PGP is the most widely used hybrid cryptosystem around today. There
have been AMPLE rumours regarding it's security (or lack there of). There
have been rumours ranging from PRZ was coerced by the Gov't into placing
backdoors into PGP, that the NSA has the ability to break RSA or IDEA in a
reasonable amount of time, and so on. While I cannot confirm or deny these
rumours with 100% certianty, I really doubt that either is true. This FAQ
while not in the 'traditional FAQ format' answers some questions about the
security of PGP, and should clear up some rumours...
[ The Feasibility of Breaking PGP ]
[ The PGP attack FAQ ]
2/96 v1.0
by infiNity [daemon9@netcom.com / route@infonexus.com]
-- [Brief introduction] --
This FAQ is a small side project I have decided to undertake.
It was originally just going to be a rather lengthy spur-of-the
moment post to alt.2600 in order to clear up some incorrect
assumptions and perceptions people had about the security of
PGP. It has grown well beyond that...
There are a great many misconceptions out there about how
vulnerable Pretty Good Privacy is to attack. This FAQ is designed
to shed some light on the subject. It is not an introduction to
PGP or cryptography. If you are not at least conversationally
versed in either topic, readers are directed to The Infinity Concept
issue 1, and the sci.crypt FAQ. Both documents are available via
ftp from infonexus.com. This document can be found there
as well (/pub/Philes/Cryptography/PGP/PGPattackFAQ.gz).
PGP is a hybrid cryptosystem. It is made up of 4 cryptographic
elements: It contains a symmetric cipher (IDEA), an asymmetric cipher
(RSA), a one-way hash (MD5), and a random number generator (Which is
two-headed, actually: it samples entropy from the user and then
uses that to seed a PRNG). Each is subject to a different form of
attack.
1 -- [The Symmetric Cipher] -- 1
IDEA, finalized in 1992 by Lai and Massey is a block cipher that
operates on 64-bit blocks of data. There have be no advances
in the cryptanalysis of standard IDEA that are publically known.
(I know nothing of what the NSA has done, nor does most anyone.)
The only method of attack, therefore, is brute force.
-- Brute Force of IDEA --
As we all know the keyspace of IDEA is 128-bits. In base 10
notation that is:
340,282,366,920,938,463,463,374,607,431,768,211,456.
To recover a particular key, one must, on average, search half the
keyspace. That is 127 bits:
170,141,183,460,469,231,731,687,303715,884,105,728.
If you had 1,000,000,000 machines that could try 1,000,000,000
keys/sec, it would still take all these machines longer than the
universe as we know it has existed and then some, to find the key.
IDEA, as far as present technology is concerned, is not vulnerable to
brute-force attack, pure and simple.
-- Other attacks against IDEA --
If we cannot crack the cipher, and we cannot brute force the
key-space, what if we can find some weakness in the PRNG used
by PGP to generate the psuedo-random IDEA session keys? This
topic is covered in more detail in section 4.
2 -- [The Asymmetric Cipher] -- 2
RSA, the first full fledged public key cryptosystem was designed
by Rivest, Shamir, and Adleman in 1977. RSA gets it's security from
the apparent difficulty in factoring very large composites.
However, nothing has been proven with RSA. It is not proved
that factoring the public modulous is the only (best) way to
break RSA. There may be an as yet undiscovered way to break it.
It is also not proven that factoring *has* to be as hard as it is.
There exists the possiblity that an advance in number theory may lead
to the discovery of a polynomial time factoring algorithm. But, none
of these things has happened, and no current research points in that
direction. However, 3 things that are happening and will continue
to happen that take away from the security of RSA are: the advances
in factoring technique, computing power and the decrease in the
cost of computing hardware. These things, especially the first one,
work against the security of RSA. However, as computing power
increases, so does the ability to generate larger keys. It is *much*
easier to multiply very large primes than it is to factor the
resulting composite (given today's understanding of number theory).
-- The math of RSA in 7 fun-filled steps --
To understand the attacks on RSA, it is important to understand
how RSA works. Briefly:
- Find 2 very large primes, p and q.
- Find n=pq (the public modulous).
- Choose e, such that e<n and relatively prime to (p-1)(q-1).
- Compute d=e^-1 mod[(p1-)(q-1)] OR ed=1[mod (p-1)(q-1)].
- e is the public exponent and d is the private one.
- The public-key is (n,e), and the private key is (n,d).
- p and q should never be revealed, preferably destroyed (PGP
keeps p and q to speed operations by use of the Chinese Remainder
Theorem, but they are kept encrypted)
Encryption is done by dividing the target message into blocks
smaller than n and by doing modular exponentiation:
c=m^e mod n
Decryption is simply the inverse operation:
m=c^d mod n
-- Brute Force RSA Factoring --
An attacker has access to the public-key. In other words, the
attacker has e and n. The attacker wants the private key. In
other words the attacker wants d. To get d, n needs to be
factored (which will yield p and q, which can then be used to
calculate d). Factoring n is the best known attack against RSA to
date. (Attacking RSA by trying to deduce (p-1)(q-1) is no easier
than factoring n, and executing an exhaustive search for values of d
is harder than factoring n.) Some of the algorithms used for
factoring are as follows:
- Trial division: The oldest and least efficient. Exponential
running time. Try all the prime numbers <= sqrt(n).
- Quadratic Sieve (QS): The fastest algorithm for numbers smaller
than 110 digits.
- Multiple Polynomial Quadratic Sieve (MPQS): Faster version of QS.
- Double Large Prime Variation of the MPQS: Faster still.
- Number Field Sieve (NFS): Currently the fastest algorithm known for
numbers larger than 110 digits. Was used to factor the ninth Fermat
number.
These algorithms represent the state of the art in warfare against
large composite numbers (therefore agianst RSA). The best algorithms
have a super-polynomial (sub-exponential) running time, with the NFS
having an asypmtotic time estimate closest to polynomial behaivior.
Still, factoring large numbers is hard. However, with the advances
in number theory and computing power, it is getting easier. In 1977
Ron Rivest said that factoring a 125-digit number would take
40 quadrillion years. In 1994 RSA129 was factored using about
5000 MIPS-years of effort from idle CPU cycles on computers across
the Internet for eight months. In 1995 the Blacknet key (116 digits)
was factored using about 400 MIPS-years of effort (1 MIPS-year is
a 1,000,000 instruction per second computer running for one year)
from several dozen workstations and a MasPar for about three months.
Given current trends the keysize that can be factored will only
increase as time goes on. The table below estimates the effort
required to factor some common PGP-based RSA public-key modulous
lengths using the General Number Field Sieve:
KeySize MIPS-years required to factor
-----------------------------------------------------------------
512 30,000
768 200,000,000
1024 300,000,000,000
2048 300,000,000,000,000,000,000
The next chart shows some estimates for the equivalences in brute
force key searches of symmetric keys and brute force factoring
of asymmetric keys, using the NFS.
Symmetric Asymmetric
------------------------------------------------------------------
56-bits 384-bits
64-bits 512-bits
80-bits 768-bits
112-bits 1792-bits
128-bits 2304-bits
It was said by the 4 men who factored the Blacknet key that
"Organizations with 'more modest' resources can almost certainly
break 512-bit keys in secret right now." This is not to say
that such an organization would be interested in devoting so
much computing power to break just anyone's messages. However, most
people using cryptography do not rest comfortably knowing the
system they trust their secrets to can be broken...
My advice as always is to use the largest key allowable by the
implementation. If the implementation does not allow for large
enough keys to satisfy your paranoia, do not use that implementation.
-- Esoteric RSA attacks --
These attacks do not exhibit any profound weakness in RSA itself,
just in certian implementations of the protocol. Most are not
issues in PGP.
-- Choosen cipher-text attack --
An attacker listens in on the insecure channel in which RSA
messages are passed. The attacker collects an encrypted message
c, from the target (destined for some other party). The attacker
wants to be able to read this message without having to mount a
serious factoring effort. In other words, she wants m=c^d.
To recover m, the attacker first chooses a random number, r<n.
(The attacker has the public-key (e,n).) The attacker computes:
x=r^e mod n (She encrypts r with the target's public-key)
y=xc mod n (Multiplies the target ciphertext with the temp)
t=r^-1 mod n (Multiplicative inverse of r mod n)
The attacker counts on the fact property that:
If x=r^e mod n, Then r=x^d mod n
The attacker then gets the target to sign y with her private-key,
(which actually decrypts y) and sends u=y^d mod n to the
attacker. The attacker simply computes:
tu mod n = (r^-1)(y^d) mod n = (r^-1)(x^d)(c^d) mod n = (c^d) mod n
= m
To foil this attack do not sign some random document presented to
you. Sign a one-way hash of the message instead.
-- Low encryption exponent e --
As it turns out, e being a small number does not take away from the
security of RSA. If the encryption exponent is small (common values
are 3,17, and 65537) then public-key operations are significantly
faster. The only problem in using small values for e as a public
exponent is in encrypting small messages. If we have 3 as our e
and we have an m smaller than the cubic root of n, then the message
can be recovered simply by taking the cubic root of m beacuse:
m [for m<= 3rdroot(n)]^3 mod n will be equivalent to m^3
and therefore:
3rdroot(m^3) = m.
To defend against this attack, simply pad the message with a nonce
before encryption, such that m^3 will always be reduced mod n.
PGP uses a small e for the encryption exponent, by default it tries
to use 17. If it cannot compute d with e being 17, PGP will iterate
e to 19, and try again... PGP also makes sure to pad m with a random
value so m > n.
-- Timing attack against RSA --
A very new area of attack publically discovered by Paul Kocher deals
with the fact that different cryptographic operations (in this case
the modular exponentiation operations in RSA) take discretely different
amounts of time to process. If the RSA computations are done without
the Chinese Remainder theorem, the following applies:
An attacker can exploit slight timing differences in RSA computations
to, in many cases, recover d. The attack is a passive one where the
attacker sits on a network and is able to observe the RSA operations.
The attacker passively observes k operations measuring the time t
it takes to compute each modular exponentation operation:
m=c^d mod n. The attacker also knows c and n. The psuedo code of
the attack:
Algorithm to compute m=c^d mod n:
Let m0 = 1.
Let c0 = x.
For i=0 upto (bits in d-1):
If (bit i of d) is 1 then
Let mi+1 = (mi * ci) mod n.
Else
Let mi+1 = mi.
Let di+1 = di^2 mod n.
End.
This is very new (the public announcement was made on 12/7/95)
and intense scrutiny of the attack has not been possible. However,
Ron Rivest had this to say about countering it:
-------------------------------------------BEGIN INCLUDED TEXT---------------
From: Ron Rivest <rivest>
Newsgroups: sci.crypt
Subject: Re: Announce: Timing cryptanalysis of RSA, DH, DSS
Date: 11 Dec 1995 20:17:01 GMT
Organization: MIT Laboratory for Computer Science
The simplest way to defeat Kocher's timing attack is to ensure that the
cryptographic computations take an amount of time that does not depend on the
data being operated on. For example, for RSA it suffices to ensure that
a modular multiplication always takes the same amount of time, independent of
the operands.
A second way to defeat Kocher's attack is to use blinding: you "blind" the
data beforehand, perform the cryptographic computation, and then unblind
afterwards. For RSA, this is quite simple to do. (The blinding and
unblinding operations still need to take a fixed amount of time.) This doesn't
give a fixed overall computation time, but the computation time is then a
random variable that is independent of the operands.
-
==============================================================================
Ronald L. Rivest 617-253-5880 617-253-8682(Fax) rivest@theory.lcs.mit.edu
==============================================================================
---------------------------------------------END INCLUDED TEXT---------------
The blinding Rivest speaks of simply introduces a random value into
the decryption process. So,
m = c^d mod n
becomes:
m = r^-1(cr^e)^d mod n
r is the random value, and r^-1 is it's inverse.
PGP is not vulnerable to the timing attack as it uses the CRT to
speed RSA operations. Also, since the timing attack requires an
attacker to observe the cryptographic operations in real time (ie:
snoop the decryption process from start to finish) and most people
encrypt and decrypt off-line, it is further made inpractical.
While the attack is definitly something to be wary of, it is
theorectical in nature, and has not been done in practice as of
yet.
-- Other RSA attacks --
There are other attacks against RSA, such as the common modulous
attack in which several users share n, but have different values
for e and d. Sharing a common modulous with several users, can
enable an attacker to recover a message without factoring n. PGP
does not share public-key modulous' among users.
If d is up to one quarter the size of n and e is less than n, d
can be recovered without factoring. PGP does not choose small
values for the decryption exponent. (If d were too small it might
make a brute force sweep of d values feasible which is obviously a
bad thing.)
-- Keysizes --
It is pointless to make predictions for recommended keysizes.
The breakneck speed at which technology is advancing makes it
difficult and dangerous. Respected cryptographers will not make
predictions past 10 years and I won't embarass myself trying to
make any. For today's secrets, a 1024-bit is probably safe and
a 2048-bit key definitely is. I wouldn't trust these numbers
past the end of the century. However, it is worth mentioning that
RSA would not have lastest this long if it was as fallible as some
crackpots with middle initials would like you to believe.
3 -- [The one-way hash] -- 3
MD5 is the one-way hash used to hash the passphrase into the IDEA
key and to sign documents. Message Digest 5 was designed by Rivest
as a sucessor to MD4 (which was found to be weakened with reduced
rounds). It is slower but more secure. Like all one-way hash
functions, MD5 takes an arbitrary-length input and generates a unique
output.
-- Brute Force of MD5 --
The strength of any one-way hash is defined by how well it can
randomize an arbitrary message and produce a unique output. There
are two types of brute force attacks against a one-way hash
function, pure brute force (my own terminolgy) and the birthday
attack.
-- Pure Brute Force Attack against MD5 --
The output of MD5 is 128-bits. In a pure brute force attack,
the attacker has access to the hash of message H(m). She wants
to find another message m' such that:
H(m) = H(m').
To find such message (assuming it exists) it would take a machine
that could try 1,000,000,000 messages per second about 1.07E22
years. (To find m would require the same amount of time.)
-- The birthday attack against MD5 --
Find two messages that hash to the same value is known as a collision
and is exploited by the birthday attack.
The birthday attack is a statistical probability problem. Given
n inputs and k possible outputs, (MD5 being the function to take
n -> k) there are n(n-1)/2 pairs of inputs. For each pair, there
is a probability of 1/k of both inputs producing the same output.
So, if you take k/2 pairs, the probability will be 50% that a
matching pair will be found. If n > sqrt(k), there is a good chance
of finding a collision. In MD5's case, 2^64 messages need to be
tryed. This is not a feasible attack given today's technology. If
you could try 1,000,000 messages per second, it would take 584,942
years to find a collision. (A machine that could try 1,000,000,000
messages per second would take 585 years, on average.)
For a successful account of the birthday against crypt(3), see:
url:
ftp://ftp.infonexus.com/pub/Philes/Cryptography/crypt3Collision.txt.gz
-- Other attacks against MD5 --
Differential cryptanalysis has proven to be effective against one
round of MD5, but not against all 4 (differential cryptanalysis
looks at ciphertext pairs whose plaintexts has specfic differences
and analyzes these differences as they propagate through the cipher).
There was successful attack at the compression function itself that
produces collsions, but this attack has no practical impact the
security. If your copy of PGP has had the MD5 code altered to
cause these collisions, it would fail the message digest
verification and you would reject it as altered... Right?
-- Passphrase Length and Information Theory --
According to conventional information theory, the English language
has about 1.3 bits of entropy (information) per 8-bit character.
If the pass phrase entered is long enough, the reuslting MD5 hash will
be statisically random. For the 128-bit output of MD5, a pass phrase
of about 98 characters will provide a random key:
(8/1.3) * (128/8) = (128/1.3) = 98.46 characters
How many people use a 98 character passphrase for you secret-key
in PGP? Below is 98 characters...
123456789012345678901234567890123456789012356789012345678901234567\
\890123456789012345678
1.3 comes from the fact that an arbitrary readable English sentence
is usually going to consist of certian letters, (e,r,s, and t are
statiscally very common) thereby reducing it's entropy. If any of the
26 letters in the Latin alphabet were equally possible and likely
(which is seldom the case) the entropy increases. The so-called
absolute rate would, in this case, be:
log(26) / log(2) = 4.7 bits
In this case of increased entropy, a password with a truly random
sequence of English characters will only need to be:
(8/4.7) * (128/8) = (128/4.7) = 27.23 characters
For more info on passphrase length, see the PGP passphrase FAQ:
http://www.stack.urc.tue.nl/~galactus/remailers/passphrase-faq.html
ftp://ftp.infonexus.com/pub/Philes/Cryptography/PGP/PGPpassphraseFAQ.gz
4 -- [The PRNG] -- 4
PGP employs 2 PRNG's to generate and manipulate (psuedo) random data.
The ANSI X9.17 generator and a function which measures the entropy
from the latency in a user's keystrokes. The random pool (which is
the randseed.bin file) is used to seed the ANSI X9.17 PRNG (which uses
IDEA, not 3DES). Randseed.bin is initially generated from trueRand
which is the keystroke timer. The X9.17 generator is pre-washed with
an MD5 hash of the plaintext and postwashed with some random data
which is used to generate the next randseed.bin file. The process is
broken up and discussd below.
-- ANSI X9.17 (cryptRand) --
The ANSI X9.17 is the method of key generation PGP uses. It is
oficially specified using 3DES, but was easily converted to IDEA.
X9.17 requires 24 bytes of random data from randseed.bin. (PGP
keeps an extra 384 bytes of state information for other uses...)
When cryptRand starts, the randseed.bin file is washed (see below)
and the first 24-bytes are used to initialize X9.17. It works as
follows:
E() = an IDEA encryption, with a reusable key used for key generation
T = timestamp (data from randseed.bin used in place of timestamp)
V = Initialization Vector, from randseed.bin
R = random session key to be generated
R = E[E(T) XOR V]
the next V is generated thusly:
V = E[E(T) XOR R]
-- Latency Timer (trueRand) --
The trueRand generator attempts to measure entropy from the latency
of a user's keystrokes every time the user types on the keyboard. It
is used to generate the initial randseed.bin which is in turn used to
seed to X9.17 generator.
The quality of the output of trueRand is dependent upon it's input.
If the input has a low amount of entropy, the output will not be as
random as possible. In order to maxmize the entropy, the keypresses
should be spaced as randomly as possible.
-- X9.17 Prewash with MD5 --
In most situations, the attacker does not know the content of the
plaintext being encrypted by PGP. So, in most cases, washing the
X9.17 generator with an MD5 hash of the plaintext, simply adds to
security. This is based on the assumption that this added unknown
information will add to the entropy of the generator.
If, in the event that the attacker has some information about the
plaintext (perhaps the attacker knows which file was encrypted, and
wishes to prove this fact) the attacker may be able to execute a
known-plaintext attack against X9.17. However, it is not likely
that, with all the other precautions taken, that this would weaken
the generator.
-- Randseed.bin wash --
The randseed is washed before and after each use. In PGP's case
a wash is an IDEA encryption in cipher-feedback mode. Since IDEA
is considered secure (see section 1), it should be just as hard to
determine the 128-bit IDEA key as it is to glean any information from
the wash. The IDEA key used is the MD5 hash of the plaintext and an
initialization vector of zero. The IDEA session key is then generated
as is an IV.
The postwash is considered more secure. More random bytes are
generated to reinitialize randseed.bin. These are encrypted with the
same key as the PGP encrypted message. The reason for this is that if
the attacker knows the session key, she can decrypt the PGP message
directly and would have no need to attack the randseed.bin. (A note,
the attacker might be more interested in the state of the
randseed.bin, if they were attacking all messages, or the message
that the user is expected to send next).
5 -- [Practical Attacks] -- 5
Most of the attacks outlined above are either not possible or not
feasable by the average adversary. So, what can the average cracker
do to subvert the otherwise stalwart security of PGP? As it turns
out, there are several "doable" attacks that can be launched by the
typical cracker. They do not attack the cryptosystem protocols
themselves, (which have shown to be secure) but rather system
specific implementations of PGP.
-- Passive Attacks (Snooping) --
These attacks do not do need to do anything proactive and can easily
go undetected.
-- Keypress Snooping --
Still a very effective method of attack, keypress snooping can subvert
the security of the strongest cryptosystem. If an attacker can
install a keylogger, and capture the passphrase of an unwary target,
then no cryptanalysis whatsoever is necessary. The attacker has the
passphrase to unlock the RSA private key. The system is completely
compromised.
The methods vary from system to system, but I would say DOS-based PGP
would be the most vulnerable. DOS is the easiest OS to subvert, and
has the most key-press snooping tools that I am aware of. All an
attacker would have to do would be gain access to the machine for
under 5 minutes on two seperate occasions and the attack would be
complete. The first time to install the snooping software, the second
time, to remove it, and recover the goods. (If the machine is on a
network, this can all be done *remotely* and the ease of the attack
increases greatly.) Even if the target boots clean, not loading any
TSR's, a boot sector virus could still do the job, transparently.
Just recently, the author has discovered a key logging utility for
Windows, which expands this attack to work under Windows-based PGP
shells (this logger is available from the infonexus via ftp, BTW).
ftp://ftp.infonexus.com/pub/ToolsOfTheTrade/DOS/KeyLoggers/
Keypress snooping under Unix is a bit more complicated, as root
access is needed, unless the target is entering her passphrase from
an X-Windows GUI. There are numerous key loggers available to
passively observe keypresses from an X-Windows session. Check:
ftp://ftp.infonexus.com/pub/SourceAndShell/Xwindows/
-- Van Eck Snooping --
The original invisible threat. Below is a clip from a posting by
noted information warfare guru Winn Schwartau describing a Van Eck
attack:
-------------------------------------------BEGIN INCLUDED TEXT---------------
Van Eck Radiation Helps Catch Spies
"Winn Schwartau" < p00506@psilink.com >
Thu, 24 Feb 94 14:13:19 -0500
Van Eck in Action
Over the last several years, I have discussed in great detail how the
electromagnetic emissions from personal computers (and electronic gear in
general) can be remotely detected without a hard connection and the
information on the computers reconstructed. Electromagnetic eavesdropping is
about insidious as you can get: the victim doesn't and can't know that anyone
is 'listening' to his computer. To the eavesdropper, this provides an ideal
means of surveillance: he can place his eavesdropping equipment a fair
distance away to avoid detection and get a clear representation of what is
being processed on the computer in question. (Please see previous issues of
Security Insider Report for complete technical descriptions of the
techniques.)
The problem, though, is that too many so called security experts, (some
prominent ones who really should know better) pooh-pooh the whole concept,
maintaining they've never seen it work. Well, I'm sorry that none of them
came to my demonstrations over the years, but Van Eck radiation IS real and
does work. In fact, the recent headline grabbing spy case illuminates the
point.
Exploitation of Van Eck radiation appears to be responsible, at least in part,
for the arrest of senior CIA intelligence officer Aldrich Hazen Ames on
charges of being a Soviet/Russian mole. According to the Affidavit in support
of Arrest Warrant, the FBI used "electronic surveillance of Ames' personal
computer and software within his residence," in their search for evidence
against him. On October 9, 1993, the FBI "placed an electronic monitor in his
(Ames') computer," suggesting that a Van Eck receiver and transmitter was used
to gather information on a real-time basis. Obviously, then, this is an ideal
tool for criminal investigation - one that apparently works quite well. (From
the Affidavit and from David Johnston, "Tailed Cars and Tapped Telephones: How
US Drew Net on Spy Suspects," New York Times, February 24, 1994.)
>From what we can gather at this point, the FBI black-bagged Ames' house and
installed a number of surveillance devices. We have a high confidence factor
that one of them was a small Van Eck detector which captured either CRT
signals or keyboard strokes or both. The device would work like this:
A small receiver operating in the 22MHz range (pixel frequency) would detect
the video signals minus the horizontal and vertical sync signals. Since the
device would be inside the computer itself, the signal strength would be more
than adequate to provide a quality source. The little device would then
retransmit the collected data in real-time to a remote surveillance vehicle or
site where the video/keyboard data was stored on a video or digital storage
medium.
At a forensic laboratory, technicians would recreate the original screens and
data that Mr. Ames entered into his computer. The technicians would add a
vertical sync signal of about 59.94 Hz, and a horizontal sync signal of about
27KHz. This would stabilize the roll of the picture. In addition, the
captured data would be subject to "cleansing" - meaning that the spurious
noise in the signal would be stripped using Fast Fourier Transform techniques
in either hardware or software. It is likely, though, that the FBI's device
contained within it an FFT chip designed by the NSA a couple of years ago to
make the laboratory process even easier.
I spoke to the FBI and US Attorney's Office about the technology used for
this, and none of them would confirm or deny the technology used "on an active
case."
Of course it is possible that the FBI did not place a monitoring device within
the computer itself, but merely focused an external antenna at Mr. Ames'
residence to "listen" to his computer from afar, but this presents additional
complexities for law enforcement.
1. The farther from the source the detection equipment sits means that
the detected information is "noisier" and requires additional forensic
analysis to derive usable information.
2. Depending upon the electromagnetic sewage content of the immediate
area around Mr. Ames' neighborhood, the FBI surveillance team would be limited
as to what distances this technique would still be viable. Distance squared
attenuation holds true.
3. The closer the surveillance team sits to the target, the more likely
it is that their activities will be discovered.
In either case, the technology is real and was apparently used in this
investigation. But now, a few questions arise.
1. Does a court surveillance order include the right to remotely
eavesdrop upon the unintentional emanations from a suspect's electronic
equipment? Did the warrants specify this technique or were they shrouded
under a more general surveillance authorization? Interesting question for the
defense.
2. Is the information garnered in this manner admissible in court? I
have read papers that claim defending against this method is illegal in the
United States, but I have been unable to substantiate that supposition.
3. If this case goes to court, it would seem that the investigators would
have to admit HOW they intercepted signals, and a smart lawyer (contradictory
allegory :-) would attempt to pry out the relevant details. This is important
because the techniques are generally classified within the intelligence
community even though they are well understood and explained in open source
materials. How will the veil of national security be dropped here?
To the best of my knowledge, this is the first time that the Government had
admitted the use of Van Eck (Tempest Busting etc.) in public. If anyone
knows of any others, I would love to know about it.
---------------------------------------------END INCLUDED TEXT---------------
The relevance to PGP is obvious, and the threat is real. Snooping
the passphrase from the keyboard, and even whole messages from
the screen are viable attacks. This attack, however exotic it may
seem, is not beyond the capability of anyone with some technical
know-how and the desire to read PGP encrypted files.
-- Memory Space Snooping --
In a multi-user system such as Unix, the physical memory of the
machine can be examined by anyone with the proper privaleges (usally
root). In comparsion with factoring a huge composite number, opening
up the virtual memory of the system (/dev/kmem) and seeking to a
user's page and directly reading it, is trivial.
-- Disk Cache Snooping --
In multitasking environments such as Windows, the OS has a nasty habit
of paging the contents of memory to disk, usally transparently to the
user, whenever it feels the need to free up some RAM. This
information can sit, in the clear, in the swapfile for varying lengths
of time, just waiting for some one to come along and recover it.
Again, in a networked environment where machine access can be done with
relative impunity, this file can be stolen without the owner's consent
or knowledge.
-- Packet Sniffing --
If you use PGP on a host which you access remotely, you can be
vulnerable to this attack. Unless you use some sort of session
encrypting utility, such as SSH, DESlogin, or some sort of network
protocol stack encryption (end to end or link by link) you are sending
your passphrase, and messages across in the clear. A packet sniffer
sitting at a intermediate point between your terminal can capture all
this information quietly and efficiently. Packet sniffers are
available at the infonexus:
ftp://ftp.infonexus.com/pub/SourceAndShell/Sniffers/
-- Active Attacks --
These attacks are more proactive in nature and tend to be a bit more
difficult to wage.
-- Trojan Horse --
The age old trojan horse attack is still a very effective means
of compromise. The concept of a trojan horse should not be foriegn
to anyone. An apparently harmless program that in reality is evil
and does potentially malicious things to your computer. How does
this sound...:
Some 31it3 coder has come up with a k3wl new Windows front-end to
PGP. All the newbies run out and ftp a copy. It works great, with
a host of buttons and scrollbars, and it even comes with a bunch
of *.wav files and support for a SB AWE 32 so you can have the
16-bit CD quality sound of a safe locking when you encrypt your
files. It runs in a tiny amount of memory, coded such that nothing
leaks, it intercepts OS calls that would otherwise have it's contents
paged to disk and makes sure all the info stays in volatile memory.
It works great (the first Windows app that does). Trouble is, this
program actually has a few lines of malevolent code that record your
secret-key passphrase, and if it finds a modem (who doesn't have a
modem these days?) it 'atm0's the modem and dials up a hard coded
number to some compromised computer or modem bank and sends the info
through.
Possible? Yes. Likely? No.
-- Reworked Code --
The code to PGP is publically available. Therefore it is easy to
modify. If someone were to modify the sourcecode to PGP inserting
a sneaky backdoor and leave it at some distribution point, it could
be disasterous. However, it is also very easy to detect. Simply
verify the checksums. Patching the MD5 module to report a false
checksum is also possible, so verify using a known good copy.
A more devious attack would be to modify the code, compile it and
surreptitouly plant in the target system. In a networked environment
this can be done without ever having physical access to the machine.
-- [Closing Comments] --
I have presented factual data, statistical data, and projected data.
Form your own conclusions. Perhaps the NSA has found a polynomial-time
(read: *fast*) factoring algorithm. But we cannot dismiss an
otherwise secure cryptosystem due to paranoia. Of course, on the
same token, we cannot trust cryptosystems on hearsay or assumptions of
security. Bottom line is this: in the field of computer security, it
pays to be cautious. But it doens't pay to be un-informed or
needlessly paranoid. Know the facts.
-- [Thank You's (in no particular order)] --
PRZ, Collin Plumb, Paul Kocher, Bruce Schneier, Paul Rubin, Stephen
McCluskey, Adam Back, Bill Unruh, Ben Cantrick, and the readers of
sci.crypt, alt.security.pgp, and the comp.security.* groups for thier
suggestions and proofreading.
Jeffrey Goldberg braved the dark, cold unknown with:
|
| On Sat, 10 Feb 1996, Route wrote:
|
| > Almost out of BETA. This should be the last UNofficial posting.
| > As always, comments, corrections, and suggestions are WELCOMED and
| > encouraged. I am looking for someone to 'html'ize this document, or
| > at least have links to it. If you wish to do either, please mail
| > me.
|
| I would be more than happy to have links to it, or put up a copy here
| (if you need a UK site for it.) Here is
| http://www.cranfield.ac.uk/docs/Email/PGP/
|
| Below are specific comments.
|
| > -- [Brief introduction] --
| >
| > This FAQ is a small side project I have decided to undertake.
| > It was originally just going to be a rather lengthy spur-of-the
| > moment post to alt.2600 in order to clear up some incorrect
| > assumptions and perceptions people had about the security of
| > PGP. It has grown well beyond that...
|
| That is okay for Beta, but leave the above out of the final version.
|
| >
| > - Find 2 very large primes, p and q.
| > - Find n=pq (the public modulous).
| > - Choose e, such that e<n and relatively prime to (p-1)(q-1).
| > - Compute d=e^-1 mod[(p1-)(q-1)] OR ed=1[mod (p-1)(q-1)].
| mod[(p-1)(q-1]
|
| > It was said by the 4 men who factored the Blacknet key that
|
| Sorry for the political correctness, but you might consider changing
| "men" to "people" since thier maleness has nothing to do with it.
|
| > My advice as always is to use the largest key allowable by the
| > implementation. If the implementation does not allow for large
| > enough keys to satisfy your paranoia, do not use that implementation.
|
| This is controversial advice. As someone said, having a 1024 bit key
| and wondering about increasing it to 2048 bits is like having a meter
| think vault door on your house, wondering about doubling it's thickness,
| while still just keeping glass in the windows.
|
| Keep in mind that the time it takes to decrypt or sign goes up with
| at least the square (I'm guessing) of the length of the key. Thus
| a key stays in memory unencrypted for a longer time.
|
| > -- Timing attack against RSA --
| >
| > [...]
| > PGP is not vulnerable to the timing attack as it uses the CRT to
| > speed RSA operations. Also, since the timing attack requires an
| > attacker to observe the cryptographic operations in real time (ie:
| > snoop the decryption process from start to finish) and most people
| > encrypt and decrypt off-line, it is further made inpractical.
|
| Make that "encrypt and sign off-line". Some people may get confused
| and mistakenly think that time encryption could be informative.
|
| Also add that some automatic decryption or signing systems (like a
| timestamper) may be vulnerable, but if someone could get close enough
| to their operation to time them precisely enough, they could probably
| grab the secret key from memory anyway.
|
| >
| > -- Keysizes --
| >
| > It is pointless to make predictions for recommended keysizes.
| > The breakneck speed at which technology is advancing makes it
| > difficult and dangerous. Respected cryptographers will not make
| > make any. For today's secrets, a 1024-bit is probably safe and
| > a 2048-bit key definitely is. I wouldn't trust these numbers
| > past the end of the century. However, it is worth mentioning that
| > RSA would not have lastest this long if it was as fallible as some
| > crackpots with middle initials would like you to believe.
| >
| >
| >
| > -- The birthday attack against MD5 --
| >
| > Find two messages that hash to the same value is known as a collision
| Finding
|
|
|
| > How many people use a 98 character passphrase for you secret-key
| their
|
| > For more info on passphrase length, see the PGP passphrase FAQ:
| >
| > http://www.stack.urc.tue.nl/~galactus/remailers/passphrase-faq.html
| > ftp://ftp.infonexus.com/pub/Philes/Cryptography/PGP/PGPpassphraseFAQ.gz
|
| I think that it would be worth pointing very explicitly that the
| passphrase is the weakest point (other than maintaining physical security
| of the machine where decryption is done). You basically tell people, but
| stating clearly in one sentence would be helpful.
|
| >
| > The relevance to PGP is obvious, and the threat is real. Snooping
| > the passphrase from the keyboard, and even whole messages from
| > the screen are viable attacks. This attack, however exotic it may
| > seem, is not beyond the capability of anyone with some technical
| > know-how and the desire to read PGP encrypted files.
|
| And is far cheaper than cryptanalytic attacks.
|
| > It works great (the first Windows app thar does). Trouble is, this
| that
|
|
| My general comments.
|
| Anyway, it's a great and much needed FAQ.
|
| It might be worthwhile adding a section on more real frequently asked
| questions like ``What if someone made a list of all the prime numbers ...''
| sort of thing that shows up on a.s.pgp every now and then.
| That is, I think you FAQ is wonderful, but it isn't very FAQ-like. It
| may be worthwhile adding a more FAQ-like begining portion with
| lots of cross references. And before you ask, no, I don't volunteer.
|
| I think it might be useful added references to some of the literature
| in places as well.
|
| -jeff
|
| --
| Jeffrey Goldberg +44 (0)1234 750 111 x 2826
| Cranfield Computer Centre FAX 751 814
| J.Goldberg@Cranfield.ac.uk http://WWW.Cranfield.ac.uk/public/cc/cc047/
|
--
infiNity .oOo. Member of the infamous Guild | spreading information
route .oOo. Use strong Cryptography | like it was going
daemon9 .oOo. Finger for info | out of style