# Thread: Little simple doubt in Handbook of Applied Cryptography...

1. ## Little simple doubt in Handbook of Applied Cryptography...

In Page 7 there's an example (1.4) that says:
"Take X = {1,2,3,...,10} and let f be the rule that for
each x E X, f(x)=rx where rx is the remainder when x^2 is divided
by 11. Explicitly then

f(1)=1 , f(2)=4 , f(3)=9 , f(4)=5 , f(5)=3
f(6)=3 , f(7)=5 , f(8)=9 , f(9)=4 , f(10)=1
"

f(x)=remainder_of(x^2/11) right?

f(3)=remainder_of(3^2/11)
then f(3) would be equal to 0.82 which the remainder is 8 and not 9,
the same happens in f(8). I dont know much this cryptography maths
but I still have interest in learn cryptography..

cya

f(3)=remainder_of(3^2/11)
f(3)=remainder_of(9/11)

9/11 = 0, with remainder of 9
That is, 11 "goes into" 9, 0 times (that is, it won't go at all), then there is 9 more to add back in = remainder of 9.

There aren't any fractional parts (.82) in remainders.

?????

Sarge

3. mmh

Originally Posted by sarge
f(3)=remainder_of(3^2/11)
f(3)=remainder_of(9/11)
9/11 = 0, with remainder of 9
I agree , however 9/11 (on my TI-83) is 0.81818...

which rounded to one decimal digit is 0.8 and not 0.9
(from what I know)

Hoppe u understand now what I mean

4. so what?

the fact that 9/11 is a decimal number that can be rounded is nothing special; ANY fraction is actually a decimal number that can be rounded.

the question was: What is the remainder of 9/11...the answer is 9. This has NOTHING to do with the decimal number .818, or the fact that is can be rounded to .8....and nobody said that it was supposed to be rounded to .9 anyway!

Just check the MOD function.

Sarge

5. Originally Posted by sarge
so what?

the fact that 9/11 is a decimal number that can be rounded is nothing special; ANY fraction is actually a decimal number that can be rounded.

the question was: What is the remainder of 9/11...the answer is 9. This has NOTHING to do with the decimal number .818, or the fact that is can be rounded to .8....and nobody said that it was supposed to be rounded to .9 anyway!

Just check the MOD function.

Sarge
I will

6. all scientific calculators have an [a b/c] button.

say 145 Modulo 11
145 [a b/c] 11 [=]

will give you-> 13/2/11 (a=13, b=2, c=11)
modulo is b, i.e: 2

anyhow, i'm not quite sure what you are doing with a crypto book if you don't know what modulo is ...

7. Originally Posted by doug
anyhow, i'm not quite sure what you are doing with a crypto book if you don't know what modulo is ...
He's *learning*! Don't dis him for that...

8. I've understood what was wrong when I calc the f(x)....

Originally Posted by doug
anyhow, i'm not quite sure what you are doing with a crypto book if you don't know what modulo is ...
Well, is it possible to learn crypto with this crypto book
just having 11 grade (no algebra and other stuff) ?

Its not the first time it happens the requirement of knowledge about
stuff I havent learnt on school..not meaning that I was successfuly
in all

9. Originally Posted by acoder
Well, is it possible to learn crypto with this crypto book
just having 11 grade (no algebra and other stuff) ?
No algebra? How do you write code without variables? You're going to have to learn algebra to understand anything on this site, and that's especially true for crypto. (Whether you learn it in school or not is irrelevant, though.)

10. Hi,

I know almost nothing about crypto and its workings.

Perhaps acoder does not know the word algebra ?
Maybe he knows Trig or Calc.
Anyway, if he can understand page 7, he MUST have
more then remedial math skills. Else he likes to view crypto
sites and quote other peoples observations.

This is just my observation.

Woodmann

11. Originally Posted by Woodmann
Hi,

I know almost nothing about crypto and its workings.

Perhaps acoder does not know the word algebra ?
Maybe he knows Trig or Calc.
Anyway, if he can understand page 7, he MUST have
more then remedial math skills. Else he likes to view crypto
sites and quote other peoples observations.

This is just my observation.

Woodmann
I agree. I think acoder must know some algebra, or like you said, he wouldn't be here or reading a book on crypto. I just meant to say that it's a necessary skill for everything on this site.

12. heya

I know the word algebra but I dont know what it exacly is,
and from what I've saw its a very different thing of Bool Algebra
Anyway, sometimes I dont know the name ppl give to stuff I

I saw things on this book which I understood and/or seen seemed
to programming stuff, but in algebra have different names and 'look'
like permutations,etc. I've understood what involutions is, but didn't knew

Also founded interesting the Vigenère cipher, to lower char frequencies.
Until now I've understood most of the stuff(I havent read much
tho), (the topic which sounded more confusing was the digital signatures), I know its probably
basic stuff, but important to continue (?)..
I read it slowly...depeding on my free time

"Else he likes to view crypto
sites and quote other peoples observations."
Nah, anyway I respect others opinion's

13. No,

Thank you

Woodmann

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