This is a proposal for a simple factoring algorithm which actually performs better than regular attacks based on lattice-type algorithms. Although more efficient algorithms exists, this one is very simple to implement, while the more efficient ones are not. Do not quote me on this, though ;-D

## Finding a good approximation

The first step is to compute an *good* approximation of . Denote this . One such *good* approximation is . Let us see why. First, assuming that , note that

Let us assume there exists a small such that and . So, we have that

Now, let . Then,

.

Since is assumed to be small (), we can approximate it as

Great! So the running time is bounded by if we use pure brute force. Now, we are going to be just a little more clever than that, aren’t we?

## Searching efficiently

We will now show how to search for a solution faster than brute force. First, we pick an arbitrary invertible element and compute all powers of up to a certain parameter , i.e.,

Obviously, this requires time and memory. A reference implementation in Python (with ) would look something like.

# create a look-up table look_up = {} z = 1 for i in range(0, b + 1): look_up[z] = i z = (z * 2) % n

Now, compute

Note that the exponent of is

.

We now do a BSGS-type of algorithm (as noted by hellman in the comments, we can use Pollard’s kangaroo algorithm to avoid exhausting precious memory without sacrificing computational effort, up to polynomial factors). Generate all powers for and multiply with forming . Again, we get an exponent which is

For each such computed, check against is it exists. If so, we have found .

# check the table mu = gmpy.invert(pow(2, phi_approx, n), n) fac = pow(2, b, n) j = 0 while True: mu = (mu * fac) % n j += b if mu in look_up: phi = phi_approx + (look_up[mu] - j) break if j > b * b: return

This is in time. Once has been found, we can trivially compute the factors in negligible time as

# compute roots m = n - phi + 1 roots = (m - gmpy.sqrt(m ** 2 - 4 * n)) / 2, \ (m + gmpy.sqrt(m ** 2 - 4 * n)) / 2

Cool! Since we require that for a solution to be found, the running time is and with equal amount of memory required.

## A real-world example (sort of)

This little finding inspired me to create a CTF challenge. During the SEC-T CTF 2017, one of the challenges (qproximity) could be solved using this method.

**Description**

We are given a public key and an encrypted message. The key is

-----BEGIN PUBLIC KEY----- MIGeMA0GCSqGSIb3DQEBAQUAA4GMADCBiAKBgAOBxiQviVpL4G5d0TmVmjDn51zu iravDlD4vUlVk9XK79/fwptVzYsjimO42+ZW5VmHF2AUXaPhDC3jBaoNIoa78CXO ft030bR1S0hGcffcDFMm/tZxwu2/AAXCHoLdjHSwL7gxtXulFxbWoWOdSq+qxtak zBSZ7R1QlDmbnpwdAgMDEzc= -----END PUBLIC KEY-----

Extracting the modulus, we find that

The factors are found as follows

def close_factor(n, b): # approximate phi phi_approx = n - 2 * gmpy.sqrt(n) + 1 # create a look-up table look_up = {} z = 1 for i in range(0, b + 1): look_up[z] = i z = (z * 2) % n # check the table mu = gmpy.invert(pow(2, phi_approx, n), n) fac = pow(2, b, n) j = 0 while True: mu = (mu * fac) % n j += b if mu in look_up: phi = phi_approx + (look_up[mu] - j) break if j > b * b: return m = n - phi + 1 roots = (m - gmpy.sqrt(m ** 2 - 4 * n)) / 2, \ (m + gmpy.sqrt(m ** 2 - 4 * n)) / 2 return roots n = 2462649746477364143454082050368305440553491900304388646893610847386194301369924385009730987303651345060031438478297733694036327257723431468649220444397635127530301992505638291521092898714917678389314956983918603221732358628680256253537449204312287724750669208856634711056863315465220853759428826555838536733 b = 10000000 close_factor(n, b)

The code runs in matter of seconds and gives the factors

and

Using these, we construct the flag

SECT{w3ll_th4t_wasnt_2_h4rd?}

Interesting! What are the “more efficient” algorithms that you mention?

There are algorithms which use bivariate Coppersmith to achieve this (e.g. https://hsbp.org/tiki-download_wiki_attachment.php?attId=174). Basically, it allows a smaller such that , so that still is factorable in polynomial time.

I think instead of BSGS you can use Pollard’s Kangaroo (lambda), no memory used. Though slightly more code 😉

Definitely! I will add it to the text!

What are the other, “more efficient” methods you mention?