[N1CTF 2022] solve_pow,baby_N1ES

周六作了两个N1CTF感觉很难就没做，有些东西只是不懂，不是很难。

solve_pow

这个题就是其一

题目会给出一道2乘幂的题

POW: 2^(2^168650911) mod 14410422756791856006977221261653848398173123908149127966358804284026606184669 = ?

一开始想最后是取模，根据费小有a**(p-1)=1 (mod p)，然后用公式求解

powmod(2,pow(2,bit,p-1),p)
#4661732238801194169405982400350387428895918512377690837157075128714991112731

但是这个事有点麻烦，p不是素数,如果乘幂又太大了，后来看别人作的，用了bit_set这个函数，原来都没听说过，因为底是2，直接置bit位就行了

所以正确的解法是

powmod(2,bit_set(0,bit),p)
#36808033546212382710607382805842490500296061506256519355047811358611503260

不过只是当时卡在这了，不有另外一个方法，不用这个函数一样

powmod(2,1<
#36808033546212382710607382805842490500296061506256519355047811358611503260

 

baby_N1ES

这个题都没看到，其实这个题很简单，而且还有更简单的解法。

原题

from N1ES import N1ES
import base64
key = "wxy191iss00000000000cute"
n1es = N1ES(key)
flag = "N1CTF{*****************************************}"
cipher = n1es.encrypt(flag)
print base64.b64encode(cipher) # HRlgC2ReHW1/WRk2DikfNBo1dl1XZBJrRR9qECMNOjNHDktBJSxcI1hZIz07YjVx

和一个N1ES.py的库

# -*- coding: utf-8 -*-
def round_add(a, b):
    f = lambda x, y: x + y - 2 * (x & y)
    res = ''
    for i in range(len(a)):
        res += chr(f(ord(a[i]), ord(b[i])))
    return res
 
 
def permutate(table, block):
    return list(map(lambda x: block[x], table))
 
 
def string_to_bits(data):
    data = [ord(c) for c in data]
    l = len(data) * 8
    result = [0] * l
    pos = 0
    for ch in data:
        for i in range(0, 8):
            result[(pos << 3) + i] = (ch >> i) & 1
        pos += 1
    return result
 
 
s_box = [54, 132, 138, 83, 16, 73, 187, 84, 146, 30, 95, 21, 148, 63, 65, 189, 188, 151, 72, 161, 116, 63, 161, 91, 37,
         24, 126, 107, 87, 30, 117, 185, 98, 90, 0, 42, 140, 70, 86, 0, 42, 150, 54, 22, 144, 153, 36, 90, 149, 54, 156,
         8, 59, 40, 110, 56, 1, 84, 103, 22, 65, 17, 190, 41, 99, 151, 119, 124, 68, 17, 166, 125, 95, 65, 105, 133, 49,
         19, 138, 29, 110, 7, 81, 134, 70, 87, 180, 78, 175, 108, 26, 121, 74, 29, 68, 162, 142, 177, 143, 86, 129, 101,
         117, 41, 57, 34, 177, 103, 61, 135, 191, 74, 69, 147, 90, 49, 135, 124, 106, 19, 89, 38, 21, 41, 17, 155, 83,
         38, 159, 179, 19, 157, 68, 105, 151, 166, 171, 122, 179, 114, 52, 183, 89, 107, 113, 65, 161, 141, 18, 121, 95,
         4, 95, 101, 81, 156, 17, 190, 38, 84, 9, 171, 180, 59, 45, 15, 34, 89, 75, 164, 190, 140, 6, 41, 188, 77, 165,
         105, 5, 107, 31, 183, 107, 141, 66, 63, 10, 9, 125, 50, 2, 153, 156, 162, 186, 76, 158, 153, 117, 9, 77, 156,
         11, 145, 12, 169, 52, 57, 161, 7, 158, 110, 191, 43, 82, 186, 49, 102, 166, 31, 41, 5, 189, 27]
 
 
def generate(o):
    k = permutate(s_box, o)
    b = []
    for i in range(0, len(k), 7):
        b.append(k[i:i + 7] + [1])
    c = []
    for i in range(32):
        pos = 0
        x = 0
        for j in b[i]:
            x += (j << pos)
            pos += 1
        c.append((0x10001 ** x) % (0x7f))
    return c
 
 
class N1ES:
    def __init__(self, key):
        if (len(key) != 24 or isinstance(key, bytes) == False):
            raise Exception("key must be 24 bytes long")
        self.key = key
        self.gen_subkey()
 
    def gen_subkey(self):
        o = string_to_bits(self.key)
        k = []
        for i in range(8):
            o = generate(o)
            k.extend(o)
            o = string_to_bits([chr(c) for c in o[0:24]])
        self.Kn = []
        for i in range(32):
            self.Kn.append(map(chr, k[i * 8: i * 8 + 8]))
        return
 
    def encrypt(self, plaintext):
        if (len(plaintext) % 16 != 0 or isinstance(plaintext, bytes) == False):
            raise Exception("plaintext must be a multiple of 16 in length")
        res = ''
        for i in range(len(plaintext) / 16):
            block = plaintext[i * 16:(i + 1) * 16]
            L = block[:8]
            R = block[8:]
            for round_cnt in range(32):
                L, R = R, (round_add(L, self.Kn[round_cnt]))
            L, R = R, L
            res += L + R
        return res

先读题，就是个分段加密，每次原来的左与Kn异或然后左右互换，作32轮。由于Kn是固定的，而且加密是异或，所以round_add并不需要处理，只要反过来作个decrypt就行了

这有个小坑，python 2的程序需要改一下

# -*- coding: utf-8 -*-
def round_add(a, b):   #xor
    f = lambda x, y: x + y - 2 * (x & y)
    res = b''
    for i in range(len(a)):
        #res += chr(f(ord(a[i]), ord(b[i])))
        res += bytes([f(a[i], b[i])])
    return res
 
 
def permutate(table, block):
    return list(map(lambda x: block[x], table))
 
 
def string_to_bits(data):
    #data = [ord(c) for c in data]
    data = [c for c in data]
    l = len(data) * 8
    result = [0] * l
    pos = 0
    for ch in data:
        for i in range(0, 8):
            result[(pos << 3) + i] = (ch >> i) & 1
        pos += 1
    return result
 
 
s_box = [54, 132, 138, 83, 16, 73, 187, 84, 146, 30, 95, 21, 148, 63, 65, 189, 188, 151, 72, 161, 116, 63, 161, 91, 37,
         24, 126, 107, 87, 30, 117, 185, 98, 90, 0, 42, 140, 70, 86, 0, 42, 150, 54, 22, 144, 153, 36, 90, 149, 54, 156,
         8, 59, 40, 110, 56, 1, 84, 103, 22, 65, 17, 190, 41, 99, 151, 119, 124, 68, 17, 166, 125, 95, 65, 105, 133, 49,
         19, 138, 29, 110, 7, 81, 134, 70, 87, 180, 78, 175, 108, 26, 121, 74, 29, 68, 162, 142, 177, 143, 86, 129, 101,
         117, 41, 57, 34, 177, 103, 61, 135, 191, 74, 69, 147, 90, 49, 135, 124, 106, 19, 89, 38, 21, 41, 17, 155, 83,
         38, 159, 179, 19, 157, 68, 105, 151, 166, 171, 122, 179, 114, 52, 183, 89, 107, 113, 65, 161, 141, 18, 121, 95,
         4, 95, 101, 81, 156, 17, 190, 38, 84, 9, 171, 180, 59, 45, 15, 34, 89, 75, 164, 190, 140, 6, 41, 188, 77, 165,
         105, 5, 107, 31, 183, 107, 141, 66, 63, 10, 9, 125, 50, 2, 153, 156, 162, 186, 76, 158, 153, 117, 9, 77, 156,
         11, 145, 12, 169, 52, 57, 161, 7, 158, 110, 191, 43, 82, 186, 49, 102, 166, 31, 41, 5, 189, 27]
 
 
def generate(o):
    k = permutate(s_box, o)
    b = []
    for i in range(0, len(k), 7):
        b.append(k[i:i + 7] + [1])
    c = []
    for i in range(32):
        pos = 0
        x = 0
        for j in b[i]:
            x += (j << pos)
            pos += 1
        c.append((0x10001 ** x) % (0x7f))
    return c
 
 
class N1ES:
    def __init__(self, key):
        #if (len(key) != 24 or isinstance(key, bytes) == False):
        #    raise Exception("key must be 24 bytes long")
        self.key = key
        self.gen_subkey()
 
    def gen_subkey(self):
        o = string_to_bits(self.key)
        k = []
        for i in range(8):
            o = generate(o)
            k.extend(o)
            #o = string_to_bits([chr(c) for c in o[0:24]])
            o = string_to_bits([c for c in o[0:24]])
        self.Kn = []
        for i in range(32):
            #self.Kn.append(map(chr, k[i * 8: i * 8 + 8]))
            self.Kn.append(k[i * 8: i * 8 + 8])
        return
 
    def encrypt(self, plaintext):
        #if (len(plaintext) % 16 != 0 or isinstance(plaintext, bytes) == False):
        #    raise Exception("plaintext must be a multiple of 16 in length")
        res = b''
        for i in range(len(plaintext) // 16):
            block = plaintext[i * 16:(i + 1) * 16]
            L = block[:8]
            R = block[8:]
            for round_cnt in range(32):
                L, R = R, (round_add(L, self.Kn[round_cnt]))
            L, R = R, L
            res += L + R
        return res
 
    def decrypt(self, plaintext):
        #if (len(plaintext) % 16 != 0 or isinstance(plaintext, bytes) == False):
        #    raise Exception("plaintext must be a multiple of 16 in length")
        res = b''
        for i in range(len(plaintext) // 16):
            block = plaintext[i * 16:(i + 1) * 16]
            L = block[:8]
            R = block[8:]
            for round_cnt in range(32):
                L, R = (round_add(R, self.Kn[round_cnt])), L
            L, R = R, L            
            res += L + R
        return res
 

然后直接解密

from N1ES import N1ES
import base64
 
key = b"wxy191iss00000000000cute"
n1es = N1ES(key)
t = base64.b64decode('HRlgC2ReHW1/WRk2DikfNBo1dl1XZBJrRR9qECMNOjNHDktBJSxcI1hZIz07YjVx')
print(n1es.decrypt(t))
#b'N1CTF{F3istel_n3tw0rk_c4n_b3_ea5i1y_s0lv3d_/--/}'

不过今天看了个WP，还有更简单的方法，这个东西是个循环，直接再加密，而且3次就循环，太容易了

for i in range(3):
    t = n1es.encrypt(t)
    print(t)

没作，也没保存确实可惜了，只是那个checkin没作出来。