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                            RSA: Attacking the cryptosystem [V. 1.3]

            

                            Written by The Death, February 8th 2001

                (I can be found on the BoxNetwork's IRC server under ThyDeath)



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Table of Content

1.0..............................................................Introduction

1.1..................................Basic Math & Functions Used In This Part

1.2................................................Basic Statements Used Here

1.3............................................................Trial Division

1.4..................................The Idea Behind The Described Algorithms

1.5........................................................Fermat's Factoring

1.6.............................................................Pollard's P-1

1.7....................................................Initial Segment Attack



2.0............................................................Random Attacks

2.1...................................................Basic Randomness Attack

2.2.............................................................Pollard's RHO



3.0.......................................................Real-Life Scenarios

3.1..............................................................Known phi(N)

3.2............................................................Common Modulus

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>>>  1.0 Introduction  <<<

The RSA cryptosystem is the most widely-used Asymmetric Cryptosystem. In this toturial I will

explain some vulnerabilities of this Cryptosystem, for people to learn from and protect themself

or attack others.



I will not explain the Cryptosystem itself, As it is described in many pages in the Internet.

Furthermore, i will expect you to know Cryptosystem yourself.



If you have any questions you can find your answers at the sci.crypt newsgroup, or you can ask 

me on the IRC or Email (thedeadh@netvision.net.il)



This part is dealing with one aspect: Finding the factors of the modulus. next parts will deal 

other aspects of attacking this Cryptosystem.



The Death





>>>  1.1 Basic Math & Functions Used In This Part  <<<



* Length(N) returns N's number of digits

* The ^ symbol will be used here for Exponent.

* M is the Modulus of the key.

* P & Q are the two primes, which their product is M

* [R] Denotes the greatest integer not exceeding R. Example: [4] = 4, [4.6] = 4, [-2.1] = -3. 

* The GCD(a,b) function returns the Greatest Common Divisor of a & b. 

  Example: GCD(100, 30) = 10, GCD(7,17) = 1.

* Euler's Phi(N) Function returns the greatest B that comply: B =< N, GCD(B,N) = 1





>>>  1.2 Basic Statements Used Here  <<<



* X^2 - Y^2 = (X+Y)(X-Y)

  Proof:

  (X+Y)(X-Y) = X^2 - XY + XY - Y^2 = X^2 - Y^2



* M is odd, unless P or Q = 2

  Proof:

  M = PQ. P & Q are primes, Therefore are not divisible by 2 unless they are 2.

  The product of two primes is divisible only by these primes.



* Fermat's factoring theorem: Every odd number can be written as the difference of two squares,

  X^2 - Y^2 (X,Y : Natural).

  Proof:

  WTF?! I Don't care what the proof is! Go dig him up and ask him if you wish!



* Conclusions from the Phi function:

  * If P is a prime, then Phi(P^K) = (P^(K-1))*(P-1)

  * If P is a prime, then Phi(P) = P-1

  * If GCD(A,B) = 1 Then Phi(A*B) = Phi(A)*Phi(B)



(Note: Fermat was a jackass guy who enjoyed solving mathematical sentences, then ask his friends

 to solve them themself, just to annoy them)





>>>  1.3 Trial Division  <<<



This is the lesser algorithm there is for attacking the RSA cryptosystem. I put it here because

 hmmm.... hmmmmm..... I am not sure. Maybe i inhaled too much CO2 from the Coke bottle.

Anyways, It is simple:



1) Assign X = 2

2) Is X a divisor of M ?

  * Yes - You found the P factor! Q is M/X, and now you cracked the modulus!

  * No  - Assign X with the next prime and do this step again



This algorithm is very slow when dealing high numbers. I still can't recall why did i put it

 here.





>>>  1.4 The Idea Behind The Described Algorithms  <<<



The next algorithms are called specific factorization algorithms. They can theoreticlly factor

 every modulus, but work best on a key with primes that weren't chosen properly.

Each of them has a short description of the element it attacks, and how to avoid it.





>>>  1.5 Fermat's Factoring  <<<



This attack is based on Fermat's Factoring Theorem (No shit...). 

If X^2 - Y^2 = M, then X^2 - M = Y^2.

M is known, so we will brute-force X to find a pair X & Y that comply being Integers.



The algorithm is as follows:

1) Assign X with [SquareRoot(M)]

2) Is SquareRoot(X^2 - M) = [SquareRoot(X^2 - M)] ?

  * No - Increase X by 1 and do this step again

  * Yes - Proceed to step 3

3) Assign Y with SquareRoot(X^2 - M)

4) The factorisation of M is (X+Y)(X-Y), meaning P = X-Y and Q = X+Y



This algorithm is very effective against keys with modulus produced from two close primes.

The bigger ((sqrt(P) - sqrt(Q))^2)/2 is, the greater is the number of steps needed for Fermat's

factoring attack to find P and Q.





>>>  1.6 Pollard's P-1  <<<



The algorithm is as follows:

1) Choose a number A that comply 1 < A < M

2) Choose a number K

3) Is GCD(A,M) = 1 ?

  * Yes - You found a factor, GCD(A,M)

  * No - Proceed to step 4

4) Assign L = A^K mod M

5) Assign D = GCD(L-1, M)

6) Is D a factor of M ?

  * Yes - You found a factor, D. P = D, Q = M/D.

  * No  - Change K and/or A and return to step 3



This algorithm is effective on M such that P or Q has small prime factors.





>>>  1.7 Initial Segment Attack  <<<

There is no exact algorithm, but this what i got: (M#3, M#1... will be used to reffer to the 

  # digit of M. Say M = 72342, then M#1 = 2, M#5 = 7...)



1) Assign A = 2

2) Assign T with the minimum number of 0s to check the segment

3) Is M#A, M#(A+1)...M#(A+T) = 0 ?

  * Yes - Proceed to step 4

  * No  - If A < Length(M) Then increase A by 1 and return to step 2, else exit

4) Assign B with the segment from M#A to M#1 (Initial segment starting from M#A)

5) Is GCD(B,M) > 1 ? 

  * Yes - You found a factor! P = GCD(B,M), Q = M/P

  * No - If A < Length(M) Then increase A by 1 and return to step 2, else exit



This algorithm is effective if Length(P) < Length(Q) by much, and Q has alot of 0s in it.





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>>>  2.0 Random Attacks  <<<

These attacks are based some statistic theories which i am not fimilliar with.

The basic idea is taking some algorithms (some described in the 1st section) and adding some

randomness to them.



Additional function used here is Random, which denotes a random number.



>>>  2.1 Basic Randomness Attack  <<<

This is the simplest attack on this field. Like Trial Devision on the 1st section, this is the

most basic algorithms and slow algorithm might be created...



1) Assign A = Random Mod sqrt(N)

2) Is A a divisor of N ?

   * Yes - Well good for you!

   * No  - Go back to step 1...





>>>  2.2 Pollard's RHO  <<<

A better algorithm (for some statistical reasons...)



1) Assign X = Random Mod N

2) Assign Y = Random Mod N

3) Is X - Y a divisor of N ?

   * Yes - Good!

   * No  - You know what to do...



This is the most basic idea. It can be improved using a polynomial. Example:

1) Assign X = Random Mod N

2) Assign Y = Random Mod N

3) Set X = (X^2 +1) Mod N

3) Set Y = (Y^2 +1) Mod N

4) Set Y = (Y^2 +1) Mod N Again

5) Set Z = GCD(X-Y, N)

6) Is 1 < Z < N ?

   * Yes - You found a factor (Z) !

   * No - go back to 1...





*** More to come soon! ***





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>>>  3.0 Real-Life Scenarios  <<<

Here I will show some situations that might occur if the application or person making/using

the RSA Cryptosystem is not fully aware of them.

Sometimes trivial knowledge of an organization's security architecture might reveal some flaws

that can be explotied to crack a key, or find the plaintext of a message.





>>>  3.1 Known phi(N)  <<<

Take a look at the RSA Cryptosystem's Key-Generation proccess. There is a great part relying 

on (P-1)(Q-1). The 3rd conclusion of the Phi() function states that Phi(N) is equivalent to 

(P-1)(Q-1) (Because N = PQ. P,Q = Primes).

The fact that Phi(N) is used in the proccess is useful to us if the app saves a log of the 

proccess, or we can find Phi(N) in any other way.

Once Phi(N) is found, we can find P & Q. Here is how:

Phi(N) = PQ - (P+Q) +1 = N -P -Q +1

Therefore: Q = N -Phi(N) +1 -P

Now, N = PQ. Instead of Q we put the value from the last equation. Therefore:

N = P*(N -Phi(N) +1 -P), thus P^2 -P*(N -Phi(N) +1) +N = 0. 

This is a typical quadratic equation, which you can solve using the quadratic formula.

Once solved, P is exposed, Therefore Q is exposed, Therefore key is factored.





>>>  3.2 Common Modulus  <<<

The next scenario will relay on this basic one:

Suppose there is a modulus S which is shared among several people. Each has it's own private/public 

exponents set, but all based on the same modulus. None of the men know the factors. (This scenerio 

may occur if a company chooses to issue all of the employees with keys, and for optimization uses a 

common modulus).

If one sends a message to two people sharing the same modulus, and an evesdropper is listening and 

reading the communication, the evesdropper can get the original message out of the two.

* M - The original message

* J, K - The two ciphertexts

* S - The common modulus

* A, B - The public exponents of the two recipents



GCD(J,K) = 1, the evesdropper uses the extended euclidian algorithm to find two numbers such that 

a*x + b*y = 1. Using x and y, the evesdropper computes:

M = (J^x)*(K^y) Mod S



And there you go...







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That is all for now. I will add this part other algorithms as I find & learn them.

Expect preceeding parts to come!



The Death



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