Abstract: By utilizing the balanceable difference weight of high order Data Dependent Operations (DDO) and the high probability differentials of SPN structures, two 5-round related-key differentials of Eagle-128 are constructed. A full round related-key rectangle distinguisher of Eagle-128 is constructed by connecting two 5-round related-key differentials, and a related-key rectangle attack is proposed on the cipher to recover 64 bit of the master key. The corresponding data complexity is about 281.5 related-key chosen-plain-text, the computation complexity is about 2106.7 encryptions of the cipher, and the storage complexity is about 250 Byte of storage space. The success rate of the attack is about 0.954. The analysis results show that the practical length of Eagle-128s master key is 192 bit.