5l1v3r1 / rsa

Cracking RSA

Cracking RSA

Cracking RSA means finding the private key from a given public key. This code extracts the components from a public key, performs factorization, and if successfull, constructs the private key.

Author:

• sganis

Created:

• 30/11/2008

Updated:

• 01/12/2008, tested in Ubuntu 8.04.
• 31/10/2012, using latest msieve 1.50, tested in Mac 1.6.
• 28/03/2014, using mpi with msive 1.52, tested in Ubuntu 12.10.
• 20/04/2015, uploaded to github, tested in Ubuntu 14.04.

RSA Concepts

Key generation:

• generate 2 random prime numbers (p,q)
• n=pq is the modulus
• relative prime count is f(n) = (p-1)(q-1)
• e is chosen such that 1<e<f(n) and gcd(f(n),e)=1, in openssl this is fixed: 2^16=65537
• d = e^-1 mod f(n)
• (e,n) is the public key, usually stored in a file in PEM format
• (d,p,q) is the private key, same format, but usually encrypted

Encryption:

M is the message in clear text, then C = M^e mod n

Decryption:

M = C^d mod n. The proof is not here, but M = (M^e)^d mod n

Breaking RSA:

We have the public key (e,n) and we need to find the private key (d,p,q).

d = e^-1 mod (p-1)(q-1), so the unkown data is p and q.

We have n = pq.

Solution: we must factorize n. It seems quite easy, but...

• Problem 1: n is really big, 1024 bit number is about 300 digits.

• Problem 2: even worse, n has only 2 factors, p and q, also big prime numbers.

Usage:

run ./test.sh [key-length]

To get an idea of time:

$ for (( i=32; i<=256; i+=8 ));do ./test.sh $i;done 2>&1| grep Success

How it works:

• First, generates a private key using openssl. Any tool can be used. The reason I am using my own implementation is because openssl does not provide an option to create a private key with 2 given prime numbers.
• Then, generates the public key using the private key.
• After that, get the public key components: modulus n and exponent e. With only this information, we are supposed to get the private key :)
• Use msieve to factorize n, a big number, Msive is an implementation of the fastest factorization algorithm known today, GNFS. If successful, we have p and q.
• Use my openssl implementation to create a private key with p and q
• Finally, compares the found key with the original to verify success