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[TJCTF2016]One-Time Subtraction – 20

問題:

I encrypted this flag with a one time subtraction, but myfriend says it's not secure because my key is only one byte. Canyou check if this is secure?

Cipher.txt: 241 231 224 241 227 248 173 235 176 220 223 246241 176 220 174 240 220 235 173 241 220 176 235 173 242 228 229 250135

解法:

數字堆中有2個241, 正好對應 tjctf 中的t

所以可以得出 241 = t

而以數字加減加密文字的很可能是使用ascii

在ascii中, t的值為116

241 - 116 = 125

跟據這 , 我得出以下算法

{TJCTF2016}[Cryptography]One-Time Subtraction - 20

tjctf{0n3_byt3_1s_n0t_3n0ugh}

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