One document matched: draft-ietf-msec-tesla-intro-01.txt
Differences from draft-ietf-msec-tesla-intro-00.txt
Internet Engineering Task Force IETF MSEC
Internet Draft Perrig, Canetti, Song, Tygar, Briscoe
draft-ietf-msec-tesla-intro-01.txt UC Berkeley/Digital Fountain/IBM/BT
27 October 2002
Expires: 27 April 2002
TESLA: Multicast Source Authentication Transform Introduction
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference mate
rial or to cite them other than as "work in progress".
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Abstract
Data authentication is an important component for many applications,
for example audio and video Internet broadcasts, or data distribution
by satellite. This document introduces TESLA, a secure source authen
tication mechanism for multicast or broadcast data streams. This doc
ument provides an algorithmic description of the scheme for informa
tional purposes, and in particular, it is intended to assist in writ
ing standardizable and secure specifications for protocols based on
TESLA in different contexts.
The main deterrents so far for a data authentication mechanism for
multicast were the seemingly conflicting requirements: loss toler
ance, high efficiency, no per-receiver state at the sender. The prob
lem is particularly hard in settings with high packet loss rates and
where lost packets are not retransmitted, and where the receiver
wants to authenticate each packet it receives.
TESLA provides authentication of individual data packets, regardless
of the packet loss rate. In addition, TESLA features low overhead for
Perrig, Canetti, Song, Tygar, Briscoe [Page 1]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
both sender and receiver, and does not require per-receiver state at
the sender. TESLA is secure as long as the sender and receiver are
loosely time synchronized.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . 2
2 Functionality . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Threat Model and Security Guarantee . . . . . . . . . . . 4
2.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 5
3 Notation . . . . . . . . . . . . . . . . . . . . . . . . 5
4 The Basic TESLA Protocol . . . . . . . . . . . . . . . . 5
4.1 Sketch of protocol . . . . . . . . . . . . . . . . . . . 6
4.2 Sender Setup . . . . . . . . . . . . . . . . . . . . . . 7
4.3 Bootstrapping Receivers . . . . . . . . . . . . . . . . . 7
4.4 Broadcasting Authenticated Messages . . . . . . . . . . . 8
4.5 Authentication at Receiver . . . . . . . . . . . . . . . 8
4.6 Determining the Key Disclosure Delay . . . . . . . . . . 9
4.7 Some extenstions. . . . . . . . . . . . . . . . . . . . . 10
5 Layer placement . . . . . . . . . . . . . . . . . . . . . 10
6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . 10
7 Bibliography . . . . . . . . . . . . . . . . . . . . . . 10
A Author Contact Information . . . . . . . . . . . . . . . 12
B Full Copyright Statement . . . . . . . . . . . . . . . . 13
1 Introduction
The power of multicast is that one packet can reach millions of
receivers. This great property is unfortunately also a great danger:
an attacker that sends one malicious packet can also potentially
reach millions of receivers. Receivers need multicast source authen
tication to ensure that a given packet originates from the correct
source.
In unicast communication, we can achieve data authentication through
a purely symmetric mechanism: the sender and the receiver share a
secret key to compute a message authentication code (MAC) of all com
municated data. When a message with a correct MAC arrives, the
receiver is assured that the sender generated that message. Standard
mechanisms achieve unicast authentication this way, for example TLS
or IPsec [1,2].
The symmetric MAC authentication is not secure in a broadcast set
ting. Consider a sender that broadcasts authentic data to mutually
untrusted receivers. The symmetric MAC is not secure: every receiver
Perrig, Canetti, Song, Tygar, Briscoe [Page 2]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
knows the MAC key, and hence could impersonate the sender and forge
messages to other receivers. Intuitively, we need an asymmetric mech
anism to achieve authenticated broadcast, such that every receiver
can verify the authenticity of messages it receives, without being
able to generate authentic messages. Achieving this in an efficient
way is a challenging problem [3].
The standard approach to achieve such asymmetry for authentication is
to use asymmetric cryptography, for instance a digital signature.
Digital signatures have the required asymmetric property: the sender
generates the signature with its private key, and all receivers can
verify the signature with the sender's public key, but a receiver
with the public key alone cannot generate a digital signature for a
new message. A digital signature provides non-repudiation, which is a
stronger property than authentication. Unfortunately, digital signa
tures have a high cost: they have a high computation overhead for
both the sender and the receiver, as well as a high communication
overhead. Since we assume broadcast settings where the sender does
not retransmit lost packets, and the receiver still wants to immedi
ately authenticate each packet it receives, we would need to attach a
digital signature to each message. Because of the high overhead of
asymmetric cryptography, this approach would restrict us to low-rate
streams, and to senders and receivers with powerful workstations. To
deal with the high overhead of asymmetric cryptography, we can try to
amortize one digital signature over multiple messages. However, such
an approach is still expensive in contrast to symmetric cryptography,
since symmetric cryptography is in general 3 to 5 orders of magnitude
more efficient than asymmetric cryptography. In addition, the
straight-forward amortization of one digital signature over multiple
packets requires reliability, as the receiver needs to receive all
packets to verify the signature. A number of schemes that follow this
approach are [4,5,6,7,8]. See [9] for more details.
This draft presents the Timed Efficient Stream Loss-tolerant Authen
tication protocol (TESLA). TESLA uses mainly symmetric cryptography,
and uses time delayed key disclosure to achieve the required asymme
try property. However, TESLA requires loosely synchronized clocks
between the sender and the receivers. See more details in Section 4.
Other schemes that follow a similar approach to TESLA are [10,11,12].
2 Functionality
TESLA provides delayed per-packet data authentication. The key idea
to providing both efficiency and security is a delayed disclosure of
keys. The delayed key disclosure results in an authentication delay.
In practice, the delay is on the order of one RTT (Round-trip-time).
Perrig, Canetti, Song, Tygar, Briscoe [Page 3]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
TESLA has the following properties:
· Low computation overhead for generation and verification of
authentication information
· Low communication overhead
· Limited buffering required for the sender and the receiver, hence
timely authentication for each individual packet
· Strong robustness to packet loss
· Scales to a large number of receivers
· Security is guaranteed as long as the sender and recipients are
loosely time synchronized, where synchronization can take place
at session set-up.
TESLA can be used both in the network layer or in the application
layer. The delayed authentication, however, requires buffering of
packets until authentication is completed.
2.1 Threat Model and Security Guarantee
We design TESLA to be secure against a powerful adversary with the
following capabilities:
· Full control over the network. The adversary can eavesdrop, cap
ture, drop, resend, delay, and alter packets.
· Access to a fast network with negligible delay.
· The adversary's computational resources may be very large, but
not unbounded. In particular, this means that the adversary can
perform efficient computations, such as computing a reasonable
number of pseudo-random function applications and MACs with neg
ligible delay. Nonetheless, the adversary cannot find the key of
a pseudorandom function (or distinguish it from a random func
tion) with non-negligible probability.
The security property of TESLA guarantees that the receiver never
accepts M_i as an authentic message unless the sender really sent
M_i. A scheme that provides this guarantee is called a secure broad
cast authentication scheme.
Since TESLA requires the receiver to buffer packets before authenti
cation, the receiver needs to protect itself from a potential denial-
of-service (DOS) attack due to a flood of bogus packets.
Perrig, Canetti, Song, Tygar, Briscoe [Page 4]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
2.2 Assumptions
TESLA makes the following assumptions in order to provide security:
1. The sender and the receiver MUST be loosely time synchronized.
Loosely time synchronized means that the synchronization does
not need to be precise, but the receiver MUST know an upper
bound on the dispersion (the maximum clock offset). For the
purposes of this draft, we assume that the receiver knows the
maximum clock offset between its clock and the sender's clock,
which we denote with D_t. We stress that the sender and
receiver's clock do not need to be synchronized a-priori.
Instead, the receiver can easily achieve the required synchro
nization through a two-round message exchange with the sender.
(This stands in contrast with authentication protocols based
on timestamps. In those protocols, the participants are
assumed to have the same global time a-priori.)
2. TESLA MUST be bootstrapped at session set-up through a regular
data authentication system. We recommend to use a digital sig
nature algorithm for this purpose, in which case the receiver
is REQUIRED to have an authentic copy of either the sender's
public key certificate or a root key certificate in case of a
PKI (public-key infrastructure).
3. TESLA uses cryptographic MAC and PRF (pseudo-random func
tions). These MUST be cryptographically secure. Further
details on the instantiation of the MAC and PRF are in Section
4.2.
4. We would like to emphasize that the security of TESLA does NOT
rely on any assumptions on network propagation delay.
3 Notation
To denote the subscript or an index of a variable, we use the under
score between the variable name and the index, e.g. the key K with
index i is K_i, the key K with index i+d is K_{i+d}. To write a
superscript we use the caret, e.g. the function F with the argument x
executed i times is F^i(x), executed j-1 times we write F^{j-1}(x).
4 The Basic TESLA Protocol
TESLA is described in several academic publications: A book on broad
cast security [13], a journal paper [14], and two conference papers
Perrig, Canetti, Song, Tygar, Briscoe [Page 5]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
[8,15]. Please refer to these publications for an in-depth treatment.
4.1 Sketch of protocol
We first outline the main ideas behind TESLA.
As we argue in the introduction, broadcast authentication requires a
source of asymmetry. TESLA uses time for asymmetry. We assume that
the sender and receivers are all loosely time synchronized -- up to
some D_t value, all parties agree on the current time. The sender
forms a one-way chain, where each such value is associated with a
time interval (say, a second). Here is the basic approach:
· The sender attaches a MAC to each packet. The MAC is computed
over the contents of the packet. For each packet, the sender uses
the current value from the one-way chain as a cryptographic key
to compute the MAC.
· Each receiver receives the packet. Each receiver knows the sched
ule for disclosing keys and, since the clocks are loosely syn
chronized, can check that the key used to compute the MAC is
still secret by determining that the sender could not have yet
reached the time for disclosing it. If the MAC key is still
secret, then the receiver buffers the packet.
· According to a schedule, the sender discloses the key from the
one-way chain.
· Each receiver checks that the disclosed key is correct (using
previously released keys) and then checks the correctness of the
MAC. If the MAC is correct, the receiver accepts the packet.
Note that one way chains have the property that if intermediate val
ues of the one-way chain are lost, they can be recomputed using the
following values. So, even if some key disclosures are lost, a
receiver can recover the key chain and check the correctness of ear
lier packets.
The sender distributes a stream of messages {M_i}, and the sender
sends each message M_i in a network packet P_i along with authentica
tion information. The broadcast channel may be lossy, but in many
broadcast applications the sender does not retransmit lost packets.
Despite packet loss, each receiver needs to authenticate every mes
sage it receives.
We now describe the stages of the basic TESLA protocol in this order:
sender setup, receiver bootstrap, sender transmission of authenti
cated broadcast messages, and receiver authentication of broadcast
Perrig, Canetti, Song, Tygar, Briscoe [Page 6]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
messages.
4.2 Sender Setup
The sender divides the time into uniform intervals of duration T_int.
The sender assigns one key from the one-way chain to each time inter
val in sequence.
The sender determines the length N of the one-way chain K_0, K_1,
..., K_N, and this length limits the maximum transmission duration
before a new one-way chain must be created. The sender picks a random
value for K_N. Using a pseudo-random function (PRF) f, the sender
constructs the one-way function F: F(k) = f_k(0). The rest of the
chain is computed recursively using K_i = F(K_{i+1}). Note that this
gives us K_i = F^{N-i}(K_N), so the receiver can compute any value in
the key chain from K_N even if is does not have intermediate values.
The key K_i will be used to authenticate packets sent in time inter
val i.
4.3 Bootstrapping Receivers
Before a receiver can authenticate messages with TESLA, it needs to
be loosely time synchronized with the sender, know the disclosure
schedule of keys, and receive an authenticated key of the one-way key
chain.
Various approaches exist for time synchronization [16,17,18,19].
TESLA, however, only requires loose time synchronization between the
sender and the receivers, so a simple algorithm is sufficient. The
time synchronization property that TESLA requires is that each
receiver can place an upper bound of the senders local time. TESLA
offers direct, indirect, and delayed synchronization as three default
options, which we will describe in the TESLA technical draft.
The sender sends the key disclosure schedule by transmitting the fol
lowing information to the receivers over an authenticated channel
(either via a digitally signed broadcast message, or over an authen
ticated unicast channel with each receiver):
· Time interval schedule: interval duration T_int, start time and
index of interval i, length of one-way key chain.
· Key disclosure delay d (number of intervals).
· A key commitment to the key chain K_i (i < j - d + 1, where j is
the current interval index).
Perrig, Canetti, Song, Tygar, Briscoe [Page 7]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
The receiver can perform the time synchronization and getting the
authenticated TESLA parameters in a two-round message exchange, which
we will describe in the technical TESLA draft. Time synchronization
can be performed as part of the registration protocol between member
and sender.
4.4 Broadcasting Authenticated Messages
Each key in the one-way key chain corresponds to a time interval.
Every time a sender broadcasts a message, it appends a MAC to the
message, using the key corresponding to the current time interval.
The key remains secret for the next d-1 intervals, so messages a
sender broadcasts in interval j effectively disclose key K_j-d. We
call d the key disclosure delay.
We do not want to use the same key multiple times in different cryp
tographic operations, that is, to use key K_j to derive the previous
key of the one-way key chain K_{j-1}, and to use the same key K_j as
the key to compute the MACs in time interval j may potentially lead
to a cryptographic weakness. Using a pseudo-random function (PRF)
f', we construct the one-way function F': F'(k) = f'_k(1). We use F'
to derive the key to compute the MAC of messages in each interval.
The sender derives the MAC key as follows: K'_i = F'(K_i). Figure 1
depicts the one-way key chain construction and MAC key derivation. To
broadcast message M_j in interval i the sender constructs packet P_j
= {M_j || MAC(K'_i,M_j) || K_{i-d}}, where || denotes concatenation.
F(K_i) F(K_{i+1}) F(K_{i+2})
K_{i-1} <------- K_i <--------- Ki+1 <-------
| | |
| F'(K_{i-1}) | F'(K_i) | F'(K_{i+1})
| | |
V V V
K'_{i-1} K'_i K'_{i+1}
Figure 1: At the top of the figure, we see the one-way key chain
(derived using the one-way function F), and the derived MAC keys
(derived using the one-way function F').
4.5 Authentication at Receiver
Perrig, Canetti, Song, Tygar, Briscoe [Page 8]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
Once a sender discloses a key, we must assume that all parties might
have access to that key. An adversary could create a bogus message
and forge a MAC using the disclosed key. So whenever a packet
arrives, the receiver must verify that the MAC is based on a safe
key; a safe key is one that is still secret (only known by the
sender). We define a safe packet or safe message to be one with a MAC
that is computed with a safe key.
If the packet is not safe, the receiver must discard that packet,
because the authenticity is not assured any more.
We now explain the TESLA authentication in more detail. When the
receiver receives packet P_j sent in interval i, the receiver com
putes an upper bound on the sender's clock: t_j. To test whether the
packet is safe, the receiver computes the highest interval x the
sender could possibly be in, namely x = floor((t_j - T_0) / T_int).
The receiver verifies that x < i + d (where i is the interval index),
which implies that the sender is not yet in the interval during which
it discloses the key K_i.
The receiver cannot yet verify the authenticity of packets sent in
interval i without key K_i. Instead, it adds the triplet ( i, M_j,
MAC( K'_i, M_j) ) to a buffer, and verifies the authenticity after it
learns K'_i.
What does a receiver do when it receives the disclosed key K_i?
First, it checks whether it already knows K_i or a later key K_j
(j>i). If K_i is the latest key received to date, the receiver checks
the legitimacy of K_i by verifying, for some earlier key K_v (v<i)
that K_v = F^{i-v}(K_i). The receiver then computes K'_i = F'(K_i)
and verifies the authenticity of packets of interval i.
Using a disclosed key, we can calculate all previous disclosed keys,
so even if packets are lost, we will still be able to verify
buffered, safe packets from earlier time intervals. Thus, if i-v>1,
the receiver can also verify the authenticity of the stored packets
of intervals v+1 ... i-1.
Note that the security of TESLA does not rely on any assumptions on
network propagation delay.
4.6 Determining the Key Disclosure Delay
An important TESLA parameter is the key disclosure delay d. Although
the choice of the disclosure delay does not affect the security of
the system, it is an important performance factor. A short disclosure
delay will cause packets to loose their safety property, so receivers
will discard them; but a long disclosure delay leads to a long
Perrig, Canetti, Song, Tygar, Briscoe [Page 9]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
authentication delay for receivers. We recommend choosing the disclo
sure delay as follows: in direct time synchronization let the RTT be
a reasonable upper bound on the round trip time between the sender
and the receiver; then choose d = ceil( RTT / T_int) + 1. Note that
rounding up the quotient ensures that d >= 2. Also note that a dis
closure delay of one time interval (d=1) does not work. Consider
packets sent close to the boundary of the time interval: after the
network propagation delay and the receiver time synchronization
error, a receiver will need to discard the packet, because the sender
will already be in the next time interval, when it discloses the cor
responding key.
4.7 Some extenstions
Let us mention two salient extenstions of the basic TESLA scheme.
A first extension allows having multiple TESLA authentication chains
for a single stream, where each chain uses a different delay for
disclosing the keys. This extension is typically used to deal with
heterogenous network delays withing a single multicast transmission.
A second extension allows having most of the buffering of packets
at the sender side (rather than at the receiver side). Both
extensions are described in [15].
5 Layer placement
The TESLA authentication can be performed at any layer in the net
working stack. The two logical places are in the network or the
application layer. We list some considerations regarding the choice
of layer:
· Performing TESLA in the network layer has the advantage that the
transport or application layer only receives authenticated data,
potentially aiding a reliability protocol and preventing denial-
of-service attacks. (Indeed, reliable multicast tools based on
forward error correction are highly susceptible to denial of ser
vice due to bogus packets.)
· Performing TESLA in the application layer has the advantage that
the network layer remains unchanged; but it has the drawback that
packets are obtained by the application layer only after being
processed by the transport layer. Consequently, if TCP is used
then this may introduce additional and unpredictable delays on
top of the unavoidable network delays. (However, if UDP is used
then this is not a problem.)
6 Acknowledgments
We would like to thank Mike Luby for his feedback and support.
7 Bibliography
[1] T. Dierks and C. Allen, "The TLS protocol version 1.0." Internet
Request for Comments RFC 2246, January 1999. Proposed standard.
[2] Ipsec, "IP Security Protocol, IETF working group."
http://www.ietf.org/html.charters/ipsec-charter.html.
Perrig, Canetti, Song, Tygar, Briscoe [Page 10]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
[3] D. Boneh, G. Durfee, and M. Franklin, "Lower bounds for multicast
message authentication," in Advances in Cryptology -- EUROCRYPT '2001
(B. Pfitzmann, ed.), vol. 2045 of Lecture Notes in Computer Science ,
(Innsbruck, Austria), pp. 434--450, Springer-Verlag, Berlin Germany,
2001.
[4] R. Gennaro and P. Rohatgi, "How to Sign Digital Streams," tech.
rep., IBM T.J.Watson Research Center, 1997.
[5] P. Rohatgi, "A compact and fast hybrid signature scheme for mul
ticast packet authentication," in 6th ACM Conference on Computer and
Communications Security , November 1999.
[6] P. Rohatgi, "A hybrid signature scheme for multicast source
authentication," Internet Draft, Internet Engineering Task Force,
June 1999. Work in progress.
[7] C. K. Wong and S. S. Lam, "Digital signatures for flows and mul
ticasts," in Proc. IEEE ICNP `98 , 1998.
[8] A. Perrig, R. Canetti, J. Tygar, and D. X. Song, "Efficient
authentication and signing of multicast streams over lossy channels,"
in IEEE Symposium on Security and Privacy , May 2000.
[9] R. Canetti, J. Garay, G. Itkis, D. Micciancio, M. Naor, and B.
Pinkas, "Multicast security: A taxonomy and some efficient construc
tions," in Infocom '99 , 1999.
[10] S. Cheung, "An efficient message authentication scheme for link
state routing," in 13th Annual Computer Security Applications Confer
ence , 1997.
[11] F. Bergadano, D. Cavagnino, and B. Crispo, "Chained stream
authentication," in Selected Areas in Cryptography 2000 , (Waterloo,
Canada), August 2000. A talk describing this scheme was given at IBM
Watson in August 1998.
[12] F. Bergadano, D. Cavalino, and B. Crispo, "Individual single
source authentication on the mbone," in ICME 2000 , Aug 2000. A talk
containing this work was given at IBM Watson, August 1998.
[13] A. Perrig and J. D. Tygar, Secure Broadcast Communication in
Wired and Wireless Networks Kluwer Academic Publishers, Oct. 2002.
ISBN 0792376501.
[14] A. Perrig, R. Canetti, J. D. Tygar, and D. Song, "The tesla
broadcast authentication protocol," RSA CryptoBytes , vol. 5, no.
Summer, 2002.
Perrig, Canetti, Song, Tygar, Briscoe [Page 11]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
[15] A. Perrig, R. Canetti, D. Song, and J. D. Tygar, "Efficient and
secure source authentication for multicast," in Network and Dis
tributed System Security Symposium, NDSS '01 , pp. 35--46, February
2001.
[16] D. L. Mills, "Network Time Protocol (Version 3) Specification,
Implementation and Analysis." Internet Request for Comments, March
1992. RFC 1305.
[17] B. Simons, J. Lundelius-Welch, and N. Lynch, "An overview of
clock synchronization," in Fault-Tolerant Distributed Computing (B.
Simons and A. Spector, eds.), no. 448 in LNCS, pp. 84--96, Springer-
Verlag, Berlin Germany, 1990.
[18] D. Mills, "Improved algorithms for synchronizing computer net
work clocks," in Proceedings of the conference on Communications
architectures, protocols and applications, SIGCOMM 94 , (London, Eng
land), pp. 317--327, 1994.
[19] L. Lamport and P. Melliar-Smith, "Synchronizing clocks in the
presence of faults," J. ACM , vol. 32, no. 1, pp. 52--78, 1985.
A Author Contact Information
Adrian Perrig
UC Berkeley / Digital Fountain
102 South Hall
Berkeley, CA 94720
US
perrig@cs.berkeley.edu
Ran Canetti
IBM Research
30 Saw Mill River Rd
Hawthorne, NY 10532
US
canetti@watson.ibm.com
Dawn Song
UC Berkeley
387 Soda Hall, 1776
Berkeley, CA 94720-1776
US
dawnsong@cs.berkeley.edu
Doug Tygar
UC Berkeley
Perrig, Canetti, Song, Tygar, Briscoe [Page 12]
Internet Draft draft-msec-tesla-intro-01 27 October 2002
102 South Hall, 4600
Berkeley, CA 94720-4600
US
tygar@cs.berkeley.edu
Bob Briscoe
BT Research
B54/74, BT Labs
Martlesham Heath
Ipswich, IP5 3RE
UK
bob.briscoe@bt.com
B Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this doc
ument itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of develop
ing Internet standards in which case the procedures for copyrights
defined in the Internet languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER
CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Perrig, Canetti, Song, Tygar, Briscoe [Page 13]
| PAFTECH AB 2003-2026 | 2026-04-23 05:13:08 |