mirror of https://github.com/zcash/pasta.git
201 lines
7.1 KiB
Python
201 lines
7.1 KiB
Python
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# -*- coding: utf-8 -*-
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import sys
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from multiprocessing import Pool, cpu_count
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from traceback import print_exc
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from math import ceil
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from itertools import combinations
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PROCESSES = None # auto-detect
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# Let Ep/Fp : y^2 = x^3 + bp
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# Let Eq/Fq : y^2 = x^3 + bq
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# p and q should each be ~ L bits.
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DEFAULT_TWOADICITY = 21
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DEFAULT_STRETCH = 0
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# <https://eprint.iacr.org/2011/465.pdf>
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# It is well known that if g is neither a square nor a cube in Fp, then all
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# possible group orders an elliptic curve E : y^2 = x^3 + b can have over Fp
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# occur as the order of one of the 6 twists with b \in {1, g, g^2, g^3, g^4, g^5}.
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# <https://math.stackexchange.com/questions/127251/when-is-a-not-a-cube-mod-p>:
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# If p = 2 (mod 3) then all elements are cubes.
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# If p = 1 (mod 3) then a is a cube iff a^((p-1)/3) = 1.
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# <https://cryptojedi.org/papers/pfcpo.pdf> section 2:
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# [...] the order of a curve satisfying the norm equation 3V^2 = 4p - t^2 has one
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# of the six forms {p+1 +/- t, p+1 +/- (t +/- 3V)/2} [IEEE Std 1363-2000, section
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# A.14.2.3, item 6].
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#
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# We choose 4p = 3V^2 + t^2, where (V-1)/2 and (t-1)/2 are both multiples of 2^twoadicity.
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#
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# Then 4p = (3(V-1)^2 + 6(V-1) + 3) + ((t-1)^2 + 2(t-1) + 1)
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# = 3(V-1)^2 + 6(V-1) + (t-1)^2 + 2(t-1) + 4
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# p = 3((V-1)/2)^2 + 3(V-1)/2 + ((t-1)/2)^2 + (t-1)/2 + 1
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#
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# So p-1 will be a multiple of 2^twoadicity, and so will (p+1-t)-1 = (p-1)-(t-1).
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#
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# We'd also like both p and q to be 1 (mod 3), so that we have efficient endomorphisms
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# on both curves. We explicitly check p = 1 (mod 3), and then if t is chosen to be
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# 1 (mod 3) then p+1-t will be 1 (mod 3) (but we must still check q since it
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# is not necessarily that order).
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def low_hamming_order(L, twoadicity, wid, processes):
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Vlen = (L-1)//2 + 1
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Vbase = 1 << Vlen
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tlen = (L-1)//4
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tbase = 1 << tlen
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trailing_zeros = twoadicity+1
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for w in xrange(wid, tlen-trailing_zeros, processes):
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for Vc in combinations(xrange(trailing_zeros, Vlen), w):
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V = Vbase + sum([1 << i for i in Vc]) + 1
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assert(((V-1)/2) % (1<<twoadicity) == 0)
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for tw in xrange(1, w+1):
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for tc in combinations(xrange(trailing_zeros, tlen), tw):
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t = tbase + sum([1 << i for i in tc]) + 1
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assert(((t-1)/2) % (1<<twoadicity) == 0)
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if t % 3 != 1:
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continue
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p4 = 3*V^2 + t^2
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assert(p4 % 4 == 0)
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p = p4//4
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assert(p % (1<<twoadicity) == 1)
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if p % 3 == 1 and is_prime(p):
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yield p
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def near_powerof2_order(L, twoadicity, wid, processes):
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trailing_zeros = twoadicity+1
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Vbase = isqrt((1<<(L+2))//3) >> trailing_zeros
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for Voffset in symmetric_range(10000, base=wid, step=processes):
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V = ((Vbase + Voffset) << trailing_zeros) + 1
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assert(((V-1)/2) % (1 << twoadicity) == 0)
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tmp = (1<<(L+2)) - 3*V^2
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if tmp < 0: continue
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tbase = isqrt(tmp) >> trailing_zeros
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for toffset in symmetric_range(10000):
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t = ((tbase + toffset) << trailing_zeros) + 1
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assert(((t-1)/2) % (1<<twoadicity) == 0)
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if t % 3 != 1:
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continue
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p4 = 3*V^2 + t^2
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assert(p4 % 4 == 0)
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p = p4//4
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assert(p % (1<<twoadicity) == 1)
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if p % 3 == 1 and is_prime(p):
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yield p
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def find_nonsquare_noncube(p):
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for g_int in xrange(2, 100):
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g = Mod(g_int, p)
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if g^((p-1)//3) != 1 and g^((p-1)//2) != 1:
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return g
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return None
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def symmetric_range(n, base=0, step=1):
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for i in xrange(base, n, step):
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yield -i
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yield i+1
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def find_nice_curves(strategy, L, twoadicity, stretch, wid, processes):
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for p in strategy(L, max(0, twoadicity-stretch), wid, processes):
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sys.stdout.write('.')
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sys.stdout.flush()
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if p % (1<<twoadicity) != 1: continue
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gp = find_nonsquare_noncube(p)
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if gp is None: continue
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for i in xrange(6):
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bp = gp^i
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Ep = EllipticCurve(GF(p), [0, bp])
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q = Ep.count_points()
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if q % (1<<twoadicity) == 1 and q % 3 == 1 and is_prime(q):
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bp = find_coefficient(p, q)
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if bp is not None:
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bq = find_coefficient(q, p)
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if bq is not None:
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gq = find_nonsquare_noncube(q)
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aq = gq^((q-1)//3)
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assert(aq^3 == Mod(1, q))
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ap = gp^((p-1)//3)
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assert(ap^3 == Mod(1, p))
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yield (p, q, bp, bq, ap, aq)
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def find_coefficient(p, q):
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for b in xrange(1, 10000):
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E = EllipticCurve(GF(p), [0, b])
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if E.count_points() == q:
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return b
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return None
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def find_lowest_prime(p):
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for r in Primes():
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if gcd(p-1, r) == 1:
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return r
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def format_weight(x, detail=True):
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X = format(abs(x), 'b')
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if detail:
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assert(X.endswith('1'))
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detailstr = " (bitlength %d, weight %d, 2-adicity %d)" % (len(X), sum([int(c) for c in X]),
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len(X) - len(X[:-1].rstrip('0')))
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else:
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detailstr = " (bitlength %d)" % (len(X),)
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return "%s0b%s%s" % ("-" if x < 0 else "", X, detailstr)
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def main():
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strategy = near_powerof2_order if "--nearpowerof2" in sys.argv[1:] else low_hamming_order
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args = [arg for arg in sys.argv[1:] if not arg.startswith("--")]
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if len(args) < 1:
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print("Usage: sage amicable.sage [--nearpowerof2] <min-bitlength> [<min-2adicity> [<stretch]]\n")
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return
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L = int(args[0])
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twoadicity = int(args[1]) if len(args) > 1 else DEFAULT_TWOADICITY
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stretch = int(args[2]) if len(args) > 2 else DEFAULT_STRETCH
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processes = PROCESSES or cpu_count()
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print("Using %d processes." % (processes,))
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pool = Pool(processes=processes)
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try:
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for wid in xrange(processes):
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pool.apply_async(worker, (strategy, L, twoadicity, stretch, wid, processes))
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while True:
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sleep(1000)
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except (KeyboardInterrupt, SystemExit):
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pass
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finally:
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pool.terminate()
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def worker(*args):
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try:
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real_worker(*args)
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except (KeyboardInterrupt, SystemExit):
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pass
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except:
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print_exc()
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def real_worker(*args):
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for (p, q, bp, bq, ap, aq) in find_nice_curves(*args):
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output = "\n"
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output += "p = %s\n" % format_weight(p)
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output += "q = %s\n" % format_weight(q)
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output += "α_p = %s (mod p)\n" % format_weight(int(ap), detail=False)
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output += "α_q = %s (mod q)\n" % format_weight(int(aq), detail=False)
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output += "Ep/Fp : y^2 = x^3 + %d (%ssquare)\n" % (bp, "" if Mod(bp, p).is_square() else "non")
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output += "Eq/Fq : y^2 = x^3 + %d (%ssquare)\n" % (bq, "" if Mod(bq, q).is_square() else "non")
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output += "gcd(p-1, %d) = 1\n" % find_lowest_prime(p)
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output += "gcd(q-1, %d) = 1\n" % find_lowest_prime(q)
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print(output) # one syscall to minimize tearing
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main()
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