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# Integer Factorization

## Quadratic Sieve

Bill Hart’s quadratic sieve is included with Sage. The quadratic sieve is the best algorithm for factoring numbers of the form \(pq\) up to around 100 digits. It involves searching for relations, solving a linear algebra problem modulo \(2\), then factoring \(n\) using a relation \(x^2 \equiv y^2 \mod n\).

    sage: qsieve(next_prime(2^90)*next_prime(2^91), time=True)   # not tested
    ([1237940039285380274899124357, 2475880078570760549798248507],
     '14.94user 0.53system 0:15.72elapsed 98%CPU (0avgtext+0avgdata 0maxresident)k')

Using qsieve is twice as fast as Sage’s general factor command in this example. Note that Sage’s general factor command does nothing but call Pari’s factor C library function.

    sage: time factor(next_prime(2^90)*next_prime(2^91))     # not tested
    CPU times: user 28.71 s, sys: 0.28 s, total: 28.98 s
    Wall time: 29.38 s
    1237940039285380274899124357 * 2475880078570760549798248507

Obviously, Sage’s factor command should not just call Pari, but nobody has gotten around to rewriting it yet.

## GMP-ECM

Paul Zimmerman’s GMP-ECM is included in Sage. The elliptic curve factorization (ECM) algorithm is the best algorithm for factoring numbers of the form \(n=pm\), where \(p\) is not “too big”. ECM is an algorithm due to Hendrik Lenstra, which works by “pretending” that \(n\) is prime, choosing a random elliptic curve over \(\ZZ/n\ZZ\), and doing arithmetic on that curve–if something goes wrong when doing arithmetic, we factor \(n\).

In the following example, GMP-ECM is over 10 times faster than Sage’s generic factor function. Again, this emphasizes that Sage’s generic factor command would benefit from a rewrite that uses GMP-ECM and qsieve.

    sage: time ecm.factor(next_prime(2^40) * next_prime(2^300))    # not tested
    CPU times: user 0.85 s, sys: 0.01 s, total: 0.86 s
    Wall time: 1.73 s
    [1099511627791,
     2037035976334486086268445688409378161051468393665936250636140449354381299763336706183397533]
    sage: time factor(next_prime(2^40) * next_prime(2^300))        # not tested
    CPU times: user 23.82 s, sys: 0.04 s, total: 23.86 s
    Wall time: 24.35 s
    1099511627791 * 2037035976334486086268445688409378161051468393665936250636140449354381299763336706183397533

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