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[原创]DASCTF复盘之密码学(2023.07)
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发表于: 2024-1-29 19:24 3574
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DASCTF复盘之密码学(2023.07)
DASCTF 2023年7月 密码学题解共3题
快过年了闲下来发现一点存货,遂记录之。
同期题目 DASCTF逆向2023.07
ezRSA
RSA is not easy, huh?
这是两道RSA的缝合题。
第一关:知道P^(Q>>16),要分解N。
第二关:知道前后缀、密文以及带前后缀的密文,求明文。
一开始注意到P的高位字节已知,想用coppersmith求,但发现未知的位数太多,遂放弃。
从第0位一直到第495位每一位的组合只有两种情况,并且可以部分检验(l是部分bit长):
Pˉ∗Qˉ(mod2l)=P∗Q(mod2l)=N(mod2l)
直接进行一个深度优先的搜索。
Q的低16位完全未知,只能纯粹枚举。
然后是一个明文相关的板子题,使用多项式求gcd的方式求解,明文的长度需要枚举哈。
注意这里有一点比较坑,n求解之后长度只有1020完全不够,需要加上N,卡了我很久(如果加上2*N长度就超过了)。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 | from sage. all import * from Crypto.Util.number import * N = 75000029602085996700582008490482326525611947919932949726582734167668021800854674616074297109962078048435714672088452939300776268788888016125632084529419230038436738761550906906671010312930801751000022200360857089338231002088730471277277319253053479367509575754258003761447489654232217266317081318035524086377 gift = 8006730615575401350470175601463518481685396114003290299131469001242636369747855817476589805833427855228149768949773065563676033514362512835553274555294034 cn = 14183763184495367653522884147951054630177015952745593358354098952173965560488104213517563098676028516541915855754066719475487503348914181674929072472238449853082118064823835322313680705889432313419976738694317594843046001448855575986413338142129464525633835911168202553914150009081557835620953018542067857943 def nextP(Pstr, Qstr): l = len (Pstr) if l = = 512 : P = int (Pstr, 2 ) Q = int (Qstr, 2 ) if P * Q = = N: print (f "P={P}\nQ={Q}" ) elif l = = 0 : assert ( len (Qstr) = = 16 ) Qbar = int (Qstr, 2 ) b = (gift >> l) & 1 if b = = 0 : # Pbar[l] == Qbar[16+l] nextP( '1' , '1' + Qstr) nextP( '0' , '0' + Qstr) else : # Pbar[l] == ~Qbar[16+l] nextP( '1' , '0' + Qstr) nextP( '0' , '1' + Qstr) else : Pbar = int (Pstr, 2 ) # Pbar = P % (2**l) Qbar = int (Qstr, 2 ) # Qbar = Q % (2**(l+16)) if (Pbar * Qbar) % ( 2 * * l) = = N % ( 2 * * l): b = (gift >> l) & 1 if b = = 0 : nextP( '1' + Pstr, '1' + Qstr) nextP( '0' + Pstr, '0' + Qstr) else : nextP( '1' + Pstr, '0' + Qstr) nextP( '0' + Pstr, '1' + Qstr) for Qlow in range ( 1 << 16 , 1 << 17 ): Qbar = bin (Qlow)[ 3 :] # 0b1???????????????? nextP('', Qbar) e = 11 P = 8006847171912577069085166877758626954304824756138758266557706391662987806065132448544117840031499707938227955094109779732609035310252723066470330862622641 Q = 9366986529377069783394625848920106951220134111548343265311677163992169555436421569730703291128771472885865288798344038000984911921843088200997725324682297 Phi = (P - 1 ) * (Q - 1 ) D = int (inverse_mod(e, Phi)) n = pow (cn, D, N) #print(len(bin(n)[2:])) # 1020 < 1022, not qualified, have to add N # n + 2*N will produce len=1024 n + = N #print(f'n={n}') ################### prefix = b "dasctf{" suffix = b "}" n = 83410392685813224685786027640778560521035854332627839979281105731457044069408118952629284089869335506983096270269822559619624906180108256504440296527471536363057103101146262613593336072556587341466840510200003498265457285439149541137127199088938421905041387224795918868443175561632999479925818053898100117419 e = 11 c1 = 69307306970629523181683439240748426263979206546157895088924929426911355406769672385984829784804673821643976780928024209092360092670457978154309402591145689825571209515868435608753923870043647892816574684663993415796465074027369407799009929334083395577490711236614662941070610575313972839165233651342137645009 c2 = 46997465834324781573963709865566777091686340553483507705539161842460528999282057880362259416654012854237739527277448599755805614622531827257136959664035098209206110290879482726083191005164961200125296999449598766201435057091624225218351537278712880859703730566080874333989361396420522357001928540408351500991 ################### def rsa_cpa(c1, c2, m0, e, n): x = PolynomialRing(Zmod(n), 'x' ).gen() shift = 8 * len (suffix) g1 = x * * e - c1 g2 = (x * 2 * * shift + m0) * * e - c2 while g2: g1, g2 = g2, g1 % g2 f = g1.monic() return int ( - f[ 0 ]) ################### # bruteforce message lenth for msglen in range ( 10 , 50 ): m0 = bytes_to_long(prefix + b '\x00' * msglen + suffix) m = rsa_cpa(c1, c2, m0, e, n) try : flag = long_to_bytes(m).decode() assert msglen = = len (flag) print (prefix.decode() + flag + suffix.decode()) except : continue # dasctf{C0pper_Sm1th_Mak3s_T1ng5_Bet4er} |
dasctf{C0pper_Sm1th_Mak3s_T1ng5_Bet4er}
ezDHKE
DHKE is easy, huh?
Diffie Hellman密钥交换的模数任选,令 p=gk−1,有
gx≡gxmodk(modp)。
所以
loggCx=logggxmodk=x
loggCy=logggymodk=y
(k取的比较大)。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | from Crypto.Util.number import * from Crypto.Cipher import AES from hashlib import sha256 from random import randbytes, getrandbits #P = 0 #for i in range(1024, 2048, 1): # P = 2 ** i - 1 # if (isPrime(P)): # print(f'P = 2 ** {i} = {P}') # #Cx = int(input('Cx = ')) #Cy = int(input('Cy = ')) P = 10407932194664399081925240327364085538615262247266704805319112350403608059673360298012239441732324184842421613954281007791383566248323464908139906605677320762924129509389220345773183349661583550472959420547689811211693677147548478866962501384438260291732348885311160828538416585028255604666224831890918801847068222203140521026698435488732958028878050869736186900714720710555703168729087 Cx = 2630067950774186753620494941440064332775169901411586929749140451534366077148540411056833268138794225613491484428089108856509716125091901931563907385325940424977611835564222299095831878942161358635646625867890688 Cy = 18014398509481984 x0 = len ( bin (Cx)) - 3 y0 = len ( bin (Cy)) - 3 key = sha256(long_to_bytes( pow ( 2 , x0 * y0, P))).digest() iv = b "dasctfdasctfdasc" aes = AES.new(key, AES.MODE_CBC, iv) enc = b '\x9d\n\x8f\xb1\xb3YO\x94;1\xb1r\xa0f\x015K>\xb4\xe0t\x04\xdd\xb9\x12\x9a[I\xc6\xd1\xdc\xb4\xbbj1\x1b\xde\xcd\xc2\x18\xd408CCM\xa5\xcc' flag = aes.decrypt(enc) print (flag) |
DASCTF{eefa6be7-57e6-4a48-99d6-936feff93108}
ezAlgebra
Algebra is not easy!
算法如下:
取3个质数pqr,p长512bit,q长256bit,r长256bit,乘积为n,n给定。
随机摇一个32bit的t,输出密文:
c1=1997+(p+t)+(p+t)2+(p+t)3+(p+t)4(modn)
c2=1997+(mt)+(mt)2+...+(mt)19(modq)
c3=1997+(m+t)+(m+t)2+...+(m+t)19(modq)
注意到
f(t)=1997+t+t2+t3+t4−c1=0(modp)
可以求小根,根据coppersmith方法有
t0<=21nδβ2−ϵ,ϵ=γβ<=71β,p>=nβ
δ是次数,取4。
β就是p的估计,p512bit,取0.5。
t有32bit,n有1024bit,大约解得γ>=16.5,取17。
得到t后,带入f(t)的公式和n作gcd,得到p,将n缩为qr的乘积。
令
g1(m)=1997+(mt)+(mt)2+...+(mt)19−c2
g2(m)=1997+(m+t)+(m+t)2+...+(m+t)19−c3
现在,在Z/nZ[x]寻找g1和g2的线性组合,使得其次数尽可能小。
g1ˉ(m)=g1(m)−g2(m)
g2ˉ(m)=g2(m)−mg1ˉ(m)
如果g1,g2同次数且都是首一多项式,进行该过程后次数均递减。
不断地进行这个过程,最后要么出现g1为常数且是q的倍数,
要么在化归首一的时候出现无逆元的最高次系数。(因为g1,g2都是q的倍式)
无论哪种情况,将其和n作gcd得到q和r。
有了q之后,在Z/pZ[x]上求g2和g3的gcd,此时一定可以找到一次式m+m0。
然后,令m=−m0+iq爆破明文,当i取到24bit的时候得到了flag。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 | from sage. all import * from Crypto.Util.number import * n = 119156144845956004769507478085325079414190248780654060840257869477965140304727088685316579445017214576182010373548273474121727778923582544853293534996805340795355149795694121455249972628980952137874014208209750135683003125079012121116063371902985706907482988687895813788980275896804461285403779036508897592103 c1 = 185012145382155564763088060801282407144264652101028110644849089283749320447842262397065972319766119386744305208284231153853897076876529326779092899879401876069911627013491974285327376378421323298147156687497709488102574369005495618201253946225697404932436143348932178069698091761601958275626264379615139864425 c2 = 722022978284031841958768129010024257235769706227005483829360633993196299360813 c3 = 999691052172645326792013409327337026208773437618455136594949462410165608463231 # step1: get t, p t = Zmod(n)[ 't' ].gen() f1 = 1997 + t + t * * 2 + t * * 3 + t * * 4 - c1 # % p (p>=n**beta) t0 = f1.small_roots(beta = 0.5 , epsilon = 0.5 / 17 )[ 0 ] assert len ( bin (t0)[ 2 :]) = = 32 #print(f"t = {t0}") p0 = gcd( int (f1(t0)), n) assert len ( bin (p0)[ 2 :]) = = 512 and n % p0 = = 0 #print(f"p = {p0}") t = 2915836867 p = 12674045065380963936369006640913707206092165254372327547575287589116533343005456615635388048683828685030860153531390706945709086518510926980644275915726413 # step2: get q, r n = n / / p m = Zmod(n)[ 'm' ].gen() # n = qr f2 = 0 * m + 1997 - c2 f3 = 0 * m + 1997 - c3 for i in range ( 1 , 20 ): f2 + = (t * m) * * i f3 + = (t + m) * * i f2, f3 = f2.monic(), f3.monic() try : while True : f2 = f2 - f3 f2 = f2.monic() f3 = f3 - m * f2 f3 = f3.monic() except : a = int ( list (f2)[ - 1 ]) q = gcd(a, n) if q = = 1 : a = int ( list (f3)[ - 1 ]) q = gcd(a, n) assert len ( bin (q)[ 2 :]) = = 256 and n % q = = 0 r = n / / q assert len ( bin (r)[ 2 :]) = = 256 and n % r = = 0 # print(f"q = {q}") # print(f"r = {r}") q = 87038069032840052005520908272237788908169043580221040711149494083975743478969 r = 108016959681605076830977486416860687442307791371372085930591884056258926759899 # step3: get m m = Zmod(q)[ 'm' ].gen() f2 = 0 * m + 1997 - c2 f3 = 0 * m + 1997 - c3 for i in range ( 1 , 20 ): f2 + = (t * m) * * i f3 + = (t + m) * * i while f3: f2, f3 = f3, f2 % f3 fm = f2.monic() m0 = int ( - fm[ 0 ]) #print(f"m = {m0}") m = 56985796272753226120469211992443340429346162287195965942430959147227534853120 assert (m < q) for _ in range ( 2 * * 24 ): try : print (long_to_bytes(m).decode()) break except : m + = q continue #length of flag: 35 #dasctf{ShangPoXiaPoYaSiLeYiQianDuo} |
dasctf{ShangPoXiaPoYaSiLeYiQianDuo}
如果你知道这个梗,甚至可以猜出flag
完结撒花
[招生]科锐逆向工程师培训(2024年11月15日实地,远程教学同时开班, 第51期)
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