18/09/28-3-bugku-Reverse-easycrack-100 (NJCTF)

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Title Link: Https://pan.baidu.com/s/12RGpSCcRVSu-tyreTqecaA
Extract code: 9XYV

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Java Layer Analysis

1) The MessageMe method first obtains the package name, and then iterates with 51 or.

1      PublicString MessageMe () {2String v3 = "";3         intV4 = 51;4string[] V1 = This. Getapplicationcontext (). Getpackagename (). split ("\ \");5         Char[] V6 = V1[v1.length-1].tochararray ();6         intV7 =v6.length;7         intv5;8          for(v5 = 0; v5 < V7; + +v5) {9V4 ^=V6[V5];TenV3 = v3 + ((Char) (v4)) ; One         } A  -         returnv3; -}

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2) ParseText is the native layer function, and the passed in parameter is the input string.

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Natice Layer Analysis

1)

1V4 = (* (int(__cdecl *) (int,Const Char*)) (* (_dword *) A1 + -)) (A1,"com/njctf/mobile/easycrack/mainactivity");2V5 = (* (int(__cdecl *) (int,int,Const Char*)) (* (_dword *) v3 + the)) (V3, V4,"MessageMe");3V6 = _jnienv::callobjectmethod (A1, A2, V5, (unsignedint)"() ljava/lang/string;");

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The returned string is obtained here for the MessageMe method of the callback Java layer.

2)

The string that is passed into the native layer function is followed or manipulated by the string returned by the MessageMe method.

3)

Then according to multiple%256 can be analyzed is the RC4 encryption, the key is "I_am_the_key", clear text is 2) after the operation of the results, ciphertext for " C8e4ef0e4dcca683088134f8635e970eead9e277f314869f7ef5198a2aa4 ".

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Reverse the algorithm, write out the decryption script

1 defre0 ():2     """Reverse Algorithm"""3STR0 ="Easycrack"4STR1 = [Ord (i) forIinchSTR0]5num = 516 7      forIinchRange (len (str1)):8Num ^=Str1[i]9Str1[i] =num;TenSTR1 *= 10 One     returnstr1 A  - defRC4 (data,key): -     """RC4 Algorithm""" thex =0 -box = Range (256) -      forIinchRange (256): -x = (x + box[i] + ord (key[i% len (key)))% 256 +Box[i], box[x] =box[x], Box[i] -x = y =0 +out = [] A      forCharinchData: atx = (x + 1)% 256 -y = (y + box[x])% 256 -BOX[X], box[y] =Box[y], box[x] -Out.append (Ord (char) ^ box[(box[x] + box[y])% 256])) -     return "'. Join (out) -  in """Main algorithm""" -res ='C8E4EF0E4DCCA683088134F8635E970EEAD9E277F314869F7EF5198A2AA4' toKey ='I_am_the_key' +ciphertext = Res.decode ('Hex') -STR0 =re0 () theSTR1 =RC4 (Ciphertext,key) *STR2 = [Ord (i) forIinchSTR1] $Flag ="'Panax Notoginseng  forIinchRange (len (str1)): -Str2[i] ^=Str0[i] theFlag + =chr (Str2[i]) + PrintFlag

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18/09/28-3-bugku-Reverse-easycrack-100 (NJCTF)