SPN，其中文是代换-置换网络，其实就是将一个比特串主要经过两种变换，分别是代换和置换，得到另一个比特串，从而实现加密的效果。但是前提是，可以经过类似的手段将这个比特串还原，这也就是解密了。(It's a substitution permutation network, in fact, a bit string is mainly transformed by two kinds, which are substitution and substitution, and obtain another bit string, so as to achieve the effect of encryption. But the premise is that this bit string can be restored by a similar means, which is the decryption.)