Created
April 26, 2020 17:04
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Solver for YOU wa SHOCKWAVE (re 250) from PlaidCTF 2020
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from struct import pack, unpack | |
# reversed from the challenge | |
def zz_helper(x, y, z): | |
if y > z: | |
return [1, z - x] | |
a, b = zz_helper(y, x + y, z) | |
if b >= x: | |
return [2 * a + 1, b - x] | |
return [2 * a, b] | |
def zz(x): | |
return zz_helper(1, 1, x)[0] | |
# inverse transformation | |
def rev_zz(x): | |
if x == 1: | |
return 0 | |
x //= 2 | |
res = 0 | |
a = 1 | |
b = 1 | |
i = 0 | |
while (1 << i) <= x: | |
if x & (1 << i): | |
res += b | |
a, b = b, a+b | |
i += 1 | |
return res | |
for i in range(1000): | |
assert rev_zz(zz(i)) == i | |
assert zz(rev_zz(zz(i))) == zz(i) | |
check_data = [ | |
[2, 5, 12, 19, 3749774], | |
[2, 9, 12, 17, 694990], | |
[1, 3, 4, 13, 5764], | |
[5, 7, 11, 12, 299886], | |
[4, 5, 13, 14, 5713094], | |
[0, 6, 8, 14, 430088], | |
[7, 9, 10, 17, 3676754], | |
[0, 11, 16, 17, 7288576], | |
[5, 9, 10, 12, 5569582], | |
[7, 12, 14, 20, 7883270], | |
[0, 2, 6, 18, 5277110], | |
[3, 8, 12, 14, 437608], | |
[4, 7, 12, 16, 3184334], | |
[3, 12, 13, 20, 2821934], | |
[3, 5, 14, 16, 5306888], | |
[4, 13, 16, 18, 5634450], | |
[11, 14, 17, 18, 6221894], | |
[1, 4, 9, 18, 5290664], | |
[2, 9, 13, 15, 6404568], | |
[2, 5, 9, 12, 3390622] | |
] | |
m = [] | |
for (i, j, k, l, target) in check_data: | |
row = [0] * 21 + [target] | |
row[i] = 1 | |
row[j] = 1 | |
row[k] = 1 | |
row[l] = 1 | |
m.append(row) | |
m.append([1]*21 + [5803878]) | |
for i in range(21): | |
# select | |
for j in range(i, 21): | |
if m[j][i]: | |
m[j], m[i] = m[i], m[j] | |
break | |
else: | |
raise RuntimeError('fail') | |
# clear others | |
for j in range(21): | |
if j != i and m[j][i]: | |
for k in range(22): | |
m[j][k] ^= m[i][k] | |
for row in m: | |
print(row) | |
flag = b'' | |
for i in range(21): | |
r = rev_zz(m[i][-1]) | |
assert(zz(r) == m[i][-1]) | |
flag += pack('>H', r) | |
assert len(flag) == 42 | |
print(flag.decode()) |
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