EXT_AUTH header extension.
The fields of the TESLA EXT_AUTH header extension are:
"ASID" (Authentication Scheme Identifier) field (4 bits):
The "ASID" identifies the source authentication scheme or protocol
in use. The association between the "ASID" value and the actual
authentication scheme is defined out-of-band, at session startup.
"Type" field (4 bits):
The "Type" field identifies the type of TESLA information carried
in this header extension. This specification defines the
following types:
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* 0: bootstrap information, sent by the sender periodically or
after a direct time synchronization request;
* 1: standard authentication tag for the on-going key chain, sent
by the sender along with a packet;
* 2: authentication tag without key disclosure, sent by the
sender along with a packet;
* 3: authentication tag with a new key chain commitment, sent by
the sender when approaching the end of a key chain;
* 4: authentication tag with an old chain last key disclosure,
sent by the sender some time after moving to a new key chain;
* 5: compact (i.e. that contains the last byte of the interval
index) authentication tag for the on-going key chain, sent by
the sender along with a packet;
* 6: compact (i.e. that contains the last byte of the interval
index) authentication tag without any key disclosure, sent by
the sender along with a packet;
* 7: compact (i.e. that contains the last byte of the interval
index) authentication tag with a new key chain commitment, sent
by the sender when approaching the end of a key chain;
* 8: compact (i.e. that contains the last byte of the interval
index) authentication tag with an old chain last key
disclosure, sent by the sender some time after moving to a new
key chain;
* 9: direct time synchronization request, sent by a NORM
receiver. This type of message is invalid in case of an ALC
session since ALC is restricted to unidirectional
transmissions. Yet an external mechanism may provide the
direct time synchronization functionality. How this is done is
out of the scope of this document;
* 10: direct time synchronization response, sent by a NORM
sender. This type of message is invalid in case of an ALC
session since ALC is restricted to unidirectional
transmissions. Yet an external mechanism may provide the
direct time synchronization functionality. How this is done is
out of the scope of this document;
"Content" field (variable length):
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This is the TESLA information carried in the header extension,
whose type is given by the "Type" field.
6.2. Use of Authentication Header Extensions
Each packet sent by the session's sender MUST contain exactly one
TESLA EXT_AUTH header extension.
All receivers MUST recognize EXT_AUTH but MAY not be able to parse
its content, for instance because they do not support TESLA. In that
case these receivers MUST ignore the TESLA EXT_AUTH extensions. In
case of NORM, the packets sent by receivers MAY contain a direct
synchronization request but MUST NOT contain any of the other five
TESLA EXT_AUTH header extensions.
6.2.1. EXT_AUTH Header Extension of Type Bootstrap Information
The "bootstrap information" TESLA EXT_AUTH (Type==0) MUST be sent in
a stand-alone control packet, rather than in a packet containing
application data. The reason for that is the large size of this
bootstrap information. By using stand-alone packets, the maximum
payload size of data packets is only affected by the (mandatory)
authentication information header extension.
With ALC, the "bootstrap information" TESLA EXT_AUTH MUST be sent in
a control packet, i.e. containing no encoding symbol.
With NORM, the "bootstrap information" TESLA EXT_AUTH MUST be sent in
a NORM_CMD(APPLICATION) message.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
| HET (=1) | HEL (=45) | ASID | 0 | Reserved |1|0| ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| 1 | 1 | 1 | 128 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| 1 | d | T_int | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
+ T_0 (NTP timestamp) + | 4
| | | 8
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| N (Number of Keys in chain) | | b
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | y
| i (Interval Index of K_i) | | t
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | e
| | | s
+ + |
| | |
+ K_i + |
| (20 bytes) | |
+ + |
| | |
+ + |
| | v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
| | ^ 1
+ + | 2
| | | 8
. . |
. Signature . | b
. (128 bytes) . | y
| | | t
+ + | e
| | v s
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
| Weak Group MAC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Example: Format of the bootstrap information message (Type
0), using 1024 bit signatures, the default HMAC-SHA-1 and a Weak
Group MAC.
For instance Figure 15 shows the bootstrap information message when
using the HMAC-SHA-1 transform for the PRF, MAC, and Weak Group MAC
functions, along with 128 byte (1024 bit) key digital signatures
(which also means that the signature field is 128 byte long). The
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TESLA EXT_AUTH header extension is then 180 byte long (i.e. 45 words
of 32 bits).
6.2.2. EXT_AUTH Header Extension of Type Authentication Tag
The eight "authentication tag" TESLA EXT_AUTH (Type 1, 2, 3, 4, 5, 6,
7 and 8) MUST be attached to the ALC or NORM packet (data or control
packet) that they protect.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET (=1) | HEL (=9) | ASID | 5 | i_LSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Disclosed Key K_{i-d} +
| (20 bytes) |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ MAC(K'_i, M) +
| (10 bytes) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | i_NSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: Example: Format of the standard authentication tag (Type
5), using the default HMAC-SHA-1.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET (=1) | HEL (=4) | ASID | 6 | i_LSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ MAC(K'_i, M) +
| (10 bytes) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | i_NSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: Example: Format of the compact authentication tag without
key disclosure (Type 6), using the default HMAC-SHA-1.
For instance, Figure 16 and Figure 17 show the format of the compact
authentication tags, respectively with and without the K_{i-d} key
disclosure, when using the (default) HMAC-SHA-1 transform for the PRF
and MAC functions. In this example, the Weak Group MAC feature is
not used.
6.2.3. EXT_AUTH Header Extension of Type Direct Time Synchronization
Request
With NORM, the "direct time synchronization request" TESLA EXT_AUTH
(Type==7) MUST be sent by a receiver in a NORM_CMD(APPLICATION) NORM
packet.
With ALC, the "direct time synchronization request" TESLA EXT_AUTH
cannot be included in an ALC packet, since ALC is restricted to
unidirectional transmissions, from the session's sender to the
receivers. An external mechanism, out of the scope of this document,
must be used with ALC for carrying direct time synchronization
requests to the session's sender.
In case of direct time synchronization, it is RECOMMENDED that the
receivers spread the transmission of direct time synchronization
requests over the time (Section 2.2.1).
6.2.4. EXT_AUTH Header Extension of Type Direct Time Synchronization
Response
With NORM, the "direct time synchronization response" TESLA EXT_AUTH
(Type==8) MUST be sent by the sender in a NORM_CMD(APPLICATION)
message.
With ALC, the "direct time synchronization response" TESLA EXT_AUTH
can be sent in an ALC control packet (i.e. containing no encoding
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symbol) or through the external mechanism use to carry the direct
time synchronization request.
6.3. Managing Silent Periods
An ALC or NORM sender may stop transmitting packet for some time, for
various reasons. It can be the end of the session and all packets
have already been sent, or the use-case may consist in a succession
of busy periods (when fresh objects are available) followed by silent
periods. In both cases, this is an issue since the authentication of
the packets sent during the last d intervals requires that the
associated keys be revealed, which can only take place after d
additional intervals.
To solve this problem, it is recommended that the sender transmit
null packets containing the TESLA EXT_AUTH header extension along
with a standard authentication tag (Type==1) during at least d
intervals after the end of the regular ALC or NORM packet
transmissions. The number of such packets and the duration during
which they are sent must be sufficient for all receivers to receive,
which a high probability, at least one packet disclosing the last
useful key.
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7. IANA Considerations
This document requires a IANA registration for the following
attributes:
Cryptographic Pseudo-Random Function, TESLA-PRF: All implementations
MUST support HMAC-SHA-1 (default).
+----------------------+-------+---------------------+
| PRF name | Value | n_p and n_f |
+----------------------+-------+---------------------+
| INVALID | 0 | N/A |
| | | |
| HMAC-SHA-1 (default) | 1 | 160 bits (20 bytes) |
| | | |
| HMAC-SHA-224 | 2 | 224 bits (28 bytes) |
| | | |
| HMAC-SHA-256 | 3 | 256 bits (32 bytes) |
| | | |
| HMAC-SHA-384 | 4 | 384 bits (48 bytes) |
| | | |
| HMAC-SHA-512 | 5 | 512 bits (64 bytes) |
+----------------------+-------+---------------------+
Cryptographic Message Authentication Code (MAC): All implementations
MUST support HMAC-SHA-1 (default).
+---------------------+-------+------------------+------------------+
| MAC name | Value | n_m | n_w |
+---------------------+-------+------------------+------------------+
| INVALID | 0 | N/A | N/A |
| | | | |
| HMAC-SHA-1 | 1 | 80 bits (10 | 32 bits (4 |
| (default) | | bytes) | bytes) |
| | | | |
| HMAC-SHA-224 | 2 | 112 bits (14 | 32 bits (4 |
| | | bytes) | bytes) |
| | | | |
| HMAC-SHA-256 | 3 | 128 bits (16 | 32 bits (4 |
| | | bytes) | bytes) |
| | | | |
| HMAC-SHA-384 | 4 | 192 bits (24 | 32 bits (4 |
| | | bytes) | bytes) |
| | | | |
| HMAC-SHA-512 | 5 | 256 bits (32 | 32 bits (4 |
| | | bytes) | bytes) |
+---------------------+-------+------------------+------------------+
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Signature Encoding Algorithm: All implementations MUST support
RSASSA-PKCS1-v1_5 (default).
+-----------------------------+-------+
| Signature Algorithm Name | Value |
+-----------------------------+-------+
| INVALID | 0 |
| | |
| RSASSA-PKCS1-v1_5 (default) | 1 |
| | |
| RSASSA-PSS | 2 |
+-----------------------------+-------+
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8. Security Considerations
[RFC4082] discusses the security of TESLA in general. These
considerations apply to the present specification, namely:
o great care must be taken to the timing aspects. In particular the
D_t parameter is critical and must be initialized correctly,
depending on the use-case;
o if the key disclosure schedule is to be changed (e.g. because the
sender realizes that the parameters do not meet the receiver
requirements), then this change MUST NOT be announced in-line,
within the session. Indeed, a receiver that missed the
announcement would be vulnerable to attacks. Note that in the
current specification, the parameters that define the key
disclosure schedule MUST be fixed during the whole session
(Section 4.1.3).
o when the verifier that authenticates the incoming packets and the
application that uses the data are two different components, there
is a risk that an attacker located between these components inject
faked data. Similarly, when the verifier and the secure timing
system are two different components, there is a risk that an
attacker located between these components inject faked timing
information. For instance, when the verifier reads the local time
by means of a dedicated system call (e.g. gettimeofday()), if an
attacker controls the host, he may catch the system call and
return a faked time information.
The current specification discusses additional aspects with more
details.
8.1. Dealing With DoS Attacks
TESLA introduces new opportunities for an attacker to mount DoS
attacks: for instance by saturating the processing capabilities of
the receiver (faked packets are easy to create but checking them
requires to compute a MAC over the packet), or by saturating its
memory (since authentication is delayed), or by making the receiver
believe that a congestion has happened (since congestion control MUST
be performed before authenticating incoming packets, Section 5.2).
In order to mitigate these attacks, when it is believed that
attackers do not belong to the group, it is RECOMMENDED to use the
Weak Group MAC scheme (Section 4.2.4).
Generally, it is RECOMMENDED that the amount of memory used to store
incoming packets waiting to be authenticated be limited to a
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reasonable value.
8.2. Dealing With Replay Attacks
Replay attacks, whereby an attacker stores a valid message and
replays it later on, can have significant impacts, depending on the
message type. Two levels of impacts must be distinguished:
o within the TESLA protocol, and
o within the ALC or NORM protocol.
8.2.1. Impacts of Replay Attacks on TESLA
Replay attacks can impact the TESLA component itself. We review
here, type by type, the potential impacts of such an attack depending
on the TESLA message type:
o bootstrap information: since most parameters contained in a
bootstrap information message are static, replay attacks have no
consequences. The fact that the "i" and "K_i" fields can be
updated in subsequent bootstrap information messages does not
create a problem either, since all "i" and "K_i" fields sent
remain valid. Finally, a receiver that successfully initialized
its TESLA component should ignore the following messages
(Section 5.1.1), which voids replay attacks.
o direct time synchronization request: If the Weak Group MAC scheme
is used, an attacker that is not member of the group can replay a
packet and oblige the sender to to respond, which requires to
digitally sign the response, a time-consuming process. If the
Weak Group MAC scheme is not used, an attack can anyway easily
forge a request. In both cases, the attack will not compromise
TESLA component, but might create a DoS. If this is a concern, it
is RECOMMENDED, when the Weak Group MAC scheme is used, that the
sender verify the "t_r" NTP timestamp contained in the request and
respond only if this value is strictly larger than the previous
one received from this receiver. When the Weak Group MAC scheme
is not used, this attack can be mitigated by limiting the number
of requests per second that will be processed.
o direct time synchronization response: Upon receiving a response, a
receiver who has no pending request MUST immediately drop the
packet. If this receiver that previously issued a request, he
first checks the Weak Group MAC (if applicable), then the "t_r"
field, to be sure it is a response to his request, and finally the
digital signature. A replayed packet will be dropped during these
verifications, without compromising the TESLA component.
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o other messages, containing an authentication tag: Replaying a
packet containing a TESLA authentication tag will never compromise
the TESLA component itself (but perhaps the underlying ALC or NORM
component, see below).
To conclude, TESLA itself is robust in front of replay attacks.
8.2.2. Impacts of Replay Attacks on NORM
We review here the potential impacts of a replay attack on the NORM
component.
First, let us consider replay attacks within a given NORM session.
NORM defines a "sequence" field that can be used to protect against
replay attacks [draft-ietf-rmt-pi-norm-revised] within a given NORM
session. This "sequence" field is a 16-bit value that is set by the
message originator (sender or receiver) as a monotonically increasing
number incremented with each NORM message transmitted. It is
RECOMMENDED that a receiver check this sequence field and drop
messages considered as replayed. Similarly, it is RECOMMENDED that a
sender check this sequence, for each known receiver, and drop
messages considered as replayed. This analysis shows that NORM
itself is robust in front of replay attacks within the same session.
Now let us consider replay attacks across several NORM sessions.
Since the key chain used in each session MUST differ, a packet
replayed in a subsequent session will be identified as unauthentic.
Therefore NORM is robust in front of replay attacks across different
sessions.
8.2.3. Impacts of Replay Attacks on ALC
We review here the potential impacts of a replay attack on the ALC
component. Note that we do not consider here the protocols that
could be used along with ALC, for instance the layered or wave based
congestion control protocols.
First, let us consider replay attacks within a given ALC session:
o Regular packets containing an authentication tag: a replayed
message containing an encoding symbol will be detected once
authenticated, thanks to the object/block/symbol identifiers, and
will be silently discarded. This kind of replay attack is only
penalizing in terms of memory and processing load, but does not
compromise the ALC behavior.
o Control packets containing an authentication tag: ALC control
packets, by definition, do not include any encoding symbol and
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therefore do not include any object/block/symbol identifier that
would enable a receiver to identify duplicates. However, a sender
has a very limited number of reasons to send control packets.
More precisely:
* At the end of the session, a "close session" packet is sent.
Replaying this packet has no impact since the receivers already
left.
* The same remark can be done for the "close object" packets.
This analysis shows that ALC itself is robust in front of replay
attacks within the same session.
Now let us consider replay attacks across several ALC sessions.
Since the key chain used in each session MUST differ, a packet
replayed in a subsequent session will be identified as unauthentic.
Therefore ALC is robust in front of replay attacks across different
sessions.
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9. Acknowledgments
The authors are grateful to Ran Canetti, David L. Mills and Lionel
Giraud for their valuable comments while preparing this document.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC3926] Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh,
"FLUTE - File Delivery over Unidirectional Transport",
RFC 3926, October 2004.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
[draft-ietf-rmt-bb-lct-revised]
Luby, M., Watson, M., and L. Vicisano, "Layered Coding
Transport (LCT) Building Block",
draft-ietf-rmt-bb-lct-revised-06.txt (work in progress),
November 2007.
[draft-ietf-rmt-pi-alc-revised]
Luby, M., Watson, M., and L. Vicisano, "Asynchronous
Layered Coding (ALC) Protocol Instantiation",
draft-ietf-rmt-pi-alc-revised-05.txt (work in progress),
November 2007.
[draft-ietf-rmt-pi-norm-revised]
Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Negative-acknowledgment (NACK)-Oriented Reliable
Multicast (NORM) Protocol",
draft-ietf-rmt-pi-norm-revised-05.txt (work in progress),
March 2007.
10.2. Informative References
[Perrig04]
Perrig, A. and J. Tygar, "Secure Broadcast Communication
in Wired and Wireless Networks", Kluwer Academic
Publishers ISBN 0-7923-7650-1, 2004.
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 4330, January 2006.
[RFC4359] Weis, B., "The Use of RSA/SHA-1 Signatures within
Encapsulating Security Payload (ESP) and Authentication
Header (AH)", RFC 4359, January 2006.
[RFC4383] Baugher, M. and E. Carrara, "The Use of Timed Efficient
Stream Loss-Tolerant Authentication (TESLA) in the Secure
Real-time Transport Protocol (SRTP)", RFC 4383,
February 2006.
[RFC4442] Fries, S. and H. Tschofenig, "Bootstrapping Timed
Efficient Stream Loss-Tolerant Authentication (TESLA)",
RFC 4442, March 2006.
[draft-ietf-ntp-ntpv4-proto]
Burbank, J., Kasch, W., Martin, J., and D. Mills, "The
Network Time Protocol Version 4 Protocol Specification",
draft-ietf-ntp-ntpv4-proto-07.txt (work in progress),
May 2007.
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Authors' Addresses
Vincent Roca
INRIA
655, av. de l'Europe
Zirst; Montbonnot
ST ISMIER cedex 38334
France
Email: vincent.roca@inria.fr
URI: http://planete.inrialpes.fr/~roca/
Aurelien Francillon
INRIA
655, av. de l'Europe
Zirst; Montbonnot
ST ISMIER cedex 38334
France
Email: aurelien.francillon@inria.fr
URI: http://planete.inrialpes.fr/~francill/
Sebastien Faurite
INRIA
655, av. de l'Europe
Zirst; Montbonnot
ST ISMIER cedex 38334
France
Email: faurite@lcpc.fr
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