Square block ciphers algorithm is the data encryption standard AES algorithm predecessor, the size of block, masterkey and round keys of it are all 128-bit. This article employs a four round distinguisher. Through the distinguisher find out that the third cipher of Square can use some few constants under conditions,to decrease the attack' s computations. Use this distinguisher to accomplish a meeting -in-middle attack for five-round Square. This attack is faster than other attacks at the expense of an increase in the complexities of memory and precomputation. The attack's precomputation stage of time complexity is 234 and space complexity is 272, time complexity of the attack is 272.%Square分组密码算法是美国数据加密标准AES算法的前身,它的分组长度、主密钥长度和轮密钥长度都是128比特.文中给出了一个4轮的Square区分器.通过这个区分器找到Square第三轮的密文可以在某些条件下用比较少的参数来表示,减少攻击的运算量.运用这个区分器成功地实现了对5轮Square的中间相遇攻击.这个攻击比其他的攻击的准备阶段和空间复杂度在花费上都少,攻击的先前准备阶段的时间复杂度为234,空间复杂度为272,攻击的时间复杂度为272.
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