Cryptography-Digest Digest #149, Volume #9 Fri, 26 Feb 99 19:13:03 EST
Contents:
Re: Testing Algorithms (R. Knauer)
Re: New high-security 56-bit DES: Less-DES (Bryan Olson)
Re: Crypt for FTP Protocol (Jeremy Nysen)
Re: Define Randomness (R. Knauer)
Re: Define Randomness (R. Knauer)
Re: True Randomness - DOES NOT EXIST!!! (R. Knauer)
Re: True Randomness - DOES NOT EXIST!!! (R. Knauer)
Re: Testing Algorithms [moving off-topic] (R. Knauer)
Re: True Randomness - DOES NOT EXIST!!! (John Briggs)
One-Time-Pad program for Win85/98 or DOS (HyperReal-Anon)
Re: A question-Mag ID Cards (karl malbrain)
A question-Mag ID Cards (JEANNINE)
Re: Testing Algorithms (R. Knauer)
Re: Testing Algorithms (Patrick Juola)
Re: Testing Algorithms [moving off-topic] (Patrick Juola)
Re: Testing Algorithms [moving off-topic] (Patrick Juola)
----------------------------------------------------------------------------
From: [EMAIL PROTECTED] (R. Knauer)
Subject: Re: Testing Algorithms
Date: Fri, 26 Feb 1999 21:13:40 GMT
Reply-To: [EMAIL PROTECTED]
On Fri, 26 Feb 1999 07:49:46 -1000, Somniac <[EMAIL PROTECTED]> wrote:
>This is good news. Where can I buy a computer or logic gate that takes no
>energy? I want one. For example, I want to buy one XOR gate that gives
>back one energy unit when it goes to a zero, and takes one energy unit
>when its output goes to a one. Is it made with one molecule? Is it solid
>state or gas? I want to manufacture a 64 bit wide IC XOR gate using this
>technology. Where can I purchase a license? What is the patent number?
>Which journal describes its behaviour? I hope that you will not say one
>has never been built. Do I need to build a black hole to make it work?
>Does it need to travel near the speed of light to function efficiently?
>Does it need to be at absolute zero temperature to give back the energy
>it uses? Please explain its principles or give a reference book citation.
>I have heard of a theory like that but it was never built. Such an
>attractive logic gate should be built, unless it is impractical.
It is my understanding that quantum computers can operate with
reversible logic circuits, in which case they can operate without
expenditure of energy.
Bob Knauer
"Did you ever notice that when a politician does get an idea
he usually gets it all wrong?"
------------------------------
From: Bryan Olson <[EMAIL PROTECTED]>
Subject: Re: New high-security 56-bit DES: Less-DES
Date: Fri, 26 Feb 1999 13:13:11 -0800
Note: Lines beginning "|" are re-included for context.
[EMAIL PROTECTED] wrote:
> Bryan Olson wrote:
> > If you think no secrecy system could produce such an example, then I
> > welcome you to re-follow-up my Jan 16'th post, in which I argued the
> > opposite. You snipped all mention of it in your reply.
>
> That "example" also was not quantitative and did not lead *to the numbers*
> you posted before. Remember, another poster also complained that it was
> obviously wrong to quote such probabilities.
The other poster was Dave Scott. The post speaks for itself;
you are welcome to follow it up.
> > > I can't help but recall Humpty-Dumpty, so I will stop reading your msg here.
> >
> > I don't think one's reading choices call for publicly posting.
| > What
| > this really tells us is that you will not _respond_ to other points
| > > "When I use a word," Humpty Dumpty said in a rather a scornful tone, "it means
| > > just what I choose it to mean -- neither more nor less."
| > Do you remember which of us is using the term accepted in the discipline
| > and which is using one he made up?
> Sorry... that is the rule. When I have to invoke Humpty-Dumpty then it is time
> to stop reading -- if it is a technical matter, of course.
>
> But, you still did not answer my didactical question. I will pose it again:
> "What is the distance of your hand?"
I believe the question is nonsense. Do you know what a term of art
is? I gave some examples, but of course you stopped reading just
before that.
--Bryan
------------------------------
From: Jeremy Nysen <[EMAIL PROTECTED]>
Subject: Re: Crypt for FTP Protocol
Date: Sat, 27 Feb 1999 08:07:02 +1100
[EMAIL PROTECTED] wrote:
> > My users have repeatedly asked if I could add crypt/cipher to my FTPD so I've
> > finally got around to looking at it ;) So basically I've trying to find if
> > there is some easy to use crypt library that would do something usable in this
> > case.
>
> SRP looked really good, but unfortunately it needs a big key(salt?) stored
> for each user which doesn't merge too well if you've already got thousands of
> users and only have their username and DES crypted passwords. Unless someone
> can think of a solutions using SRP? Some way to generate the salt/key on the
> fly once user has sent their login?
If you don't mind a changeover time you can try the following. If you
setup the SRP PAM modules (as described under the /base/pam source
tree), you can use your existing passwd/shadow file to authenticate user
login, but allow passwd to change the password/hash/etc in both
passwd/shadow AND in tpasswd. Tell everyone to change their passwords
(they can change them to what they had before if they really want
(probably no a good idea, as these 'old' passwords may have already been
sniffed.)
When users want to use SRP ftp they just connect using the srp ftp
client. If their password(hash) exists in tpasswd they can use a secure
session. If not, ftp tries to reopen the connection, but this time in
'non-secure' mode where the system will use PAM to authenticate (ie.
your passwd/shadow file). Note: Users can still telnet in and be
properly authenticated with their passwd/shadow password.
Once all users are in tpasswd, you can then completely change
authentication over to the SRP EPS PAM module and have long/secure
passwords. This will allow passwords that are greater than 8 characters
and make it much more difficult for dictionary programs to crack you
database.
Warning: Even though your system is using the PAM EPS password modules
to authenticate, they do not 'secure' the network. This means that it is
possible to login using a non-SRP version of telnet/ftp and send your
plaintext password directly over the net - this is what happens with the
SRP suite. But the users should be warned about the consequence of what
happens if they ignore the 'This sesssion is not using secure
authentication message.' (This usually happens if they use and old
non-SRP telnet/ftp client.)
Hope that helps,
Jeremy
------------------------------
From: [EMAIL PROTECTED] (R. Knauer)
Subject: Re: Define Randomness
Date: Fri, 26 Feb 1999 21:34:58 GMT
Reply-To: [EMAIL PROTECTED]
On 26 Feb 1999 12:10:41 -0500, [EMAIL PROTECTED] (Patrick Juola)
wrote:
>The problem with pac learning is that, as stated on the label, it's
>only "probably approximately correct"; the probability and degree of
>approximation are usually quantifiable, but at the end they end up
>being no more than the statistical guesses with error bars that you
>were objecting to in a different context.
I was never objecting to error bars in a different context. I
appreciate the utility of statistical measurements as far as they go.
What I was objecting was the use of statistical measurements to
characterize something that cannot be characterized on its own, namely
random number generation per se.
But you can characterize the security of a cryptosystem using those
random numbers - that is an entirely diffetent matter altogether.
Whether the keystream is ideally random or not is not the issue any
more - the issue now becomes the empirical strength of the ciphers
produced using that keystream. I don't care if the keystream passes
all statistical tests, or fails all statistical tests - as long as it
passes THE test, Unrebakability, it has served its purpose.
>You're not being consistent here. On the one hand, you ask for
>*proof* that a given cryptoystem is 100% unbreakable or that there
>are firm bounds on the risk; when I point out that statements like
>"a uniform Bernoulli process will generate uniform strings" are
>provable to any desired degree of belief, you object because there
>is some possibility that in a billion billion years, you might see
>one violate the conditions of the test.
MY point is that trying to characterize the suitability of a
particular TRNG to crypto is futile and is fraught with perils of all
sorts. It is much more significant to use proababilistic tests like
Bayesian inference to try to break actual ciphers. That is a
fundamentally different thing from trying to infer some property from
the raw keystream itself.
We know that the set of all numbers that pass all statistical tests
for randomnes is an empty set. Therefore there will always have to be
some level of imperfection in the keystream. The problem is that we do
not know how to quantify that by testing the keystream itself. If you
focus on bit bias, how are you going to handle the case where a long
run of highly biased bits are output. Such runs do happen - in fact
they MUST happen if the generator is to be capable of generating a
normal number in the infinite limit.
We do not know if such runs are really detrimental until we use them
to produce test ciphers and then see if they leak too much information
to be considered useable for secure crypto. Unless you can show that
the Bayesian attack is *identially* the same procedure as statistical
testing on keystreams, then there is a fundamental shift in emphasis
with this proosed method of characterizing crypto security for stream
ciphers. For one thing, you have introduced regularity into the system
by the intelligible test messages you encrypted, and they can be used
to leak information. You cannot expect to leak any pertinent
information from a keystream itself. But if you leak intelligible
information - English text - from a cipher you have something positive
to grab ahold of. With the raw keystream all it is to get ahold of is
a bunch of negative properties - lack of bias, lack of correlation,
etc.
>But this is exactly the same thing that we see with measurements of the
>speed of light. It *might* well vary; all we can really say is that
>the degree of variation is (with extremely high probability) less than
>a few zillionths of a percent.
I have no problem with that since the speed of light is a positive
thing being measured.
Bob Knauer
"Did you ever notice that when a politician does get an idea
he usually gets it all wrong?"
------------------------------
From: [EMAIL PROTECTED] (R. Knauer)
Subject: Re: Define Randomness
Date: Fri, 26 Feb 1999 21:42:10 GMT
Reply-To: [EMAIL PROTECTED]
On Fri, 26 Feb 1999 10:55:53 -0600, [EMAIL PROTECTED] (wtshaw) wrote:
>A local resident, Frank, called me and asked me to come over. He had
>carefully recorded and tabulated all of the Lottery winners for several
>months, and noticed that there seemed to be a pattern of some numbers
>coming up less or more frequently, and some more together than others. He
>said he would pay lots of money for a program to predict the next winning
>numbers. He got mad when I told his that it would be a waste of my time
>and his money.
You may have over-reacted. What if he planned to sell that program for
a profit? I was approached in a similar manner when the Texas lottery
first got popular, but the people did not want to pay my going rate,
so I blew it off.
If people want to bet on streaks, that is there business. Building a
streak identifying machine is not unethical either - there many that
try to identify the "trend" in some market.
Most gamblers wait for a streak and start betting heavily when they
spot one. Most successful commodity traders do that.
We know that for a uniform Bernoulli process with *fixed* wager, you
will break even eventually. But successful gamblers do not bet fixed
amounts. The fact that streaks emerge in a Bernoulli process is just
the nature of the beast.
Bob Knauer
"Did you ever notice that when a politician does get an idea
he usually gets it all wrong?"
------------------------------
From: [EMAIL PROTECTED] (R. Knauer)
Subject: Re: True Randomness - DOES NOT EXIST!!!
Date: Fri, 26 Feb 1999 22:01:33 GMT
Reply-To: [EMAIL PROTECTED]
On Fri, 26 Feb 1999 20:26:13 GMT, BRAD KRANE <[EMAIL PROTECTED]>
wrote:
> Ever wonder why there are prophits that can tell you whats going to happen
>thousands of
>years from now with surprising certanty and hay there even right exactly right about
>whats going
>to happen in lets say 1000 years.
> I kindly ask you to explain this.
The human brain is a quantum mechanical device, and is therefore
capable of doing things that are outside of classical computation. See
Roger Penrose's take on that, but don't pay much attention to the
microtubules.
We have little notion of what this new era of quantum computing is
going to usher in. All we can say is that we are fortunate enough to
be in on its beginnings.
Bob Knauer
"Did you ever notice that when a politician does get an idea
he usually gets it all wrong?"
------------------------------
From: [EMAIL PROTECTED] (R. Knauer)
Subject: Re: True Randomness - DOES NOT EXIST!!!
Date: Fri, 26 Feb 1999 22:02:45 GMT
Reply-To: [EMAIL PROTECTED]
On 26 Feb 99 16:32:55 -0400, [EMAIL PROTECTED] (John Briggs)
wrote:
>> Ever wonder why there are prophits that can tell you whats going to happen
>thousands of
>> years from now with surprising certanty and hay there even right exactly right
>about whats going
>> to happen in lets say 1000 years.
>> I kindly ask you to explain this.
>Sorry. I didn't mean to fling stones at your religion. Calm down
>and I promise not to do it again.
I was unable to detect any religious notion in the original poster's
comments.
What religion are you talking about?
Bob Knauer
"Did you ever notice that when a politician does get an idea
he usually gets it all wrong?"
------------------------------
From: [EMAIL PROTECTED] (R. Knauer)
Subject: Re: Testing Algorithms [moving off-topic]
Date: Fri, 26 Feb 1999 22:05:13 GMT
Reply-To: [EMAIL PROTECTED]
On Fri, 26 Feb 1999 19:46:37 GMT, Doggmatic <[EMAIL PROTECTED]>
wrote:
> ^^^^^^^^^^^^^^^^ There's the key ... a force has to BE APPLIED. It may be
>small, but there is a force. I believe the limit is asymptotic, i.e. the
>zero value can be approached, but never reached. I'm also guessing that
>moving that brick actually has an energy requirement greater-than oh let's
>say 1e(-18) ergs. But I will look up this "reversible computing." For such a
>great idea researched 30 years ago, you think I'd have my Free-Energy
>computer by now. I'll assume that this "reversible computing" is on the
>level of great physical phenomena like superconduction and the EPR paradox
>and black-hole-radiation.
Classical reversible computing requires infinite perfection of the
components, so it is only conceptual. It is like the ideal wheel -
frictionless only if it is perfectly circular.
Bob Knauer
"Did you ever notice that when a politician does get an idea
he usually gets it all wrong?"
------------------------------
From: [EMAIL PROTECTED] (John Briggs)
Subject: Re: True Randomness - DOES NOT EXIST!!!
Date: 26 Feb 99 16:32:55 -0400
In article <[EMAIL PROTECTED]>, BRAD KRANE <[EMAIL PROTECTED]> writes:
>> Now Brad Krane seems to be claiming that the universe evolves in a
>> deterministic fashion from some starting state so that everything
>> that happens is, in principle, completely determined by that starting
>> state. While I disagree with this view, it is both self-consistent
>> and consisent with the experimental evidence. (It's hard to falsify
>> non-local hidden-variable theories).
>
> Ever wonder why there are prophits that can tell you whats going to happen
>thousands of
> years from now with surprising certanty and hay there even right exactly right about
>whats going
> to happen in lets say 1000 years.
>
> I kindly ask you to explain this.
Sorry. I didn't mean to fling stones at your religion. Calm down
and I promise not to do it again.
John Briggs [EMAIL PROTECTED]
------------------------------
Date: 26 Feb 1999 22:55:05 -0000
From: HyperReal-Anon <[EMAIL PROTECTED]>
Subject: One-Time-Pad program for Win85/98 or DOS
Crossposted-To: alt.security,alt.privacy
Found a basic one-time-pad program which uses the XOR function.
It can be found at http://surf.to/hookah or http://hookah.ddns.org
depending on which service is working. The site is up most weekends,
round the clock. Look for "V-OTP".
------------------------------
From: [EMAIL PROTECTED] (karl malbrain)
Subject: Re: A question-Mag ID Cards
Date: Fri, 26 Feb 1999 13:41:55 -0800
In article <[EMAIL PROTECTED]>,
[EMAIL PROTECTED] says...
> Hi, I am writing again, because i have not receieved any
> responses. I am a student at Bloomsburg University and I have a project
> on Magnetic Stripe ID cards. I need to find out what is stored on them,
> how and how the reader identifies the person. I don't know where to begin
> researching adn i don't know any of these facts. Any help would really be
> appreciated, thank you
>
>
Very GENERALLY: The magnetic stripe contains an ENCRYPTED version of your Personal
Identification Number, PIN, which is compared to the PIN you enter to AUTHORIZE your
identity to the reader. Karl M
------------------------------
From: JEANNINE <[EMAIL PROTECTED]>
Subject: A question-Mag ID Cards
Date: Fri, 26 Feb 1999 15:27:31 -0500
Hi, I am writing again, because i have not receieved any
responses. I am a student at Bloomsburg University and I have a project
on Magnetic Stripe ID cards. I need to find out what is stored on them,
how and how the reader identifies the person. I don't know where to begin
researching adn i don't know any of these facts. Any help would really be
appreciated, thank you
------------------------------
From: [EMAIL PROTECTED] (R. Knauer)
Subject: Re: Testing Algorithms
Date: Fri, 26 Feb 1999 21:12:02 GMT
Reply-To: [EMAIL PROTECTED]
On 26 Feb 1999 08:57:44 -0500, [EMAIL PROTECTED] (Patrick Juola)
wrote:
>This point is incorrect, no matter how many times Schneier's book is
>quoted. There is *no* minimum energy required for computation.
I thought that the minimum logic gates needed for computation were the
AND gate and the NOT gate. The AND gate is not reversible, so it costs
energy to operate it. According to Bennett it takes energy to erase
bits in a memory register.
Bob Knauer
"Did you ever notice that when a politician does get an idea
he usually gets it all wrong?"
------------------------------
From: [EMAIL PROTECTED] (Patrick Juola)
Subject: Re: Testing Algorithms
Date: 26 Feb 1999 15:25:40 -0500
In article <7b6sog$nsm$[EMAIL PROTECTED]>,
Doggmatic <[EMAIL PROTECTED]> wrote:
>In article <7b698o$a4$[EMAIL PROTECTED]>,
> [EMAIL PROTECTED] (Patrick Juola) wrote:
>> In article <7b53f6$649$[EMAIL PROTECTED]>,
>> Doggmatic <[EMAIL PROTECTED]> wrote:
>> >
>> > Okay .. given all that, as long as your computer is made of matter (or
>> >even, anti-matter, I conjecture), it will conform (nasty word!) to the laws
>> >of thermodynamics. Bruce Schneier brought up the point in his book. Every
>> >action takes a discrete amount of energy to perform and thus, even if your
>> >computer can load registers at speeds approaching light-speed, you still have
>> >to power it.
>>
>> This point is incorrect, no matter how many times Schneier's book is
>> quoted. There is *no* minimum energy required for computation.
>
> That isn't "Schneier's Law" buddy .... that's the 2nd Law of Thermodynamics,
>which gives time a direction and describes this universe (on the assumption
>that it is a closed system). The assumption may be debatable, but I don't
>too often see dropped plates leaping back onto the table mending themselves,
>no matter how many times I bump the pieces after they are on the floor. Any
>state change in this universe causes entropy to increase and takes a GREATER
>THAN ZERO amount of energy.
You're misstating the second law. First of all, entropy isn't necessarily
strictly increasing; its simply non-decreasing. Second, Lots of state
changes don't result in any entropy change at all. Think for a moment about
an oscillating pendulum -- the change from kinetic to potential energy
and back *IS NOT ACCOMPANIED BY A CORRESPONDING CHANGE IN ENTROPY* and
the state-change can be completely reversed back to the original
starting point.
Computation *can* be done using only reversible operations of the
second type; once you've done your computation, you reverse the
machine, plug the outputs back into the inputs and recover the input
state exactly. You'll not do it with traditional NAND or NOR gates -- but
there are lots of other primitives you can use to construct Turing machines,
and some of these primitives are reversible.
Now, a "real" pendulum won't swing forever; you'll get air resistance
and stress on the pendulum, for example. But these parasitic losses can
be made as small as you like. If you don't like the pendulum to have
air resistance, then construct your apparatus in a vacuum chamber and
you'll lower the parasitic energy loss. The lower limit of the parasitic
losses are ZERO; there's no "fundamental" energy change that occurs
in a pendulum swinging, and there's no bound in basic physics, even
allowing for quantum effects, on the length of time that a pendulum can
swing.
Ditto : there is *NO* bound on the minimum amount of energy it takes to
perform a computation.
>As it has been stated many times in this thread,
>if you're assuming that the intrinsic physical limits of this universe can be
>broken, then there is little point in this debate.
And I suggest that you don't know what the physical limits *are*; a
Turing-machine equivalent can operate on an arbitrarily small amount of
power.
-kitten
------------------------------
From: [EMAIL PROTECTED] (Patrick Juola)
Subject: Re: Testing Algorithms [moving off-topic]
Date: 26 Feb 1999 15:32:19 -0500
In article <7b6tmq$ojt$[EMAIL PROTECTED]>,
Doggmatic <[EMAIL PROTECTED]> wrote:
>In article <7b69tr$au$[EMAIL PROTECTED]>,
> [EMAIL PROTECTED] (Patrick Juola) wrote:
>> In article <7b56hg$8m8$[EMAIL PROTECTED]>,
>> Doggmatic <[EMAIL PROTECTED]> wrote:
>> >In article <7b101l$q4v$[EMAIL PROTECTED]>,
>> > [EMAIL PROTECTED] (Patrick Juola) wrote:
>> >> In article <7avprg$jvm$[EMAIL PROTECTED]>, <[EMAIL PROTECTED]> wrote:
>> >>
>> >> The fundamental limit of powering a computer processor is *ZERO*.
>> >>
>> >> Power provides *NO* limitation on how big you can make a computer.
>> >>
>> >
>> > Uhh.... who told you that lie? Any process (the smallest possible relevent
>> >one of which is counting) requires a discrete (more than zero) amount of
>> >energy. So, unless your processor does *ZERO* work, it will consume more
>> >than ZERO energy. So, unless this computer is processing in another universe
>> >which is not subject to the physical of this one, there IS a limit.
>>
>> Wrong. Look up "reversible computing" sometime; hell, this point was
>> explored at length in a Scientific American article sometime in the 70's.
>>
>> Think of it this way -- what's the minimum amount of energy necessary
>> to move a brick five feet (horizontally)? On a frictionless surface
>> (there's those damned parasitic costs again), it will move, albiet
>> slowly, no matter how little force is applied.
>
> ^^^^^^^^^^^^^^^^ There's the key ... a force has to BE APPLIED. It may be
>small, but there is a force. I believe the limit is asymptotic, i.e. the
>zero value can be approached, but never reached. I'm also guessing that
>moving that brick actually has an energy requirement greater-than oh let's
>say 1e(-18) ergs.
Wrong.
The force can be arbitrarily small (at least under the classical physics
approximation appropriate to this oversimplification).
Dragging out freshman physics :
Force * time = mass * velocity change
Applying any non-zero force *will* result in a velocity change, which
is sufficient to cause the brick to move (absent friction).
The limiting case of a sufficiently small non-zero number is zero.
Q.e.d.
>But I will look up this "reversible computing." For such a
>great idea researched 30 years ago, you think I'd have my Free-Energy
>computer by now.
I'll build one for you. Just buy me a frictionless surface.
Oh, you mean you think that lubrication technology will never improve
over 1999 levels?
-kitten
------------------------------
From: [EMAIL PROTECTED] (Patrick Juola)
Subject: Re: Testing Algorithms [moving off-topic]
Date: 26 Feb 1999 15:46:36 -0500
In article <[EMAIL PROTECTED]>,
R. Knauer <[EMAIL PROTECTED]> wrote:
>On 26 Feb 1999 09:08:59 -0500, [EMAIL PROTECTED] (Patrick Juola)
>wrote:
>
>>Wrong. Look up "reversible computing" sometime; hell, this point was
>>explored at length in a Scientific American article sometime in the 70's.
>
>Bennett at IBM did the pioneering work on that.
Thanks. 8-)
>>Think of it this way -- what's the minimum amount of energy necessary
>>to move a brick five feet (horizontally)? On a frictionless surface
>>(there's those damned parasitic costs again), it will move, albiet
>>slowly, no matter how little force is applied.
>
>>So the fundamental limit to the amount of energy is zero.
>
>Any logic gate that is reversible uses no energy in principle to do
>its logic operation. The friction you speak of is equivalent to
>resistive losses in ordinary semiconductor gates.
>
>A NOT gate is reversible in that you can infer the input from the
>output. You cannot do that with an AND gate, so it is not reversible
>and therefore energy is consumed.
>
>I believe Bennett identified energy dissipation (beyond resistive
>losses) with erasing bits in memory registers. Also I believe that
>only quantum computers can support reversible logic computations, so
>classical computers are always going to require energy to run them
>beyond purely resistive losses.
No, classical computers designed with an appropriate set of primitives
will also suffice. A good example is a three by three gate :
c
____|____
| |
A- | | -a
| |
| |
B- | | -b
| |
_________
|
C
Signals A, B, and C are inputs, a,b,c, are outputs. The operation is
fairly simple; if C is zero, A -> a and B ->b. if C is nonzero, A->b
and B->a. In either case, C -> c.
I can easily turn this into an AND gate; hold A constant and zero.
If A is zero, then a is nonzero if-and-only-if both B and C are one.
Creating the rest of the "normal" primitives is an interesting
challenge I here omit for brevity. Because in theory the triple (A,B,C)
is reconstructable from (a,b,c), there's no irreversible state change.
Constructing a simple flip-flop from these gates is an exercise usually
assigned in a freshman EE class; I built dozens of J/K flip flops and
their friends and have long-since forgotten the details. Of course,
at every time step, the computer will be announcing the current situation
of every memory cell via one of the "ignorable" outputs such as 'c' in
the AND gate above, but presumably someone will be keeping track of them
and make them available when you decide to reverse the procedure.
So there's no reason in principle that reversible computing wouldn't
work. The question then becomes one of reducing (or eliminating) parasitic
losses; just how good *can* we get our conductors? And at this point
we're back in the realm of technology and engineering instead of
physics -- I see no reason to believe that we've hit fundamental limits
in superconductor technology.
-kitten
------------------------------
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